Severe storms likely early Saturday morning - what to expect

[ Updated at 5:00PM - timing of storms, increasing flooding risk, updated graphics ]

A surface low pressure system with a connecting cold front is situated over north central Texas and southern Oklahoma. This low will deepen and dig toward Arkansas Friday night before it begins lifting out toward the northeast. Most of the models agree that the low will move right up the Mississippi River. This may serve to prolong the period of thunderstorms some, as the line will not move east as fast while the low pressure center is moving parallel to it. 

Detailed outlook for the metro

Thanks to a strong surface high pressure system situated to our east, winds are fairly strong and from the south. In fact, a  Wind Advisory has been posted for the entire area as gusts could reach 25-30 mph this afternoon and 30-40 mph tonight. This will allow abundant moisture from the Gulf of Mexico to reach the Mid-South. With abundant moisture and a strong front to lift the moist air, a line of convective storms will form ahead of the cold front in eastern Texas and propagate through northern Louisiana and Arkansas before crossing the Mississippi River into western Tennessee and Mississippi. The main concern with this event will be the squall line as most models show little in the way of discrete storms ahead of the line. Models have focused in on a potential severe weather window of 3:00-9:00am Saturday morning.

The high-resolution NAM3 model forecast radar valid from midnight tonight through 6pm Saturday gives us an idea of what to expect as the line crosses Memphis proper in the 5-9am window. (WeatherModels.com)

Flooding threat

Over the past couple weeks, we have received 1.5-3" inches of rain across the area. This means our ground is pretty wet and a Flash Flood Watch has been issued due to the expectation of additional heavy rain. Ahead of the line, showers are possible, which will only aid in saturating the ground. However, this system may produce 2-3" of rainfall by mid-morning Saturday. A significant concern with this event will be flooding. Low lying areas should be avoided or monitored closely to ensure your safety. It is especially important that you exercise extreme caution when driving at night in or after heavy rain. The condition of the roadways is difficult to gauge in the dark! Remember: Turn Around, Don’t Drown!

Straight-line wind threat

The wind energy associated with this system is very significant. With surface wind gusts over 30 mph and wind of 80-90 mph at just 5,000' above us, the stage is set for a potential damaging event as wind with this system could reach hurricane strength in spots. That means we may be seeing surface wind on the magnitude of 70+ mph! A Severe Thunderstorm Warning with winds this high should be taken as seriously as a Tornado Warning! Take action and take cover in an interior room on the lowest floor, away from windows, if a Severe Thunderstorm Warning is issued. With saturated ground, it won’t take much for trees or possibly power poles to be knocked over, so make sure to charge all devices are charging overnight to ensure you can still receive weather updates and other information if the power goes out.

Tornado threat

The tornado threat with this event appears to be limited to a low potential for spin-ups, however these cannot be ruled out. Higher tornado threats exist to our southwest in southern AR, northern Louisiana, and western Mississippi. However, with storms moving at highway speeds due to the magnitude of the wind, even a spin-up tornado can move very quickly. Be ready to act if a warning is issued!

We also highly recommend you have multiple ways of receiving warning information. That includes commercial TV and radio, NOAA Weather Radio, Wireless Emergency Alerts on your cell phone, as well as a configurable mobile warning app like SW+ Alerts in the MemphisWeather.net app. SW+ Alerts is a low-cost, highly-customizable method of receiving warnings for specific locations you program in. It will even wake you up for the most severe warnings overnight! Learn more and download the app at App.MemphisWeather.net.

After the storm

Despite all the excitement Saturday morning, Saturday afternoon looks much better! There may be trees down or power outages, but weather conditions themselves improve. A few straggling showers are possible and it will remain breezy, but clouds will begin to decrease into the evening hours with temperatures hanging out in the mid 50s during the afternoon before falling into the 30s overnight. 

Paige Davide
MWN Meteorologist Intern

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: MemphisWeather.net on the mobile web or via the MWN mobile app
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

MWN is a NOAA Weather Ready Nation Ambassador

Meteorologist Erik Proseus is an NWA Digital Seal Holder

Storm recap: wind damage in Bartlett, TN Sunday morning (4/30/2017)

As many folks were in worship services this past Sunday morning, a series of storms moved across the metro with high wind and torrential rain. Two in particular hit portions of Bartlett very hard with severe winds.

To set the stage, a great deal of rain fell over the previous week, including over 3 inches the previous weekend, another storm that dropped more than an inch mid-week, and finally another inch-plus over the weekend on which the storm damage occurred, leaving the ground saturated and vulnerable to falling trees in the high wind.

Early morning storm

The first round of strong storms in Bartlett occurred between 8:30-9:00pm.  Here's a radar loop that ends around 8:15, just prior to the storms reaching Bartlett.

8:20 AM - The main line of heavy rain and thunderstorms is now pushing through Memphis proper. More to follow the line too. #memstorm /AH pic.twitter.com/hTeBKoE5zo

— MemphisWeather.net (@memphisweather1) April 30, 2017

Those storms produced straight-line wind that uprooted 5-6 mature trees, including the very large oak shown below, near and just southwest of the intersection of Billy Maher Road and Old Brownsville on the north side of Bartlett. No warning was in effect at the time.

An uprooted tree that fell at the corner of Billy Maher and Old Brownsville in north Bartlett during the 8:30am storm. There were several others in the same area that also feel during that storm. Photo credit: Erik Proseus.

Late morning storm

Behind that storm, another severe storm embedded in a larger area of heavy rain raced north at 55 mph through east Memphis and into Bartlett between 11:30-12:00pm. This storm was warned in advance for the likelihood of high wind. Here is the radar wind velocity loop from that storm, which shows the light blue colors (wind of about 60 mph) moving over Bartlett at 11:40am:

60+MPH winds pushing north thru Bartlett, TN right now. Take cover! #Tnwx pic.twitter.com/R3Xc75g9oU

— NWS Memphis (@NWSMemphis) April 30, 2017

This storm cause more extensive damage. In fact, I was able to trace an approximate path based on fallen trees from Stage Road and Kirby-Whitten in the middle of Bartlett north to Old Brownsville Road east of Oak Road, a distance of nearly 4 miles. The map below shows the areas where fallen trees and damage occurred, with the most extensive damage on Broadway Street south of Shadowlawn Road, where 4-5 mature, large oak trees were uprooted. In addition, in the Easthill subdivision south of the Broadway damage, multiple large trees were also uprooted, including one that fell over a house, splitting it in two. Just south of the Easthill subdivision, in the wooded Bartlett-Ellendale Cemetery, one large oak tree was snapped off about 30 feet above the ground. Pictures of this damage are shown below.

An approximation of the wind damage path through Bartlett around 11:45am Sunday morning. Highlighted areas are where downed trees and/or damage was noted, with the direction they fell indicated by the arrows.

Photos taken of tree damage during the late morning storms in Bartlett. These photos were taken at Bartlett-Ellendale Cemetery, Easthill, Broadway Street, and Old Brownsville. Photos credit: Erik Proseus.

An analysis of the direction that trees fell (as shown in the map above) and the lack of damage to any structures other than those hit by falling trees indicates that the damage was likely the result of straight line wind, produced by a microburst (or perhaps multiple bursts). A microburst is produced by strong downdrafts from thunderstorms. The fact that the ground was saturated made it possible for lower than normal wind speeds to uproot the trees, although a couple of trees that were snapped well up the trunk indicate that there were some very severe winds in the storms.

Based on damage indicators and degree of damage used by the National Weather Service, in my estimation the straight-line wind associated with each event was likely severe (60 mph or stronger) with roughly 75 mph wind gusts required to uproot trees of the size that were toppled. The trees that were snapped off could have seen wind gusts of 80+ mph. This wind is equivalent to an EF-0 to low-end EF-1 rating had it been a tornado. However, there are no indications from my unofficial assessment that a tornado occurred in these cases.

Tipton County Tornado

Finally, I'll end with the official NWS survey of a tornado that occurred nearly at the same time as the early damage in Bartlett. An EF-0 caused minor damage in Covington, TN, in Tipton County. Here is the NWS survey report:

.Tornado in Covington Tennessee...

Rating: EF0
Estimated Peak Wind: 75 MPH 
Path Length /Statute/: 2.1 miles 
Path Width /Maximum/: 100 yards
Fatalities: None
Injuries: None

Start Date: Apr 30 2017
Start Time: 0846 AM CDT 
Start Location: 1.6 S Covington
Start Lat/Lon: 35.5403/-89.6447

End Date: Apr 30 2017
End Time: 0850 AM CDT
End Location: 0.9 ENE Covington 
End Lat/Lon: 35.5699/-89.6341

SURVEY SUMMARY:
Damage primarily to trees and power poles.
Minor damage to Crestview Middle School. 

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

MWN is a NOAA Weather Ready Nation Ambassador

Meteorologist Erik Proseus is an NWA Digital Seal Holder

Sunday Update on Severe Weather Potential Later Today #memstorm

With one weather-maker in the form of low pressure and a trough having departed this morning, we now focus our eyes to the west where our next chance of rain will arrive from later today. Temperatures should easily make it to the mid 70's with help from peaks of sunshine. That energy will help fuel storms later this afternoon and evening along an approaching cold front. Lets take a closer look at what to expect from storms.

The Storm Prediction Center outlook from 11:30 AM has the entire metro in an Enhanced Risk zone (3 out of 5). The expected threats from this evening's severe weather are mainly straight-line wind above 60 mph (a 30% chance of this occurring within 25 miles of you) and large hail (a 15% chance), though an isolated tornado is also possible (5% chance within 25 miles). Lets shift toward the most likely timing of the event.

HRRR model simulated radar valid at 10pm, showing the main storms over the metro. Graphic courtesy WxBell.

The latest data from our short term, high resolution weather model (the HRRR, above) shows the main line of thunderstorms across the metro at 10 PM tonight. While this is not an exact forecast (it is just one model run among many), it still gives us an idea of approximate arrival of the line of potentially severe storms. Often times frontal systems will be a little bit faster than the model predicts, so it would not be surprising if we saw this set-up an hour earlier. Additionally there is a chance of a few showers this afternoon and a strong isolated storm or two could get going prior to the arrival of the squall line itself (late this afternoon and evening), though that scenario doesn't appear to be as likely as the early afternoon shower chance.

Enjoy the afternoon, but be prepared for scattered showers and thunderstorms by early evening, with the arrival of the main line of storms between 8-11 PM. For those of you headed to the first Grizzles playoff game, be prepared for severe weather to be coming in right around the end of the game and remain weather aware as you depart.

As always, make sure you have multiple sources for warning information and plan accordingly. While this is not expected to be a significant severe weather outbreak, it only takes one severe storm for you to be impacted. We recommend StormWatch+ within the MemphisWeather.net app as one means of receiving precise warnings for your specific location and stay tuned to our social media channels for the latest throughout the day.

William Churchill
MWN Social Media Intern

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

Wednesday update on severe weather potential Thursday evening

As temperatures climb into the lower and mid 80s this afternoon, southerly wind continues to push moist, unstable Gulf air across the area. This pattern will continue right through tomorrow, priming the pump ahead of a strong spring cold front that is expected Thursday evening.

Dry weather is expected through the overnight hours tonight before we start to see a small chance of showers Thursday morning. That chance of showers becomes a chance of thunderstorms in the afternoon, but these will be scattered at best and likely not severe. By evening, we'll be watching as storms over AR increase in intensity as a cold front moves across the state. The storms should merge and develop into a squall line just ahead of the cold front before arriving in the metro.

According to this morning's GFS model, the cold front is nearly overhead at 1am Friday with a large area of thunderstorms accompanying it. Graphic courtesy WxBell Analytics.

The Storm Prediction Center has placed a huge area from the southern Great Lakes into the Mississippi and Ohio River Valleys and the Southern Plains in an Enhanced risk (category 3 of 5) of severe weather with a 45% chance of severe weather within 25 miles of any given location.

Storm timing and impacts

While timing of storms is not certain, it appears the most likely impact from the line moving through will be between 9pm-2am Thursday night. The main hazards will be strong to damaging straight-line wind, small hail, lightning, and torrential rain that could cause some flash flooding. An isolated tornado could spin up in the squall line, but the chances are fairly low and typically these types of tornadoes are weak and short-lived. We can't rule out the chance though.

Plan ahead

We encourage you to plan ahead, starting today, for the severe weather risk. Know where you will be, which can guide your planning. With a good chance of strong wind, secure outdoor objects that could take flight, plan to garage your vehicles if possible to protect from hail and wind-blown objects, and make sure your weather radios have fresh batteries. It's also a good idea to set up StormWatch+ in your MemphisWeather.net mobile app so that you will receive severe weather watches and warnings instantly if and when they are issued by the NWS. A video on how to set that up can be found here. Additional safety tips can be found below.

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

Severe thunderstorms expected this afternoon in the metro

The first in possibly a series of mesoscale convective systems (MCS, or "macho thunderstorm complex") will move through the Mid-South this afternoon. This system is currently in the Ozarks and moving east-southeast towards the region at 50-60 mph.

Radar image at 10:20am. Green square icons in and behind the line are damaging wind reports. Yellow boxes are Severe T'storm Warnings.

The Storm Prediction Center (SPC) has issued a Severe Thunderstorm Watch for the entire metro until 5pm.  They provide probabilities of various severe weather events within the watch box. For this one, there is a 70% chance of 10 or more wind events of 60+ mph, a 50% chance of a 75 mph wind report, a 30% chance of 10 or more 1" hail reports, and only a 10% chance of 2 tornadoes. Thus, the main threat with these storms will be straight-line damaging wind of 50-60 mph.

Severe Thunderstorm Watch in pink. A Flash Flood Watch is also in effect.

SPC has also placed AR and points along the river in west TN under a Moderate Risk for severe weather today. I expect this to be extended eastward a bit later this morning.

A Moderate Risk zone extends from the Ozarks to the Mid-South at mid-morning. This could be extended east by late morning.

The estimated time of arrival for this complex of storms is between 2-4 pm in the metro with rain continuing for a few hours behind the line. The worst of the severe storms will hopefully be done by the time rush hour kicks into full gear, but expect wet streets, including residual ponding or minor flash flooding, and some traffic light failures due to power outages from high wind. If your car windows are rolled down at work, roll them up at lunch time. If you're attending the FESJC golf tournament, find suitable shelter WELL BEFORE the line arrives this afternoon. It will hit quickly and pack a significant punch when it gets here.

With any luck, most of the rain will be done by 7-8 pm tonight, but I make no promises on that. If you have outdoor plans tonight (such as a concert at Snowden Grove or an outdoor party), be prepared for possible rain and some thunder through at least mid-evening.  If you have outdoor objects that are not tied down and you have the ability to move them in or secure them prior to 2pm, we recommend doing that.

Stay with MWN on our social media channels below for the latest info. We make no guarantees of seeing everything we post to Facebook due to the way Facebook filters posts from Fan Pages like ours. Twitter is always a great option. Of course, we also highly recommend multiple ways of getting severe weather warnings, including the MWN apps with StormWatch+. A one-time upgrade fee for StormWatch+ (in the MWN app) will provide peace of mind with instant weather alerts for the locations you care about.

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

Severe weather likely Sunday-Monday - latest information from MWN

A multi-day severe weather event remains on track and appears likely at this time.

If you read the blog yesterday and follow us on social media, you are aware that the Mid-South and Lower Mississippi Valley are facing what could be the most significant severe weather event(s) since spring 2011. (NOTE that I am not comparing this to April/May 2011, just saying that we haven't seen anything like this SINCE then).  I'll handle this day-by-day in an effort to be coherent and for you to glean the most information, bypassing the nitty-gritty details of the state of the atmosphere to focus on threats and possible timing.

At a high-level, the main setup is a developing upper level low of huge proportions setting up over the Plains and moving ever-so-slowly east into the Ohio River Valley the next few days, strong jet stream wind and screaming mid-level wind enhancing storm intensity, and abundant of moisture and instability (especially during peak heating) creating the environment capable of producing severe weather.

Sunday and Sunday Night

The day starts quiet, but becomes stormy pretty quickly - probably by late morning - as the first area of lift associated with an upper-level disturbance brings in enough instability to create thunder. This round may last into mid-afternoon and will probably NOT be severe, but it primes the pump. Main risks: lightning, gusty wind.

A break will probably occur during the late afternoon/evening hours as storms explode over a Moderate Risk area in southwest MO, west AR, and the ArkLaTex. This area could be upgraded to a High Risk tomorrow. During the evening these storms, which will be capable of strong tornadoes, large hail, and high wind, move east and may coalesce into a squall line or MCS (mesoscale convective system) as they move towards the metro. We expect these storms during the overnight hours Sunday into Monday - perhaps late overnight (after midnight). The Moderate Risk currently extends to just west of the metro; we are in a Slight Risk.

Some weakening is expected as they move into our area due to less heating from the sun and more organized line(s) rather than individual supercells, but they definitely still bear watching. Main risks: damaging wind, large hail, isolated tornado, heavy rain, copious lightning. The probability of severe storms in the metro Sunday/Sunday night is 30% according to SPC (see below), with a 45% area just to our west and a 10% risk of significant severe weather (EF-2 tornadoes, 2" hail, 75 mph wind), thus the hatched area in the graphic.

Monday

Monday is the day of biggest concern, particularly areas along and east of the Mississippi River, which includes the entire metro. Once overnight storms move out, another break is expected in the morning, which (with some sun) will recharge the atmosphere. The impressive (or awful if you hate severe weather) dynamics of the atmosphere shift slightly east and are expected to be overhead the Mid-South. A Moderate Risk of severe weather exists along/east of the river, including the metro. By afternoon, perhaps early in the afternoon depending on morning heating, storms will begin firing right over our area. As opposed to Sunday night when storms will probably be more linear, Monday's storms will likely be dangerous supercells, capable of large hail, high straight-line wind, and perhaps strong tornadoes. These storms are most likely Monday afternoon and evening, probably ending by midnight.

According to SPC, the severe weather probabilities Monday are 45% for severe weather and 10%+ for significant severe (EF-2 tornadoes, 2" hail, 75 mph wind).  These probabilities could change. Anything much higher than this and a rare High Risk zone would be required. The potential limiting factor is overnight storms the previous night. If they linger into Monday morning and cloud cover sticks around, the threat could shift just to our east a bit. For now, we're in the Moderate Risk.

Tuesday

The risk on Tuesday appears to be lower than thought yesterday.  It now appears that the main risk of severe weather will be to our east, more towards middle TN and AL, though a few showers or storms are possible Tuesday morning. The threat on this day is fairly uncertain though and will need to be updated as time gets closer. If the cold front that ends all of the severe weather threats comes through early Tuesday, we'll be fine. If it is delayed into the afternoon and more heating can occur, we'll be more at risk.

Flash Flood Potential

A Flash Flood Watch has been issued from 7pm Sunday to 7pm Tuesday for the metro. Multiple rounds of storms, each capable of producing heavy rain, could compound issues related to excess water. Sunday starts with ground and streams fairly dry and capable of taking on some water. By Monday though, any downpours could cause flooding concerns. The Weather Prediction Center's graphic below for projected total rainfall through Tuesday morning shows widespread 2-4" rainfall totals and places that see several storms pass over in that period could see 4-6".

Safety Preparations

Being that this is the best chance of widespread severe weather we have seen in a few years, brushing up on severe weather safety is a necessity. See the graphic below for our suggestions, including what a safety kit should include.

Always understand your risk and know what you will do no matter where you are during the period of potential severe weather. Plan it ahead of time.

Have multiple ways of receiving warnings. Don't count on sirens, especially if you typically have trouble hearing them indoors at your home/work. We highly encourage the use of smartphone apps as one of your "layers" of information. Of course, we highly recommend ours because we built it for you! Get more info on it's capabilities and then download it here.

In the MWN app, the most important thing you can do is purchase (one-time) StormWatch+ severe weather notifications. Program your location(s) and then leave it next to the bed overnight unsilenced. It WILL wake you up if a Severe T'storm or Tornado Warning activates while you are sleeping, but only if your programmed location(s) are in the warning polygon. For 1/3 the price of a Weather Radio (which we also recommend but isn't as precise), it could save your life and it definitely provides peace of mind.

If you have outdoor plans, games, practices, etc. Sunday PM or Monday PM, don't be complacent. They will likely be affected by storms. Don't be afraid to make the safe and wise move. We're delaying assembling our new trampoline until at least Tuesday, maybe Wednesday, because it would be stupid to have it destroyed by a storm in it's first week! Yes, the kids are upset. They'll get over it. :-)

Finally, we will bring your the absolute latest info, including nowcasting live during storms (without interrupting your favorite TV program) on Facebook and Twitter. We've talked about the issues with Facebook not showing you everything before. We aren't responsible if you miss something - it's best to go to directly to our FB page and refresh regularly or click "Get Notifications" on the Like button on our page if you want to see everything.

As always - be prepared, not scared. Though the risk is elevated, the chances of a tornado hitting your house, even in an outbreak, are small. With proper preparation, heeding all warnings appropriately, and taking cover if necessary, chances are you'll be in good shape.

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

First Day of Winter, or Severe Weather Awareness Day? Be prepared today!

UPDATE - 12:40pm Saturday

A Tornado Watch has been issued for the Memphis metro until 8pm. SPC says that within the Tornado Watch area, there is a 50% chance of 2 or more tornadoes, 40% chance of an EF-2+ tornado, and a 60% chance of 10 or more high wind events.  Map (showing radar at issuance time) below.

UPDATE - 11:00am Saturday

The Storm Prediction Center has issued updated severe weather risk maps as of 10:30am.  The new overview map (below) has lowered the Moderate Risk to a Slight Risk for a portion of west TN (north of I-40 from Memphis to Jackson).  This is due to the presence of showers and more extensive cloud cover over this area, which is limiting instability.

What's instability?  Think of a pot of water on a stove.  The higher you turn up the heat, the warmer the water becomes. Turn down the heat a touch  and the water temp decreases.  Instability is the heat from the stove and the water is the atmosphere. In a primed atmosphere like we have today, it doesn't take a lot of heat (instability) to fire storms. A little less instability lowers the risk just a bit.

Even though instability is a bit lower, wind shear is still high so any storms that fire CAN still produce tornadoes. The chance of strong tornadoes still exists along and east of the Mississippi River in the latest SPC update. Damaging wind (up to 75 mph) is the primary threat. Timing has not changed from our original post below.  We expect a Tornado Watch to be issued in the next hour or two at most.  DO NOT LET YOU GUARD DOWN BECAUSE OF A MINOR CHANGE IN THE RISK AREA.


Originally issued 8:25am Saturday: 

The Storm Prediction Center, this morning, has expanded the Moderate Risk area - the most likely area to see severe storms - well to the northeast (see above), but the entire metro is still included.  The details of the storm system, including it's threats and timing, remain nearly the same as we posted in our blog on the topic late yesterday afternoon.

Below are the probability maps for today, showing the chance of tornadoes, severe wind (58 mph+), and large hail (1" or larger) within 25 miles of you. Click the graphic for a better view. According to SPC - the worldwide leaders in severe weather prediction - we have a 15% chance of a tornado (a few possibly strong), a  45% chance of severe wind (with potential for very damaging gusts of 75 mph+ in storms), and only a very small chance of hail.

Given the high probabilities of severe weather, SPC has issued a "Public Severe Weather Outlook" (below) to draw attention to a potential outbreak of severe storms.

PUBLIC SEVERE WEATHER OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0237 AM CST SAT DEC 21 2013

   ...SEVERE THUNDERSTORMS EXPECTED OVER PARTS OF THE LOWER MISSISSIPPI
   AND TENNESSEE VALLEYS TODAY THROUGH TONIGHT...

   The NWS Storm Prediction Center in Norman, Oklahoma is forecasting
   the development of a few strong tornadoes and swaths of damaging
   wind over parts of the lower Mississippi and Tennessee Valleys today
   through tonight.

   The areas most likely to experience this activity include:

          Northwest Alabama
          Central and Eastern Arkansas
          Western Kentucky
          Much of Louisiana
          Far Southeast Missouri
          Western and Northern Mississippi
          Western and Middle Tennessee

   Surrounding the greatest risk area, severe storms are also possible
   from eastern Texas and the central Gulf Coast region into the Ohio
   Valley.

   A potent jet stream disturbance now over west Texas will track
   northeast into Red River Valley later today, before accelerating
   northeast into the mid-Mississippi Valley tonight and early Sunday.

   Associated with this disturbance, an area of low pressure now
   forming over eastern Texas will become better organized as it moves
   northeast into southern Arkansas this afternoon, northwest Tennessee
   this evening, and the lower Ohio Valley early Sunday. 

   Ahead of the low, a broad flow of unseasonably warm and humid air
   will surge north from the western Gulf of Mexico into the lower
   Mississippi, the lower Tennessee, and the Ohio Valleys through early
   Sunday. 

   The increasing moisture and strong wind field that will accompany
   the jet stream impulse will create an environment favorable for
   bands of rotating thunderstorms capable of both potentially strong
   tornadoes and damaging wind over a broad swath extending from
   eastern Texas and southern Louisiana northeastward into the lower
   Mississippi and lower Tennessee Valleys.

   Although the severe weather threat is expected to be greatest this
   afternoon through early tonight over parts of Louisiana, Arkansas,
   Mississippi, and Tennessee, a more conditional risk for tornadoes
   and damaging winds will persist into early Sunday from the central
   Gulf Coast northeast into the mid-Ohio Valley.

   State and local emergency managers are monitoring this developing
   situation. Those in the threatened area are urged to review severe
   weather safety rules and to listen to radio, television, and NOAA
   Weather Radio for possible watches, warnings, and statements later
   today.

   ..Corfidi.. 12/21/2013

Today's scenario is not one to be taken lightly. The best chance for severe storms is after 2-3pm and the threat ends before midnight.  Tornado Watches will be issued.  If your safe place is not ready for you, get it ready!  You don't want to be cleaning it out when storms moving 60-70 mph are headed your way.  We can't guarantee you'll need it, but it's better to be safe than sorry.

The most likely severe weather scenario is a squall line, or perhaps a couple, containing very strong wind, however the tornado threat is not to be disregarded.  If a warning is issued, seek shelter immediately. Storms will be moving fast and wind of 60-70 mph or higher will pick up objects and can break the window you're staring out - even without a tornado.

We can't stress enough the need for multiple ways to receive severe weather warnings and paying attention to them today.  NOAA Weather Radio, local TV/radio, and smartphone apps are all good ways of receiving info. We strongly suggest getting StormWatch+ activated in your MemphisWeather.net mobile apps this morning and set your location(s) that you want warnings for. Then, don't silence your phone. it could be the best $8 you spend this Christmas season. Links to download are below.

Follows us on social media throughout the day for the latest (links below). We'll strive to keep you informed and safe, but it's up to you to know what to do if severe weather happens, and then to DO IT, if and when it becomes necessary. Stay safe.

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

Severe weather possible Saturday - our thoughts on how it unfolds

As most people are aware by now, a risk of severe weather exists across area on Saturday.  All one has to do is step outside and feel the humidity, gusty south wind, and warm temperatures, then check the calendar, to know things aren't quite right in the atmosphere!

First, what we expect, for those who just want to know when it's going to rain and how bad it's going to get!

Tonight: As the MWN Forecast indicates, showers and thunderstorms are expected overnight tonight with unseasonably mild and breezy conditions.  The precipitation is expected to move in after 9pm and last intermittently throughout the night. No severe weather will occur tonight, though a few rounds of thunder are likely.

Saturday: A break in the precipitation will likely occur in the immediate metro during a chunk of the morning Saturday with rain and storms continuing to our northwest in AR.  No promises on the Ugly Sweater Run in the morning however! This will allow temperatures to climb to near-record levels in the mid 70s by noon.  By late morning or early afternoon, we expect scattered storms to again develop over the area, fueled by the rising morning temps. The severe weather risk with these cells increases as the afternoon goes along.  By mid to late afternoon, the storms should start organizing into waves or clusters of storms, perhaps with a "main round" of storms, or squall line, by late afternoon to early evening. These afternoon storms will pose a damaging wind threat, as well as a tornado threat.  In addition, outside of storms, it will be very windy during the day Saturday with gusts to 40-45 mph possible.  As my friend David in Nashville said, loosen any objects in your yard that you don't want to keep!

Below is a loop of one high-resolution model's IDEA on how storms will play out overnight tonight and through tomorrow evening. This is NOT a forecast, just a possibility. We wouldn't show it though if we didn't like the general idea it renders.

Finally, it should be noted that tomorrow's storms (due to the wind pushing them) will be moving at 50-60 mph.  If a warning is issued for your area, don't wait to see what happens - take action! At that speed, you don't have time to think about it or go look for the storm.  It could be on you before you know it.

A Moderate Risk of severe storms exists Saturday for the Memphis metro and points southwest.

Saturday night: By evening, or behind the first "main line" of storms, additional rainfall is a good bet and models are hinting at another fairly substantial line of storms during the late evening or early overnight hours.  Damaging wind will again be possible and a tornado can't be ruled out.  We expect all precipitation to move east of the metro between 10pm-midnight.  When all is said and done, we should see rainfall totals in the 1.5-2" range in the immediate metro. However, just to our northwest, in northeast AR and southeast MO, totals will be 4-6" with locally higher amounts, Flash Flood Watches are posted for these areas. After the rain ends tomorrow, we begin a stretch of cooler and dry weather that lasts through the Christmas holiday.

Rainfall totals from 6pm tonight through 6pm Sunday. 2" or so is expected in the metro while flash flooding is possible to our northwest where 4-6" is possible.

Turning a little more to the technical side... 

First, tornado threat: This event will be a typical "high shear, low instability" winter event.  While the winds at the surface and aloft are impressive and sufficient shear could be present for tornadoes (even strong ones), we'll have to watch closely if to see if instability can form. The best (or worst) case for this would be a dry morning with peaks of sunshine and temps in the mid 70s by noon.  Most other ingredients are in place, including surface low pressure that moves across AR (a classic transition season severe weather track).  If it can deepen enough, surface wind may back just enough to increase low-level helicity values sufficiently to produce tornadoes.  You'll note that this is not quite my usual spiel in which I completely downplay the tornado threat. Moderate risk areas on day 2 are not common and this setup is not "garden variety." While damaging wind (up to 70 mph) is a more likely threat due to low-level flow of 60+ knots as low as 2000-3000' above us tomorrow, enough shear could be present to spin up tornadoes. The most likely time for tornadoes is mid to late afternoon.

Second, flash flooding: we do not believe the flash flood threat is high in the immediate metro. Though precipitable water amounts are very high (over 1.5" tomorrow), the heaviest and most persistent precipitation will occur to our northwest along a stalled front that low pressure will move along and storms will be moving fast enough that we don't expect copious rainfall over a long period from any one storm or line.  However, all storms can and may produce torrential rainfall for brief periods, temporarily filling low-water crossings, ditches, etc.  If training storms develop, localized areas could see an inch or more of rain in a short period of time.

Preparedness
Plan ahead for what you'll do if you need to seek shelter quickly between early afternoon and late evening.  Where will you go and how will you know you need to take action?  Again, with storms moving at freeway speeds, you may not get much notice.  We HIGHLY recommend you get yourself an early Christmas present and purchase our MemphisWeather.net mobile app (only $0.99 for all of our great info, radar, forecast, etc.) and THEN activate StormWatch+ on the Alerts tab for about 1/4 the cost of a weather radio. A small one-time fee gets you personalized watch and warning information with alerts that will even wake you if you are sleeping - but only if YOU are in the path of the storm. Put the MWN meteorologist on your phone and fill the preparedness void with information you can count on!  Links to the apps are below.

We'll be providing rapid and timely updates throughout the day tomorrow on our social feeds below - which means you'll know exactly what to expect and when to expect it.  It's called social media nowcasting and you won't find it anywhere else in Memphis, with a focus on JUST the Memphis metro.

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

Re-visiting the summer storm of July 22, 2003 - "Hurricane Elvis"

Monday, July 22, 2013 marks the 10 year anniversary of the "Mid-South Summer Storm of 2003," or as it is now called in Bluff City lore - "Hurricane Elvis."  Like the Ice Storm of 1994 and Super Tuesday tornado that struck Hickory Hill, Hurricane Elvis is a weather event that is ingrained in the psyche of anyone who lived in the Memphis metro that summer.  In meteorological-speak, Hurricane Elvis was a "derecho" that formed from a mesoscale convective system.

A massive tree was uprooted on Belvedere Street. Photo credit Mike Maple, The Commericial Appeal.

What is a derecho?

A derecho is, in it's most basic form, a very long-lived severe wind event.  To be deemed a derecho, a severe wind event must meet stringent requirements set by Johns and Hirt (1987) and re-stated by McNeil, et al (2003) from NWS-Memphis in their report on the storm.  The requirements are:

1. A concentrated area of thunderstorm-induced wind reports or damage resulting from wind gusts of at least 58 mph over a path of 249 statute miles (400 km)
2. The reports in this area must exhibit a "non-random  pattern of occurrence" as a singular (serial) or multiple (progressive) swaths
3. Within the area, there must be at least 3 reports, separated by 40 statute miles (64 km), of F1 damage on the Fujita scale or gusts of 74 mph
4. No more than 3 hours may elapse between successive wind events

In other words, a line of thunderstorms with some damaging wind reports is NOT a derecho in and of itself.  In addition, a line of storms cannot be called a derecho until it has met ALL of the above criteria (which is not when it first forms, as some media outlets are known to do, but hours into the event).

The atmospheric setup

The environment across the Mid-South on that fateful Tuesday morning was primed to support severe thunderstorms, despite being at diurnal minimum (lowest heat content in the atmosphere, which is typically just after dawn).  At the surface, a weak stationary front was draped across the metro and had stalled, as fronts typically do in this part of the country in the summer.  Low-level moisture had pooled along and ahead of the front as surface dewpoints were in the mid 70s. The atmosphere over the metro was highly unstable for 6am (CAPE was 3400 J/kg per McNeil, et al).

Surface map at the time of the derecho showing a stationary front over the region (click for larger map)

Analysis of CAPE values at 09Z (4am CDT) indicating an extreme amount of instability for this time of day

Aloft, drier air was present in the mid levels of the atmosphere (a recipe for downbursts) and the Mid-South was positioned in a northwest flow regime. In northwest flow, winds aloft blow from northwest to southeast - a prime pattern for mesoscale convective systems (or large areas of thunderstorms) to affect the region as upper-level disturbances rode the northwest wind into the region.  An upper level trough was located to our north and northwest and the jet stream was to our north, positioning the Mid-South in a favorable environment for convection with diverging wind (a pattern in which the wind separates, or diverges, from a common point) over the area at jet stream level.  All of these were ingredients for severe weather that morning.

The mid levels of the atmosphere (500mb, or ~18,000') showing the northwest flow over the region

Jet stream level chart showing a jet streak (light blue) over the Ohio Valley and divergence over the Mid-South

Meanwhile, one of those pesky upper-level disturbances had induced thunderstorms over the Ozarks in the wee hours of Tuesday morning and they strengthened as they rode the northwest flow into the area and encountered a very unstable airmass.  As they did so, the storms, now a series of supercells with a history of producing damaging wind across northern AR, coalesced into a bow echo with strong inflow wind on the rear side of the line causing supercells to merge just across the river from Memphis.  When they merged, the inflow wind was aided by convective downbursts and hit the ground just before 7am with a force Memphians will never forget.

Radar reflectivity as the bow echo/derecho raced through Midtown, where much of the most significant damage occurred. Courtesy McNeil, et al (2003).

The storm began producing severe wind gusts over northern AR around 3am, peaked in the Memphis metro and along the TN/MS state line between 6-8am, then continued east into northwest AL and far southern middle TN from 9-10am.  Once it was all said and done, there was plenty of evidence to rate the storm not only a derecho, but likely one of the worst to ever hit Memphis.  The derecho's path length was 400 miles and it traveled this distance in a period of 7 hours - a forward speed of nearly 60 miles per hour!

Seven-hour path of the Mid-South Derecho, courtesy NWS-Memphis.

Courtesy NWS-Memphis - axis of heaviest wind damage highlighted

The immediate effects

The "Mid-South Summer Storm of 2003" caused extensive damage across the region (estimates were more than $500 million in 2003 dollars), but was concentrated in Shelby County. Other areas hit hard were northern Tunica County, DeSoto County, Fayette County, and points east along the TN/MS state line. The derecho brought torrential rain and extreme wind reports that topped out at 102 mph from a weather station at AutoZone Park and unofficially 108 mph from a barge on the Mississippi River.  Other gusts included 84 mph at WREG-TV on the riverbank south of downtown, 77 mph at the Agricenter, and 61 mph at Memphis International Airport.  One death was directly attributed to the storm (from a falling tree), while six others died in the aftermath from various related accidents, fires, and carbon monoxide poisoning.

Damage to the Gibson Guitar Factory and a leaning crane on the FedExForum construction project. Photo credit Jeremy Atherton.

In the storm's path, many trees were torn down causing the most damage by falling on houses and cars, in the streets, and most of all on power lines. More than 1.1 million cubic yards of tree debris was collected in the weeks following the storm. The wind and falling trees left more than 1,300 power poles damaged, caused 2,000 transformers to blow, and downed 76 miles of wire. At the end of the storm, some 750,000 people (~338,000 of Memphis Light Gas and Water's [MLGW] customers) had lost electricity in Shelby County alone, as 279 of their 435 circuits were knocked out. MLGW worked diligently, including doubling its workforce through contract workers, to re-build the electrical grid and restore power. Over 100,000 people were still without power after ten days and many residents had to wait as long as 16 days in the blistering summer heat to have their power restored.  The utility alone suffered $30 million in damage.

Linden Avenue looked like many other Midtown streets following the storm. Photo by Mike Maple, Commercial Appeal.

Also in Midtown, Scott McNeil of the NWS took this picture on Central Avenue.

The derecho caused 600 of the 800 traffic lights in Memphis to malfunction. Therefore resident's commutes to and from work took much longer. The powerful wind also caused a crane that was a part of the FedExForum construction project to bend, which resulted in Beale Street being shut down temporarily due to the hazardous conditions overhead. Some areas experienced flash flooding as well. For example, Idlewood Elementary had its entire cafeteria flood and ten classrooms were heavily damaged as part of its roof was blown off.  Memphis International Airport was closed for a time after the storm and President Bush declared a federal disaster area for Shelby County, clearing the way for federal assistance during the recovery effort.

In the suburbs, 80% of the Germantown area experienced power outages. Bartlett had 1,000 homes and businesses with damage. Lakeland had a total of 500 houses with damage. The Collierville area had 469 homes damaged.

Lingering effects

During the time after the storm, the crime rate rose significantly. Between July 22 and August 4, residential burglaries rose from 431 the year before to 514 in 2003. Business burglaries also rose from 113 to 218.  Because there was no power, many citizens resorted to using candles as an alternate source of light. Due to the use of candles, some fires broke out, one of which caused the death of a two month old baby.

Because it was summer in Memphis, many invested in a generator to power air conditioners and fans. Although this may have helped them cool off, carbon monoxide poisoning became a problem. In one case, ten people were found unconscious in their home and a toddler died.  Even though Memphians without power will remember sweltering conditions, records indicate that in the 16 days starting on July 22 (the period of time it took to restore all power), the average high was only 87 degrees and only 6 of those days saw a high of 90.  A couple of days did reach the mid 90s however, and overnight lows in the 70s much of that period contributed to the heat issue, as it didn't cool off enough at night to provide some even a few hours of relief on many nights.

Advance warning

The National Weather Service report (McNeil, et al) points out that, given radar systems in place at the time and the angle that the storm was coming into the county, the NWS was prevented from seeing the full magnitude of the wind velocity before the damage reports started rolling in.  However even with this limitation, a Severe Thunderstorm Warning was issued for Crittenden County at 6:15am and for Shelby County at 6:29am, about 12 minutes prior to the storm reaching downtown.  That warning, and a subsequent re-issuance, were in effect for Shelby County until 7:30am.  Warnings were also issued for counties downstream prior to the storm arriving.

A warning issued at 7:02am for DeSoto and Marshall County was the first to mention a "history of wind damage," though the magnitude of wind damage was not given.  At 7:16am, the Shelby County warning was re-issued with the wording "wind damage has been reported across Shelby County," but there was still no enhanced language indicating the extreme nature of the damage.  Three minutes later, a Fayette County warning indicated "widespread wind damage across Shelby County."  Obviously, the extent of the damage was arriving in the NWS office as the storm moved through Shelby County and efforts were made to increase the strength of the wording in subsequent warnings.

The warning process since 2003

From having personally read weather warnings and follow-up statements for the past decade, great advancements have been made in the warning process, including more use of enhanced language, regular use of severe weather statements that follow a warning with additional information on the threats posed, and certainly the 2007 transition to polygon warnings for specific areas independent of political boundaries.  In addition, new tools and radars are providing even more information and data to NWS warning forecasters.  However, there is still work to be done in this area.  We are currently in an era of modernizing the warning process even further, including the use of social scientists and surveys to determine what the best way is to get a strong message out to the public to minimize apathy and increase awareness.

A drawing created by Sher Stewart after the storm

MWN blog entries on other recent Memphis area derechos:

June 12, 2009 - Event analysis of Mid-South Derecho - June 12, 2009  and Follow-up to the June 12 Mid-South derecho
April 4, 2011 - Early April severe weather events foreshadow a busy season (Top 11 of '11, #6)
June 11, 2012 - Derecho sweeps through the Memphis metro on June 11

References:
McNeil, S.J., et al, 2003: The Mid South Derecho - 22 July 2003. (Also, PowerPoint in PDF format)
Johns, R. H. and W. D. Hirt, 1987: Derechos: widespread convectively induced windstorms. Weather Forecasting, 2, 32-49.
Storm Prediction Center (SPC) Derecho page
SPC Page on Mid-South Summer Storm of 2003
WMC-TV special, "Destruction at Dawn", August 2003.
Commercial Appeal 10th Anniversary Retrospective
Wikipedia - Memphis Summer Storm of 2003

Erik Proseus,
MWN Meteorologist

Abigail Topham,
MWN Student Intern

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

Gorgeous early summer days give way to a slight risk of severe storms today

After enjoying a couple of near-perfect early summer days on Friday and Saturday, southerly wind has brought Gulf moisture back into the region as a weak cold front and fairly potent upper-level disturbance approach the Mid-South.  This combination will set the groundwork for storms this afternoon and evening, a few of which could become severe.  Below is the severe weather outlook for today, with graphic from the Storm Prediction Center.

The main threat today will be damaging winds produced by cluster of storms that are likely to form by mid-afternoon and move across the area through mid-evening.  The probability of 60+ mph wind over the metro is 15%, while there is a 60% chance of storms this afternoon into this evening.

We'll keep you abreast throughout the day via our social channels listed below, as well as via our website and mobile apps.  Watch for further updates as the day goes on.

--Erik, Meteorologist/MWN

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring a fresh new interface and StormWatch+ severe weather alerts!

Enhanced risk of severe storms later today

The Storm Prediction Center (SPC) in Norman, OK has placed the entire region in a Slight Risk for severe weather today.  At 10:45am, a weakening convective system is moving across northern AR and southern MO, driven by an upper-level disturbance ahead of a cold front.  Storms to our east are associated with a separate disturbance and pose no threat to the Memphis metro.

Latest Lower MS River Valley regional radar imagery.  (Subtract 5 hrs from UTC time on the image for CDT)

As the disturbance gets closer, and perhaps with the aid of an outflow from the storms to our west, storms are expected to develop in an unstable airmass over the Mid-South.  A few thunderstorms are possible by mid-afternoon in the heating of the day as temperatures reach the upper 80s, however storms triggered by the disturbance are more likely from rush hour through the evening hours. The main threat will be straight-line wind, though some hail is also possible. Tornadoes are not expected.

We do not believe there is an overly high chance of severe weather, but sporadic reports of 60 mph wind and small hail are quite possible. Below are graphics from SPC, which will automatically update as new graphics are released.

The Storm Prediction Center has the entire Mid-South in a Slight Risk for late this afternoon and evening.

SPC severe wind probabilities for today. There is a 15% chance of damaging straight -line wind.

SPC severe hail probabilities for today. There is a 15% chance of large hail.

MWN will nowcast any storms that move through the region on our Facebook and Twitter accounts, linked below. Any watches or warnings will be pushed to StormWatch+ in our mobile apps as soon as they are issued by the NWS.

----
Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring a fresh new interface and StormWatch+ severe weather alerts!