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

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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

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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.

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St. Patty's Day storms possible in the metro

Warm spring days to end  the week have given way to a frontal boundary that has draped itself across the Memphis metro today. While northern areas of the metro (Tipton County) are still sitting in the 40s at 2pm, north MS is nearing 70 degrees!

1pm temperatures across the metro as a stationary front lingers overhead

Low pressure over the southern Plains will move just north of the front across AR and into the Ohio Valley by tonight.  As it does, the stationary front will lift north as a warm front and a cold front will move towards the region from the west.  As the stationary/warm front lifts north, more unstable and warm air will also move north, placing the metro in the "warm sector" of the approaching low pressure system.

Green line = stationary front draped over the metro at 1pm. Low pressure to the west will move NE, lifting the front north and bringing a cold front into the region late tonight.

With the low moving in close proximity to the region and warm and moist air just to our south, the Mid-South will be in a corridor for potential severe weather.  The stationary front will start lifting north this afternoon.  As the sun sets, instability tends to wane - so we have a dilemma regarding the magnitude of the unstable air, which will have a direct effect on the strength of any storms.

It does appear that storms will be likely, however, and there is a risk of a few severe storms south of the path of the low, in the warm sector, and ahead of the approaching cold front. The area delineated by the Storm Prediction Center is the Slight Risk area (shown below) , or that zone where the possibility of strong to severe storms is enhanced.  It includes the Memphis and Little Rock metro areas down to Texarkana and north to far western KY.

Slight Risk area as defined by the Storm Prediction Center. This area has an enhanced risk of severe weather.

The most likely time for thunderstorms in the metro will be during the overnight hours, or after 10pm until about 4-6am (which doesn't technically make most of these storms St. Patrick's Day storms I guess).  The cold front will arrive around dawn on Monday with the strongest storms in the hours leading up to that.  Therefore, scattered strong storms are possible after 10pm, but more likely time for storms (with a few being severe) will be after midnight through dawn.  The overall risk appears to be conditional - meaning that if certain atmospheric conditions are not met (namely sufficient instability), severe weather is not a given.

As for threats in the local area from any severe storms, there is a 15% chance of large hail (1" or larger), about a 10% chance of damaging wind (60 mph or greater), and  less than a 5% risk of a tornado.  So, while hail and strong wind are possible in the stronger storms, an isolated tornado cannot be ruled out.

(Top to bottom) Large hail, damaging wind, and tornado risk maps from SPC.

MemphisWeather.net encourages you to have fresh batteries in your NOAA Weather Radio and your cellphone charged BEFORE you head to bed tonight as preparation for possible severe weather. For those with iPhones or Android devices, now is a great time to download the MemphisWeather.net mobile app (mobile link) and sign up for our StormWatch+ severe weather alerts on the Alerts tab in-app.  StormWatch+ will wake you up if severe weather is imminent - but only if it's imminent at YOUR LOCATION (or any location you register).  While NOAA Weather Radio alerts by county, StormWatch+ alerts are sent for your house!  You don't necessarily need to be woken up for a storm 20 miles away on the other side of the county, but you should if it's bearing down on you.

We will effort to provide complete overnight coverage of this event on our Facebook and Twitter feeds listed below. Our nowcasting is our bread-and-butter product - and it's free.  Tell your friends and stay safe!

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Recap of today's thunderstorms and a pleasant pattern ahead!

We began the day in a Slight Risk area for the possibility of some severe weather thanks to a potent upper-level low pressure system that was traverse the metro this afternoon. During this time of year, with some heating ahead of one of these lows, it is possible to get large hail in any thunderstorms that form thanks to the cold air aloft and lower freezing levels. Instead of large hail, some areas saw buckets of small hail!  Below are a couple of pictures from Summer Avenue near White Station around 10:10am where so much hail fell it covered the ground!

Small hail covered the ground on Summer and White Station around 10am. Photo courtesy @bengalkatlady (Twitter)

A couple of hours later, this was the scene as the hail was washed into a low lying area in the parking lot:

Remnants of the hail 30 minutes later. Photo courtesy @bengalkatlady (Twitter)

This storm developed very quickly over the city and moved slowly east. Below is a sequence of cross-sections of the storm put together by Weather Channel meteorologist Stu Ostro.  Note the cell on the right size of the storm, which went from a small green dot in the top frame to a large red core held aloft by a developing updraft in the middle frame, and finally collapsed to the ground as pinks and reds (hail) in the bottom frame.

Time sequence cross-sections of the Memphis hail storm. Courtesy Stu Ostro. 

From the south (backside) of these storms, mammatus clouds were present on the underside of the thunderstorm anvil clouds.  The picture below was taken from Memphis International Airport looking north. Mammatus clouds are typically indicative of severe weather and extreme turbulence.

Mammatus clouds on the storm producing hail to the north of this location. Photo credit MWN.

After a brief respite, a line of storms moved into the metro from the west after the lunch hour. As the storms approached, they pushed out an outflow boundary, or gust front, that produced a photogenic shelf cloud.  The pictures below show the shelf as it approached southeast Memphis and Collierville, respectively.  Though no severe weather was associated with this storm, there were reports of near-severe size hail in north Mississippi in association with the line of storms.

Shelf cloud as it approaches Mendenhall south of Mt. Moriah. Photo courtesy @nas38117 (Twitter)

Shelf cloud from a distance, taken in Collierville by Michael Hardeman.

Precipitation rapidly diminished in the metro by 2pm as the upper-level low responsible for the day's weather moved nearly overhead and conditions stabilized behind the previous storms.

Looking ahead, a seasonal and very welcome pattern change is expected.  Thanks to the rain today, for only the second time since March 11, temperatures remained below 70 degrees!  As we head into the Easter weekend, high pressure will dominate, bringing very pleasant conditions with abundant sunshine, reduced humidity, and high temperatures in the upper 60s Friday and lower to mid 70s this weekend. A weak cold front will pass through early Sunday morning but only an increase in cloud cover is expected.

Another cold front that brings a shot of much cooler air arrives around Tuesday, further reducing temperatures to the 60s for the middle of next week. Overnight lows will be in the 40s Friday night and again for the middle of next week! Models are hinting at more rain chances late next week, but it's too early to say for sure now.  In the meantime, enjoy some truly SPRINGLIKE conditions. A detailed look at the extended forecast can be found on the MWN Forecast page.

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For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.

"See Text" - what text? Thunderstorm discussions on marginal days

UPDATE 2 (2:30pm): The Slight Risk area has been expanded and now includes the Memphis metro area (see map below).  The main threat is large hail through about 8pm. For more info, click here.

Updated slight risk issued at 2pm by the Storm Prediction Center.

UPDATE (10:30am): A Slight Risk area has been added for Arkansas, roughly south of I-40. The threat for hailstorms is higher in that area. The risk has not increased significantly in the metro. For more info, click here. 

Original Post:

As of 8am, the Memphis metro is under what meteorologists often refer to as a "SEE TEXT" area for this afternoon with regards to thunderstorm development. What the heck does that mean? And what text am I supposed to see, and what does it say?? First of all, check out the first graphic below from the Storm Prediction Center. These are the folks that issue the Slight/Moderate/High risk areas for severe thunderstorm potential. See the words "SEE TEXT" over the Mid-South in the graphic? This means that SPC believes there is a small risk for some type of strong storms, but not enough to post a Slight Risk. The accompanying graphics show where that risk lies and what the threats are. The "text" being referred to is the technical discussion from SPC, which can be found on their website at http://www.spc.noaa.gov/products/outlook/ - click on the outlook that includes the "See Text" area.

The SPC "See Text" area includes the Mid-South for Wednesday afternoon

So what about today's threat specifically? An upper-level disturbance and cold front will move into the area this afternoon. With plenty of low level moisture (dewpoints approaching 60) and a modest amount of instability, especially if we see some sunshine this afternoon that could boost temperatures to 70 degrees, the upper-level impulse will be enough to trigger some scattered convection. According to the graphics below, some storms could pose a threat of large hail and a damaging wind gust. The tornado threat graphic is not shown as it indicates there is almost no threat of tornadoes.

Hail outlook for today - 5% chance of severe hail

Wind outlook for today - 5% chance of severe wind

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For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.

First potential severe weather episode of 2012

Parts of the Mid-South are in line to possibly receive their first severe weather of 2012, and the first round in a few months.  The possible severe weather will be the result of a fairly potent low pressure system that will move almost directly over Memphis around midnight tonight (see graphic below).  Ahead of the low and it's accompanying cold front, atmospheric parameters will come together to support thunderstorms with some possibly strong to severe.

Low pressure will track over the Memphis metro overnight bringing a potential round of severe weather

The main threat of severe weather will be over MS, where large hail, damaging straight line wind and isolated tornadoes are possible. Closer to the track of the low - in the immediate metro - the main threat will be large hail, though damaging wind is possible in individual storms near the low pressure track.  The convective outlook from the Storm Prediction Center (below) indicates a slight risk of severe weather with this system.

The SPC convective outlook for today indicates a Slight Risk for severe weather

As for timing, showers and non-severe thunderstorms will be likely beginning this afternoon.  The best chance for any severe weather though appears to be a few hours ahead of the arrival of the low until it passes - which would be during the evening hours, or roughly from 6pm to midnight.

Though the threat of severe weather in the greater Memphis area is low, remain aware of changing weather conditions and stay with MemphisWeather.net for the latest forecasts and severe weather information.  As thunderstorms approach and cross through the area, we'll be nowcasting the event on our Facebook and Twitter streams (linked below) to keep you abreast of the situation.  We also encourage you to check out our MWN Android app, which now includes the option of adding StormWatch+ - our personalized severe weather alert service that delivers watches and warnings directly to your Android device as they occur.

Behind this system, a strong Arctic cold front will move through the area Wednesday night, which will bring the first blast of winter to the area since the New Year. High temperatures will occur early Thursday with temperatures falling through the 30s on a gusty northwest wind during the day.  For the complete forecast, visit MWN.

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For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our Android app, and visit us on Facebook and Twitter.

More borderline severe weather possible tonight, and results of a Shelby Co. damage survey from last night

More strong thunderstorms will likely affect portions of the Memphis metro area this evening as a disturbance moves east across AR along a stationary front to our south.  The front is the remnants of the cold front that moved through overnight, bringing severe straight-line wind, some hail, and tornado warnings to the area. The primary threat with tonight's storms will be large (possibly very large) hail.  A Slight Risk of severe weather is posted for north MS, where the majority of the storms are expected. However, storms could also move through the Bluff City this evening and into the early nighttime hours.

Below is a statement released by the NWS regarding damage and survey results from last night's storm that left over 50,000 customers without power in Shelby County alone.


...PRELIMINARY DAMAGE SURVEY OF SHELBY COUNTY TENNESSEE...

A NATIONAL WEATHER SERVICE SURVEY TEAM DETERMINED THAT VARIOUS AREAS OF SHELBY COUNTY TENNESSEE HAD STRAIGHT LINE WIND DAMAGE FROM SEVERE THUNDERSTORMS THAT MOVED ACROSS THE COUNTY BETWEEN 1030 PM AND 1115 PM ON TUEDAY APRIL 19, 2011.

DAMAGE IN THE FRAYSER AND RALEIGH AREAS CONSISTED OF DAMAGE TO SHINGLES ON HOMES, FENCES AND DOWNED TREES. WIND ESTIMATES IN THAT AREA ARE FROM 60 TO 70 MILES AN HOUR.

DAMAGE IN THE BOLTON AREA, NEAR THE HIGH SCHOOL, CONSISTED OF DAMAGE TO ROOFS, BUILDINGS, AND DOWNED TREES. THE WIND WAS  ESTIMATED AROUND 75 MILES AN HOUR.

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Anatomy of a hail storm - Gallaway, TN - 3/26/11

Last night, a severe thunderstorm rumbled through northern Shelby and Fayette Counties between 4-5am. Fortunately, I was able to grab some screenshots of cross-sectional radar data to go along with a picture sent to me by a Twitter follower in Gallaway, TN (extreme northwest Fayette County, near I-40) that will allow me to demonstrate some of the principles of hail formation and what meteorologists look for when predicting hail. (Note that a Severe Thunderstorm Warning had been issued for the affected areas before the hail storm arrived.)

First off, in traditional situations, there must be a thunderstorm and it's rising currents of air present for hail to be produced.  The dynamics within a thunderstorm, including the upward motion of air (or lift) is a necessary ingredient.  Hail begins small in storms and grows through a process of ascending and descending in the storm through sub-freezing air. Rising air in a storm, sometimes at a very fast rate, throws the hail higher into the storm. It subsequently gets heavier due to accumulating more ice around the core of the hailstone and descends. Stronger gusts of rising air can push it back up which cause it to accumulate another layer of ice, and the process it repeated.  The more times the process is repeated, the larger the hailstone.  A good animation of the process can be found here.

Another important factor is the freezing level, or what height above the ground the air drops below 32F (0C).  Liquid droplets must be present above this levels to produce ice, and hailstones must rise above this level to continue to accumulate more ice around the core.  In addition, if the freezing level is very high, the hail will melt on it's way to the ground and shrink in size, or melt completely.  For the largest hail to reach the surface, optimal conditions are a strong thunderstorm with strong updrafts and a low freezing level (near 10,000 feet or less in the Mid-South).

Let's take a look at the storm from last night.  The first image below is a cross-section of the storm at 4:22am from the Millington NEXRAD radar.  Left to right is roughly west to east (so we are looking north in this image) and bottom to top in the image is increasing altitude (or going up in the sky).  Each horizontal white line is 10,000 feet.  The brightest colors (reds and purples) are the heaviest rain, or in this case rain and hail. The  freezing level last night was fairly favorable at about 11,000 feet.  So, any heavy precipitation you see above that level is likely freezing/frozen.

In the image below, hail resides in the white circled area, and actually are being thrown up to even higher levels in the reds between 20,000-30,000 feet.  Gallaway is labelled and the storm is moving from left to right, so the storm has not quite reached there yet.  Note that nearest the ground, the color above Gallaway is green, or light rain, but the storm is tilted from west to east by upper-level wind and the strongest precipitation (reflectivity) aloft is almost over Gallaway.

In the next image, taken at 4:31am, the color nearest the surface directly over Gallaway is red and that red extends up to about 25,000 feet.  Hail is now falling at Gallaway, though the storm has weakened a little, as evidenced by the pack of purples.  The darkest reds are now below 10,000 feet and there is much less red between 10,000-25,000 feet, indicating that the storm is in the process of collapsing, the updrafts are weakening, and the hail suspended aloft is falling to the ground.

Twenty minutes later (4:50am), in the last image below, the process appears to be starting over again as plenty of heavy rain and probably some hail is reaching the ground, while another core of heavier precipitation is forming between 5,000-20,000 feet above the ground. This is the nature of the thunderstorm beast, continuing to mature and dissipate repeatedly until an unfavorable atmosphere is encountered.

So what did this storm produce?  The picture below, sent by Cam Fondren of Gallaway, tells the story - hail, some of it qurater-sized (1" diameter), which verifies the Severe Thunderstorm Warning that was is effect for Fayette County when the hail fell.  Special thanks to Cam for sending the picture!

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Extreme weather video and a note on hurricane season

I thought I'd share a couple of videos of extreme weather with you today.

First up, check out this video from KMBC's news chopper as a weak tornado touches down near Sedalia in central MO (east of KC). Thanks to Eric Sorensen, whose weather blog for NBC-13 in Rockford, IL, brought it to my attention. Here's a screenshot:

In addition, this AMAZING video was taken in the midst of the Oklahoma City hailstorm earlier this week. This video was brought to my attention by Chris Spears with the CoCoRaHS blog. Again, a screenshot, showing the baseball-sized stones pelting this guy's pool:

Also of note, the Atlantic Hurricane Season gets underway on June 1. Already, computer models are hinting at development offshore the Mid-Atlantic coastline by the middle of next week. With the end of El Nino conditions and record warm waters being found in the favored tropical development regions, all signs point to what could be a very busy 2010 season - much moreso than 2009. Stay tuned!

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Severe weather increasingly likely on Thursday

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE MEMPHIS TN
442 PM CDT TUE MAY 18 2010

...SEVERE WEATHER OUTBREAK APPEARS INCREASINGLY LIKELY ACROSS THE MID-SOUTH THURSDAY AND THURSDAY NIGHT...
...FLASH FLOODING WILL BE POSSIBLE AS EARLY AS WEDNESDAY NIGHT...

WARM AND MOIST AIR WILL BUILD INTO THE MID-SOUTH WEDNESDAY NIGHT BEHIND A NORTHWARD MOVING WARM FRONT. STRONG TO SEVERE THUNDERSTORMS AND FLASH FLOODING WILL BE POSSIBLE NEAR THIS FRONT WEDNESDAY NIGHT... ESPECIALLY OVER EASTERN ARKANSAS... WEST TENNESSEE AND THE MISSOURI BOOTHEEL.

THE ATMOSPHERE WILL BECOME VERY UNSTABLE BY THURSDAY AFTERNOON... IN ADVANCE OF DEEPENING SURFACE LOW LIFTING NORTH THROUGH ARKANSAS. THIS INSTABILITY... COMBINED WITH STRONG WIND SHEAR... WILL PROVIDE FAVORABLE CONDITIONS FOR RAPID DEVELOPMENT OF SUPERCELL THUNDERSTORMS OVER THE MID-SOUTH ON THURSDAY. TORNADOES WILL BE POSSIBLE... A FEW OF WHICH COULD BE PARTICULARLY STRONG AND LONG LIVED. IN ADDITION TO THE TORNADO THREAT... VERY LARGE HAIL AND STRAIGHT-LINE WIND GUSTS WELL IN EXCESS OF 60 MPH WILL BE POSSIBLE THROUGH THE EARLY MORNING HOURS ON FRIDAY.

THE FLASH FLOOD THREAT WILL CONTINUE THROUGH THE DAY THURSDAY... EXPANDING TO INCLUDE ALL OF THE MID-SOUTH THURSDAY AFTERNOON THROUGH THE EARLY PART OF FRIDAY.

THIS IS A POTENTIALLY DANGEROUS WEATHER SITUATION. IF YOU DO NOT HAVE A WEATHER RADIO... THIS IS A GOOD TIME TO PURCHASE ONE. IN ADDITION... YOU SHOULD REVIEW SEVERE WEATHER SAFETY RULES. KNOW THE DIFFERENCE BETWEEN A WATCH AND A WARNING AND BE READY TO PUT YOUR SAFETY PLANS INTO ACTION WHEN SEVERE WEATHER THREATENS.

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PWB

Public Severe Outlook from the Storm Prediction Center

PUBLIC SEVERE WEATHER OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1152 AM CDT SAT MAY 01 2010

...SEVERE THUNDERSTORMS EXPECTED OVER PARTS OF THE ARKALTEX REGION
NORTHEASTWARD ACROSS AR INTO THE WESTERN TENNESSEE VALLEY LATER
TODAY AND TONIGHT...

THE NWS STORM PREDICTION CENTER IN NORMAN OK IS FORECASTING THE
DEVELOPMENT OF A FEW STRONG...LONG-TRACK TORNADOES OVER PARTS OF THE
ARKALTEX REGION NEWD ACROSS AR INTO THE WESTERN TENNESSEE VALLEY
LATER TODAY AND TONIGHT.

THE AREAS MOST LIKELY TO EXPERIENCE THIS ACTIVITY INCLUDE

SOUTHERN AND EASTERN ARKANSAS
FAR NORTHERN LOUISIANA
MISSOURI BOOTHEEL
NORTHWEST MISSISSIPPI
WESTERN TENNESSEE

ELSEWHERE...SEVERE STORMS WITH TORNADOES...AS WELL AS WIND DAMAGE
AND HAIL...ARE ALSO EXPECTED ACROSS THE LOWER MISSISSIPPI AND
TENNESSEE VALLEYS THIS AFTERNOON AND TONIGHT...SPREADING INTO THE
OHIO VALLY OVERNIGHT.

A FEW TORNADIC STORMS ARE POSSIBLE ACROSS PORTIONS OF THE LOWER
MISSISSIPPI VALLEY THROUGH THE AFTERNOON/EVENING HOURS. HOWEVER...
THE MOST INTENSE STORMS...WITH THE GREATEST POTENTIAL FOR LONG TRACK
AND VERY DAMAGING TORNADOES...ARE EXPECTED TO DEVELOP IN THE
ARKLATEX REGION AROUND MID AFTERNOON...AS AN UPPER LEVEL IMPULSE
APPROACHES FROM THE SOUTHWEST. THESE STORMS SHOULD THEN SPREAD AND
DEVELOP NORTHEASTWARD ACROSS ARKANSAS AND INTO PORTIONS OF
NORTHWESTERN MISSISSIPPI...WESTERN TENNESSEE AND THE MISSOURI
BOOTHEEL LATE THIS AFTERNOON AND INTO THE NIGHTTIME HOURS.

STATE AND LOCAL EMERGENCY MANAGERS ARE MONITORING THIS POTENTIALLY
VERY DANGEROUS 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.

..IMY.. 05/01/2010

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Upgraded severe weather risk for the metro area tonight

Supercell thunderstorms, many producing tornadoes and some of which could be long-track, are lining up over western and central AR at the time of this writing (8:15pm). The graphics above show the upgraded risk area as of 8:00pm tonight. Much of AR has been raised to a High Risk of severe weather with long-track significant tornadoes a distinct threat. A Tornado Watch is in effect until 2am Saturday for these areas.

Surrounding the High Risk, including extreme eastern AR up to the Mississippi River (including West Memphis and counties bordering the river), a Moderate Risk of severe weather has been posted as supercells are likely to track into eastern Arkansas later tonight with the possibility of tornadoes.

A large portion of west Tennessee and all of northwest Mississippi, including the TN/MS portions of the metro region, are included in a Slight Risk for severe weather as storms could bring large hail, damaging wind gusts, and the possibility of a tornado overnight. Graphics showing the outlook areas, as well as the probabilities of tornadoes, large hail, and damaging wind, are shown above.

Stay with MemphisWeather.net for the latest on this developing weather scenario. Our newest addition - the MWN Storm Center - has the latest severe weather outlooks, county-by-county warning information, and updated radar for the Mid-South and maintains the latest information 24x7.

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Update on severe weather potential over the next 48 hours

A potentially significant severe weather outbreak is setting up for the Lower Mississippi Valley into the Tennessee Valley, including the Mid-South. A combination of factors, including extreme wind shear, abundant moisture, and necessary atmospheric lifting will all come together to create ideal conditions for the development of severe weather across the Mid-South. The event will likely unfold during two periods - one on Friday evening and the other Saturday morning and early afternoon.

On Friday afternoon, scattered thunderstorms are expected to form to our southwest over northern LA and southern AR, as well as the ArkLaTex region. These storms could quickly become severe as they go supercellular. The storms will move rapidly northeast and are expected to mainly affect areas near and west of the Mississippi River, with a few expected in the metro, sometime after 4pm and lasting through the evening until after dark (see graphic above). The main threats are large hail and damaging wind, though a few tornadoes are possible with these storms. In fact, the Storm Prediction Center has included areas just to our southwest under a Moderate Risk of severe weather tomorrow, while the metro area is under a Slight Risk (see SPC graphic at top).

There will likely be a break in the action early Saturday morning before the next, potentially more concerning, wave of severe weather hits. A low pressure system is expected to develop to our south and move across eastern AR during the morning and early afternoon hours as it gains strength, driven by very strong jet stream (see NWS graphic above). As it does, storms will quickly develop and become severe in areas near and east of the Mississippi River, including the metropolitan region. The timing of this severe weather episode is expected to be from early Saturday (after 7am) through early afternoon.

Currently, the Mid-South is under a Slight Risk of severe storms for Saturday, but I fully expect that to be upgraded to a Moderate Risk tomorrow. There is the potential for these storms to produce very large hail, damaging wind, and tornadoes - some possibly strong and long-lived. In addition, flash flooding will be an issue as rain amounts could be upwards of 2-3", locally higher, during the day Saturday.

Now is the time to review your severe weather safety plan. The most important tool during a severe weather event is a method of getting current information. The best way is via NOAA Weather Radio - make sure it works and is programmed (if applicable) and has good batteries, or get one if you don't have one. Other great sources of information are local media outlets (TV and radio), as well as MemphisWeather.net. Excellent severe weather safety information can be found here. Tornado tips, courtesy of West Tennessee Weather Online, are listed below.

MemphisWeather.net will continue to update you with the latest information. More frequent updates will be posted on Facebook and Twitter as the event unfolds.

TORNADO SAFETY RULES FROM WTWO

1. IN HOMES OR SMALL BUILDINGS, GO TO THE BASEMENT OR TO AN INTERIOR ROOM, SUCH AS A CLOSET OR BATHROOM, ON THE LOWEST LEVEL. GET UNDER SOMETHING STURDY SUCH AS A HEAVY TABLE OR A BED.

2. IN MOBILE HOMES AND VEHICLES, ABANDON THEM AND GO TO A STURDY STRUCTURE. IF THERE IS NO SUCH STRUCTURE NEARBY, LIE FLAT IN A DITCH, RAVINE, GULLY, CULVERT OR LOW SPOT WITH YOUR ARMS AND HANDS SHIELDING YOUR HEAD.

3. IN LARGE BUILDINGS, SUCH AS SCHOOLS, FACTORIES, HOSPITALS, NURSING HOMES AND SHOPPING CENTERS, GO TO THE PREDESIGNATED SHELTER AREA. INTERIOR HALLWAYS ON THE LOWEST FLOOR ARE USUALLY BEST. STAY AWAY FROM ROOMS THAT ARE LARGE IN AREA BECAUSE THEY HAVE WEAKLY SUPPORTED ROOFS.

4. IN HIGH-RISE BUILDINGS, GO TO AN INTERIOR SMALL ROOM OR HALLWAY.

5. STAY AWAY FROM WINDOWS. DON'T BOTHER OPENING OR CLOSING THEM. IT WON'T MAKE ANY DIFFERENCE TO THE STRUCTURE AND YOU'LL JUST WASTE TIME OR PUT YOURSELF AT RISK SHOULD GLASS BREAK AS YOU ARE NEAR.

Slight risk of severe storms Thursday

There is a slight risk of severe thunderstorms across the Memphis metro area later today as a spring low pressure system moves just north of the region. The time of greatest threat for severe thunderstorms, some of which could contain large hail or damaging thunderstorm wind, is between 10am and 4pm in the metro area, extending later for points east of the metro.

Stay on top of the weather situation today with radar, severe weather maps, and warning notification from MemphisWeather.net.

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Slight risk of severe weather tonight

Severe Weather Outlook

Tornado probability

Hail Probability

Damaging Wind Probability

A severe weather threat exists for the entire region beginning late this afternoon and continuing tonight. The graphics above outline the slight risk area and depict the probability of (top to bottom) tornadoes, large hail, and damaging wind within 25 miles of any point. Click each image for a larger version. More info from NWS-Memphis:

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM LATE THIS AFTERNOON THROUGH TONIGHT ACROSS THE ENTIRE MID-SOUTH. THE GREATEST POTENTIAL FOR SEVERE WEATHER WILL OCCUR ACROSS EASTERN ARKANSAS BETWEEN 4 PM CST AND MIDNIGHT. THE SEVERE WEATHER POTENTIAL WILL INCREASE ACROSS WEST TENNESSEE AND NORTH MISSISSIPPI AFTER 9 PM CST AND CONTINUE THROUGH THE EARLY MORNING HOURS OF THURSDAY.

THERE REMAINS THE POTENTIAL FOR DAMAGING WINDS IN EXCESS OF 60 MPH... LARGE HAIL UP TO THE SIZE OF QUARTERS... AND ISOLATED TORNADOES. IN ADDITION... HEAVY RAINFALL OF 1 TO 2 INCHES MAY CAUSE LOCALIZED FLOODING.

Stay tuned to MemphisWeather.net and commercial TV and radio outlets, as well as NOAA Weather Radio, for the latest on the possibility of severe weather.

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Severe hail criteria changing in January 2010

How big does hail have to be to be considered "severe?" Well, the official definition that most people know well will be changing on January 5, 2010 nationwide.

Following a public comment period and operational testing in the Central and Western Regions of the National Weather Service (NWS), the NWS will officially raise the criteria required for hail to be considered severe from at least 0.75" (penny-sized) to at least 1.00" (quarter-sized) in about a month. The criteria for thunderstorm wind to be considered severe (50 knots, or 58 mph) is not changing. During the public review and comment period, fully 86% of the comments favored raising the criteria.

As to the expected impact, the NWS estimates that approximately 40% of Severe Thunderstorm Warnings will no longer be required - that many are based solely on hail of less than quarter size. (In fact, Eastern Region testing indicated that only 35% of their hail reports are of quarter-size or greater (Steinbugl, 2009).) In addition, damage assessments indicate that hail must typically be 1.00" or larger to produce damage to structures or automobiles (Marshall, et al.). In the tested locations, the public, media, and emergency managers liked the change.

My opinion is this: The reduction in number of warnings due to the new criteria should make for more meaningful warnings and should eventually (after a period of education) cause the warnings that are issued to carry more weight. The fact of the matter is, currently the vast majority of the public is complacent and has become de-sensitized when a Severe Thunderstorm Warning is issued. That is due in large part to the fact that, much of the time, the weather doesn't get strong enough in Joe Q. Public's eyes to justify taking action when a "severe thunderstorms" rolls through. We've gotten used to these marginal warnings and have let our guard down. I am in favor of the change and hope that this change, in addition to the recently-implemented storm-based warnings (issuing a thunderstorm warning not by county, but along the path of the storm), will eventually cause heightened awareness of, and greater credibility to, thunderstorms that pose a threat to life and property.

References:
Marshall, Timothy P., et al, Haag Engineering Co. "Hail Damage Threshold Sizes for Common Roofing Materials."
Steinbugl, Matt, NWS/State College, PA. "Issues with Transitioning to a New Severe Hail Criteria," 2009.
Runk, Kim, NWS/Central Region. "One-Inch Hail Podcast," 2009.

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