Thursday, October 30, 2014

First fall freeze on tap this weekend, maybe the second as well

As promised, a big time chill is on deck, and is slated to move in behind tonight's cold front. The only good news is that it won't last long, but the magnitude of the cold will be quite a shock after a couple of 70 degree days and last weekend's 80s.

We'll first feel the effects of the front tomorrow during the day. Despite temperatures only falling to 50 overnight, a brisk north wind will pick up by mid-morning under cold air advection stratus clouds, gusting at times to near 30 mph during the day. Coupled with the clouds (which depart by late afternoon), temperatures will remain in the 50s all day. It could be our coldest high temperature since April.

For trick-or-treating, Tiger football, and Friday Night Lights, "layer up" will be the action plan as temperatures fall through the 40s in the evening with wind chills near 40. Doesn't sound bad until you get out and walk a few blocks with Elsa, who will discover she isn't as prepared for Frozen as the real princess! By dawn, temperatures will reach the freezing mark for nearly everyone in the metro with wind chills early Saturday morning in the 20s. A Freeze Watch  Warning (as of early Fri AM) is in effect and will be upgraded to a Freeze Warning tomorrow. (The average date of first freeze at Memphis International Airport is November 12, while it is November 5 at the Agricenter, which is representative of the suburbs.)

Overnight lows Friday night will be below freezing across a huge portion of the eastern U.S.

Saturday will be downright cold for November 1. Though it won't set a cool record, temps in the 40s nearly all afternoon (the high will be just above 50) with a north wind at 10-15 mph will feel more like mid-December than the day after Halloween. Sunny skies will be little comfort. Saturday night will again see temperatures plummet to near freezing. The is a little more uncertainty with regard to reaching 32 in the city, but with calm wind, most areas outside the city will fall to freezing and outlying areas have the potential for upper 20s early Sunday morning. A Freeze Watch has been issued for early Sunday morning.

Another very cold night Saturday night with low temperatures Sunday morning shown above.

Temperatures start to moderate Sunday with highs nearing 60.  Another 10 degrees of warmth can be expected on Monday as we return to a more normal pattern.

If you have outdoor plants or other vegetation that could succumb to frost or a freeze, plan to have them covered Friday and Saturday nights, especially if they are not within a couple of feet of a heated structure like a home. There is not a lot of concern over water or pipes freezing as the duration of freezing temperatures will be a couple of hours or less. Also, bring in your outdoor pets or provide a warm spot for them each night. Finally, keep space heaters away from any flammable items, such as drapes or upholstery that could cause a fire.

Now to get back to inciting panic...

Erik Proseus
MWN Meteorologist

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Tuesday, October 28, 2014

Two cold fronts result in a cool Halloween and chilly start to November!

Autumn weather can be fickle and this week is proving to be exhibit A, at least in the temperature department. This past weekend, Mid-South air conditioners cranked back up as highs reach the 80s with more humidity than we have been accustomed to the past couple of weeks. For the first weekend of November though, it appears there will be more shivering than sweating!

A cold front moves through the Mid-South today, bringing with it scattered showers and a chance of afternoon thunderstorms. The showers will linger through the evening hours, but clouds and precipitation will hold temps down in the 70s, which is still a bit above average. The Storm Prediction Center has placed the metro under its first-ever "Marginal" (category 1 of 5) risk of severe weather today ahead of the cold front, but we're rather skeptical that any storms will get anywhere near severe limits. (For a review of the new SPC outlook categories, click here.)

SPC places areas ahead of the cold front in a Marginal risk of severe storms this afternoon. We think the threat of anything approaching severe will be very isolated.
Tonight, rain moves out and by early Wednesday morning clouds follow suit, leaving a mostly sunny day with cooler but not unpleasant temperatures near average for late October - 70 degrees. Thursday will be similar to Wednesday with highs near 70, though a reinforcing (dry) cold front will approach the area. This front moves through early Friday, resulting in a cooler and breezy day on Halloween Friday. Trick-or-treaters those heading to the Liberty Bowl for Memphis Tiger football will need to plan on cool conditions (50s) with a noticeable north breeze on Friday night.

A reinforcing cold front moves through early Friday, with cold Canadian-origin high pressure building in at the surface behind it. This will result in cool temperatures for the weekend.
Behind the secondary cold front, a large upper-level trough forms over the eastern U.S., promising below normal temperatures throughout this first weekend in November. In fact, some areas outside the urban core could see their first frost Saturday and/or Sunday mornings as lows look like they'll drop into the 30s! (For those curious, we're well past the earliest recorded fall freeze dates. The average first freeze in Memphis is November 12. It's a week earlier - November 5 - at the Agricenter which is reflective of conditions in the suburbs. Today is the average first freeze date in Jackson, TN.)

Saturday's high with full sunshine will remain in the 50s before temperatures start to rebound a bit Sunday into early next week. For those who dislike cold weather, this weekend's cool spell looks to be brief as warmer air moves back in next week, along with more rain chances by mid-week. Click here for the complete MWN Forecast.

According to modeled temperatures from the GFS model for the coming couple of weeks (this is NOT forecast data), trends favor a return to warmer weather after this weekend's cool conditions. Other sources agree with this general trend. 

P.S. Don't forget that it's also the end of Daylight Saving Time this Saturday night! "Fall back" to Standard Time when low sun angles occur during rush hour and evenings are dark. Not my favorite time of year...

Erik Proseus
MWN Meteorologist

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Thursday, October 23, 2014

A warm weekend in store; rain chances hold off until next week

A rather cool afternoon is underway with temps in the mid to lower 60's as a partial solar eclipse will hopefully be visible in the Mid-South. The only issue at this time is some high clouds over the area, but these are showing some signs of breaking up and we're hoping we'll be lucky enough to catch a break for the eclipse beginning at 4:52 PM (more info in our previous blog post). (If we aren't able to view it locally, you can always tune into the Slooh Observatory live broadcast.) However, our main story concerns the fairly drastic change in temperatures for this weekend and into next week.

GFS model valid Friday night showing temperatures (in °C) about 5000' feet above the surface
Warmer temperatures will begin streaming into the Mid-South starting tomorrow mainly due to a shift in the upper-level atmospheric pattern. A ridge (area of higher pressure) to our west will bring northwesterly flow aloft which will advect warmer air into the Mid-South from the lower plains. This may seem like a bit of an unusual pattern, but warmer air is often brought into the plains earlier then the south after several rounds of cold fronts. The ridge to our west and trough to our east promote this atmospheric pattern.

GFS model Friday night showing temperatures (in °F) at the surface
At the surface the pattern is more of what you would expect, with winds shifting towards the south with a weakening surface high over the area (which was responsible for the cooler air). Expect temps to begin increasing tomorrow with highs in the mid 70's. Warming continues into Saturday and Sunday with temps getting all the way into the lower 80's. It will feel quite warm during the day, but by Saturday night temps will only get into the 60's at night, making it relatively mild even in the mornings to end the weekend.

GFS model Tuesday afternoon showing Precipitable Water Values
Rain chances will return early next week as the next cold front moves through. Models are indicating a strengthening low pressure system to our north with a developed cold front sweeping through the Mid-South. The band of high precipitable water values along the cold front represent deep moisture which could bring thunderstorms along with it. We'll keep an eye on this threat and will let you know if we expect anything significant. As always you can check out the full MWN Forecast here.

William Churchill
MWN Social Media Intern

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Wednesday, October 22, 2014

Partial solar eclipse visible to Mid-Southerners on Thursday

In an event that won't be seen again for nearly 3 years, a partial solar eclipse will grace the sky over North America on Thursday, October 23. Partial solar eclipses happen when a new moon comes between the sun and the Earth, but they don't align in a perfectly straight line. Therefore, the moon only partially covers the sun's disc. 

Diagram of how a solar eclipse occurs, as a new moon passes between viewers on Earth and the sun.
Graphic credit:
In the Memphis metro, the partial eclipse will begin at 4:52pm Thursday afternoon as a small shadow on the right side of the sun. As the shadow moves across the top of the sun, a maximum eclipse (shown below) will occur at 5:53pm low on the western horizon. The sun will slip below the horizon at 6:14pm, thus ending the viewing opportunity prior to the end of the eclipse.

How the sun will appear at maximum eclipse (5:53pm Thursday) just prior to setting. For an animation of the complete eclipse cycle, see

The next solar eclipse opportunity won't be until August 21, 2017 when a total eclipse takes place. While we look forward to that opportunity, don't miss the chance to see this partial eclipse or you'll have to wait another 3 years to see it again!

As to viewing an eclipse, remember the #1 rule is to NEVER look directly at the sun, eclipsed or otherwise, without protective eyewear! The sun’s UV radiation can burn the retinas in the eyes leading to permanent damage or even blindness. The only way to safely see a solar eclipse is to wear protective eclipse glasses, look through welder's goggles with a rating of 14 or higher, or to project an image of the eclipsed sun using a pinhole camera. Here's info on making a simple camera from Mr
One safe way of enjoying the Sun during a partial eclipse--or anytime--is a "pinhole camera," which allows you to view a projected image of the Sun. There are fancy pinhole cameras you can make out of cardboard boxes, but a perfectly adequate (and portable) version can be made out of two thin but stiff pieces of white cardboard. Punch a small clean pinhole in one piece of cardboard and let the sunlight fall through that hole onto the second piece of cardboard, which serves as a screen, held below it. An inverted image of the Sun is formed. To make the image larger, move the screen farther from the pinhole. To make the image brighter, move the screen closer to the pinhole. Do not make the pinhole wide or you will only have a shaft of sunlight rather than an image of the crescent Sun. Remember, this instrument is used with your back to the Sun. The sunlight passes over your shoulder, through the pinhole, and forms an image on the cardboard screen beneath it. Do not look through the pinhole at the Sun.
Be safe and have fun watching the eclipse on Thursday!

Erik Proseus
MWN Meteorologist

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New severe weather outlook categories roll out

Beginning up to eight days ahead of potential severe weather, the Storm Prediction Center (SPC), a part of NOAA/NWS and the worldwide leaders in severe weather prediction, begins issuing convective outlooks (a.k.a. "risk areas") for the possibility of severe storms.  For many years, these outlooks have included a "general thunderstorm" area, as well as Slight, Moderate, and High Risk areas.


In recent years, especially with the widespread use of social media and availability of information around the clock and at our fingertips, the general public has become familiar with these outlooks and the terms used.  There has also been a call to update the outlooks to provide better delineation of the risk that the public faces from severe storms. In particular, "Slight Risk days" seem to cover a wide gamut of severe weather possibilities, from a small threat for a damaging wind gust or large hail to something just short of a tornado outbreak.

Responding to the call to better define the risk, take into consideration the research and advice of social scientists who specialize in communicating risk or threat to the general public, and to provide better consistency with other NWS products, SPC is using a new classification system for their severe weather outlooks.  The new system can be found in the tables below (pay particular attention to the first table).  The probability of tornadoes, 1"+ hail, and/or 58+ mph wind in a particular area defines the risk outlook category issued by SPC. These probability-to-outlook category conversion tables are shown below the new outlook classification system.  NOTE: There will be NO changes to the watches or warnings issued as a result of this modification.

SPC outlook categories prior to October 22, 2014 ("Old") and since October 22 ("New"), as well as the numeric scale that will accompany the categories in SPC outlooks and MWN postings.

Probability matrix SPC uses for determining outlook category on the Day 1 outlooks. For instance, a 10% risk of a tornado or 30% risk of severe wind or hail warrants an Enhanced risk. "Significant Severe" means EF-2 tornadoes, 74 mph wind, or 2 inch hail.

Probability matrix SPC uses for determining outlook category on the Day 2 outlooks

Probability matrix SPC uses for determining outlook category on the Day 3 outlooks. High Risks are not issued on Day 3.


The biggest change users will see will be the addition of two new risk areas - "Marginal" and "Enhanced."  The Marginal Risk indicates that the chance of severe weather is very low but not non-existent (or marginal). The Marginal Risk would replace the current "See Text" areas in the outlooks. According to first table above, areas under a marginal risk of severe weather have a low chance of severe storms - less than the slight risk of previous years.  Enhanced Risk indicates a more significant chance of severe storms and it will be used for "high-end" slight risk areas.  In other words, the current slight risk category will be split between "slight" and "enhanced."  Enhanced risk indicates a higher chance of severe weather than slight risk, but not quite up to a moderate risk.  There will be no changes to the moderate or high risk areas.

In addition, each Severe Weather Outlook text bulletin that accompanies the maps will contain a "public discussion" section that describes the weather risks for that day in non-meteorological jargon, so that the general public can understand the threat. An example of a day which had a high risk of severe weather (May 24, 2011) is shown below, followed by what the outlook areas would look like under the new classification system (click each for a larger image).

Example showing the classification system for severe weather outlooks as used on May 24, 2011

How the convective outlook would look for May 24, 2011 using the new classification system

How MWN will handle the change

We understand that this change seems to make things more complicated. We will employ a supplemental numerical scale (1 to 5), in addition to the new categories, to help better define the risk. These numerical categories are similar to the Saffir-Simpson Hurricane Wind Scale and the Enhanced Fujita Scale for tornadoes with a higher number indicating a greater risk of severe storms. (We have learned since deciding to use the numerical scale that SPC actually intends to do the same thing. They must've thought we were on to something there! :-)  The scales will be identical.)

More important than the number or name of an outlooked area are the threats posed. As a Weather-Ready Nation Ambassador that promotes information and education, we'll be sure that potential impacts are the key ideas in blog and social media posts, just as we have always done!  If you have any comments or questions, feel free to send them to us via our social media feeds or as a comment on this blog.

Erik Proseus
MWN Meteorologist

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Wednesday, October 15, 2014

Join us for a "Severe Weather Roundtable" Google+ video Hangout #wxchat

Secondary severe weather season is quickly approaching and, in fact, the southeast U.S. (where secondary season is centered) has just experienced a regional severe weather event. It is a good time to bring severe weather preparedness and education back into the forefront.

To that end, on Thursday night, October 16, at 9pm will host a "Severe Weather Roundtable" discussion via live video-based Google+ Hangout. Joining MWN meteorologist Erik Proseus and other members of the MWN team on the panel will be local weather enthusiast and veteran of MWN hangouts, John Maddox, and special guest Rick Smith. Rick is the Warning Coordination Meteorologist at the National Weather Service office in Norman (Oklahoma City), OK and has roots in the Mid-South. He knows Mid-South weather.

For one hour, panelists will discuss topics related to severe weather policy, procedure, and practice, answering some of your questions, such as:

  • "Why does a watch extend for so many hours when the severe weather will be occurring very soon?"
  • "Why do warnings not cover entire counties, and if they don't, why do I still hear sirens across my entire county?"
  • "I hear that the traditional Slight, Moderate, and High risk outlooks are changing. What does that mean, how does it affect me, and how will I know how bad the weather is going to be?"
  • "I always hear people say to have multiple ways of receiving warnings. What are the best ways to ensure that my family remains safe?"
  • And finally, "Why do some schools dismiss early on severe weather days and is that really a good idea?"

You will be able to watch the broadcast live, and ask your questions online, via our MWN Hangout page and our Google+ page. In addition, we'll be live-tweeting using the #wxchat hashtag on Twitter. You may also ask your questions via Twitter using the #wxchat tag. If you can't watch the broadcast live, a recording will be available on YouTube via the links above after the broadcast ends.

This discussion promises to be educational, informative, and perhaps even a bit controversial. We hope you'll be able to join us Thursday night at 9pm!

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Monday, October 13, 2014

Monday morning update on today's severe weather threat

UPDATE -- 9:00am Monday

As suspected last evening, the Mid-South has been upgraded to a Moderate Risk for severe storms today. Probability of damaging wind within 25 miles of your location is now at 45% with a 10% chance of wind over 75 mph. The tornado risk is also at 10%.

Current SPC severe weather outlook for Monday - a Moderate Risk is in effect for the metro.

Current SPC probabilities for damaging wind. Pink is 45% and the black hatching indicates a 10% or greater risk of wind over 75 mph.

Current SPC probabilities for tornadoes. Yellow is 10% and any black hatching indicates a 10% or greater risk of strong (EF-2+) tornadoes.

As far as timing, supercells capable of high wind and a few tornadoes will be possible by late this morning, while the squall line itself is on track for an afternoon (1-4pm) arrival. Everyone in the metro should prepare for the possibility of severe storms from late morning through early evening.

The remainder of the blog, posted last night, is still applicable, particularly the severe weather preparations discussed near the bottom.

--Erik Proseus,, MWN Meteorologist

ORIGINAL POST -- 8:25pm Sunday

As we blogged about yesterday, severe weather looks likely on Monday for a large portion of the middle and lower Mississippi Valley. In fact, the Storm Prediction Center has indicated as many as 42 million people from the Midwest to the Gulf Coast are under a risk of severe weather.

The greatest likelihood, where there is currently a 30% risk of severe storms within 25 miles, exists from St. Louis across the Mid-South to northern Louisiana. This same area also is under a threat of "significant severe" weather, which includes EF-2 or stronger tornadoes, 75+ mph wind, or 2"+ hail. The Memphis area is in both the 30% area, as well as 10% risk of significant severe storms. SPC has indicated that there is a decent chance parts of the 30% area could be upgraded to a Moderate Risk tomorrow, the first for our area since June 5 when a derecho caused widespread damage.

The Mid-South is currently forecast for at least a 30% chance of severe weather within 25 miles, as well as a 10% risk of "significant severe" weather. These probabilities could go up tomorrow morning.  
The area is currently sitting under a warm and moisture-laden southerly flow of air from the Gulf of Mexico, evident by dewpoints in the upper 60s this evening and south wind. Overnight, wind will begin increasing from the south and southwest aloft, further priming the pump for severe weather as moisture and wind shear increase. This will bring about a chance of a few thunderstorms tonight, though no severe weather is expected.

Forecast dewpoints as of 7pm Monday evening are still near 70 degrees. This amount of low level moisture is more than sufficient for strong to severe storms and is very muggy for October when dewpoints typically are in the 40s and 50s.
By tomorrow, low-level wind will really start to crank up, and in fact, wind at the surface ahead of the potent cold front will increase to 25-35 mph during late morning and afternoon, prompting the issuance of a Wind Advisory for the area from 10am - 7pm. If you need to, tie down any loose objects outdoors tonight or before leaving for work or school in the morning. Wind in the lowest 5000' of the atmosphere will also pick up to more than 50 mph during the afternoon, setting up a low-level wind shear scenario that is favorable for rotating storms and possibly tornadoes.

Forecast wind at about 3,000' early Monday afternoon. Values are in knots, which can be converted to mph by adding 15%. Eastern AR has values approaching 60 mph, while they are near 50 mph over Memphis. Strong storms can push this wind to the surface, creating a damaging wind scenario. The wind, coupled with changes in direction or speed above and below this layer, can create a wind shear scenario, which is necessary for severe storms.

Forecast wind shear values as of 4pm Monday afternoon. Values near 50 knots (present over the metro) are more than sufficient to allow storms to rotate, which is a precondition for supercells.
With an abundance of warm air and a period of dry weather expected during the day tomorrow, temperatures will warm to at least 80, more likely the lower 80s, by mid-afternoon. This heating will add fuel to the fire in the form of instability, another necessary ingredient for strong to severe storms. However, even if temperatures don't quite reach 80, there are plenty of other factors in play that could allow for storms to become severe.

Forecast CAPE values at 1pm Monday are near 2000 J/kg, which is sufficient for strong storms, given other factors which are also forecast to be present.
Given all of the above parameters - instability, moisture, shear - the final ingredient, lifting of the air into the primed airmass, will be provided by the cold front, or perhaps more likely, a trough out ahead of the front. Storms will move into western AR early Monday and then travel across the state during the day ahead of the front. A squall line of storms (or using meteorological jargon, a QLCS) will sweep across the metro most likely during the late afternoon to early evening hours. As of now, we're expecting this line between 3pm-8pm, but are favoring a rush hour impact in the metro. In fact, one of the high-resolution models (below) agrees.

Simulated radar (not a forecast), shows a squall line pushing into the city at 5pm. This type of forecast product is used for TRENDS. Radar is NOT expected to look exactly like this at that time, but gives us a good idea what to expect.

The main threat will be damaging straight-line thunderstorm wind of up to 60-75 mph with the squall line. However, as we saw with the last big squall line, "loops" or notches in the line are favored spots for rotation and a few tornadoes can NOT be ruled out.

The better chance for any rotating severe storms capable of tornadoes, however, will be in any renegade cells that form ahead of the line during the afternoon in a sheared environment. If these storms form, and can tap into the shear and become supercells, tornadoes would be possible, as well as some large hail. These threats are CONDITIONAL though, meaning we're not positive that this scenario plays out. If it does, the threat becomes more likely.

Once the line moves through, expect a quick drop in temperatures and a few hours of rain, perhaps heavy. The heavy rain along and behind the squall line could be capable of producing some flash flooding, so be aware of that threat during rush hour and into the evening as well.

Note a quick drop in temps behind the line, as shown by the high-res NAM model valid at 5pm. Temps in the 80s fall quickly into the 50s/60s behind the line.

Bottom line: prepare for the likelihood of severe storms tomorrow afternoon and evening, possibly first as supercells, then as a squall line around rush hour. Damaging wind is the main threat and appears to be a good possibility. Tornadoes and large hail are a secondary risk. A Tornado Watch is expected during the afternoon tomorrow. Know your plan for wherever you will be during the afternoon and evening should a warning be issued. Avoid windows even as a line of storms moves through due to the threat of high wind and lofted objects.

We'll bring you the latest all day tomorrow, including nowcasting of the events as they unfold, on our social feeds below. We highly recommend having a severe weather app to alert you to severe weather conditions. We prefer ours (links to info and download below), but honestly don't care as long as you have a reliable way to get warning information.

Also, remember that outdoor tornado warning sirens are for OUTDOOR use and the policies vary by municipality and county. If you hear one and aren't sure of the policy, take cover. In general (in Shelby County), Bartlett, Collierville, and Germantown only sound their sirens when the respective municipalities are under a threat. Memphis and Shelby County sound their sirens for any threat in Shelby County.

Click here for the latest forecast, or get it on our apps.  Stay safe!

Erik Proseus,
MWN Meteorologist

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Saturday, October 11, 2014

Threat of Severe Weather on Monday

The atmosphere has been unsettled over the Mid-South for the last several days, with continued rain and thunderstorm chances as a frontal boundary has meandered across the region. This front passed through the metro area Friday night, leaving behind much cooler temperatures and occasional showers Saturday. However, it will lift back north Saturday night, as a warm front, putting the Mid-South back into the warmer, muggier, and more unstable airmass. While a chance of showers and t'storms exists Sunday, the best chances will arrive Monday as a powerful surface and upper-level system takes shape across the Plains states. This will set the stage for what increasingly looks like a severe weather event for the Mid-South Monday afternoon and evening.

Thunderstorms are likely to begin organizing over Central and Western Arkansas Monday morning ahead of a cold front pushing eastward through the state. A strong low pressure system in Missouri will travel northeast during the day, dragging the cold front into the Mid-South This should push what will become an extensive line of thunderstorms into the metro area sometime between Monday afternoon and early evening. With the timing of storms currently expected at what we call "peak heating", the amount of instability in place should be maximized and sufficient for severe weather. This is especially true considering this system will carry a high amount of wind energy (or wind shear) aloft, which combined with the instability will aid the line of storms to become well organized and sustained, and more easily able to transfer those high wind speeds aloft to the surface.
GFS model forecast at 4 PM Monday, October 13, depicts a line of strong to severe thunderstorms entering
the Memphis metro area at that time.
In anticipation of this threat, the Storm Prediction Center has outlooked a large area under a "Slight Risk" of severe thunderstorms for Monday, with the entire Memphis metro area centered on an enhanced 30% probability of severe weather expected within 25 miles of a given point. At this time, the greatest severe weather risk is expected to be damaging wind, which could be rather widespread along the line of storms. However, with the amount of wind energy and instability forecast, a risk of a few tornadoes developing within the line is likely to be present as well. A risk of hail cannot be ruled out, but is lower. Additionally, torrential rain falling in a short amount of time could create some flash flooding issues. The threat should end by early evening as the cold front exits the region, but look for more specific timing and threat details Sunday as computer models continue to come into better agreement. Behind this system, a return to cooler temperatures and drier weather is expected.
Storm Prediction Center Day 3 Convective Outlook valid for Monday. The entire metro area is outlooked
in a 30% probability of severe weather within 25 miles of a given point.
With the chance of severe storms increasing, we highly recommend you check the settings on StormWatch+ in your app to be sure you will be alerted to any weather alerts that may be issued Monday. If you don't have the MWN app for Android for iPhone/iPad, or haven't upgraded to add the precision warning technology of StormWatch+, links below will provide more information. Also, now is a good time to review your severe weather safety and preparedness plans so you know where to go and what to do if severe weather threatens your area.

Kevin Terry, MWN

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Sunday, October 5, 2014

September 2014 Weather Recap and MWN Forecast Accuracy

September Recap

A ten month streak of below normal temperatures was broken in August. September saw those above normal temperatures continue, averaging 1.0 degree above normal.  Despite the warmer than average month, the average temperature for the year through September ranks 30th coolest on record (a span of 140 years).

Precipitation-wise, it was a rather odd month. The summer dry pattern continued on 29 of the 30 days, but historic flooding in the metro and record rainfall at the airport on September 11th left the month wetter than an average September. However, a total of just 0.30" of rain fell for the month if one disregarded that single day. And after September 11, only a trace of rain was recorded. For the year, precipitation is still 126% of normal, or exactly 10" above normal. Severe weather was limited to the significant flooding event on the morning of September 11. We posted a thorough recap of the event previously on the blog. You can find it here.

Precipitation totals from the September 11, 2014 Flash Flood event.

Memphis International Airport, Memphis, TN

Average temperature: 76.2 degrees (1.0 degrees above average)
Average high temperature: 85.6 degrees
Average low temperature: 66.7 degrees
Warmest temperature: 95 degrees (4th, 5th)
Coolest temperature: 54 degrees (23rd)
Records set or tied: None
Comments: 10 days had maximum temperatures at or above 90, two above the climatological average. For the year, the average temperature at Memphis is 64.0 degrees, which is 2.1 degrees below average.

Monthly total: 4.75" (1.66" above average)
Days with measurable precipitation: 4
Wettest 24-hour period: 4.45" (September 11)
Records set or tied: 4.45" on September 11 broke the previous record of 1.87" in 1965.
Comments: Besides September 11, only one other day had more than 0.02" (0.27" on the 6th). As of the end of September, the yearly precipitation has been 48.47", which is 10.00" above (or 126% of) average.

Peak wind: Southwest/35 mph (11th)
Average wind: 6.0 mph
Average relative humidity: 67%
Average sky cover: 50%

Click here for a daily statistical recap for Memphis International Airport.

Cirrus Weather Solutions /, Bartlett, TN

Average temperature: 73.2 degrees
Average high temperature: 85.8 degrees
Average low temperature: 62.7 degrees
Warmest temperature: 96.5 degrees (4th)
Coolest temperature: 46.7 degrees (23rd)
Comments: 9 days had high temperatures at or above 90. The warmest low temperature was 77.1 on the 10th.

Monthly total: 8.39" (automated rain gauge), 8.46" (manual CoCoRaHS rain gauge)
Days with measurable precipitation: 5
Wettest date: 7.62" (11th) (via automated gauge)
Comments: As in Memphis, the vast majority of the rain fell on September 11 (91%). There was only one other day with more than 0.03" (0.71" on the 6th). Over 7" of rain fell in 3.5 hours on the morning of the 11th, with Bartlett one of the epicenters of historic flooding. For the period July-September, excluding September 11, Bartlett received only 4.25" of rain, or 40% of the normal Memphis average for that period.

Peak wind: Southwest/16 mph (10th)
Average relative humidity: 80%

Click here for a daily statistical recap for Bartlett, TN.

MWN Forecast Accuracy

MWN average temperature error: 1.64 degrees
MWN forecast temperatures within 2 degrees of actual: 81%
MWN average dewpoint error: 2.02 degrees
MWN forecast dewpoints within 2 degrees of actual: 70%

MWN's forecasts extend out five periods (2.5 days, or roughly 60 hours). Historical accuracy statistics can be found here.

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Unsettled weather week starts with a chance of severe storms Monday

A gorgeous, if breezy, weekend is in the books with warming temperatures and increasing rain chances this week. The main discussion point is the potential for severe storms on Monday.

Clouds increase overnight as southwesterly wind continues to bring in additional moisture. Lows will be much warmer than the 40s of the past couple of mornings, only dropping to near 60. On Monday, mid-level energy will rotate around a large trough north of the region. As temperatures warm to near 80 by afternoon, temperatures aloft will cool due to the approaching mid-level disturbance. The disparity in temperatures creates a scenario with high lapse rates (which is the rate at which temperatures decrease with height). In addition, wind energy will increase as the mid-level wind picks up with the approach of the disturbance,resulting in increasing wind shear. All of these ingredients together will create a scenario which promotes the formation of thunderstorms, some of which could become severe.

The primary severe weather mode will be individual storms or clusters/short lines of storms with the main threats being large hail and damaging wind during the afternoon and early evening hours (coincident with peak heating). A tornado or two will be possible in any strong isolated cells that can become supercellular. We expect storms will be most likely and strongest roughly near a line from near Nashville to Corinth to Clarksdale, MS, but severe weather will be possible throughout the TN Valley, west TN, and north MS. The entire metro is outlooked under a Slight Risk tomorrow with a 15% chance of severe weather within 25 miles.

Severe weather probability for Monday afternoon and evening, centered on the TN Valley/Mid-South.

Preparations should include monitoring the latest forecasts from your favorite local sources, securing loose objects ahead of time (if they haven't already blown away from the wind the past few days!), and protecting anything outdoors that could suffer from hail damage. Have your severe weather apps,including StormWatch+, setup and ready to receive any watches and warnings that are issued.

For the rest of the week, the weather looks somewhat unsettled as we're squeezed between high pressure over the Midwest/Great Lakes and the Bermuda High over the Southeast. Temperatures should average near normal during the day (mid 70s to low 80s) and above normal at night (60s).

Erik Proseus, MWN Meteorologist

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Friday, October 3, 2014

Recap of Oct. 2 severe weather, challenges faced, and a word on siren policy

Until last night, it had been a long time since widespread severe weather (not including biblical flooding events) had affected the Memphis metro. Tornado Warnings were last issued in late April. A well-advertised, and well-forecast, squall line moved through the region during the late evening hours bringing many reports of wind damage and several areas of rotation on radar. No reports of tornado touchdowns were received in the immediate area, though possible tornado damage occurred in northeast AR.

Every county in the metro was under a Tornado Watch and Severe Thunderstorm Warning as the line moved through and several areas were also under Tornado Warnings due to radar-indicated rotation in the fast-moving line. An early recap of the night from NWS-Memphis can be found here. They also discuss areas they intend to survey for storm damage.

Severe weather reports received by NWS as of mid-morning Friday. Most were wind or wind damage, though a possible tornado struck east of Jonesboro.
There are two main issues I want to address in this blog: 1) technology challenges and 2) outdoor warning siren policy.

Technology challenges

Technology. We love it for many good reasons. It allows us to do many things quickly and more efficiently than ever before. It allows us to push specific warning information to you via your smartphone almost as soon as it is issued by the NWS. But it also has its pitfalls. Namely when it doesn't work. Fortunately our critical warnings worked flawlessly last night, but there were other issues.

MWN StormView Radar on the website and mobile apps was one such problem. Due to multiple malicious attacks on multiple weather vendors' servers yesterday, including at least one big name and ours (though not targeting MWN directly), our radar stopped updating. The timing was awful. We apologize for and sincerely regret this major inconvenience; unfortunately some idiots like to "be funny" at other's expense. We're not laughing. If they can be tracked down, they ought to be prosecuted. We will be following up with our vendor on best practices going forward, but the outage was (and remains) completely outside of our control, though we take responsibility for providing the service to you.

In addition to radar issues, MWN headquarters lost internet connectivity as the squall line was moving across the river and Tornado Warnings were being issued (it also remains down this morning). This outage hampered our ability to disseminate information (including critical social media posts and data from our Bartlett weather station), monitor incoming data (including radar), troubleshoot the ongoing issues with our web radar, and communicate with the NWS. I'm not turning this into a bash-fest on our internet provider. This is the first outage we have had in months, perhaps over a year, and the connection handles everything we throw at it on a routine basis. The point is, once again, technology is not flawless and is subject to outage.

I want to take just a minute though to recognize MWN interns Kevin Terry and William Churchill for exemplary service last night as MWN's internet feed went down. Working from Mississippi State (there are advantages to working from disparate locations during severe weather events!), Kevin and William immediately took over nowcasting responsibilities in the middle of the event and didn't miss a beat. I'm guessing that most people watching our Facebook or Twitter feeds last night didn't know a major link in the chain had failed. That is a sign of a well-oiled machine and I thank them for their awesome work and dedication to all of you! My wife loaned her laptop to the cause (thanks honey!) and, courtesy of a smartphone hotspot, I was able to re-engage on a limited basis. Sometimes it takes a team effort and I've got one of the best in #TeamMWN!

Tornado Warnings and outdoor siren policy

The other discussion that needs to be had is regarding outdoor warning siren policies. Truth be told, it's been a relatively quiet couple of years for severe weather in the metro. Sometimes that "lack of practice" results in lack of preparation and awareness. The main issue we saw last night was in Shelby County, but the general principles apply most anywhere. Recall that Tornado and Severe Thunderstorm Warnings are not issued by county. They are "storm-based polygons," meaning they are drawn to reflect the forecast path of the suspect storm. Also recall that outdoor warnings sirens are for OUTDOOR use and aren't designed to be heard inside well-insulated homes with TVs and air conditioners on and storms overhead, and definitely not while you are sleeping. That being said, we recognize that many people are close enough to a siren to hear it indoors.

Siren policy differs by county, and in Shelby County it varies by municipality. The best siren policy is one that limits the activation to times when the community in the path of the storm is the only one alerted. To review, these are the policies in place in Shelby County as of October 2:

  • Bartlett, Collierville, and Germantown sound sirens for their cities (the entire city) only if the warning polygon intersects their respective municipal boundaries.
  • Shelby County sirens (including Millington, Arlington, Lakeland, and unincorporated areas of the county) sound for the entire county when any part of the county is warned.
  • City of Memphis sirens also sound for the entire city when any part of the county is warned.
  • However, Memphis is nearing implementation of a brand new system (the change should probably occur in November) that will activate sirens in Memphis only for those areas that are within the warning polygon, plus any sirens outside the polygon that can be heard within the polygon. Details on this policy change can be found here.

A Tornado Warning was issued at 11:13pm last night (shown below) that clipped the eastern side of Collierville. No other part of Shelby County was affected. In fact, the storms had exited all but the southeast corner of the county. However, given the above-stated current policies, sirens should have sounded in Memphis, Shelby County, and Collierville.  Bartlett and Germantown were not affected.

This was a case in which, under Memphis' new policy, sirens would NOT have sounded for any part of Memphis, which is exactly the way it should be. In our opinion, Shelby County should also move to a policy that allows segmentation of the sirens. There really was no reason to warn Shelby Forest, Millington, Arlington, or Lakeland about the storm in Collierville. I checked with Bartlett officials and sirens were not sounded in their city - well done! I also heard multiple reports last night that Collierville siren activation may have been delayed, and then continued after the warning for Shelby County was cancelled at 11:23pm (10 minutes after issuance). We can't corroborate that firsthand however.

Despite the sometimes untimely pitfalls of technology and some initial setup, the best way to receive warning information specific to YOUR location is smartphone weather apps with location-based warning technology. They can be set to warn specific points, regardless of county, and provide the most pertinent information to your location. Because of potential for technology issues, we also strongly advise a secondary means of receiving warning information that will wake you. The best secondary source is NOAA Weather Radio. Despite being county-based (all radios in Shelby County would have alarmed with the Collierville warning above), it's better than nothing, especially when you're asleep and local media is turned off in your home. Outdoor sirens are not to be relied on to wake you up.

We appreciate those who have trusted MWN and our StormWatch+ technology to provide the precision warnings described above. If you're interested in checking it out, click the appropriate link below. It's a fraction of the cost of a weather radio and only alerts you when necessary - plus it will wake you up in the event of the most severe storms. The precision is best demonstrated by an individual who lives on the west side of Collierville and indicated that StormWatch+ did not sound last night. As it turns out, he was less than a mile west of the warned area and not in harm's way. The technology worked as advertised.

To conclude, we thank you for your patronage and support of MWN, we apologize for the radar issues, and we greatly appreciate the dedication of #TeamMWN! If you have any questions, comments, or issues to raise, we would love to hear from you! Comment on this blog, hit us up on social media, or drop an e-mail via our contact form.

Erik Proseus, MWN Meteorologist

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Thursday, October 2, 2014

UPDATED: Possible severe storms tonight, then much cooler this weekend

UPDATED 7:00pm:
A Tornado Watch has been posted for much of the metro, as well as a good part of AR, until 2am (see map below). We've been asked "why a tornado watch? I thought this was a straight-line wind threat?" Two reasons:

  1. The same watch box covers areas to our west in AR where the tornado threat is a bit higher than here in the metro. Therefore, a Tornado Watch was issued.
  2. The tornado threat, while very low, is not zero in the metro. Out of an abundance of caution, a Tornado Watch was issued. We expect MAINLY a damaging wind threat in the metro, however, wind shear is present and any supercells that form in the squall line later this evening could briefly spin up tornadoes.
Current watches and warnings in the Mid-South. Yellow fill is a Tornado Watch, orange hatch is a Severe T'storm Warning.
Our thoughts on timing and specific threats have not changed from our earlier post this afternoon. You'll find those below, as well as our fabulous weekend outlook and ways you can stay informed throughout the evening. Now is also a good time to charge your devices and prepare for possible power outages due to high wind later tonight.


ORIGINAL POST (1:30pm Thursday):
The atmosphere is quite unsettled in the Mid-South this afternoon. Warm and muggy with scattered showers and thunderstorms is not exactly the type of weather you think of for early October! However, this will quickly change as a cold front sweeps through the Mid-South early tomorrow, ushering in much cooler, drier air that is more expected in early October. However, before we look forward to the beautiful weather this weekend we'll have to focus on the chance for strong to severe storms tonight.

The Storm Prediction Center is indicating damaging winds as the primary severe weather threat heading into tonight. If you're in the red area (including the metro), there is a 30% chance of 60+ mph wind within 25 miles of you.

For the remainder of today you can expect scattered showers and t'storms to continue with no severe weather expected. After 9 PM tonight, that will change as a squall line approaches the area. The primary severe threat associated with this line of thunderstorms will be damaging straight line winds. Although we cannot rule out hail or an isolated tornado, it appears the best chance of these threats will remain west of the metro. Expect the strong to severe line of storms to be departing the eastern side of the metropolitan area no later than 1 AM. We expect the storms to last no more than a couple of hours at any one point and the squall line itself will be pretty fast moving.

Modeled reflectivity at 11 PM tonight from the HRRR weather model, or what the radar MAY look like. This is not an official forecast, just one possible solution.

Once the cold front has passed through in the early morning hours, expect those warm and muggy conditions to be quickly shoved out, bringing in much-welcomed crisp October air. Temperatures tomorrow will struggle to warm to 80 degrees with a gusty northwest wind continuing to usher in cooler, drier air throughout the day. With such dry air in place this weekend we're expecting temperatures to dip all the way down to the mid 40's in many areas Saturday and Sunday mornings! You'll need to grab a jacket if you're heading out early anytime this weekend, as it will feel quite chilly! In fact, the afternoon hours Saturday will feature temps in the 60s and a cool breeze. Sunday will see temps begin to rebound back into the 70s as wind becomes southerly.

Weather Prediction Center Forecast for Friday, showing the cold front through the Mid-South
With a chance of severe storms after dark tonight, we highly recommend you check the settings on StormWatch+ in your app to be sure you will be alerted in case warnings are issued. If you don't have the MWN app for Android or iPhone/iPad, or haven't upgraded to add the precision warning technology of StormWatch+, links below will provide more information. We'll provide frequent updates throughout the evening and until the severe weather threat departs on Facebook and Twitter, links listed below.

Be safe tonight, then get ready for a fabulous fall weekend!

Erik Proseus, MWN Meteorologist
William Churchill, MWN Social Media Intern

Follow MWN on Facebook, Twitter, and Google+
Visit on the web or on your mobile phone.
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