Brief Tornado Update – Early Wed

…SEVERE WEATHER OUTBREAK…INCLUDING TORNADOES AND HIGH WINDS…ON WEDNESDAY…

I already went over most of the atmospheric mechanics behind this likely tornado outbreak in a blog early yesterday morning…for that see https://ckweather.com/2021/03/16/tornado-analysis-for-wednesday/

Not a whole lot has changed in 24 hours, but we do have a little better handle on the timing and locations. Right now, the cold air wedge has come into parts of Alabama from around the Appalachians to the east. Current temperatures show this cold air region. It is 56 at my house in Trussville, but 64 at the lake cabin on The Warrior River, only 35 miles west of here.

(Weather.us)

As the large upper-level system slowly moves toward Alabama this morning, SW winds will bring in warm, moist air from the Gulf. It will take some time to push the denser, cooler air out of the way over northeast AL…but the warm and unstable air will make into BHM by noon.

2 pm CDT temperatures (NAM model, pivotalweather.com)

Even with the center of the upper-level storm in Oklahoma at mid-afternoon, there will be strong wind fields in place over Alabama…between 50 and 60 mph at 5,000 feet. With the humid, unstable air in place over much of the area during the afternoon hours, despite the lack of forcing from the distant upper system and associated cold front, scattered thunderstorms will likely develop over Mississippi and parts of west Alabama between 1 and 6 pm, and any of these may become severe and produce large hail, damaging winds, and even tornadoes. The computer model estimate of the atmospheric temperature and wind profile is from Tuscaloosa at 2100 GMT (4 pm CDT). The CAPE of 2,700 indicates instability more typical of a May or June afternoon, while the storm relative helicity (SRH) that causes storm rotation is significant, but not extreme, at 170. These combined together produce an energy-helicity index (EHI), a great meaaure of tornado potential, at 3.0. Tornadoes are generally considered possible for EHI > 1.0, so at 3.0 there will probably be a few tornadoes, mainly west of a line from Decatur to Talladega to Auburn, including TCL, BHM, and Jasper.

The main event for the entire state will come after sunset, as the upper-level system and cold front move through. Winds at 5,000 feet will increase to 75 mph. The air will not be quite as unstable as it was in the afternoon with the lack of sunshine (this is why Mississippi has the highest risk for tornadoes). However, with the forcing, high wind shear (SRH 350-400), and sufficient instability, an intense line of storms will move through the state during the evening hours, probably arriving in BHM between 11 pm and 2 am. Tornadoes and straight-line wind gusts up to 75 mph are all likely with this line of storms.

You will hear a lot of hype on TV, and a lot more hype on Facebook, often from people who are NOT meteorologists. I don’t understand why people take weather advice from a pipe fitter, architect, or salesperson, but they wouldn’t allow a lawyer to take out a gall bladder, or a dentist to build a bridge. Don’t be scared. Even during the worst outbreak in 70 years, April 27, 2011, only 1% of the land area of Alabama was affected by a tornado. The graphic below shows the percent chance of a tornado passing within 25 miles of any point in that area. In Birmingham, according to NOAA, that chance is about 20%. This means that in the ~2,000 square mile area around your house, there is a 20% chance that a tornado, typically one that covers only a few square miles of area, will occur at all.

(NOAA/SPC)

Still, you and your co-workers and family should discuss your tornado plans for work, school, home, and anywhere else you might go, and do so this morning. The best location is inside a sturdy building, in an interior room near the center of the building, away from outside walls. Stay away from doors and windows. Get down low. Protect your head with a helmet, or pillow if nothing else is available. A basement is best in a regular home, but even in the basement you need to get inside the car or under a sturdy table, etc. Falling bricks, concrete blocks, and lumber can hurt you.

(Top, home where only room left intact on main floor was small closet; middle, area under porch in basement saved 7 people; bottom, getting in cars in basement may have prevented serious injury)

Have your NOAA Weather Radio programmed and ready with battery backup, and also have a good app on your phone to receive Tornado Warnings (I recommend the app “tornado” or the Fox 6 weather app). Make sure your phones and mobile devices are charged, and that you have batteries for flashlights, etc. We will be OK. This will not be April 27, 2011. We just need to use common sense and protect ourselves.

Dr. Timothy A. Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Twitter @timbhm

Tornado Analysis for Wednesday

…Most significant threat for tornadoes in Alabama since 2020…

Warm air has been in place over Alabama for days, with beautiful sunshine and temperatures reaching 80. But it is mid-March, and these temperatures are 15 degrees above normal. Remember, 28 years ago this weekend, we had a Blizzard! Winter is not over, but with longer days and more direct sunlight over the southern U.S., the contrasts that produce severe weather are building.

With a weak upper low far to our north on Monday…we experienced isolated, but some intense, thunderstorms over Alabama. Several of these rotated, and one produced at least a wall cloud and funnel cloud just north and west of BHM, and possibly a brief tornado west of Adamsville. See radar info and pictures below. Another produced hail in the Gadsden area Monday evening.

Radar and photography of possible tornado around 2:45 pm today.

With warm, humid air in place over Alabama, we could see scattered showers and thunderstorms just about anytime today also, but most of those should not be severe.

A stronger upper-level trough with a cutoff low in the center is currently spinning near Las Vegas, NV, with a 125 mph jet at 17,000 feet to its south. The dry air being pulled in by the jet is clear on water vapor channel satellite imagery this morning. That big upper trough will cross the Rockies today and become negatively tilted over KS/OK/TX by Wednesday.

Satellite water vapor (now, top), 500 mb chart (now, middle), 500 mb chart (Wed evening, bottom)

Due to the upper-level divergence ahead of this large upper-level system, a surface low pressure area will form to the Lee of the Rockies late today, and reach Missouri by Wednesday afternoon. The air mass over the Southeast U.S. is already warm and humid in many areas (65 degrees before sunrise here in BHM) But, the deep, strong southerly flow out ahead of this surface low will bring even warmer, more humid air north from the Gulf of Mexico and Caribbean. The air in BHM on Wednesday afternoon, for example, will come from the area just west of Cuba. This will allow for dewpoints, the best measure of humidity in the atmosphere, to rise way up into the 60s over most of southern, central, and western Alabama.

NOAA Forecast Trajectories Ending in several cities in the SE US
(pivotalweather.com)

An interesting feature tknown as “cold air damming”, where very cold air in the Northeast US can’t get over the Appalachians and pours south into the Carolinas, Georgia, and eastern Alabama, will likely be present early, maybe as far west as I-65. But, the strong southerly flow will push this cold air out of the way. This may be associated with some cool, overrunning rain during the day Wednesday, delaying the threat for severe weather. However, strong thermal boundaries can serve as focal points for storm rotation later.

So how will this all play out?

Expect some rain over parts of Alabama Wednesday morning, as the main surge of warm, humid air moves in and pushes the cooler air out of the way. By Wednesday afternoon, some areas may see a bit of sunshine, warming temperatures into the 70s. Whether or not the sun comes out Wed afternoon may play a key role in the intensity of nighttime storms (the sun came out the day of the Fultondale EF-3 tornado). Either way, with the upper forcing, temperatures aloft will be cold, allowing for CAPE values of 1,000 to 2,000 J/kg. A few isolated, severe storms may develop across parts of AR/MS/and western AL Thursday afternoon in the unstable air mass with enough wind shear for storm rotation and tornadoes. This threat would mainly be west of I-65. Below is the expected atmospheric temperature and wind profile from Bluff Creek, AL, in Walker County about 25 miles west of BHM. Note the wind shear (winds increasing in speed and changing direction with height at low-levels); this causes helicity (247) that is not extreme, but enough to create storm rotation with instability in place (CAPE 2100 J/kg). This is an aggressive model…some do not have temperatures as warm as this.

(NAM model sounding for Bluff Creek, AL, 25 mi west of BHM, 7 pm Wed

The main event will likely come during the evening hours, after sunset, over north and central Alabama. The strongest forcing from the upper system will move in during the evening. Winds at 850 mb (about 5,000 feet) will increase to 60 mph, and SR helciity values may go above 300 m2/s2. With sunset and cooling, the air won’t be quite as thermodynamically unstable, but CAPE values over 1,000 J/kg are still likely. A combination of CAPE and shear that provides an indicator of tornado probability known as the Energy-Helicity Index (EHI) will still be 2-4, and only 1 is needed for tornadoes. Computer model forecasts for CAPE/SRH/EHI for 10 pm CDT Wednesday are shown below. By this time, a line of intense storms will likely be moving into Alabama ahead of the main cold front, with straight-line winds up to 75 mph, the potential for large hail, and several tornadoes. A few supercell storms may form out ahead of the line…these are often the most dangerous as they can produce strong tornadoes.

(data from pivotalweather.com)

There is still uncertainty with this system, perhaps more than normal due to 1) the closed upper low, 2) the cool Gulf waters due to our very cold February, and 3) the cold air coming in from the east. However, it looks like we will have an outbreak of severe weather, including some tornadoes, on Wednesday and Wednesday night. Even the computer models can’t agree (as shown below), with the American models more aggressive than the European model. But, they all show a risk for tornadoes and severe storms, peaking between 6 pm and 11 pm here in central Alabama.

No, this will not be April 27, 2011. The EHI that day was 11, not 3. But, it could be a dangerous weather day, and you should prepare now. Have multiple ways of getting warnings, including some that will work with no power. NOAA Weather Radio, phone apps like “Tornado” from the Red Cross, and your local TV station works best (I watch JP Dice and Wes Wyatt on Fox 6). Make sure you have batteries for flashlights, and that your phones are fully charged. Think about where you will go if a Tornado Warning is issued for your area. Remember, lowest floor, center of building, away from windows and doors, small room. Helmets, blankets, etc. to protect you from debris. And, even in a basement, get under something sturdy like a heavy pool table or in your car.

Another update tomorrow morning.

You can also follow me on Facebook, Tim Coleman, LinkedIn Tim Coleman, PhD, and Twitter @timbhm

Tornado analysis – 1235 pm

The intense weather system is rapidly developing over the southeast US The thunderstorms we are getting in central Alabama right now are elevated because they are north of a warm front that is currently over south Alabama. These storms are producing heavy rain and lightning, but because they are not surface based, they present little or no tornado threat.

We at UAH are conducting limited operations (limited because of COVID-19 restrictions). The picture below shows the data from a recent weather balloon released from SWIRLL, our primary radar and lightning lab on campus. The profile of temperature (left) shows a stable atmosphere at low levels (no CAPE), but the hodograph, right, shows winds increasing and changing direction with height very quickly, and a storm-relative helicity of 510 m2/s2.

UAH weather balloon data this morning. Note large, looping hodograph circled upper right, with winds getting stronger and veering from SE at low levels to SW at upper levels.

To look at a view of CAPE and helicity across the Southeast, we go to the SPC mesoanalysis. One can see the warm front clearly here also, as CAPE rapidly increases in the warm, humid air over south MS and south AL. CAPE values of 1500 J/kg are already as far north as Jackson, MS and Evergreen, AL. Note also the extreme wind shear in place over a large part of LA, MS, and AL, with SRH values generally 400-600 m2/s2. This means the atmosphere has a lot of the type of wind shear that will cause storms to rotate, and produce tornadoes once they become surface based.

Even though the storms over Alabama right now are noisy, they are not dangerous (except for the lightning). But, the surface low pressure area is currently centered in NE Louisiana, and it will move toward west TN over the next few hours. This will keep the wind shear high around here. The wind shear will be enhanced along the warm front itself, as these fronts and the rapid warm air advection along them cause this. As the upper-level system continues to intensify over the next 3 hours, a strong southerly wind surge will occur ahead of it, bringing the warm front into central Alabama. Warm fronts sometimes have a hard time moving north when storms are to its north, but with the dynamics of this system, I think it will make it to I-20 by 4 pm, and then to the Tennessee border by 6 pm.

Back in NE LA, south of the warm front, damaging tornadoes have already occurred this morning. There are reports of houses blown away in Monroe, LA. A tornado watch is in effect for north and central Mississippi now, and it is the rare “PDS” watch (particularly dangerous situation). Visible satellite shows a few breaks in the clouds south of the warm front over southern MS. If that happens here, warming up the air even more at low levels, it could cause additional instability.

As the warm front moves north, the atmosphere over central Alabama will become very volatile for storm development, starting around 3 or 4 pm, as the warm front gets close. Supercell storms may form over Mississippi in the next couple of hours and track into Alabama. Then, the main line of storms just now crossing the Mississippi River will move through here with the possibility of widespread, significant damaging winds (75 mph or higher) and additional tornadoes, between 8 pm and 11 pm.

Go ahead and prepare now. Please have at least two sources of weather information with you at all times. A good phone app that will alert you of tornado warnings where you are, and a properly programmed NOAA weather radio. Make sure your phone is charged and you have batteries for your radio. Also, flashlights, helmets, etc. Just stay calm, and will we get through this.

For additional safety information, see the end of my previous blog, at https://ckweather.com/2020/04/10/tornado-analysis-for-sunday/.

I will be monitoring the situation from my UAH research office here in Trussville this afternoon, helping to watch our crews’ backs and keep them safe.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Twitter @timbhm

Cold front in July!

My forecast for temps over the next 4 days

A bona fide cold front is moving southward through the United States today, and will move all the way through most of Alabama, Mississippi, Tennessee, and Georgia tomorrow!

The term “cold front” is always used to describe the front edge of cool air masses that replace warmer ones. Of course, it is not going to get “cold” here in the Southeast U.S., but it will certainly feel much more pleasant than the sauna we have felt the past 3 weeks, with morning lows dropping near 60 degrees as far south as Birmingham and Atlanta, and daytime high temperatures only in the mid 80s.

Current temperatures (1 pm CDT)

The above map shows current temperatures from SE Kansas to NW Alabama right now. Some of the lower temperatures over parts of KY and AR are being affected by rain. But, it is 79 in Joplin, MO at 1 pm, and they have sunshine! So, this is a legitimate cool air mass. By tomorrow afternoon, winds will turn out of the north along I-20 from Jackson to Birmingham to Atlanta, and the cooler air will be moving in.

The air in places like Birmingham and Nashville will be coming from the eastern Great Lakes, as the NOAA computer model trajectories show below.

We will see some rain showers and maybe a few thunderstorms as the front moves through overnight and tomorrow morning. But, from tomorrow afternoon through Thursday, temperatures will be below normal. Some locations in the colder valleys of northeast AL, north Georgia, and east TN will easily drop well into the 50s.

Another big change that will make it feel nice will be the much lower humidity. Dewpoint is a meteorologist’s favorite measure of water vapor in the atmosphere because it is not affected by temperature, it simply indicates how much water vapor is in the air. The dewpoints over much of the Southeast U.S. the past few weeks have been between around 73, indicating a water vapor content of 17.5 grams of water vapor for every kg of air. Tuesday through Friday, dewpoints will be in the 50s, indicating water vapor contents dropping by 50%, to about 9 grams per kg. You will feel the difference!

Enjoy it, because with the strong, nearly overhead sunshine angle of July and August and warm soil, it won’t last, and we should be back into the 90s by the weekend. The humidity will come back too, with a big Gulf of Mexico nearby and water temperatures in the 80s.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Tropical Storm Barry – Sunday Update

High-resolution visible satellite picture

Barry is still considered a tropical storm right now by NHC, even though its maximum winds are out over open waters of the Gulf of Mexico. These winds are far from the center of circulation and lowest pressure near Shreveport, Louisiana, where winds are slowed by friction and thunderstorms are not as intense. Winds over most of south Louisiana as of 2 pm had dropped to 25-35 mph.

The big story with Barry now is the rain it is producing. The regional radar composite above shows a near solid wall of rain over the state of Mississippi, extending into much of eastern Louisiana and eastern Arkansas. Along the eastern edge of the area of heaviest rain, bands of thunderstorms are forming over parts of western and southern Alabama, and these may spread into central Alabama later today. There are also a few bands of thunderstorms over northern Georgia.

Some parts of south MS, extreme south AL, and southwest LA have already received 3-8″ of rainfall from Barry, and some rivers are already flooding. As Barry’s pressure continues to rise and the circulation weakens over the next 48 hours, the mass of rain will slowly decrease in size and intensity. But, even by Tuesday, with tropical moisture in place over the Southeast US, scattered afternoon storms will be increased, and some heavy rainfall amounts will occur from Memphis to Nashville and Paducah. The NOAA total rainfall forecast for the next 7 days reflects this increase centered near Memphis, with amounts over 7″ centered there, and amounts over 4″ expected in Little Rock, Paducah, and Jackson, TN. Rainfall amounts will drop off dramatically to the east over AL, middle and eastern TN, and GA, where less than 2″ is expected. Given the alignment of the rain maximum, some flooding along the Mississippi River, especially from Cape Girardeau south, is likely.

NOAA forecast 7-day rainfall starting now

One more interesting thing being picked up by upper-air weather balloons and computer models is the warm air aloft associated with Barry. Even though it has weakened to tropical storm winds, one can still see that temperatures at 500 mb (about 18,000 feet) are 10 to 15 degrees warmer above Barry than at other locations even over the Southern United States and the Gulf of Mexico. It is this warm air aloft, produced by the latent heat release in thunderstorms, that causes the air aloft to be lighter, and therefore cause the low pressure at the surface that drives tropical systems like hurricanes.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Tropical Storm Barry

USAF C-130 planes found flight level winds sufficient for the tropical depression in the Gulf to be upgraded to Tropical Storm Barry, with sustained winds near 40 mph. The center of Barry, is located south of Pascagoula, MS, and most of the strong storms are to the south of the center. This, along with wind shear and some dry air coming in from Texas, are not conducive for rapid intensification. However, Barry is over warm water in the shallow part of the northern Gulf, and the National Hurricane Center expects Barry to become a minimal hurricane by landfall sometime on Friday or Saturday in Louisiana.

The computer models are all over the place, and the center of the storm is still disorganized and, to me, unimpressive. But, my PhD is not in hurricanes like those guys, so the chance of it becoming a hurricane is still there. It looks possible that it could turn north a little earlier and come into Gulfport, Mississippi then move slowly through Mississippi as a tropical depression this weekend, but we’ll know more about the track once it develops a little more out in the Gulf.

The weather along most of the beaches from Mobile to Panama City will be OK the next few days. There will be more scattered storms than normal, but still occasional sunshine and warm temperatures. And no threat of tropical storm winds or storm surge. However, there will be rough waves and rip currents, and the water itself is closed to the public for now. Have to look at it from the beach or balcony.

The big threat to Alabama is from heavy rain over the next few days, especially Sunday through Wednesday of next week. Assuming the track stays further west near the Mississippi River, we should only see an increase in afternoon storms around here and 1-3″ of rain over the next few days. But, if it turns north earlier and comes up through eastern Mississippi, we could see a flooding threat, with 3-5″ of rain, more in west Alabama.

Another update on Barry and possible effects on the weather here in central Alabama, coming up on Saturday.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Early Heat Wave for Southeast

Current weather map across SE

For late May, today is nice with mostly sunny skies and temperatures in the upper 80s to lower 90s. But, the atmosphere is already dry (as shown by the green line, a profile of moisture in the atmosphere from the morning weather balloon at NWS Birmingham), leading to the large areas of clear to scattered clouds and no rain. About the only storms today are in middle TN and just east of Atlanta.

Weather balloon data from 7 am today at BHM. Red is temperature, green is dew point
High-resolution visible satellite (top), regional radar composite (bottom)

However, changes are coming, as a ridge of high pressure moves almost directly over the SE, cutting off any cooler, more humid air off the Gulf of Mexico. Plus, temperatures in the Gulf are still cool, in the 70s across most of the northern Gulf, and only in the lower 80s at shallow beach locations like Orange Beach, AL and Panama City Beach, FL. So, humid air will have a hard time making it in here over the next week, and our atmosphere will stay dry.

Gulf of Mexico water temperatures

This causes heat because 1) we don’t have many clouds and the sun this time of year is near maximum strength due to its high angle above the horizon, 2) the afternoon thunderstorms we normally get this time of year that cool us off won’t occur in the dry atmosphere, and 3) without afternoon storms to rain on the ground, the ground will slowly dry out, allowing more of the sun’s energy to go into heating the air than drying up soil moisture.

Surface pressure and winds on Monday (pivotalweather.com)

We still have a decent south wind today and tomorrow, bringing some of that slightly cooler air off the Gulf in here, so we should mainly have high temperatures here in Birmingham between 90 and 92. But the winds calm and turn SW by Friday and the weekend.

Computer models have a hard time with the layer of air we live in near the surface, and statistics derived from models have difficulty with abnormal events. So I take a simpler approach that has worked for years. Based on that, my best shot at high temperatures in BHM for the next several days is:

Today: 91 Thursday: 92 Friday: 94 Saturday: 95 Sunday: 97 Monday: 98

The good news is that humidity levels will remain low, with dewpoints in the 60s. So, heat index values will not be much higher, if any, than the temperature. But, in a slightly drier heat like this, people can get dehydrated because they don’t realize how much they are sweating. So, stay hydrated. Since this is coming on so quickly (last Summer it never got above 97 and that occurred in September, with most of May through August being unusually mild), check on the elderly and anyone who may not have air conditioning. Close south-facing window shades and blinds. And one final note, UAH research has shown that it can often be 4-5 degrees hotter right near the ground than the official temperature (taken at 2 meters). So, where dogs and cats are, it could easily be over 100. Bring them in during the day if you can, and make sure they have plenty of water. If you are going away for the Memorial Day weekend, make sure they have a cool place to go (basement, etc.) and enough water.

Dr. Tim Coleman

Consulting Meteorologist, Coleman and Knupp LLC

Twitter @timbhm

Sun strong, days long now!

As we move through Spring and toward Summer, the days have gotten a lot longer already. As shown by the orange curve above, today, April 24, we will have 13 hours and 6 minutes of daylight. This is after the minimum on the Winter solstice, Dec 21 2018, when we only had 9 hours and 50 minutes of daylight. The blue dashed curve indicates the change in daily daylight in minutes per day. Right now, we are still gaining almost 2 minutes per day, but that daily gain is slowing down. It will go to negative on the Summer solstice, the longest day of the year, June 21. That day, we will have 14 hours and 23 minutes of daylight, and the sunset is at 8:00 pm CDT.

For you math nerds like me out there, notice the length of day is basically a periodic function, and the change per day is its derivative, another wave function.

Sun path in sky over Birmingham on various days in 2019. Courtesy Sun Position app.

The sun is also getting higher in the sky. The above shows the sun’s path across the sky (compass direction and angle above the horizon) on 4 days in 2019. Note that today, the sun rose in the ENE at 6:07 am, reached it zenith in the sky at 70 degrees above the horizon at 12:45 pm, and then sets in the WNW at 7:23 pm. It has moved a lot since March 1, when it will still rising in the SE and setting in the SW, and its highest angle above the horizon was only 49 degrees.

The pictures below show sunset on the Warrior River at Bluff Creek looking due west from roughly the same spot on two dates, Nov 30 (left) and May 4 (right). Note that the sun is the SW sky on Nov 30, and in the NW sky on May 4.

The angles above the horizon matter, because when the sun is high in the sky, its angle is much more direct, and the incoming sun rays do not get spread out across a larger area of the ground like it does in Winter. For example, back on Jan 1, at its peak even with a clear sky the sun only produced 762 Watts of energy per square meter of ground, and the total daily solar energy was 0.0049 Watt-hours per square meter. This is why the northern hemisphere gets cold in winter. Low sun angle, short days, small amounts of incoming solar energy (none in polar regions where it stays dark for weeks or months).

Today, on the other hand, we get 1271 W/m2 at solar noon, and a total of 0.0104 Wh/m2, twice as much energy as on Jan 1. At its peak on June 21, we will get 0.0119 Wh/m2.

Some people ask why the hottest weather lags about a month or two behind the peak solar energy, and the coldest weather lags about a month or two behind the minimum. This is shown for Birmingham in the chart above (length of day and average high temperature). This is because the sun is slowly heating up the ground and the atmosphere in the northern hemisphere in the spring and early Summer, while the outgoing longwave radiation and lack of sunlight in Winter slowly cools the ground and the atmosphere in Autumn and early winter. It takes time for energy to heat things up, and loss of energy to cool things down.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Severe weather analysis – 215 pm

NOAA/SPC convective risk outlook

It has been an active year for severe weather in the Southeast, and we have another system to deal with later today and tonight.

Right now, a big area of rain and thunderstorms is occurring just east of the Mississippi River. A tornado watch is in effect for much of central and south Mississippi, and several storms have tornado warnings on them right now near and south of Jackson, MS.

This entire system is reflected by a slow-moving front and a surface low pressure area forming along it in south Louisiana. Note that pressures are falling rapidly across eastern MS into northern AL, so that is where the low pressure area in Louisiana will likely track toward over the next several hours.

How far north the low tracks is critical to how far inland the tornado threat can make it. With the low pressure area intensifying and creating imbalances in the atmosphere as it moves to our NW this evening, wind shear for storm rotation will be very high. Storm relative helicity values could be over 400 m2/s2 here in central Alabama between 7 pm and midnight.

The only limiting factor may be a lack of instability for storm updrafts. First of all, the air over central Alabama is warm right now, but somewhat dry, with dewpoints ranging from upper 50s in Anniston and Montgomery to lower 60s in Tuscaloosa and Demopolis. That makes the air more stable. Higher dewpoint air will likely move in this evening as south winds pick up, destabilizing the air somewhat, but it will be at night by then and cooling off. Also, temperatures aloft are not that cold and do not get cold rapidly with height. That will also limit instability somewhat, as shown in the expected atmospheric temperature profile for Tuscaloosa at 7 pm.

Computer model temperature profile vs. height at TCL at 00 GMT (7 pm CDT)

Given current information, it looks like the greatest threat for tornadoes will be south of I-20, especially in areas like Jackson, Meridian, Montgomery, and Troy, where it will be warmer and more humid. Storms will likely move into west Alabama by 5 pm, and the main line of storms will come through Birmingham metro between 7 pm and 11 pm. Despite the more stable air, with the wind shear in place, tornadoes will be possible even as far north as Birmingham, and I expect us to be under a tornado watch this evening. Even if we don’t get tornadoes, straight-line winds over 60 mph are likely.

Review your safety plan. Lowest floor, small interior room, no outside windows or doors, wear a helmet, etc. Make sure you have a NOAA weather radio with alert feature to get warnings from, and hopefully a phone app. I use the app called “tornado” from the American Red Cross.

Dr. Tim Coleman

Twitter @timbhm

http://www.colemanandknupp.com

Dangerous storms central AL now

Reflectivity and velocity Birmingham radar 515 pm

Even though the shear is trying to pull away, temperatures have warmed into the upper 70s in Birmingham and Tuscaloosa, with 70s in Gadsden and Anniston. This is causing very unstable air, and multiple dangerous storms right now. We have already had at least one tornado touchdown in Cullman County, and two possible others.

The most dangerous storms now are near Oneonta, Fultondale, and Bankhead Lock and Dam. On their current paths they will affect areas including Altoona and Steele; Pinson, Clay, Center Point, and Trussville; and Oak Grove, Sylvan Springs, Hueytown. The only one officially under a tornado warning as of 520 pm is the one in Blount County, but all these are supercells with tornado potential.

Dr. Tim Coleman

Twitter @timbhm