The somewhat rare situation is setting up right now that may allow two tropical cyclones (tropical storms or hurricanes) to be in the Gulf of Mexico and threatening the continental U.S. at the same time. However, there is a lot of uncertainty with the two storms. The newly-designated Tropical Storm Marco is located just off the Yucatan Peninsula (SW of Cuba), and T. S. Laura is the one in the Caribbean to the east, centered just east of Puerto Rico.
Marco is better organized this morning, with USAF C-130 hurricane hunter aircraft finding flight-level winds around 41 knots, and thunderstorms developing near the center. Most of the thunderstorms associated with Laura are to the east of its low pressure center, and that will make intensification slow for a while. It is the condensation and release of latent heat by thunderstorms that lowers the surface pressure underneath those storms, so until the storms line up with the center, strengthening is difficult.
Marco is expected to move northwest toward the Texas Coast by Tuesday. Even though the waters in the Gulf of Mexico are very warm this time of year, the upper-level trough of low pressure that has been keeping our weather unsettled here in Alabama lately will produce wind shear that will keep Marco from becoming fully vertically-stacked the way an efficient hurricane would be.
Then there is Laura. The NHC track has been shifted to the left. That is good news if it holds, as there would be land interaction through Monday (Puerto Rico, Hispaniola, Cuba), keeping the storm weaker. This thing will be moving over a lot of undisturbed, warm water (84-90 F) for the next 3-5 days, and by the time it gets into the central Gulf late Monday, wind shear will be decreasing. There is still track uncertainty in the computer models, as shown below. Most now take it toward Louisiana by next Wednesday night.
The track staying south until Monday then turning northwest indicates that the models are counting on the Bermuda High spreading west into Alabama, Georgia, and the eastern Gulf ovet the next few days, with the upper trough weakening and moving into the Ohio Valley. This would allow the storm to go around the high then move north between it and the trough next week. Tropical systems generally find the path of least resistance to go north (their large-scale role in nature is to redistribute heat upward and northward).
There is also the unknown effect of the “Fujiwhara effect” where two cyclones interact if they get close enough together. They can start to rotate around each other or even converge. That is a very complex process and impossible to forecast at this point.
It will be an interesting week along the Gulf Coast. Anyone with family, friends, or property along the Gulf Coast, pretty much anywhere in the US, but especially from about Pensacola westward, needs to pay close attention to this developing situation in the coming days.
The next upper-level storm system in this active Spring is currently moving through Oklahoma, producing thunderstorms, with a few of them severe, and even some Tornado Warnings in Texas this afternoon.
A lead line of storms associated with this system will move through Alabama early tomorrow morning, likely reaching BHM by 7 am. However, these storms will not be surface-based nor produce any tornadoes, as the air mass will still be very stable. Currently, dewpoints are in the 40s, and they will not recover by tomorrow morning.
Even with elevated storms, as we saw on Sunday morning, we can get a lot of lightning and heavy rain, and even sometimes high winds and hail. A very unique event occurred from Clanton through Rockford, Dadeville, and Alex City Sunday morning, where the downdrafts from elevated storms produced large hail and an atmospheric wave that produced very high winds. But that is for another blog post! Either way, the storms tomorrow morning are unlikely to do that again, but expect heavy rain, possibly some hail, and local wind gusts up to 60 mph. Tornadoes should not be a threat in the morning.
The question comes during the day tomorrow. After the initial line of storms moves through, the main storm system will still be to our NW, and the air mass will start to recover (warm and humidify). By tomorrow afternoon, especially if we get any sunshine, the air will be unstable over central AL. Take a look at this NAM model sounding at Bluff Creek, AL, for tomorrow at 4 pm.
It has CAPE of 2,700 J/kg, so plenty of fuel for thunderstorm re-development. However, the wind shear by then will have decreased as surface winds veer to the SW, and 0-1 km helicity is only projected here to be around 100 m2/s2. Not really enough for any significant storm rotation. The instability will be moving in as the shear leaves. Now, if the system slows down and wind shear hangs around longer, we could see a few tornadic storms in central AL tomorrow afternoon. That is unlikely. The best chance for tornadoes tomorrow will be over SE Alabama and south Georgia, where the instability and shear will phase better.
We have been having difficulties with our computer models due to the lack of aircraft observations going into them, since most commercial airplanes are sitting idle right now and not flying. So there is higher than usual uncertainty in the forecast.
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.
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.
Not a lot has changed since my blog on Friday, but at least now we are not totally reliant on computer models and can start looking at some real data, as the main upper low that is causing all this trouble has opened up and moved into AZ and NM, and will move into TX this morning.
Weather balloon data and surface observations already indicate pressures falling out ahead of this trough due to divergence aloft, and as the pressures fall from west to east, this produces south to southeast low-level flow, that is starting to come off the Gulf of Mexico. Dewpoints are above 60 now roughly north of a line from Mobile to Shreveport to Wichita Falls, TX.
As this warm, moist air pushes north as a warm front this morning, this will create lift in the atmosphere, and clouds and showers will spread across Alabama by 8 am. These are already occurring to our NW. A few thunderstorms have blown up over Oklahoma.
As the low-level flow off the Gulf intensifies by afternoon, the warm front will move northward through Alabama. As noted by the local NWS and by SPC, this is a complicated pattern. In addition, we are missing hundreds of atmospheric observations that normally flow into our weather analyses from commercial airliners that are not flying right now, so the forecast has more uncertainty than normal. The warm front will likely reach I-20 by around 1 pm, and the Tennessee border by 4 pm. This warm front could be a focus for some of the earliest severe storms, as wind shear is greatly enhanced along warm fronts. And, behind the front, the air mass will become steadily more unstable with time. Any sunshine will increase the instability, but it is doubtful that we get too many breaks in the clouds tomorrow.
In the image above, the HRRR model, a high-resolution model that is initializing well with what is actually happening in TX/LA right now, the CAPE at 7 pm tomorrow in the BHM area would be a healthy (but not extreme) 864 J/kg, and the helicity, or wind shear for storm rotation, a significant 482 m2/s2. This will provide the energy for some supercell thunderstorms, capable of producing significant tornadoes, across north and central Alabama between 2 pm and 9 pm. In the model-predicted map of helicity below, note the maximum along the warm front in northern AL.
Storms will likely grow into a squall line by 10 pm CDT, decreasing the tornado threat and increasing the tornado threat.
A lot of people all over Facebook and other social media have become meteorologists lately. Check their credentials…if they have no formal meteorology training and experience, consider the source. I can’t drive a fork lift, design a bridge, or put the pipes in a house; generally, fork lift drivers, civil engineers, and plumbers don’t make the best weather forecasters. Secondly, with the high-resolution of our computer models today, some very high severe weather parameters will naturally pop out in some localized areas. If you are as afraid as some of the people posting and commenting on social media seem to be, go ahead this morning and drive to south Georgia, as if you were evacuating a hurricane.
For the rest of us, yes, this is the most significant potential for severe weather I have seen in 3 years or so. There will likely be tornadoes. Large hail, damaging winds, and lightning will also occur. But this is not going to be like April 27, 2011. Still, if it hits your house, it will be awful. It’s Easter Sunday, and we’re in the middle of a quarantine for a virus. 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.
UAH will be conducting “limited” (because of COVID-19) severe weather operations tomorrow across the northern half of Alabama, with Doppler wind lidars, multiple Doppler radars, wind profilers, and weather balloons all over the place. My job will be to watch radar and their GPS locations and direct them to the right locations and, most importantly, keep everyone safe.
Although there are still sources of uncertainty, there is potential for a significant outbreak of severe weather, including tornadoes, over Alabama on Easter Sunday. I know this is a bad prognosis, given that it will be Easter Sunday and we are all quarantined from COVID. But, I will simply lay out what I see in the data below, state what it means, and present the limitations we have. There is no reason to panic right now, as many people are starting to do. This WILL NOT be like April 27, 2011.
The storm of primary concern is still nearly stationary over the southwest U.S. right now. The water-vapor satellite image from NOAA’s western satellite shows the intense upper-level cyclone/low pressure area centered over southern California right now. According to computer models (see below), this low pressure area will move little (since it is closed in a full circle) through tomorrow afternoon, then open up and move eastward across the southern U.S. Saturday night and Sunday.
Note how the wind speeds (shaded) intensify as the trough of low pressure aloft gains a negative tilt over Missouri and Arkansas on Sunday. I will go ahead and note right here one of the things causing an extra level of uncertainty in this forecast: Normally, we get thousands of profiles of weather data throughout the atmosphere from commercial airplanes taking off, flying, and landing, across North America every day, and this data is fed into the computer models. Since air traffic is almost completely shutdown over North America right now, we are not getting all that extra data, so computer models are less reliable than normal.
That being said, the big low in southern CA will kick out eventually, and likely follow a southern track. But, there are some key differences in the computer models as to how this will play out, especially timing (that is key), even only 2 days away. As the upper low moves into the Great Plains Saturday night, causing intense divergence aloft, an intense surface low pressure area will develop over TX/OK/KS by early Sunday morning, then move rapidly NE toward AR/KY by Sunday evening. The low will likely still be deepening as it moves eastward. This will cause winds, especially at low-levels, to blow across the isobars toward the low. In addition, the winds at low-levels will become very intense, with speeds of 50-70 mph at 850 mb (5,000 ft.) over MS and AL by Sunday evening.
Because of the intensifying low pressure area and the strong southerly flow, there will be large wind shear and helicity that can cause storm rotation and tornadoes over the Southeast U.S. For example, below is the forecast hodograph, showing the winds at different altitudes around the storm motion, giving an idea of how the wind flowing into a storm would behave.
Notice the long, looping shape of the red part of the hodograph (the lowest 3 km, or about 9,000 feet). This indicates winds increasing speed and changing direction with height on Sunday afternoon. This type of wind profile, with a helicity value of 350 m2/s2, could induce significant rotation in storms.
Oddly enough for April, the question is instability (warm, humid air at the surface and cold air aloft that allows thunderstorms to form). This is where the models differ. If one looks at the 2:00 pm CDT (1900 GMT) surface map below, the cold front that has made today so cool has pushed all the way into the Gulf. The top left number is temperature, bottom left is dewpoint.
Note that dewpoints in Birmingham are in the 20s (like Winter), in the lower 40s along the Gulf Coast, and even in the 50s at some offshore oil platforms out from New Orleans. The warm, moist air will not get here until the system is arriving. A warm front will push northward across Alabama on Sunday, producing rain showers and clouds. According to the NAM model, and to a lesser extent the GFS model, warm and humid air will get here by early Sunday afternoon, and if that happens, the atmosphere will be volatile and supercell storms with tornadoes could develop. See the NAM model sounding below.
Even in this most aggressive model, the CAPE is only 1000 J/kg, and there is a modest cap of warm air that would help to suppress thunderstorm development up around 775 mb. The European model, the other most accurate model generally speaking, shows the warm front being slower to move north through Alabama, and the unstable air not arriving until after 5 pm on Sunday. In that scenario, some tornadoes are still possible, but we’d more likely be looking at a line of intense storms moving through Sunday evening with damaging winds. Either way, that line will move through between 7 pm and midnight, and the question is whether or not we get unstable air into central Alabama Sunday afternoon, out ahead of the main line, in time for supercell storms to develop. My gut says we will get some supercells with the chance for tornadoes out ahead of the main line between 2 pm and 8 pm on Sunday, but again, with model disagreement and lack of aircraft data, I don’t know for sure. Besides, the main line itself could have tornadoes in it, too.
Either way, people should be preparing for what they will do in the event of a Tornado Warning. Most people will be home, so you want to go to the lowest floor (basement if available), get near the center of the house, away from windows, doors, and outside walls, wear bicycle/batting/football helmets, cover yourself with pillows/blankets. Have at least two sources of weather info on Sunday, including a reliable cell phone app, and preferably a properly programmed NOAA Weather Radio.
I will have another update on this blog late tomorrow.
You probably noticed the big wind gusts and heavy rain that occurred across the Birmingham metro area this morning around 8:00 am. The wind gusted to 46 mph at the BHM airport, 47 mph at the NWS office in Calera, and 52 mph at the Bessemer airport. Trees and/or power lines were blown down in Tuscaloosa, Bessemer, Pelham, and Alabaster, and at least one barn was destroyed. The radar reflectivity (rainfall intensity) picture above shows what appears to be a line of thunderstorms moving through with gusty winds. But where was the thunder (or lightning that causes it)? And why didn’t the temperature cool down like it normally does when a thunderstorm passes by (at BHM it only dropped 1 degree).
It appears that some type of atmospheric wave moved through this morning instead. This wave of high pressure, in this case a type of hydraulic jump called a “bore”, caused a sudden, drastic pressure rise in a very short period of time. The atmosphere couldn’t get in balance with such a rapid increase, so the high pressure just blew the air along. One tell-tale sign of a wave or bore is in the Doppler velocity data from the same time as the picture above.
In the picture above, one can see strong winds blowing toward the radar (from the west) near the area of heaviest rain. But note behind the area of inbound winds, just 1 or 2 miles west, there is a strip of outbound velocity, indicating winds from the east. Then another couple of miles west, the winds go back to inbound again. This periodic change in winds is typical of a packet of atmospheric waves.
The atmosphere was also set up for waves, with cooler, more dense air near the surface than above it (not what you expect in thunderstorms). See the National Weather Service weather balloon data from 6 am CDT this morning in Calera.
In the charts below, 5-minute resolution data from the BHM airport shows the pressure, wind, and wind gusts vs. time. Note the wind gusts to 46 mph as the huge pressure increase moves in. This convergence of air is what created the upward motion and heavy rainband, too. Similar rapid increases in pressure were noted in dozens of official and unofficial (home) weather stations that report to the internet around central Alabama.
A rare January severe weather outbreak is likely across the Southeast U.S. today and tomorrow. As is usual for this time of year, it will primarily be driven by atmospheric dynamics (wind shear, large-scale forcing, etc.) as opposed to the very unstable air we see in Spring severe weather episodes. The dynamic setup on this one is extremely impressive, though, and I will show you a few maps and graphics to illustrate that below.
Water vapor satellite imagery already shows a deep upper-level trough in the wind field digging into AZ and NM. This trough is projected to move eastward and gain a negative tilt by tomorrow morning, which is often associated with severe weather.
Due to divergence ahead of the upper trough, surface pressures are falling rapidly over the Midwest and SE US. Meanwhile, a strong ridge of high pressure is trying to hold on over the Atlantic Coast. This is setting up a pattern of strong low-level southerly flow off the Caribbean and Gulf of Mexico, bringing warm and humid air slowly back into the Southeast.
The air that will be in central Alabama tomorrow is currently just north of the Leeward Islands in the Atlantic Ocean, near the Dominican Republic, according to computer trajectories.
This would normally provide a very unstable air mass. However, we need warm, humid air at the surface and cold temperatures aloft for that, and with the strong ridge of high pressure to our east, our upper level temperatures will not be that cold. Below are forecast temperature, dewpoint, and wind vertical profiles for Birmingham tomorrow noon. There is some instability there, but the computer models have large disagreement. The HRRR (left) shows a CAPE of 1100 J/kg, while the NAM, with a slight warm nose aloft (circled), shows almost zero.
How far north the unstable air makes it will play a large role in how intense the storms will be. The GFS model, that has been around a long time but updated constantly, goes down the middle of these two, so that is a good guess of what will happen. It shows CAPE greater than 1000 J/kg near the Gulf Coast, with values decreasing to near 800 around MGM and 500 at BHM.
Normally, these levels of CAPE would not support a severe weather outbreak. However, the dynamic nature of the upper-trough shown above, and the rapid atmospheric adjustments happening downstream from it, will create extreme wind shear over Alabama tomorrow, some of the worst I have seen in a while. It will be windy tonight and tomorrow at the ground, with winds gusting to 30 mph at times. Up at only 3,000 feet, winds will be 60-70 mph, and at 5,000 feet around 85 mph! This is very rare. The rapid increase in wind speed with height, combined with the winds changing direction with height, will produce helicity, or circulation in the winds flowing into storm updrafts that can be tilted into the vertical by the storms and produce rotation, and potentially tornadoes. A map of the 0-1 km storm-relative helicity is shown below. Only 150 m2/s2 is necessary for tornado development, and we will have values 400-500 m2/s2 from about Clanton northward, and values near 300 m2/s2 all the way to Mobile and Pensacola.
A good indicator of the right combination of instability and wind shear for tornadoes is the EHI, shown below for noon tomorrow. It indicates that despite the best wind shear over north Alabama, the best instability overrides that, placing south Alabama in the highest risk for long-track, large tornadoes. This area of EHI will move eastward during the afternoon.
Models are fairly consistent in bringing an intense line of storms into west Alabama around 10 am tomorrow morning, and this line will move rapidly across the state. It should arrive in BHM by 1 pm. With the lack of significant instability, individual supercell storms out ahead of the line (that are the most dangerous in terms of violent tornadoes), are unlikely north of I-20, but a couple of these could form in areas like Selma, Montgomery, Greenville, Troy, or Enterprise. We can’t rule it out north of I-20, especially if temperatures get into lower to middle 70s before the line comes through. So, tomorrow will be a day where everyone will need to have a plan for a Tornado Warning.
Even in the squall line, all this wind shear could produce multiple tornadoes over north and central Alabama, plus straight-line winds of 50-70 mph. This outbreak will be nothing like all the tornadoes during the afternoon of April 27, 2011, but if you recall, we had an intense squall line move through during the early morning hours that day, producing widespread damaging winds and several tornadoes (including two in the BHM metro area). I have talked to some of my colleagues at UAH and the NWS, and we all agree this squall line has the potential to be like that one. Let’s hope it is not.
Bottom line, have a severe weather plan and a source of weather information. Plan for the worst but hope for the best. You need a NOAA Weather Radio and/or a phone app properly set up to alert you of warnings, and access to local TV or radio. Since it is Saturday, most people will be home. If a tornado warning is issued for your location, go to the lowest floor, near the center of the building, away from windows, doors, and outside walls. If you have a basement, that is best. Pick the part of the basement that is underground. If you don’t have a basement, get in a closet or bathroom in the center of the house. Get under something sturdy if possible like a table or workbench. If you have bicycle, baseball, or football helmets, wear them. Many people are killed by flying debris hitting them in the head.
UAH will be doing full VORTEX SE research operations out across north Alabama tomorrow, with wind profilers, mobile radars, lidars, etc. I will be helping coordinate that from SWIRLL operations center on UAH campus, so I will not be able to do much in the way of blogs or updates tomorrow.
…TORNADO WATCHES IN EFFECT FOR LOUISIANA…MISSISSIPPI…AND WESTERN HALF OF ALABAMA…
The cold front has moved east this afternoon, and the low pressure area along it that is causing all the wind shear for storm rotation has moved just SE of Memphis. Take a look at the air swirlling around the low at the large scale, and the huge temperature difference across the front. 73 in Columbus, MS; 39 in Little Rock.
UAH SWIRLL is fully deployed with mobile radar, wind profilers, and Doppler Lidar in NW Alabama as part of VORTEX SE, and is releasing weather balloons from campus in Huntsville every 2 hours. Here is the latest. Note the cap of warm air at 700 mb is almost gone compared to what it was on the NWS Birmingham balloon data this morning. That cap has prevented supercells from forming out ahead of the main line of storms so far, and if we make it through the atmospheric changes that we have found to occur during the day-to-evening transition, it looks like we will avoid significant storm development ahead of the main line. That is good news, as lone supercell storms have a higher risk of tornadoes.
Even still, VAD wind profiles from the NEXRAD Doppler radars show that computer models underestimated the wind shear in this system. Note that winds at 4,000 feet at Columbus, MS (KGWX radar) are 55 knots (64 mph), 10-15 knots higher than predicted. This means storm-relative helicity, wind shear for rotation in storms, is also higher, around 500 m2/s2 (as opposed to the forecast 300 m2/s2).
This is causing storms within the QLCS, aka squall line, to rotate. Tornado Warnings have been issued for a significant area of rotation in the QLCS in NE MS that has now moved into NW AL, and a new area of rotation has prompted a Tornado Warning. These warnings are now in effect for Lauderdale, Colbert, Franklin, Morgan, Lawrence, and Limestone Counties, including Florence, Decatur, and Athens. A tornado has been confirmed in Colbert Heights, AL. Note that the entire line of storms has swirlled into a large MCV with a small eye near Rogersville.
Further south, in the storms approaching Hamilton, Tuscaloosa, and Jasper, there is not as much rotation at this time, so fortunately there the main risk is lightning and straight line wind. Those storms are further south and in a more unstable air mass, but clearly the wind shear and its dynamic forcing are trumping the instability, at least in north Alabama. However, tornadoes have been reported in several locations in central and southern MS, as far north as McComb, where the air is more unstable, and Tornado Warnings are in effect now in storms south of Jackson, MS moving toward Meridian. We may have to deal with a few of those in west-central Alabama before 7 pm, so be ready in Pickens, Sumter, Greene, Hale, and Tuscaloosa counties. With the higher-than-expected helicities (see below for current), we have to be on the lookout for small tornadoes within the line of storms too.
Bottom line…things look a little more scary than they did in my last blog due to the unexpected increase in wind shear. However, with the low instability over the northern half of the state, it is taking huge wind shear to spin up any tornadoes, and it looks like the shear will decrease somewhat as the line moves toward Birmingham as the low pressure area moving into Tennessee gets farther separated from the storms. It is odd, from a meteorology perspective, to see some storms with high instability and low shear, then others with low instability and high shear, both producing tornadoes, but the ones in the middle, where the mixture should be best, doing nothing. This means we will be looking at the storms very carefully the next 3-4 hours.
An intense and dynamic winter storm system is affecting the Southeast U.S. today. Most of the rain and storms are west of the MS River or north of I-40 right now. Much of the heavy precip in Missouri and Illinois is snow! But note the storms exploding over SE Texas, Louisiana, and SW Mississippi. That will be the big trouble over LA/MS/AL today. Check out the current surface map (annotated).
So, it is 70 degrees in Muscle Shoals, AL but 42 in Memphis and 28 and Springfield, MO. Very strong cold front. The surface low that has formed near the LA/AR border (note the wind flow swirlling around it) could be a big troublemaker for us today. Upper air charts below show a jet to our north at 500 mb (18,000 feet), and divergence due to atmospheric adjustment south of the jet is causing the surface low. Also, at 850 mb (about 5,000 feet), note the strong southerly flow developing, transporting warm moist air into the Southeast U.S.
The map below shows the trajectories of air that will be located at various points by 2100 GMT today (3 pm CST). Note that the air in Birmingham, AL comes from SW Florida between Tampa and Ft. Myers.
So, as the afternoon goes on, temperatures from central and south MS into north and central AL, especially with the sunshine we are getting, will warm into the 70s, and dewpoints will slowly climb into the mid 60s, setting up a prime lower atmosphere for thunderstorm development. One thing that will hold back storms over eastern MS and most of AL through late afternoon is a cap of warm air aloft, shown on this morning’s National Weather Service weather balloon at Calera, AL.
With time, as the cold front approaches, upward motion and cooling aloft, along with warming at the surface, will erode that cap. That will happen quickly over LA and western MS this afternoon, so storms will likely develop in that area quickly over the next 3 hours, and SPC has issued a Tornado Watch over that region. It will take longer over east MS, and maybe until the cold front actually arrives this evening in central AL, to fully become unstable. By then, it will be dark, surface temperatures a tad cooler, so the air over AL will not become as unstable as that in MS. The greatest risk for tornadoes today will be over Louisiana and Missisippi. There is a chance a few isolated supercell storms with a tornado threat could form west of I-65 over Alabama between 3 and 6 pm, but the widespread severe weather in Alabama will arrive after sunset.
As the cold front moves into Alabama this evening, the air will become more unstable. CAPE values will reach 500-800 J/kg, nothing like the 1500 J/kg in south MS, but enough for severe storms. The concern is the wind shear associated with the surface low. The wind will change direction and increase in speed quickly with height, creating 0-1 km SRH values near 300 m2/s2 over central Alabama. This wind shear provides the energy for storm rotation. A good combination parameter for wind shear and instability, the EHI, is shown below for 00, 03, and 06 GMT (6 pm, 9 pm, and 12 am CST).
Note the EHI decreases overall as it moves into Alabama this evening, mainly due to the wind shear pulling away to the north but the instability staying south.
The bottom line here is that there will be a threat for tornadoes, some significant, from Louisiana into Mississippi this afternoon and early evening. As the dynamics pull away to the north and it gets dark, storms will likely grow into a squall line as they move into Alabama around 7 pm and into BHM by 9 pm. However, with the high wind shear, straight-line wind damage and isolated tornadoes will likely occur along the line across west and central Alabama this evening, and in west Alabama, a couple of supercells could form ahead of the line with a higher tornado threat in places like Hamilton, Tuscaloosa, and Demopolis.
This is a dangerous weather situation, especially in Louisiana and Mississippi, but even into Alabama. Review your tornado safety plan wherever you will be. If a Tornado Warning is issued, go to the lowest floor of the building you are in, get in a small room near the center of the building (away from walls, doors, garage doors, and outside windows), get under something sturdy if possible like a desk or table, and if you have helmets (baseball, football, bike, etc) wear them.
I will have another update on this blog around 5 pm CST.
The Arctic cold front has entered north Alabama, with winds turning to NW and temperatures falling quickly into 40s. Deeper into the cold air but not far away, it is 36 in Oxford, MS and 21 in Fayetteville, AR. That air is rushing toward Alabama partially to its own weight, at wind speeds 15-35 mph. The cold front will reach BHM, TCL, and GAD around 11 pm, and temperatures will go below freezing around 6 am.
Radar shows rain along and behind the front. There has been a lot of talk about snow and ice for tomorrow morning’s rush hour in central Alabama. It is difficult for precipitation to survive in an area of cold advection, where cold air is replacing warm air and therefore flows downhill slightly, drying up precipitation. In addition, the cold air will come in shallow,, so there will be a warm nose aloft preventing snow from reaching the ground (see figure).
The problem COULD BE a brief window tomorrow morning where there is still precipitation, likely rain, falling out of it, then reaching the ground and freezing, causing some icy bridges. This is most likely in north Alabama, but is even a possibility as far SE as I-59. Another possibility is that we get 1/2″ of rain overnight, and the cold air comes in so fast some of the water puddles freeze on a few bridges. The thing helping us out on this is the very warm ground temperatures, thanks to sunshine yesterday and BHM reaching 72 degrees today. I don’t see this as a big deal, but you definitely want to check weather in the morning before leaving home, and be careful on bridges and higher elevation roads.
The cold air will be downright crazy! Temperatures will hover between 30 and 35 degrees all day tomorrow in BHM, and with northwest winds 15-30 mph, wind chills will range between 5 and 20 degrees during the day! We will drop slowly through the 20s tomorrow night, ending up near 22 degrees Tuesday morning. We will be below freezing for 24 out of 28 hours from 6 am Tue through 10 am Wed, and 17 hours straight. Remember pets, plants, pipes. And most of all, the elderly or disabled who may not have access to safe, sufficient heat.