…PARTICULARLY DANGEROUS SITUATION TORNADO WATCH REMAINS IN EFFECT FOR MOST OF NORTH AND CENTRAL ALABAMA…
Current radar shows the intense supercell in Cherokee County, a tornado-warned storm approaching Florence/MSL, and a tornadic storm about to move into Sumter County. Plus, it shows a lot of non-severe rain and storms over northeast AL. This is keeping the warm, unstable air back in south and west Alabama.
An area of concern is developing in NW Alabama as temperatures are a bit warmer, the widespread rain is moving out, and the storms are starting to look more cellular. Satellite shows there may be a little sunshine in that area over the next 2 hours.
Because the surface low moved farther east than expected, and is currently in northeast Arkansas, the highest wind shear so far has remained to the east, over Alabama, while the most unstable air has been over Mississippi where it is warmer.
As the upper trough, surface low, and 850 mb jet continues to lift northward, wind shear may increase a bit in NW Alabama and some of the heavy rain may continue to thin out north of I-59 and west of I-65. If so, some of the storms in NE MS/NW AL could become tornadic like the one near MSL has.
Eventually, this evening, the storms will form into a line and move on through HSV and BHM around 10-11 pm. There have been numerous reports of damage and injuries with the long-track supercell that hasd moved from SW AL through BHM suburbs to Anniston area.
The loud storms that have been moving through the BHM area over the past 3 hours are elevated storms, with unstable air aloft but still cool, stable air at the surface. But the line of storms over west Alabama is in a confluence zone out ahead of the main low pressure area, that is ahead of schedule and according to my non-atristic analysis, is sitting in Southeast AR now. A couple of the storms are near the warm front (circled) and are starting to acquire rotation over TCL and Walker Counties…we will have to watch those. Note the wind blowing toward the orange confluence line I drew in on both sides. I expected this zone today, but not until afternoon. But it’s ahead of schedule too.
With strong 850 mb winds (50-60 kt) blowing from the now very moist and unstable airmass over southern Mississippi, the warm front will continue to slowly move north as the surface low moves north along the Mississippi River. The storms north of the front help to slow it down due to the cooling effect of the rain (and it is a warm front after all)., but it will likely move on north slowly into the afternoon. The system moving a bit faster has helped us somewhat, and the longer we keep getting elevated storms north of the warm front, the more it will slow it down and give us less time to destabilize. But, it does look like the warm, unstable air will make its way into north and central AL just through advection by 1 or 2 pm.
You can see the shadows cast by the big storms in central Alabama on visible satellite…but there are breaks in the clouds over Mississippi, and temperatures are rising quickly over there. 70s as far north as Tupelo. Hard to say if we get sunshine here in Alabama today, but it is most likely in the high risk zone over northwest AL.
The SPC doesn’t often issue a HIGH risk at all. And they don’t often say things like they did in the above paragraph.
Despite the morning storms possibly delaying things and maybe even keeping a few areas a little less volatile, this is still a dangerous situation. The wind shear will still be very high (although not quite as high as we thought after the morning storms), but even 75% of the forecast numbers from last night still support tornadic supercells over western AL this afternoon…then a line of tornadic storms moving into central AL this evening. Don’t let your guard down…it still looks like a dangerous day. We will get a better handle on the effects of the earlier confluence zone and morning storms over the next 2 hours. That zone may stay in place from eastern MS into west AL through the afternoon…providing a focus for tornadic storms as the more unstable air moves in.
SPC is planning to issue Tornado Watches for much of AL and MS soon.
It looks like the ingredients are coming together for a significant outbreak of severe weather, including a few long-track significant tornadoes, over the Southeast US over the next 24 hours. A sharp upper trough is already apparent over NM and AZ, and it will move eastward overnight and interact with a warm front currently stretching from Austin, TX to Montgomery, AL. The dewpoints along the Gulf Coast are not that impressive right now, relative to severe weather outbreaks, as shown in the current surface map (dewpoint in green). The dewpoints in the 70s are way out at the southernmost oil platforms.
However, the upper trough will cause rapid height falls along the warm front, and a low-level jet will form, that will transport warm, humid Gulf air all the way to Tennessee. Parcel trajectories indicate that the air currently over north AL is right now over west Cuba.
The surface low will rapidly deepen, causing flow across the isobars at low levels because the atmosphere can not remain in balance. This will lead to very large wind shear and storm-relative helicity for tornadoes over the Southern U.S. tomorrow. Check out how quickly the European model deepens the surface low just between 1 pm and 7 pm tomorrow:
One complicating factor may be that showers and thunderstorms, likely not severe, will move northward through north and central AL and MS as the warm front lifts northward, much like the system last week. The European model at 1 pm (shown above) still indicates a good bit of rain north of a line from Jasper to Gadsden. How quickly this widespread rain moves out may determine how far north the warm, unstable air can go. But, there is no cold wedge holding it back this week like last week, so it will likely go farther north, at least to a line from Florence to Cullman to Anniston. Below is the expected CAPE (instability for thunderstorm growth) and storm-relative helicity (for storm rotation) expected at 22 GMT (5 pm).
CAPE values increase from around 1,200 J/kg along I-65 to about 2,000 J/kg near the AL/MS border. SRH values are very high, between 300 and 400 m2/s2, from eastern MS into central AL. This combination creates a very high energy-helicity index (EHI), a good indicator of the likelihood of tornadoes, as shown below:
At 22Z (5 pm), there is widespread EHI between 3 and 5, sufficient for intense tornadoes, centered near Columbus, MS, and including Tuscaloosa, Hamilton, and Demopolis. With upper-level diffluence and low-level convergence ahead of the dryline/surface trough, expect storms to form in those areas between noon and 3 pm, and some will quickly grow into supercells with large hail and tornadoes. That part of the state I just outlined, west of a line from Grove Hill to Jasper to Huntsville, has the greatest risk for tornadoes tomorrow afternoon. Storms will me moving quickly, from the SW (or about 230 degrees) at 55 mph, so that adds extra risk. By 04Z (11 pm), the EHI has a sharp edge at the back, and it appears that as the strongest wind shear pulls away after sunset, and surface temperatures cool a bit, lessening instability, the storms will form into a line and move through the rest of north and central AL during the evening. With EHI still 2-3, tornadoes will likely accompany some of these storms, along with straight line winds up to 70 mph. Below are computer model temperature profiles for Hackleburg, AL at 2 pm and 5 pm.
Another complicating factor is a small but potentially very significant nose of warm air near 7,000 feet, mentioned in the previous blog, that could inhibit storms from forming in the afternoon. At Hackleburg, as the strongest forcing associated with the upper trough and dryline approaches, that warm cap of air erodes, so supercell development is likely in that part of the state. Below is a profile from Bluff Creek, AL, just west of BHM. Even at 6 pm, a but of warm cap is still present.
A good way to measure the effect of these “caps” is through convective inhibition (CIN), a number similar to CAPE but in reverse, meaning how much negative energy is there to hold back updrafts at low-levels. Below, at 5 pm, one can see an area of CIN from Jefferson County northeast into Blount, Shelby, St. Clair, and Etowah Counties. This may be a big key in preventing supercell storms a bit farther east. But it is a small warm nose and tough to forecast with the atmosphere changing so fast. It could be stronger and extend further north and west, preventing supercell storms in Huntsville, Jasper, and Tuscaloosa. Or it could be a little weaker, allowing supercell storms all over central AL. We will have to keep an eye on that.
The expected EHI values of 3 to 5 are indicative of a significant severe weather event. There are still complicating factors that I mentioned that could weaken the system, including morning and early afternoon showers near the warm front, and the cap discussed immediately before this. However, we have to be ready for tornadoes and strong winds across all of north and central Alabama…because even if the supercells don’t affect you in the afternoon with potentially larger tornadoes, the squall line in the evening will still likely be intense and contain tornadoes.
Think about your usual safety plans for tomorrow afternoon and evening. Have multiple ways of getting severe weather warnings. I recommend NOAA Weather Radio and the phone app called “Tornado” that has a white icon with a picture of a red squiggly tornado with a plus at the bottom. You have to set it up for the places you want to be warned for. I will have another update tomorrow around or before lunchtime and go over more detailed safety rules, but you know what to do…lowest floor, away from windows and doors, away from outside walls, small room, get under something sturdy, helmets or pillows to protect your head, if you have cars in the basement, some recommend getting in them and getting down in the seats to protect yourself from falling debris.
UAH will be releasing weather balloons and have the mobile radars, wind profilers, lidars, and radiometers out across north Alabama, including Hackleburg, Courtland, and Huntsville, plus one weather logging vehicle driving around rogue to where we need data. To capture the warm frontal passage and destabilization from SW to NE, weather balloons and instruments will likely need to start taking data around 1700 GMT (noon) and continue until the main squall line passes Huntsville, around 10 pm. The main line will pass BHM around 11 pm. We hope to get a full picture of how the atmosphere changes going from the cool air north of the warm front to the warm, unstable air south of it, so operations may need to begin by 10 or 11 am at our SWIRLL facility at UAH.
This one has the potential to be bad, but things could still greatly slow it down, so no reason to panic. And, again, this is no April 27, 2011! EHI then was not 3 to 5, it was 8 to 11!
Earlier this evening, it got quite windy outside, with wind gusts as high as 43 mph at the BHM Airport, and likely higher on ridgetops. These high winds were not caused by a thunderstorm downdraft, nor by more typical atmospheric pressure gradients. They were caused by a phenomenon known as a “wake low”. The pressure at BHM dropped 5 mb (about 0.15″) in only about 30 minutes, then rebounded fairly quickly. The peak winds occurred as the lowest pressure went by. These wake lows sometimes can produce winds over 70 mph, cause downed trees and power lines, and even cause injuries, as one did in Atlanta several years ago.
Sometimes, at the back edge of a convective rain event, something called a rear-inflow jet (RIJ) develops due to thermodynamic imbalances. Air rushes toward the front of the rain mass, in this case at speeds around 40 mph. These RIJ’s are sometimes forced downward, by evaporating rain above them, by being suddenly decelerated in the rain area itself, etc. This downward motion warms the air, and dries up raindrops, causing a sharp back edge to the precipitation, as shown in the radar image from the NWS Birmingham radar below at 2249 GMT (5:49 pm CDT).
This downward motion aloft also caused the air to get compressed, as if putting it into a tire, and warmed. Since warm air is less dense than cool air, low pressure is produced underneath the lighter air at the surface. This warming aloft is what caused the low pressure in the wake of the rain, known as a wake low. Below are cross-sections of reflectivity (rainfall) like you see above, and Doppler radial velocity (green is toward radar, or SE, red is away from radar, or NW). These cross-sections are along the white line in the above radar picture, at that time. You are looking at these images from the side, as if you are looking from the southwest and up to about 10,000 meters (30,000 feet).
Note the RIJ in the bottom panel as a jet of high inbound velocities (blue) descending from about 5000 meters to 3000 m between roughly northern Tuscaloosa County (left side of image) to Shelby County Airport (right side). Near the ground, air is rushing away from the radar (red) toward the low pressure created by the downward motion aloft. This is the wind from the SE that we felt this evening.
…Another outbreak of severe weather, including tornadoes, possible on Thursday…
Here we go again. There is more uncertainty with this system than with the one last week, but it still looks threatening. First of all, for today (Tuesday), we have a system moving through with rain, and some thunderstorms. But, the best instability for any strong storms will be near the Gulf Coast, and no severe weather is expected north of a line from Monroeville to Opp to Enterprise.
The above full-disk image from GOES is fascinating, in that it shows that daylight is fairly evenly distributed in the northern and southern hemispheres (we just passed the equinox), the sun just came up in Hawaii, and there is a second upper-level system (circled) in the southwest US, headed our way.
A significant upper trough is currently centered over southern California, and it will move out into the Southern Plains by Thursday morning. Here is where some of the uncertainty comes in…there has been some computer model disagreement on the strength of the upper system coming in. Compared to last week, this upper system does not look as intense over OK/AR/MO/KS on the American models. But, the European model has a stronger system. Even with the weaker upper system, there will not be a wedge of cool air coming in from the east this time to weaken most storms as they get north and east of Tuscaloosa, and the wind shear in this system will be high. The American GFS model this morning has come in with a classic severe weather setup, with a surface low in Tennessee by Thursday afternoon.
The NAM, showing the weakest solution of the three major models, is still indicating a setup over north and central Alabama for tornadoes. As the upper system approaches, falling pressures will lead to a strong low-level jet stream, transporting warm, moist air from the Gulf into Alabama. This will produce unstable air over Alabama. With the surface low so close by and deepening as it moves across to our north, wind shear values will be very high.
Between 3 pm and 10 pm on Thursday afternoon, CAPE values will increase from west to east, to 1,000 to 2,000 J/kg. In addition, storm-relative helicity values, that indicate wind shear for storm rotation, will be quite high, between 300 and 500 m2/s2. These, in combination with high ground-level relative humidity, will produce a significant risk for tornadoes. This is measured by the Significant Tornado Parameter (STP), that used all three of these values. Below is a map of the STP for late Thu afternoon and Thu evening. Any value over 1 is sufficient for tornadoes, and we have some values up near 5 in some places/times.
Note that the greatest threat for tornadoes will be over Mississippi and the northwest 1/2 of Alabama, mainly north and west of I-59, including Birmingham, Tuscaloosa, Huntsville, Florence, Jasper, Cullman, Hamilton, Fayette, and Demopolis. The greatest threat for supercell tornadoes will be in the late afternoon and early evening hours. By late evening, the storms will likely form into a line, but still have a risk of 70 mph winds and tornadoes as they move through. Expect the main line of storms in the BHM area between 11 pm and 3 am.
One other factor that could keep supercells from forming in the unstable air out ahead of the main line is a small cap of warm air aloft, as shown on the computer-model generated temperature profile below. The red line shows how temperature changes with height. The jump over to the right near 800 mb (about 7,000 feet) is a warm area. When thunderstorms build, they do so primarily due to buoyancy, or by the air in the storm updraft being warmer than the air around it. If the storms encounter warm environmental air, it could suppress them until the main line comes through during the late evening. However, if this warm cap does not appear, or the sun comes out and warms the surface temperature into the 70s so that the thunderstorm air just goes right through the cap, it won’t matter and we’ll have supercell storms with possibly large tornadoes like we did last Wednesday. Just another uncertainty right now.
This system will bring storms with heavy rain, strong winds, and possibly hail and tornadoes, Thursday night. It is possible that intense supercell storms will form earlier, between 2 pm and 7 pm, and produce large hail and significant tornadoes. It could get rough. We don’t know yet. The farther west you are, the larger the risk. We will know more as the system develops and gets closer, but every severe weather system is different. I will be watching this very closely. UAH plans to deploy groups to collect weather balloon and surface data, as well as Doppler wind lidar, wind profiler, and mobile Doppler radar data, across north Alabama. Another blog from me tomorrow evening.
…Tornado outbreak still underway…more to come this evening…
It appears that we have already had tornadoes in the following locations in Alabama: Autaugaville, Demopolis, Weogufka, Moundville, and near Howton’s Camp on the Warrior River.
The above visible satellite picture, with temperatures I added on, shows a good idea of where the cold wedge remains. Because of all the rain and storms keeping the air north of it cool (see radar picture below), it hasn’t moved very far north during the day. But with some daytime heating and moderate wind shear over south-central and west-central Alabama, those storms got going in a hurry around noon. We still have several intense supercells with possible tornadoes in the western BHM metro, and also near Selma, Orrville, and west of Thomasville.
Note that, in the warmer areas over Mississippi and northwest Alabama, there is less heavy rain and the storms are more surrounded by drier air. There is less wind shear in those areas right now (SRH < 150 m2/s2). However, as the main low-level jet overspreads the area between 5 pm and 9 pm, expect those storms to intensify and possibly become tornadic as they move into Alabama. These storms could affect areas like Aliceville, Jasper, Birmingham, Tuscaloosa, Fayette, Cullman, Moulton, Florence, and Decatur. Also the two UAH VORTEX-SE observing platforms in Courtland, AL and Hackleburg, AL. It is unclear how much all the rain over northern and central Alabama will help to weaken these storms as they move east; the atmosphere is worked over from storms and stabilized by rain, but with 70 mph south winds at 5,000 feet coming in from areas where it is currently near 80 degrees, it will likely recharge the atmosphere at least somewhat. The storms will likely merge into an intense line of storms late this evening, with tornadoes still possible, in addition to damaging straight-line winds and hail.
This is a rapidly developing and complex weather situation. But, no people should let their guard down, whether tornadoes have gone near you in this first round, or if they haven’t. Keep a plan in mind for sheltering through the evening, as night tornadoes are even more dangerous because we can’t see them to confirm them.
The tornado outbreak is underway over central Alabama. The above photo was taken earlier near Autaugaville, AL in the storm that moved into Chilton and Coosa County.
The above image shows radar reflectivity (top) and Doppler velocity (bottom). The most unstable air is still struggling to move NE of BHM…but in west and south AL CAPE has gone up to nearly 3,000 J/kg with moderate wind shear of 200-250 m2/s2. This has allowed numerous supercells with tornadoes to form. Right now, potentially tornadic storms are near West Jefferson, Tuscaloosa, Moundville, Demopolis, Linden, and Sweet Water. We have already seen a large tornado today, so take Tornado Warnings very seriously. Large hail has also been reported in several locations.
With all the rain and storms and clouds (see white circle), the warm front is still having a hard time moving NE. But, it has made some progress as evidenced by an apparent tornado touchdown along the Warrior River near Franklin Ferry on the Jefferson/Walker County line. With strong SW flow, the front will move very slowly NE, potentially putting much of Tuscaloosa, Walker, Shelby, and Jefferson Counties in the danger zone, with unstable air and high shear, between now and 5 pm. Some of that depends on rain-induced cooling. But heads up Tuscaloosa, Birmingham, and west Alabama areas up toward Jasper and Hackleburg.
…Significant outbreak of severe weather including tornadoes next 12 hours…
The Storm Prediction Center has gone all in with this one…we are in the seldom-used “high risk” for severe weather, and we are under a PDS (Particularly Dangerous Situation) Tornado Watch here in much of Alabama. The warm front between the cool wedge from the NE and the extremely warm and humid air from the Gulf is sitting across central Alabama now. It shows up well in the following map of CAPE (best measure of instability). The front extends from roughly MSL to BHM to AUB.
Storm-relative helicity is generally 100-200 m2/s2 right now, but with a weak surface trough, there is a maximum in SW Alabama centered around Grove Hill and Monroeville. The entire area from there and SE Mississippi near Meridian…up to TCL…must be watched closely for tornado development over the next 2 hours. Dewpoints at several stations in eastern MS and SW AL are in the lower 70s, so any storms there will grow quickly and potentially rotate.
Visible satellite shows breaks in the clouds over MS and parts of AL…so we will get some sunshine in some areas today…especially western AL. Temps may reach 75-80 over parts of west and south AL, creating a volatile environment for storms.
With thunderstorms and heavy rain developing along and north of the warm front over central AL…the front may be slow to move further north over the next few hours due to rain reinforcing the cool air to its north.
Computer model consensus would take it to a line from DCU to ANB by 5 pm CDT…with the best chance for severe weather SW of that line over places like Hackleburg, Hamilton, Jasper, Tuscaloosa, Birmingham, Selma, Montgomery, and Grove Hill (see below).
The primary 850 mb jet and highest SRH (over 400 m2/s2 in many locations) will move through between 04Z and 08Z. Even though CAPE will be reduced by darkness, there will be enough along with the wind shear to produce a line of intense storms, some of them with tornadoes.
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.
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.
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.
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.
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.
…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.
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.
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.
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.
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.
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.
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