Severe weather analysis – 100 pm

SPC severe weather risk map for Saturday

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.

500 mb (about 17,000 ft) charts for today at noon CDT and tomorrow at noon CDT

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.

850 mb (about 5,000 ft) flow tonight over CONUS

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.

Noon skew-T log-p computer model soundings for BHM noon Saturday

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.

Adapted from pivotalweather.com

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.

Adapted from pivotalweather.com

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.

Adapted from pivotalweather.com

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.

Expected radar at 1 pm CST, pivotalweather.com

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.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

Severe weather update – 515 pm

…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.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Severe weather analysis – 1210 pm

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).

Courtesy UCAR

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.

Maps courtesy pivotalweather.com

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.

Courtesy NOAA

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).

Maps courtesy pivotalweather.com

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.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

Snow, ice brief update – 830 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).

Model sounding tomorrow 6 am Trussville. Note the area above freezing aloft (yellow circle)

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.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com (check out our new website)

Major Arctic Blast Next Week

Current North American Temoeratures

Above is a map of current temperatures across North America. The large purple area covering much of central Canada, the Arctic Ocean, and Greenland, has temperatures below zero now, and some are below -20 F. We didn’t get any significant cold air flush outs in October (when it was still going above 100 F in parts of Alabama, Mississippi, and Georgia), but the sun angle has been low up in the Arctic for a while now and the days very short (0-7 hours long). So, cold air has really built up.

Over the next 2-3 days, a large upper-level ridge will build into Alaska, and the upper-level convergence on its east side will produce a large high pressure area in western Canada. Cold air is more dense and therefore heavier than warm air, and that will intensify the developing high pressure area. The shallow cold air will start to try to flow out of the dome of high pressure, but it will be blocked to its west by the Rocky Mountains, so it will be forced southeast into the U.S. starting this weekend.

Upper-levels (500 mb) and surface weather maps for Saturday morning (pivotalweather.com)

This is a very large and cold air mass. So its own weight, combined with the favorable upper-level ridge to the west keeping pressures high, and a surface low pressure area forming near the Great Lakes, will push this Arctic air deep into the central and eastern United States. By Tuesday evening, temperatures at 850 mb (about 5,000 feet) will be as much as 21 C (nearly 40 F) below normal for mid-November across most of the U.S. east of the Rocky Mtns.

850 mb temperature anomaly projected by GFS model Tue evening (pivotalweather.com)

A significant cold front will move through the Southeast US Thursday and Thursday night, with lows dropping below freezing north of I-20 (places like Little Rock, Nashville, Huntsville, and Chattanooga), and high temperatures Friday into the weekend will only be in the 40s and 50s over many areas. But this is not the main event. That moves through next Monday and Tuesday. Below is a graph of our predicted temperatures every 6 hours at Birmingham airport.

So, here we sit today near 70 degrees, we will drop to near freezing by Friday morning, have a normal, chilly weekend, then the cold blast will come Monday night. Temperatures will drop very quickly from the 50s in the early evening to the lower 30s by Tuesday morning, stay in the the 30s all day on Tuesday (with wind chills during the day in the 20s), then drop to near 20 degrees by Wednesday morning.

We have high confidence in this cold outbreak because of the ridge over Alaska and the large pool of cold air over the Arctic and Canada. Plus, all 3 major computer models (the American GFS, the Canadian GDPS, and the European ECMWF) all show this happening. A zoom-in on ECMWF model projected temperatures around the Birmingham metro ares on Wednesday morning is shown below.

ECMWF predicted temps Wed 6 am.

We didn’t have that much cold air last Winter, and if this pattern were to set up again in December or January, we could easily see temperatures near zero here in Alabama. With this cold outbreak next week, some locations in Arkansas and Tennessee may drop into the single digits, and a little snow is even possible, especially in the mountains of east Tennessee and western NC. This will be the coldest air since January 30, 2019 here in Alabama.

Interesting to note that the temperature hit 101 degrees on October 3 in Birmingham. If we drop to 21 next week, that will be an 80 degree drop in temperature in less than 45 days. This large of a temperature drop in 45 days has only occurred 5 other times since 1900.

This is the kind of cold air outbreak than can freeze pipes, kill plants, and harm animals. Go ahead and think about winterization around the home, such as covering outdoor faucets, draining water hoses, and winterizing boats. Make sure your antifreeze is good in your car and your tire pressure is sufficient (it will drop significantly next week). Once the cold air arrives, if you want any plants to live, bring them inside or cover them. Also remember that it is sometimes up to 5 degrees colder right near the ground than the official 2 meter temperature, so bring pets inside or at least make sure they have plenty of warm stuff to use inside a dog house, and dog house doors should never face north or west. Check on any elderly or disabled people you think might not have sufficient, safe heat. And check your own heating system if it hasn’t run this year.

Dr. Timothy A. Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com (check out our new website!)

Big time cold front!

The biggest cold front of the year so far will move through Alabama on Thursday. Temperatures will drop very quickly during the day, starting out near 70 at 7 am, drop to 55 by 10 am, then into the 40s during the afternoon. It will feel even colder, with northwest winds 10 to 20 mph. It will be about 25 degrees colder when you get home tomorrow evening than when you leave tomorrow morning, so dress appropriately!

Temperatures across the Southeast US show the strong cold front right now (1:45 pm CDT). It is near 70 across much of Alabama and Mississippi, but behind the cold front it is only in the 50s in central Arkansas, 40s in Fayetteville and Fort Smith, AR, then 30s in Oklahoma.

The cold front will tighten up overnight. Take a look at the computer model based temperature chart for tomorrow morning at 7 am. That is a huge temperature change over a short distance.

We may some light rain tomorrow, but it will likely move out in time for kids’ trick or treat activities. But it will be cold, so bundle them up! At 8 pm, expect 42 degrees with a wind chill in the 30s.

Early mornings this weekend we will get close to freezing, and many locations north of I-20 will get their first freeze of the year this weekend. There will probably be enough wind to prevent frost on Friday morning, but frost is possible Saturday and Sunday mornings.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com

(See our new website!)

The heat in perspective

The above chart shows the 120-year average high temperatures in Birmingham, AL (black), along with the 1-standard deviation bands (where about 70% of all high temperatures should fall on a given date). Through most of 2019, temperatures stayed within these bands most of the time, so it was a normal year. Even the 100 degree days in August were not too far outside the normal. Note how the bands are tightly packed in the summer, meaning there is not that much variance from the normal in the Summer. The bands get much wider in the cold season. In late December, for example, high temperatures may be as low as the 20s or as high as the 70s!

But then, in September, we went into a very dry pattern, no cold fronts came through, and temperatures stayed very hot. Droughts and heat waves go together…the sun heats the ground which then heats the air, and if there is no water to evaporate, all of the sun’s energy goes into heating.

September 2019 was the 2nd warmest September on record with an average high of 94.5 and average temperature of 82.1. Only Sep 1925 was hotter. Sep 2019 was the 21st warmest month on record, period, beating out dozens of Julys and Augusts.

Using Coleman and Knupp, LLC’s 7-day moving average for all days going back decades, the expected distribution of high temperatures (and low temperatures) can be shown for any day of the year, in any city, with expanded precision. Below is the distribution for Oct 1, yesterday, for Birmingham. It is close to a normal distribution, with a median of 81 degrees, mode 83 degrees, standard deviation 7 degrees. We hit 99 yesterday, something that had never happened during the period Sep 29-Oct 4 before, so obviously it was a record high temperature.

But, using climate data from any site, we at Coleman and Knupp, LLC can custom design the probabilities of exceeding or going below any high or low temperature at any location, and the probability of given amounts of rain. This can be useful for planning weddings and other outdoor events beyond 2 weeks out, when we have essentially no weather predictive capability.

The good news is that a cool front may touch off come clouds and showers this weekend, cooling us off a few degrees, then a more significant cold front will sweep through early next week, making it feel a bit more like October. And, as the sun’s angle and the length of day are both decreasing rapidly now, the Northern Hemisphere has to cool off and more cold fronts will come to the eastern U.S.! Below I show the length of day at Birmingham across the year (solid), and the rate of change (dashed). Right now, our days are just under 12 hours long, and we are losing about 2 minutes of daylight every day. The Geochron-style image shows that the sun is illuminating more of the Southern Hemisphere than the Northern one now, too.

Dr. Timothy A. Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

http://www.colemanandknupp.com