Early Heat Wave for Southeast

Current weather map across SE

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

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

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

Gulf of Mexico water temperatures

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

Surface pressure and winds on Monday (pivotalweather.com)

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

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

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

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

Dr. Tim Coleman

Consulting Meteorologist, Coleman and Knupp LLC

Twitter @timbhm

Sun strong, days long now!

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

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

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

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

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

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

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

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

Dr. Tim Coleman

Twitter @timbhm


Severe weather analysis – 215 pm

NOAA/SPC convective risk outlook

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

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

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

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

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

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

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

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

Dr. Tim Coleman

Twitter @timbhm


Dangerous storms central AL now

Reflectivity and velocity Birmingham radar 515 pm

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

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

Dr. Tim Coleman

Twitter @timbhm

Textbook storm across US – Tornado update 401 pm

Upper-level winds (500 mb, about 18,000 ft); surface pressure, and surface temperature at 7 am CDT
Visible satellite picture over US (courtesy WU)

The storm system that has developed over the continental United States over the past 24 hours is truly remarkable. It is ironic that it developed on March 13, 2019, because the most perfect atmospheric storm/heat engine that I have seen in my career occurred on March 13, 1993 (known to most southerners and the northeast megalopolis as “The Blizzard of 1993).

This one has some different characteristics, including the way it initially developed, as a “lee cyclone”, in the lee of the Rocky Mountains. However, the surface low interacted with a strong zone of temperature gradient over the Plains, and the upper-level cyclone deepened rapidly as cold air was pulled southward and warm air was pulled northward by the surface cyclone. The positive feedback loop began, and the perfect heat engine developed, transporting warm air away from the tropics toward the poles, and cold air from the poles into the midlatitudes. These big storms keep our atmosphere in balance. This storm has caused record low surface pressures in Oklahoma, blizzards in Colorado and Nebraska, flooding, and is now causing severe weather in the east.

A cold front is moving eastward, and the upper-level low continues to produce strong wind shear and forcing for ascent from Michigan to the Gulf Coast. Warm, moist air has moved northward and has produced an unstable air mass for thunderstorm updrafts through much of Indiana, Kentucky, Tennessee, Mississippi, and Alabama, and severe storms have developed. A tornado passed very near the Paducah National Weather Service office, and blew commercial air conditioning units off a mall. There have been numerous reports of straight-line wind damage and large hail, and the storms are still going.

Zooming into Alabama, NW Alabama is under a Tornado Watch, and I expect that to be extended eastward to include Birmingham soon.

A line of severe storms extends from near Huntsville to Cullman to Jasper to Reform, then on into Mississippi. There is strong wind shear, with 0-1 km helicity above 300 m2/s2 over much of western and central Alabama. The most unstable air is over south Alabama, but with the cold upper level temperatures and surface temperatures in the 70s with dewpoints in the 60s as far north as Huntsville, CAPE values are 500-1000 J/kg all the way into Tennessee. The temperature is 77 at BHM and 82 in TCL, but the NWS balloon release at 1 pm CDT showed a weak temperature inversion at mid-levels, that may be inhibiting storm development somewhat. Still, the storms near Cullman, Jasper, and just south of Fayette show signs of large hail and all 3 are rotating.

As the afternoon goes on, the air will destabilize further over central Alabama, but the wind shear will begin to pull away as the upper storm system races toward Canada. However, there will be a time overlap between now and about 7 pm when intense storms with large hail and damaging winds are likely in central Alabama, and a couple of tornadoes are likely also. The biggest threat here in the Birmingham metro area will be around 7 pm.

This is not a big outbreak of tornadoes, but be prepared in case a tornado warning is issued for your area. Have at least two ways to get tornado warnings in addition to outdoor tornado sirens. A cell phone app and NOAA weather radio that alert you are best. Remember, lowest floor, smallest room, center of the building, away from doors, windows, protect your head with a helmet or pillows.

Dr. Tim Coleman

Twitter @timbhm

Severe weather analysis – 430 pm

Current radar

…Tornado Watch in effect for much of north and west Alabama…includes Huntsville, Birmingham, and Tuscaloosa…

Right now, we have an area of showers with some claps of thunder moving through central Alabama. One of the storms in this band earlier prompted a tornado warning for counties in extreme north Alabama, but that warning is gone and I have not heard of any reports of damage.

We have an environment today that has limited instability for updraft growth (surface temperatures are only in the low 70s in most areas, with dewpoints in the low to mid 60s). An area of dry air is parked over central Alabama, and as it gets mixed down to the ground, it limits our instability. The largest instability (CAPE values) are currently in northeast MS and extreme NW AL, where the dry air has not made it (and may not).

CAPE right now

This storm system is mainly scary at all because it has very large wind shear for storm rotation (and the dynamics created by the wind shear can help updrafts grow also). 850 mb pressure level winds (about 5,000 feet) are about 60 mph over the Birmingham area, and increase to 80 mph over NE MS in the mid-level jet max. This is producing the large wind shear. 0-1 km storm-relative helicity values, a really good measure of wind shear for storm rotation, is extremely high, with areas in NW AL near 400 m2/s2. It is near 250 m2/s2 here in BHM.

850 mb (5,000 ft) winds
0-1 km storm-relative helicity

The storms that are currently moving through central Alabama should move on out in the next hour or so without incident. It is that next line of storms in northern MS that is somewhat concerning. However, the sun sets in one hour, so as it gets dark it will be difficult for the atmosphere to destabilize any further. However, the strongest wind shear will move into NW Alabama around 6:00 pm with those storms, and the most unstable air is also up there (areas like Florence, Moulton, Russellville, Hamilton, Double Springs, and Vernon). So, the largest risk for tornadoes in Alabama will be in those areas between 5 pm and 8 pm.

As the storms move farther southeast toward Birmingham and Tuscaloosa, the instability will be weaker. However, given the strong wind shear in the environment, we will likely see some damaging winds and we can’t rule out a tornado here in central Alabama this evening. The storms will move through the BHM metro between 10 am and 2 pm.

This does not look like it will be a tornado outbreak, especially not here in central Alabama. We will have to watch the storms coming out of north Mississippi into NW Alabama very carefully over the next 2 hours. We are under a tornado watch, so have a safety plan in mind for wherever you are. Remember, lowest floor, middle of the building, away from windows and doors, protect your head.

Dr. Tim Coleman

Twitter @timbhm

Weather update – 707 pm

Tornado on US 72 near Burnsville, MS (courtesy UAH SWIRLL)


Several tornadoes have occurred over Mississippi and Alabama today.  The one above in north MS, then we have always gotten reports of severe tornado damage in Columbus, MS.  There are also reports of wind damage in parts of Lamar, Fayette, and Marion Counties in Alabama.

The initial line of storms that brought funnel clouds, wall clouds, and torrential rainfall to parts of Tuscaloosa, Walker, Jefferson, and St. Clair counties this afternoon has moved out.  The main area of storms is now moving through western and central Alabama.  Currently, there are no tornado warnings in Alabama.

The strongest storms extend from Huntsville to Decatur, through Winston County near Smith Lake, and moving into Walker County.  That Walker County storm is rotating and is moving toward Cordova and Parrish.

The instability in the atmosphere has decreased due to rainfall and sunset.  CAPE has decreased to around 500 J/kg.  But, wind shear actually increased a little bit with sunset (something we are researching at UAH), and with the man storm system approaching.  So, a few storms could still rotate, and an isolated tornado can not be ruled out this evening. 

But, the threat for tornadoes has greatly decreased and will decrease through the evening.  However, so much rain has fallen the past few days, and now with dark, flash flooding will be the biggest threat tonight.  Do not cross any roads with water over them.  It is not worth it to drown.  The governor has issued a state of emergency for the northern 1/3 of Alabama due to long-term flooding.

The whole thing will be over in Birmingham by around midnight.  Sunny and cooler weather for early week!

Dr. Tim Coleman


Twitter @timbhm