Tornado analysis for Sunday

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.

Water Vapor Satellite courtesy NASA/MSFC
NAM computer model 500 mb (18,000 ft.) flow tonight, Sun AM, Sun PM

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.

NAM model surface pressure and wind at 7 pm Sunday
NAM model 850 mb (5000 ft.) flow at 7 pm Sunday

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.

NAM hodograph at Trussville, AL at 4 pm Sunday

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.

Observed SE surface at 2 pm CDT

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.

NAM skew-T log P for 4 pm Sunday for Trussville, AL

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.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

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