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)

…TORNADO WATCH UNTIL 10 PM…

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

http://www.colemanandknupp.com

Twitter @timbhm

Update – 345 pm

Reflectivity (left), velocity (right) from BMX radar

…TORNADO WATCH UNTIL 10 PM…

…TORNADO WARNING CULLMAN COUNTY NOW…

A band of supercell storms has formed in western Alabama and is now moving into central Alabama. These storms are rotating.

The one with the most vigorous rotation, that has caused the NWS to issue Tornado Warnings for northern Walker, southern Winston, and now Cullman counties, is currently west of Cullman, approaching Trimble and possibly Exit 310 on I-65.

There is another one in southwest Jefferson county with rotation and a significant “hook echo” on radar, but so far the NWS has not issued any warnings on it. It went right over Tuscaloosa with no damage reports I am aware of.

It will be interesting to see what happens to these storms in Cullman and Jefferson Counties as they cross a thermal boundary between the cooler air trying to come in from Georgia and the warm humid air over west Alabama.

Dr. Tim Coleman

http://www.colemanandknupp.com

Twitter @timbhm

Severe weather analysis – 1241 pm

NOAA/SPC severe weather outlook for today and tonight

The Storm Prediction Center has upgraded north MS to a moderate risk of severe storms, and extended the enhanced risk into NW AL. A tornado watch has just been issued for most of Mississippi. For a discussion of the larger scale features driving this event, see yesterday’s blog at https://ckweather.com/2019/02/23/severe-weather-analysis-for-tomorrow/

GOES visible satellite image 12:07 pm CST

There are breaks in the clouds over west AL and east MS early this afternoon, allowing temperatures to really warm up. In east AL, a wedge cold front is keeping it much cooler. Currently, it is 79 degrees in TCL, 71 in BHM, but only 63 in Anniston (and 50 in Atlanta). Perhaps even more striking is that the dewpoint in TCL is already up to 67 degrees, almost like summertime, and south winds will continue to bring moist air northward through the afternoon. With all this warm, moist air coming north and the breaks in the clouds, the air is already unstable from just west of I-65 back into Mississippi. SPC mesoanalysis indicates CAPE >1500 J/kg as far east as Tuscaloosa and Hamilton.

These numbers are pretty close. A VORTEX SE balloon sounding from Starkville, MS at 11 am CST showed a CAPE of 1,370 J/kg and it has warmed up some since then. The observed 0-1 km helicity was 182 m2/s2, enough for rotating storms, but not quite as high as the computer models are putting out there, partially because the winds are so high at the surface.

Starkville, MS balloon data from 11 am CST

As warm, moist air continues to pour into MS this afternoon and the forcing for ascent gets closer as the upper system and front move east, storms will likely form over MS in the next 2-3 hours out ahead of the main line of storms that is currently just west of the Mississippi River. Some of these storms will likely be supercells, capable of producing large hail and tornadoes.

As far Alabama, with the warm, humid air in place in Tuscaloosa, Demopolis, and Greenville, and that air moving north toward Jasper and Hamilton, storms could begin to fire as early as 4 pm, and they will increase as the forcing gets closer around 6-7 pm. The highest risk for tornadoes in Alabama is north and west of a line from Huntsville to Double Springs to Jasper to Aliceville, but we can’t rule out a tornado in areas farther east like Tuscaloosa, Jefferson, and Shelby counties.

The main line of storms will move through around midnight, with damaging straight line winds as the main threat. As my colleague Nathan Burks points out, the ground is so saturated from all the rain, trees will go down easier than they normally would.

This still does not look like it will be a major outbreak of tornadoes here in Alabama, as the system will weaken slowly as it moves in here this evening and grow into a squall line, and those are less likely to produce tornadoes than the isolated supercell storms we expect to the west. But, 2 or 3 tornadoes are likely in Alabama, mainly in the areas I outlined.

Just be ready, keep an eye on the weather this evening, and have a source of weather information including tornado watches and warnings. Know your tornado safety rules.

Dr. Tim Coleman

http://www.colemanandknupp.com

Severe weather analysis for tomorrow

This has been one of the rainiest weeks in north and central Alabama in a very long time. Some rainfall totals for the week include 3.63″ in Birmingham, 3.72″ in Tuscaloosa, and 6.89″ in Huntsville. It looks like the rainy pattern will end after a cold front moves through this weekend. But, as it does, it will bring our first real chance for severe weather in north Alabama in 2019.

NOAA/SPC Severe Weather Outlook for Sat/Sat Night

A front has been stalled over Alabama and Tennessee for the past several days, with a strong ridge of high pressure at upper levels to its SE keeping it in place. This front, and upper-level disturbances moving along, it have given us all the rain and thunderstorms this past week. You can see the big upper ridge in satellite water vapor imagery.

WV channel satellite image

But, a large upper-level trough is moving through the southwest US now, and will move out into the Plains states tomorrow. The upper divergence ahead of this trough will cause a surface low pressure area to form and move to near Kansas City by late tomorrow afternoon. It looks like this upper-level system will finally be strong enough to push the cold front through Alabama early Sunday morning.

Current flow at 300 mb (about 30,000 ft.) Trough is highlighted in yellow
Surface pressure and winds tomorrow at 3 pm (2100 GMT)

With the deepening surface low to our NW, winds will become gusty out of the south tomorrow, near 30 mph at times. If the wind is strong enough to break the low clouds, temperatures could warm into the mid 70s in the Birmingham area, but even without sunshine it will be near 70 due to warm air coming from the south.

Those winds will also bring in some of the very humid air currently in place over Louisiana and the Gulf of Mexico, and with the ground as wet as it is, the moisture will not modify that much. Winds at 850 mb (about 5,000 ft) will reach 55 kt (65 mph) and help transport in the warm, humid air at low-levels, especially over Mississippi and extreme west Alabama. With the air fairly cold aloft, this warm, humid air near the ground will create some instability for thunderstorm updrafts. CAPE values over MS and western AL tomorrow afternoon will likely reach 1,000 J/kg, pretty high for February.

Computer model expected 850 mb winds, surface dewpoint, and CAPE at 6 pm CST tomorrow

The areas in blue on the last panel will have CAPE above 1,000 J/kg. With the strong upper-level system there will be plenty of wind shear to support severe storm organization and potentially rotation, with helicity values over 300 m2/s2 over most of the northern half of Alabama tomorrow.

This combination of shear and instability means thunderstorms are likely. They will develop over Mississippi during the day, and a few may move into Alabama, especially west of I-65, before sunset. These storms may contain heavy rain, damaging winds over 60 mph, hail, and dangerous lightning. The significant tornado parameter is a combination of CAPE, helicity, and cloud base, and gives us an idea of tornado potential.

The map above is for 6 pm CST tomorrow. Note the best combo of shear and instability for tornadoes will be over northern MS, but anywhere shaded in blue has a value of 1, enough for the atmosphere to produce a tornado. We have some very high-resolution computer models now that actually simulate expected storm formation (this is fairly new). One of those shows some individual storms, or supercells, over west Alabama tomorrow at 6 pm. This is concerning because these lone storms often are the ones that produce tornadoes.

Taking a look at the expected atmospheric profile of temperature, humidity, and wind at that time, one can see temperatures getting cooler rapidly with height, and the wind direction veering sharply from SE near the ground to SW aloft.

As the cold front moves in and the strongest dynamics move away tomorrow evening, the storms should grow into a squall line that can still produce damaging wind but are less likely to produce significant tornadoes.

It looks like the highest risk for tornadoes will be over northeast MS and northwest AL, basically north and west of a line from Decatur to Jasper to Aliceville. But, we can’t rule out a tornado further east here in Birmingham or Tuscaloosa. The main threat time for tornadoes in west Alabama will be 4 pm to 9 pm. The squall line will move through BHM around 1 am, then it will finally get sunny and cooler on Sunday.  The good news is that these storms will move through fairly quickly and we will not get a whole lot more rain.

This should not be a widespread outbreak of big tornadoes, especially in Alabama. So, don’t panic. However, there is a good chance that one or two tornadoes will hit in Alabama, and people west of I-65 and north of I-20 need to have the tornado plan in place. Lowest floor of your home, away from windows and outside doors, near the center of the building, in a small room like a closet if possible. Bicycle, football, or baseball helmets will protect your head from flying or falling debris. You can also get under a heavy piece of furniture or workbench to protect yourself.

I will update this blog again not later than noon tomorrow.

Dr. Tim Coleman

http://www.colemanandknupp.com