Category: Severe Weather
By Bill Murray, Saturday 120 pm
A strong thunderstorm has developed over eastern Mississippi between Philadelphia and Meridian. It is moving northeast and will impact Sumter and southern Pickens Counties starting between 1:30 and 2 p.m. They are not severe for now but are in an environment of moderate instability with decent wind shear so they will persist into Alabama. They are being triggered by an upper level disturbance that is crossing the Mississippi River out of Louisiana. There is also some enhancement from an upper level jet streak just to the west.
South of US-82, CAPE values are already at 1,000 j/kg or greater. They will continue to increase through the afternoon as sun works on a moist atmosphere to create more instability. The storms back to the southwest of this leading activity will cut off moisture inflow from the Gulf, so the storms may have to work with what they have available. But dewpoints are already in the upper 60s so that may not be much of a problem.
There will be scattered storms to severe storms over Central Alabama this afternoon. The SPC slight risk severe weather outlook boundary is shown in yellow on the graphic. Areas south of that have the best chances for severe storms to develop this afternoon and evening. I don’t expect there will be much severe activity to the north of areas within 25-50 miles of I-20. Damaging winds and hail look likely. There is a small chance of a tornado, but with southwesterly surface winds, the chance is minimized.
Stay tuned for the latest severe weather information throughout the afternoon and evening.
By Brian Peters
9:27 AM Saturday
First, I am unable to prepare a Weather Xtreme Video because I am providing weather support to the Honda Indy Grand Prix of Alabama at Barber Motorsports Park in Birmingham. My ‘office’ is an open area with numerous people checking on the latest weather, so the video would be horrible. But I thought I would post a couple of notes about the weather for Central Alabama today.
First, a large cluster of thunderstorms moved through Central Alabama early this morning helping to stabilize the atmosphere for the time being. By working over the atmosphere so much, it has helped to reduce to some extent the threat of a second round of thunderstorms. The sounding from BMX at 12Z this morning showed a CAPE value of only 6 in the wake of this cluster. In addition to this, another large cluster of storms over Southeast Texas and Southwest Louisiana was producing some additional clouds which may help to reduce the daytime heating across Central Alabama which could conceivably aid in lowering any destabilization of the atmosphere late this morning and into the afternoon. As this cluster moves eastward along the Gulf Coast, it may help to disrupt the moist southerly flow further northward.
Supporting additional development today is the fact that the upper trough axis still remains to our west and northwest. SPC has an enhanced risk of severe thunderstorms in the Ohio River Valley just ahead of this trough. And the GFS continues to project moderately high instabilities over 3,000 j/kg with scattered thunderstorms developing.
Satellite imagery this morning also shows some breaks in the clouds across Central Alabama and Central Mississippi. These breaks will allow for heating of the near surface layer and help with destabilization. I have noticed, however, that the lower clouds have closed off some of the breaks which may slow the destabilizing process.
The HRRR model shows additional thunderstorms developing into the early afternoon, so we will need to be vigilant for this development. The HRRR model continued to trend toward less development into the afternoon for Central Alabama while the NAM continued to be more robust in the development of thunderstorms. The best time period for additional development still remains from the early afternoon into the early evening generally from about 1 to 9 pm. While thunderstorms may not be widespread, those that do develop could pack a good punch!
As always, be weather safe and be sure to have a way to get weather warnings. Stay tuned to the Blog for further updates.
The final counties in Central Alabama have been cleared from Tornado Watch 108 but more severe weather is possible this afternoon.
the area remains under a slight risk severe weather outlook from the Storm Prediction Center for the threat of additional severe weather this afternoon and evening.
Scattered storms will form this afternoon and the environment will be conducive for severe weather with moderately high instability. The best window for storm development will be between 1-9 p.m. today. Most of the storms will form after 3 p.m. The SPC has areas south of a line from Vernon to Double Springs to Falkville to Scottsboro outlooked for severe weather through the day and into tonight.
Here is the 4km NAM’s depiction of what the radar might look like at 7 p.m. when CAPE values are still running over 3,000 j/kg over North Central Alabama:
A caveat: the HRRR model is not as robust with the instability environment or the storms, so this forecast is heavily laden with uncertainty. We will just have to keep a close eye on mesoscale trends through the afternoon. One key factor will be cloud cover and how much the lower atmosphere can heat up. Temperatures this afternoon are forecast to reach the upper 70s to near 80F.
Skies are clearing partially in a southwest to northeast swath that you can see on visible satellite this morning. Temperatures are cool, in the 60s and there is no instability north of US-80 for now but as breaks in the clouds allow things to heat up, the airmass will become unstable. There is plenty of moisture. We will be in the favorable right rear quadrant of a strong upper level wind maximum, which is favorable for diffluence aloft to support thunderstorm updrafts. There will be plenty of speed shear, or difference in wind speed with height, so the storms will be able to stay organized. There will be sufficient low level shear to produce the threat of tornadoes as well.
So, while the storms may not be widespread, the ones that do form will have the potential to produce damaging winds, hail and tornadoes.
Our first round of severe weather for today is winding down and we have been lucky so far in Central Alabama.
You can see the line of storms on radar at 6:15 a.m. looking about like what we expected it to.
A tornado watch remains in effect until noon for Autauga, Barbour, Bullock, Elmore, Lowndes, Macon, Montgomery, Pike and Russell Counties in the NWS Birmingham Warning Area. Other counties are included in South Alabama and NW Florida. Dallas County is technically in the watch until 8 a.m. but the threat has ended for there and the NWS just has to get around to canceling the watch for our friends there.
There are tornadoes warnings now for parts of Covington County Alabama as well as Escambia, Santa Rosa and Oklaoosa Counties in Florida.
The severe weather threat will end as the line of storms pass southeast.
But a second, more dangerous round of severe weather is possible later today across the area. Scroll down for more information on this potentially hazardous severe weather threat and Brian Peters will have a new morning map discussion video shortly.
Here is the SPC Day One Severe Weather Outlook with the individual threat probabilities on the left and the categorical outlook map on the right.
As you can see, most of Alabama is in the standard severe weather risk, which is called “slight”. There is an “enhanced” risk area to the north over northern Tennessee into Kentucky, where dynamics will be better later this afternoon and the airmass hasn’t been worked over my morning convection.
There is a 2% chance of a tornado within 25 miles of any spot over much of Alabama as you can see in the top left panel. To the north, there is an area of 5% probabilities.
The severe hail probability is 15% over much of Alabama as is the severe wind (>58 mph) probability.
Stay weather aware over South Central Alabama this morning in the tornado watch area until 8 a.m. then everyone needs to pay attention this afternoon to see if another round of severe thunderstorms can develop.
BULLETIN – IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH OUTLINE UPDATE FOR WT 108
NWS STORM PREDICTION CENTER NORMAN OK
500 AM CDT SAT APR 25 2015
TORNADO WATCH 108 IS IN EFFECT UNTIL 1200 PM CDT FOR THE
. ALABAMA COUNTIES INCLUDED ARE
AUTAUGA BALDWIN BARBOUR
BULLOCK BUTLER CLARKE
COFFEE CONECUH COVINGTON
CRENSHAW DALE ELMORE
ESCAMBIA GENEVA HENRY
HOUSTON LOWNDES MACON
MOBILE MONROE MONTGOMERY
PIKE RUSSELL WILCOX