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Reports From the Birmingham Area

| 5:02 pm April 25, 2015

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Strong storms are impacting Jefferson and Shelby Counties now with heavy rain and lightning as well as gusty winds. There have been reports of some small hail.

Some reports…

…Hail and heavy rain reported in Vestavia by @AUAtty
…I picked up 0.50 inches of rain in less than 30 minutes off Altadena Road. 1.16 so far since midnight.
…Heavy rain on Highway 150 in Hoover flooded the road in 10 minutes. Strong winds too according to @ProddyLeigh
…Power lines sparking on 280 at Parc at Grandview.
…Jeff Drake reports 0.60 inches of rain in 15 minutes at Bessemer.

Severe Thunderstorm Watch Still For East Central Alabama

| 4:41 pm April 25, 2015

UPDATED MAP 6:12 PM

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UPDATE 6:02 PM
Barbour, Pike, Bullock, Macon and Russell Counties have been dropped from the Severe Thunderstorm Watch. The watch continues for Chambers, Clay, Lee, Randolph and Tallapoosa Counties until 10:00 PM CDT

ORIGINAL POST
The NWS has extended the Severe Thunderstorm Watch over South Alabama northward to include part of East Central Alabama.

THE NATIONAL WEATHER SERVICE HAS ISSUED SEVERE THUNDERSTORM WATCH
115 IN EFFECT UNTIL 10 PM CDT THIS EVENING FOR THE FOLLOWING
AREAS

IN ALABAMA THIS WATCH INCLUDES 8 COUNTIES

IN EAST CENTRAL ALABAMA

CHAMBERS CLAY LEE
MACON RANDOLPH RUSSELL
TALLAPOOSA

IN SOUTHEAST ALABAMA

BULLOCK

THIS INCLUDES THE CITIES OF…ALEXANDER CITY…ASHLAND…AUBURN…
LANETT…LINEVILLE…OPELIKA…PHENIX CITY…ROANOKE…TUSKEGEE…
UNION SPRINGS AND VALLEY.

=

Heavy Storms in Tuscaloosa County, Not Severe

| 4:04 pm April 25, 2015

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Heavy thunderstorms have developed this afternoon across Tuscaloosa County in West Alabama. They are not severe, but they do have lot sof lightning and heavy rain as well as gusty winds.

They are moving ENE and will move into Jefferson County shortly. They will reach downtown Birmingham before 4:30.

Across South Central Alabama, storms were developing across Perry and Bibb County, moving toward Clanton.

The SPC has moved the slight risk severe weather outlook southward about 50 miles, sort of as expected. It is depicted in the radar graphic as a yellow line extending from Grove Hill to Centreville to Columbiana and Talladega. The dark green line indicates the northward extent of the Marginal Risk and the light green line delineates the northern extent of the threat of storms.

South of US-82, CAPE values are over 2,000 j/kg giving the storms plenty of octane to work with. There is strong speed shear as well. Mid-level lapse rates are moderately strong as well. But there is almost no low level helicity, so tornadoes are not an issue for now. Overall moisture levels have dropped over North Central Alabama as the special balloon release from BMX indicated at 1 p.m. But low level dewpoints are still quite juicy, running in the upper 60s and lower 70s.

Storms over South Alabama are stronger, including a couple of severe ones. Two severe thunderstorm watches cover South Alabama, but one has not been issued for South Central Alabama yet. Winds gusted to 59 mph at Fairhope when the storms were over Mobile Bay.

One Severe Thunderstorm Watch Already, Maybe Another Soon

| 2:29 pm April 25, 2015

By Bill Murray, 2:25 p.m. Saturday

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An approaching upper level disturbance coming across southeastern Louisiana and southern Mississippi this afternoon. Strong thunderstorms extend now from Southwest Alabama through southern Mississippi into southeastern Louisiana. This system has weakened in the past hour or so but still pose a threat as they move into unstable air.

But a severe thunderstorm watch has been issued for South Alabama and Northwest Florida downstream of this activity.

Storms that formed on the northern edge of the area of rain and storms have weakened as well. But the SPC is monitoring conditions across South Central Alabama where instabilities have risen to over 1,500 j/kg south of US-82. Strong wind shear will produce organized thunderstorms, including supercells. Low level helicities are not high, so damaging winds and hail are the main threats, not tornadoes.

There could be an additional severe thunderstorm watch to the north of the new one over South Alabama.

Overall, the situation looks better for areas north of I-20, with little to no storm development expected. South of I-20, we will continue to be watching the latest developments all afternoon and evening.

Storms Already Developing in Eastern Mississippi

| 1:19 pm April 25, 2015

By Bill Murray, Saturday 120 pm

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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.

Stay Weather Aware: More Severe Storms Possible This Afternoon

| 9:23 am April 25, 2015

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.

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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:

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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.

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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.

The First TDS Warning

| 9:00 am April 25, 2015

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Photo from @UAHSWIRLL

In 2002, a WSR-74C radar was donated to the University of Alabama – Huntsville. The radar is called ARMOR: the Advanced Radar for Meteorological and Operational Research.

In 2004, it was upgraded to dual-polarization capability. Radars typically send out and receive their electronic signals in the horizontal plane only. Dual-pol adds the vertical plane. This gives meteorologists quite a bit more data about the targets that the radar is “seeing” in the atmosphere, including size and shape. This can be very helpful in determining precipitation type and amounts.

One thing that researchers have found is that the data can sometimes differentiate the signature of debris being lofted into the air by a tornado that is on the ground. This can help confirm that a tornado is occurring when ground truth reports aren’t available, a great thing at night and in remote locations, or where terrain and trees limit visibility. The signature is called a Tornadic Debris Signature (TDS). We use them all the time today. There was clear evidence of one last night in Central Mississippi for example.

Researchers at UAH documented the first TDS in February 2008 in a pre-dawn F4 tornado during the Super Tuesday Outbreak in North Alabama.

On this date, meteorologists at UAH provided their first real time reports of a tornadic debris signature to the NWS to help issue a tornado warning for Marshall County in North Alabama as a tornado was doing damage in Albertville.

Be Alert for More Severe Weather This Afternoon

| 6:13 am April 25, 2015

Our first round of severe weather for today is winding down and we have been lucky so far in Central Alabama.

Click image to enlarge.

Click image to enlarge.

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.