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Category: Alabama’s Weather

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.

An Update on Our Situation

| 9:27 am April 25, 2015

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.

SimuAWIPS image showing region radar (top left), current instability (bottom left) and visible satellite and surface observations (right panel).

SimuAWIPS image showing region radar (top left), current instability (bottom left) and visible satellite and surface observations (right panel).

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.

-Brian-

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.

1.26″ of Rain in 1 Hour at BHM

| 5:47 am April 25, 2015

1.26 inches of rain fell in the hour ending at 5 a.m. at the Shuttlesworth Birmingham International Airport.

A total of 1.42 had been recorded through 5:14.

We may threaten the record rainfall for April 25th in Birmingham before the day is out, It currently stands at 2.10″ in 1980.

On the right panel of the graphic below, you can see radar estimated rainfall from the BMX Doppler. It shows that over two inches has likely fallen across parts of Lamar, Fayette and Walker Counties.

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RADAR UPDATE
The left panel shows the current BMX radar image with the showers and storms lined up from Calhoun and Cleburne Counties through Clay and Randolph, Coosa and Chilton into Dallas and Wilcox Counties.

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WATCH UPDATE
A tornado watch remains in effect until 8 a.m. for Dallas County and until noon for Autauga, Barbour, Bullock, Elmore, Lowndes, Macon, Montgomery, Pike, Russell.The tornado watch is canceled for Greene, Hale, Marengo, Perry and Sumter Counties.

Storm Prediction Center Day One Severe Weather Outlook

| 5:08 am April 25, 2015

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.

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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.