Here is the latest severe weather outlook issued for the evening/overnight hours from the Storm Prediction Center. The standard severe weather risk is defined by the yellow color with the enhanced zone defined by the red. There is a low severe weather threat in the brown shaded zone.

Brown Shaded Zone: Low severe weather chance (5% chance of severe weather occurring within 25 miles of your location tonight.)

Yellow Shaded Zone: Standard severe weather chance (15% chance of severe weather occurring within 25 miles of your location tonight.)

Red Shaded Zone: Elevated severe weather chance (30% chance of severe weather occurring within 25 miles of your location tonight.) This is where the highest tornado potential is also located.

A Tornado Watch has been posted for much of South Central Texas until 3 AM. The highest tornado threat should remain along and south of Interstate 10, but severe weather is possible across all of the watch.

In addition, a Flash Flood Watch is in effect for much of the Texas Hill Country and South Texas. It is my belief that flooding will become the biggest issue overnight. We’ll post updates as warranted on our Facebook and Twitter pages, along with our blog.

Radar at 1:41 PM

At the time of this posting, several tornadic supercells were in progress in Deep South Texas. Multiple brief tornadoes have been confirmed at this time, but damage surveys will be conducted after the event to confirm the exact number and magnitude of tornadoes. This event is on-going and will continue through the afternoon and evening hours.

Chance of severe weather within 25 miles of your location

This is the latest severe weather outlook showing the probability (chance) of experiencing severe weather within 25 miles of your location this afternoon and this evening.

Brown Shading: “Low” chance of severe weather (5 percent chance of having severe weather occur within 25 miles of your location.)

Yellow Shading: Standard Risk of severe weather (15 percent chance of having severe weather occur within 25 miles of your location.)

Red Shading: Elevated chance of severe weather (30 percent chance of having severe weather occur within 25 miles of your location.)

Primary Hazards: Tornadoes (mainly in the elevated risk) and Tennis Ball Size Hail with the strongest storms

Secondary Hazards: Straight-line winds and Flash Flooding (risk of Flooding will increase tonight)

Chance of severe weather occurring within 25 miles of your location on May 10, 2012

This graphic shows the chance of severe weather occurring within 25 miles of your location today and tonight. The Storm Prediction Center has issued a standard “slight” risk of severe weather, shown by the yellow shading, for areas along and south of Interstate 10 from Fort Stockton eastward to San Antonio, then southeast to El Campo, Texas. Within this yellow shaded zone, there is a 15 percent chance of experiencing severe weather within 25 miles of your location. In the brown shaded zone, there is a low chance of severe weather, meaning a 5 percent chance of experiencing severe weather within 25 miles of your location.

Brown Shaded Zone: “Low” chance of severe weather (5 percent chance of severe weather occurring within 25 miles of your location.)

Yellow Shaded Zone: Standard Risk of severe weather (15 percent chance of severe weather occurring within 25 miles of your location.)

Primary Hazards with the strongest storms: Golfball size hail and damaging straight-line winds over 60 MPH

Second Hazards with the strongest storms: Brief tornadoes

The risk of Flooding will increase later this evening with a Flash Flood Watch already posted for much of South Texas.

The cap is a region of warmer air over a layer of cooler air that keeps a lid on convective lifting and can inhibit thunderstorm development.   Remember from Thunderstorm 101, warm air from the earth’s surface wants to rise and will continue to rise as long as it’s warmer than its surroundings. Thus, when it reaches a layer of air equal to or greater than it’s temperature at that height, it will no longer rise.  Convection, or cloud formation, is then thwarted and unable to develop any further without the aid of other lifting mechanisms such as an approaching frontal boundary or surface heating that exceeds that of the cap.

What causes caps to form?  There’s quite a few, but the loss of daytime heating from the earth’s surface is one primary cause.  The earth’s surface cools faster than the air above, so you will have warmer air aloft most nights and mornings.  Once the sun has a chance to heat the earth’s surface, that heat is reflected back up into the atmosphere heating it as well.  As a result, the cap is typically strongest during the night and early part of the day, and weakest in the afternoon during peak daytime heating.  That’s when it’s most likely that the cap will break and enable storms to form if there is enough moisture and instability in the atmosphere.   Another primary cause is warmer mid-level air masses moving over the cooler ground level air masses.  Winds can move across us from different directions at different levels in the atmosphere, and one prevailing wind we have in Texas each summer at the mid-levels is a southwest or westerly wind which brings in that warmer layer of air from the arid regions of Mexico and our southwestern states.

A cap is considered “weak” or “breakable” when there is only a small temperature increase between the air aloft and the air at ground level.  Conversely, a cap is strong when there is a large temperature increase in the air aloft over the air at ground level.  The cap can be the difference between quiet weather and a severe weather.  If the cap HOLDS, it will be a quiet weather day.  However, if the cap weakens or BREAKS, daytime convection could explode and produce damaging thunderstorms.

The Storm Prediction Center has placed a large section of the Big Bend County, the Texas Hill Country, and parts of East Texas in a standard risk of severe weather for the afternoon and evening hours.

Brown Shaded Zone means there is a “low” chance of severe weather – 5% chance of severe weather occuring within 25 miles of your location.

Yellow Shaded Zone is the “standard” severe weather risk – 15% chance of severe weather occuring within 25 miles of your location.

Red Shaded Zone is an “elevated” risk of severe weather – 30% chance of severe weather occurring within 25 miles of your location.

For weather information for your specific location, please visit your local National Weather Service office at www.weather.gov

This graphic shows the probability of experiencing a severe thunderstorm within 25 miles of your location today and tonight. The brown shaded zones are what we consider a low risk, or a 5 percent chance. Within the yellow zone, the Storm Prediction Center has issued a standard risk of severe weather which means there is a 15 percent chance of experiencing severe weather within 25 miles of any given point. Thunderstorm coverage will be isolated across South Texas with a slightly higher coverage expected across North Texas.

The strongest storms will be capable of producing golfball size hail and damaging winds in excess of 60 MPH. However, all storms will be capable of producing cloud to ground lightning. As it is a Sunday and numerous outdoor events are planned, please keep that in mind. Thunderstorms should begin firing by 4 to 5 PM to the west of Interstate 35 and begin approaching Interstate 35 by 6 to 9 PM. Remember, expect the unexpected when it comes to thunderstorms and have a way to receive weather warnings later this afternoon, especially if you’re planning to be outdoors. When thunder roars, go indoors!

We’ll be chasing northwest of Fort Worth this afternoon and plan on following the storms back into the metroplex. We’ll have live video available after 1 PM.

THE NWS STORM PREDICTION CENTER HAS ISSUED A
   SEVERE THUNDERSTORM WATCH FOR PORTIONS OF 

          CENTRAL TEXAS

   EFFECTIVE THIS SATURDAY AFTERNOON AND EVENING FROM 505 PM UNTIL
   1000 PM CDT.

   HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
   MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

  REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
   FAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
   AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
   THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
   AND POSSIBLE WARNINGS. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY
   DO PRODUCE TORNADOES.

  DISCUSSION...A CLUSTER OF THUNDERSTORMS IS EXPECTED TO DEVELOP NEAR
   AND JUST E OF A DIFFUSE DRYLINE IN CENTRAL TX.  FLOW ALOFT IS
   RELATIVELY WEAK/ANTICYCLONIC...BUT SURFACE TEMPERATURES NEAR 100 F
   AND DEWPOINTS IN THE MID 60S ARE CONTRIBUTING TO STEEP LOW-LEVEL
   LAPSE RATES AND MLCAPE NEAR 4000 J/KG.  THE WEAK VERTICAL SHEAR
   SUGGESTS THAT INDIVIDUAL STORMS WILL NOT BE LONG-LIVED...BUT THE
   STEEP LAPSE RATES/LARGE CAPE WILL SUPPORT A RISK OF BOTH STRONG
   UPDRAFTS WITH LARGE HAIL...AND STRONG DOWNDRAFTS WITH DAMAGING WIND
   POTENTIAL THROUGH LATE EVENING.

After a somewhat busy weather day on Friday, we’re expecting a more localized, isolated severe weather threat for Saturday. The Storm Prediction Center has issued a standard risk of severe weather, shown in yellow, along and about 50 miles east of a dryline expected to be located from Del Rio, to San Angelo, to Vernon line. A strong cap will help to limit overall thunderstorm coverage and wind shear is going to be a tad less then Friday. Still, any thunderstorms that develop will have the potential to become severe with large hail and downburst winds possible. The most likely zone to see isolated thunderstorms will be between Interstate 10 and Interstate 20 in the yellow shaded zone, but one or two storms cannot be ruled out north of Interstate 20.

The Storm Prediction Center has issued a large Severe Thunderstorm Watch until 11 PM from Interstate 10 in South Texas northward to the Red River counties. The primary severe weather risk will be very large hail in the strongest storms. Downburst winds could become a threat later this evening.

   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   SEVERE THUNDERSTORM WATCH FOR PORTIONS OF 

          SOUTHWEST AND SOUTH-CENTRAL OKLAHOMA
          NORTHWEST AND WEST-CENTRAL TEXAS

   EFFECTIVE THIS FRIDAY AFTERNOON AND EVENING FROM 350 PM UNTIL
   1100 PM CDT.

   HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
   MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

   REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
   FAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
   AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
   THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
   AND POSSIBLE WARNINGS. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY
   DO PRODUCE TORNADOES.

DISCUSSION...TSTMS ARE EXPECTED TO INCREASE IN COVERAGE AND
   INTENSITY THIS AFTERNOON INTO EVENING ALONG A DRYLINE WHERE THE
   BOUNDARY LAYER HAS BECOME HOT AND DEEPLY MIXED.  HERE...THE
   COMBINATION OF VERY STEEP LOW TO MIDLEVEL LAPSE RATES AND MLCAPE OF
   2000-3000 J/KG WILL PROMOTE VIGOROUS UP/DOWNDRAFTS WITH AN
   ASSOCIATED RISK FOR LARGE HAIL AND DAMAGING WIND GUSTS.