Tornadoes and Flash Flooding in Texas, April 24 to 25, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FORT WORTH TX
1102 AM CDT FRI APR 24 2015

.UPDATE...

...NORTH TEXAS MESOSCALE FORECAST UPDATE...

WATER VAPOR SATELLITE IMAGERY SHOWS A STRONG SHORTWAVE TROUGH
OVER FAR WEST TEXAS...MOVING NORTHEAST OVER EASTERN NEW MEXICO.
THIS TROUGH IS EXPECTED TO CONTINUE NORTHEAST THROUGHOUT THE DAY
TODAY...SPREADING RELATIVELY STRONG LARGE SCALE FORCING FOR ASCENT
OVER THE REGION. 12Z UPPER AIR ANALYSIS SHOWED A PLUME OF DEEP
MOISTURE IN PLACE OVER THE SOUTHERN PLAINS WITH STRONG SPEED AND
DIRECTIONAL SHEAR FROM THE SURFACE TO 500 MB. AT THE 850 MB LEVEL
A RELATIVELY STRONG TROUGH WAS ANALYZED OVER THE HIGH
PLAINS...DOWNSTREAM OF THE SHORTWAVE TROUGH MOVING NORTHEAST OVER
THE REGION. AT THE SURFACE...ONGOING THUNDERSTORM ACTIVITY LEFT A
SOMEWHAT MESSY ANALYSIS IN PLACE...BUT BASED ON THE 15Z ANALYSIS
THERE WAS A WARM FRONT LOCATED JUST SOUTH OF THE INTERSTATE 20
CORRIDOR ACROSS THE DFW AREA...MORE OR LESS ATTACHED TO A SURFACE
LOW LOCATED IN SOUTHERN PALO PINTO COUNTY.

15Z REGIONAL RADARS SHOWED SCATTERED CONVECTION NEAR THE SURFACE
LOW...AND ALL ALONG AND NORTH OF THE WARM FRONT. VISIBLE SATELLITE
IMAGERY SHOWED THAT THE THICK MORNING CLOUD COVER WAS BEGINNING TO
CLEAR OUT FROM CLEBURNE SOUTHWEST TO GOLDTHWAITE...AND SOUTH ALONG
THE INTERSTATE 35/35W CORRIDORS. FROM GRANBURY TO CLEBURNE
SOUTHWEST TOWARDS HAMILTON...LOCALLY CALCULATED CONVECTIVE
PARAMETERS WERE HIGHLIGHTING A LOCAL MAXIMA OF LOW-LEVEL (0-3 KM)
CAPE OF AROUND 100 TO 150 J/KG...AND A LOW-LEVEL EHI OF JUST UNDER
2. BOTH OF THESE PARAMETERS FAVOR THE POTENTIAL FOR STRONG LOW-
LEVEL UPDRAFTS AND THE POTENTIAL TO TILT LOW-LEVEL HORIZONTAL
VORTICITY INTO THE VERTICAL. THE PRIMARY MISSING INGREDIENT TO
ACCOMPLISH THIS THUS FAR HAS BEEN WELL ORGANIZED CONVECTION/STRONG
UPDRAFTS.

WHILE DEEP MOISTURE WAS ANALYZED ON THE 12Z UPPER AIR MAPS...THERE
WERE NO STRONG OR STEEP MID-LEVEL LAPSE RATES ANALYZED UPSTREAM OF
NORTH AND CENTRAL TEXAS. WHILE DEEP MOISTURE VALUES APPEAR TO BE
AT NEAR RECORD LEVELS FOR THIS TIME OF YEAR...THIS DEEP MOISTURE
ALSO INDICATES THAT THERE IS NO STRONG THETA-E CHANGE WITH
HEIGHT...LEADING TO A THERMODYNAMIC PROFILE CHARACTERIZED BY TALL
AND SKINNY CAPE. WHILE THIS IS A VERY FAVORABLE PROFILE FOR
THUNDERSTORM CHANCES AND EFFICIENT RAINFALL IN GENERAL...IT COULD
LIMIT THE STRENGTH OF INITIAL UPDRAFTS TODAY. IF STORMS CAN
MAINTAIN UPDRAFTS LONG ENOUGH TO INGEST SOME OF THE RELATIVELY
STRONG HORIZONTAL VORTICITY INDICATED BY CALCULATED PARAMETERS...
SUPERCELLS WILL BECOME MORE LIKELY. IF A SUPERCELL CAN BECOME
ESTABLISHED...THE INTERNAL DYNAMICS OF THE SUPERCELL WILL PROMOTE
A MUCH STRONGER UPDRAFT THAN WHAT WOULD BE SUPPORTED BY
THERMODYNAMICS ALONE.

WITH STRONG FORCING UPSTREAM...DEEP MOISTURE IN PLACE...AND ALMOST
NO CAPPING INVERSION IN PLACE...THUNDERSTORMS ARE VERY LIKELY
TODAY...AND WILL LIKELY CONTINUE INTO THE AFTERNOON HOURS OF THE
DAY. COPIOUS AMOUNTS OF MORNING CONVECTION CAN OFTEN STABILIZE THE
ENVIRONMENT MAKING IT LESS FAVORABLE FOR STORMS LATER ON. THAT
DOES NOT APPEAR TO BE THE CASE TODAY AS THE LACK OF A STRONG
THETA-E DIFFERENCE WITH HEIGHT NOT ONLY LIMITS UPDRAFT
STRENGTH...BUT IT ALSO LIMITS THE STRENGTH OF RAIN COOLED AIR...
I.E. THE RAIN IS NOT VERY COLD BECAUSE THERE IS NOT MUCH DRY AIR
ALOFT TODAY. THE PRIMARY FORECAST CHALLENGE IN DETERMINING STORM
MODE AND THEN THE ASSOCIATED HAZARDS. HERES WHAT WE'RE WATCHING
CLOSELY:

IN GENERAL...THE KINEMATIC OR WIND SHEAR AND FORCING ENVIRONMENT
ARE FAVORABLE FOR SUPERCELL THUNDERSTORMS AND TORNADOES TODAY. THE
THERMODYNAMIC ENVIRONMENT IS MORE QUESTIONABLE. WE CERTAINLY HAVE
AMPLE MOISTURE IN PLACE TO SUPPORT ANY STORM MODE...BUT LAPSE
RATES ALOFT ARE SOMEWHAT MARGINAL...LIMITING UPDRAFT STRENGTH FROM
AIR BEING LIFTED ABOVE ITS LEVEL OF FREE CONVECTION. IT LOOKS LIKE
THE MORE SUNSHINE WE GET THROUGH THIS AFTERNOON...THE MORE LIKELY
WE ARE TO SEE SUPERCELL THUNDERSTORMS. THE MORE CLOUD COVER WE
HANG ON TO...THE MORE LIKELY WE ARE TO SEE NUMEROUS...SOMEWHAT
WEAKER STORMS...THAT POSE MORE OF A HEAVY RAINFALL AND FLOODING
THREAT THAN A LARGE HAIL...DAMAGING WIND AND TORNADO THREAT.

IF WE MAINTAIN THE CLEARING SOUTH-SOUTHWEST OF THE DFW AREA FOR A
FEW HOURS HEADED INTO THIS AFTERNOON...OUR CHANCES OF SEEING A
SUPERCELL STORM MODE INCREASE QUITE A BIT. LOW-LEVEL HEATING WILL
HELP MORE OR LESS MAKE UP FOR THE MARGINAL MID-LEVEL LAPSE RATES
AND INCREASE UPDRAFT SPEED TO THE POINT THAT SUPERCELLS DOMINATE
THE INITIAL STORM MODE. IF WE GET STORMS QUICKLY...OR SIMPLY CLOUD
UP HERE AGAIN BEFORE NOON...A MORE WIDESPREAD STORM COVERAGE MAY
OCCUR WITH WEAKER STORMS IN GENERAL.

IN EITHER CASE...MOST MODEL GUIDANCE FAVORS THUNDERSTORMS GROWING
UPSCALE INTO A SQUALL LINE AND MOVING EAST ACROSS NORTH AND
CENTRAL TEXAS BY LATE THIS AFTERNOON. WITH A NEAR RECORD
PRECIPITABLE WATER VALUE OBSERVED ON THE 12Z FWD
SOUNDING...FLOODING IS ALSO A MAJOR CONCERN TODAY. ONCE DEEP
CONVECTION INITIATES...ANY TRAINING OF STORMS WILL LIKELY RESULT
IN ENOUGH LOCALLY HEAVY RAINFALL TO RESULT IN FLOODING. RAINFALL
RATES OF 2 INCHES PER HOUR ARE VERY LIKELY... AND REALLY THE ONLY
LIMITING FACTOR FOR FLOODING IS THE EXPECTED SPEED OF STORMS.
STORMS ARE EXPECTED TO MOVE EAST OR NORTHEAST AT 35 TO 40 MPH
TODAY...MAKING ANY ONE STORM UNLIKELY TO PRODUCE FLOODING.

WE WILL BE KEEPING A CLOSE EYE ON ALL OF THESE FACTORS HEADED INTO
THE AFTERNOON HOURS...AND WILL ISSUE THE APPROPRIATE WATCHES AS
VARIOUS THREAT LEVELS INCREASE. THERE IS A FAIR AMOUNT OF
UNCERTAINTY REGARDING STORM MODE THIS AFTERNOON...SO IT IS
ADVISABLE TO PAY CLOSE ATTENTION TO THE FORECAST THROUGHOUT THE
DAY AS CONDITIONS COULD BECOME MORE FAVORABLE FOR SUPERCELLS OR
HEAVY RAINFALL AND FLOODING QUICKLY. WE WILL PROVIDE SHORT TERM
FORECAST UPDATES AS QUICKLY AS POSSIBLE AS THESE CONDITIONS
EVOLVE.

CAVANAUGH

&&

.AVIATION...
/ISSUED 642 AM CDT FRI APR 24 2015/
CONVECTION TO THE NORTH OF THE METROPLEX APPEARS TO HAVE PUSHED
THE WEAK QUASI STATIONARY WARM FRONT BOUNDARY BACK TO THE SOUTH.
MOISTURE ADVECTION OVER THE TOP OF THE BOUNDARY HAS RESULTED IN
LIFR CIGS AND LIGHT FOG. THE BOUNDARY WILL HEAD BACK TO THE NORTH
IN RESPONSE TO HEIGHT FALLS TO THE NORTHWEST...WHICH WILL RESULT
IN AN IMPROVEMENT OF CIGS TO MVFR BY LATE MORNING.

TAF SITES APPEAR TO BE IN LULL FOR RAIN/STORM CHANCES FOR AT
LEAST THE NEXT COUPLE OF HOURS BASED ON RADAR TRENDS. WILL SHOW
VCTS FROM 14-17Z AS THAT IS WHEN LIFT FROM LEAD SHORTWAVE IS
STRONGEST...BUT IT IS POSSIBLE STORMS WILL REMAIN BOTH TO THE
NORTH AND ALSO WELL TO THE SOUTH. OTHERWISE NEXT WINDOW OF
HIGHER THUNDERSTORM POTENTIAL IS LATE AFTERNOON AND EARLY
EVENING. IN GENERAL THE HIGH-RES GUIDANCE IS ALL IN GOOD AGREEMENT
ON SEVERE CONVECTION FIRING ALONG THE DRY LINE AND MOVING EAST
THROUGH THE I-35 CORRIDOR DURING THIS TIME. HOWEVER THE HRRR/RAP
SOLUTIONS ARE A COUPLE HOURS FASTER...SEEMINGLY TAKING THE
CONVECTION THAT IS BETWEEN MIDLAND AND SAN ANGELO NOW AND PUSHING
THAT THROUGH MID AFTERNOON. WILL SHOW VCTS FROM 21-01Z. LATER
UPDATES TO REFINE THUNDER TIMING AND THREAT ARE LIKELY.

VFR SHOULD PREVAIL TONIGHT BEHIND ANY STORMS...WITH SOUTH WINDS
BECOMING SOUTHWEST AFTER MIDNIGHT.

TR.92

&&

.PREV DISCUSSION... /ISSUED 450 AM CDT FRI APR 24 2015/
ACTIVE WEATHER APPEARS LIKELY ACROSS PORTIONS OF NORTH/CENTRAL
TEXAS FOR TODAY WITH A COUPLE OF ROUNDS OF SHOWERS AND
THUNDERSTORMS. WATER VAPOR IMAGERY EARLY THIS MORNING SHOWS A
LARGE BAROCLINIC LEAF AHEAD OF THE MAIN UPPER LOW NOW SWINGING
THROUGH THE SOUTHWEST US. AS THIS FEATURE MOVES TO THE EAST...IT
SHOULD HELP TO BREAK DOWN THE SHORTWAVE RIDGE THAT QUELLED
WIDESPREAD CONVECTION YESTERDAY. SURFACE ANALYSIS THIS MORNING
REVEALED THAT LEE SIDE-TROUGHING CONTINUED WITH SURFACE LOW
PRESSURE CENTERS ACROSS COLORADO AND ANOTHER LOW PRESSURE CENTER
ACROSS THE SOUTHERN TX PANHANDLE. A SLOWLY ADVANCING WARM FRONT
WAS LOCATED ROUGHLY FROM NEAR CHILDRESS TO FORT WORTH TO ATHENS.

THE AFOREMENTIONED WARM FRONT WILL SERVE AS ONE FOCUS FOR THE
FIRST ROUND OF SHOWERS AND STORMS THIS MORNING. AS THE UPPER
TROUGH NEARS...A FEW THINGS WILL HAPPEN TO INCREASE THE
PROBABILITY FOR PRECIPITATION. THE FIRST IS THAT BROAD TROUGHING
ACROSS THE TX PANHANDLE WILL CONTINUE. THIS SHOULD ALLOW THE WARM
FRONT TO LIFT NORTHWARD TOWARDS THE RED RIVER. SECOND...THE
AFOREMENTIONED TROUGH NOW ACROSS THE PACIFIC SOUTHWEST WILL
CONTINUE TO MOVE CLOSER TOWARDS N/C TX. THE DIVERGENCE ALOFT AND
MID LEVEL HEIGHT FALLS SHOULD FURTHER RESULT IN CONTINUED LOW
LEVEL THETA-E ADVECTION TOWARDS THE NORTH AS WELL AS INCREASE
ASCENT. BOTH OF THESE ARE ALREADY EVIDENT PER REGIONAL RADARS THIS
MORNING WITH CONVECTION ONGOING ACROSS OUR NORTHERN AND WESTERN
ZONES AS WELL AS ACROSS THE TX HILL COUNTRY. THE MAIN AREAS FOR
PRECIPITATION WILL BE ALONG THE WARM FRONTAL BOUNDARY WHICH SHOULD
BE LOCATED NEAR THE RED RIVER BY MID-MORNING AND ACROSS CENTRAL
AND THE EASTERN TWO-THIRDS OF OUR ZONES. WHILE WIDESPREAD SEVERE
WEATHER IS NOT ANTICIPATED THIS MORNING...SOME HEAVY RAIN AND
FREQUENT CLOUD TO GROUND LIGHTNING WILL BE POSSIBLE. CANNOT RULE
OUT A FEW INSTANCES OF VERY SMALL HAIL...BUT FORECAST PROFILES
APPEAR QUITE MOIST THIS AM.

THE DIFFICULT PART OF THE FORECAST WILL BE THE AFTERNOON STORM
CHANCES. AS WE HAVE SEEN IN THE PAST WEEK...MORNING CONVECTION CAN
OFTEN SUPPRESS AFTERNOON STORM CHANCES DUE TO THE AMOUNT OF
OVERTURNING AND SUBSEQUENT SUBSIDENCE IN THE WAKE OF STORM
COMPLEXES. IN SOME INSTANCES...THE ATMOSPHERE HAS THE ABILITY TO
RAPIDLY DESTABILIZE AND RECHARGE FOR A SECOND ROUND OF SHOWERS AND
STORMS WHEN STRONG UPPER LEVEL DYNAMICS ARE IN PLACE. WITH
REGARDS TO THE POTENTIAL AFTERNOON STORM CHANCES...WE WILL CARRY A
RELATIVELY BROAD BRUSH POP FORECAST /50 PERCENT/ MAINLY DUE TO
THE FACT THAT MUCH OF THE REGION WILL BE BENEATH THE RIGHT
ENTRANCE REGION OF A 50-60 KT MID LEVEL JET WHICH SHOULD PROVIDE
LARGE SCALE ASCENT. IN ADDITION...SURFACE DEWPOINTS IN THE 60S AND
70S SHOULD STILL YIELD A HEALTHY AMOUNT OF INSTABILITY. IF
SUFFICIENT DIABATIC HEATING OCCURS...THE ABILITY TO LIFT SURFACE
BASE PARCELS UP TO THEIR LFC WILL RESULT IN THE DEVELOPMENT OF
VERY ROBUST CONVECTION ALONG THE APPROACHING DRYLINE. THE DRYLINE
SHOULD MIX EASTWARD TOWARDS OUR WESTERN ZONES MID TO LATE
AFTERNOON REACHING THE I-35 CORRIDOR AROUND 6 PM.

WITH CAPE VALUES EXPECTED TO CLIMB UP TO 2000 J/KG AND DEEP LAYER
SHEAR VALUES ON THE ORDER OF 45-50 KNOTS...SUPERCELLS WILL BE
POSSIBLE...POSSIBLY GROWING UPSCALE INTO LINE SEGMENTS. THE MAIN
HAZARDS WITH THUNDERSTORMS THIS AFTERNOON AND EVENING /SHOULD THEY
DEVELOP/ WILL BE VERY LARGE HAIL AND DAMAGING WINDS...BUT BACKED
LOW LEVEL FLOW SHOULD RESULT IN A FAVORABLE WIND PROFILE
SUFFICIENT FOR A TORNADO OR TWO AS WELL. WITH THE UNSEASONABLY
HIGH MOISTURE CONTENT...HEAVY RAINFALL WILL ALSO BE POSSIBLE. THIS
MAY LEAD TO AREAS OF LOCALIZED FLOODING.

FOR TONIGHT...THE DRYLINE/PACIFIC FRONT SHOULD MOVE EASTWARD INTO
THE PINEY WOODS OF EAST TEXAS. SKIES SHOULD CLEAR FROM WEST TO
EAST WITH WEST TO SOUTHWEST WINDS OF 5 TO 10 MPH. THE EXCEPTION
MAY BE ACROSS FAR SOUTHEAST ZONES. AS A RESULT OVERNIGHT LOW
TEMPS SHOULD BE COOLEST IN THE NW /MID 50S/ TO LOW TO MID 60S
ELSEWHERE.

FOR SATURDAY AND MOST OF SUNDAY...WARM CONDITIONS AND DRY
CONDITIONS SHOULD EXIST AS WARM SOUTHWEST TO WEST-SOUTHWEST WINDS
SHOULD OVERSPREAD THE AREA IN THE WAKE OF THE DRYLINE/PACIFIC
FRONT. THIS FAVORABLE DOWNSLOPE WIND SHOULD ALLOW TEMPERATURES TO
SOAR INTO THE MID TO UPPER 80S. A FEW SITES ACROSS THE WESTERN
ZONES MAY EVEN CLIMB INTO THE LOW 90S. SUNDAY APPEARS TO BE WARM
AS WELL...THOUGH WINDS SHOULD TURN A BIT MORE TO THE SOUTH /A
LITTLE LESS DOWNSLOPE/ AND THIS MAY RESULT IN SLIGHTLY COOLER
TEMPERATURES. THE EXCEPTION WILL BE FAR NW ZONES WHERE SW WINDS
MAY CONTINUE. MOST OF THE GLOBAL MODELS ARE IN FAIR AGREEMENT THAT
A LARGE UPPER TROUGH WILL DIVE DOWN THROUGH THE FOUR CORNERS
REGION AND INTO FAR WEST TEXAS. THE SUBSEQUENT LEE-SIDE TROUGHING
SHOULD HELP TO DRAW LOW LEVEL MOISTURE TO THE NORTH. THIS WILL
ALLOW A DRYLINE TO SHARPEN NEAR OR OUR WESTERN ZONES. WHILE IT IS
A LITTLE TOO EARLY TO GET OVERLY DETAILED...THERE DOES APPEAR TO
BE A FAIR AMOUNT OF BOTH INSTABILITY AND SHEAR FAVORABLE FOR
SEVERE WEATHER.

FOR THE LONGER TERM PERIOD...RAIN CHANCES AND THE POTENTIAL THREAT
FOR SEVERE WEATHER WILL CONTINUE THROUGH MONDAY AS THE UPPER LEVEL
LOW BECOMES SOMEWHAT CUTOFF FROM THE MAIN FLOW.
AGAIN...INSTABILITY AND SHEAR PARAMETERS STILL APPEAR FAVORABLE
FOR SOME SEVERE WEATHER. IT DOES APPEAR THAT A TRUE COLD FRONT
WILL PLOW SOUTHWARD THROUGH NORTH/CENTRAL TEXAS AROUND TUESDAY AND
THUS THE THREAT FOR SEVERE WEATHER WILL LIKELY COME TO AN
END...BUT THE RAIN CHANCES WILL CONTINUE ON TUESDAY. THE COLD
FRONT SHOULD USHER IN COOLER AND DRIER WEATHER FOR MID TO LATE
WEEK.

15-BAIN

&&

.PRELIMINARY POINT TEMPS/POPS...
DALLAS-FT. WORTH, TX  81  64  87  62  87 /  60  30   5   5  30
WACO, TX              81  63  88  62  88 /  70  40   5   5  30
PARIS, TX             74  64  85  58  85 /  70  40   5   5  10
DENTON, TX            81  61  86  58  86 /  60  30   5   5  40
MCKINNEY, TX          79  62  86  57  87 /  70  30   5   5  30
DALLAS, TX            82  65  88  65  87 /  60  30   5   5  30
TERRELL, TX           78  63  86  61  86 /  70  40   5   5  20
CORSICANA, TX         80  65  88  64  86 /  70  40   5   5  20
TEMPLE, TX            81  63  88  65  86 /  70  40  10   5  30
MINERAL WELLS, TX     80  58  88  58  88 /  60  10   5   5  40

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE FORT WORTH TX
450 AM CDT FRI APR 24 2015

TXZ091>095-100>107-115>123-129>135-141>148-156>162-174-175-251000-
MONTAGUE-COOKE-GRAYSON-FANNIN-LAMAR-YOUNG-JACK-WISE-DENTON-COLLIN-
HUNT-DELTA-HOPKINS-STEPHENS-PALO PINTO-PARKER-TARRANT-DALLAS-
ROCKWALL-KAUFMAN-VAN ZANDT-RAINS-EASTLAND-ERATH-HOOD-SOMERVELL-
JOHNSON-ELLIS-HENDERSON-COMANCHE-MILLS-HAMILTON-BOSQUE-HILL-NAVARRO-
FREESTONE-ANDERSON-LAMPASAS-CORYELL-BELL-MCLENNAN-FALLS-LIMESTONE-
LEON-MILAM-ROBERTSON-
450 AM CDT FRI APR 24 2015

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH CENTRAL TEXAS.

.DAY ONE...TODAY AND TONIGHT.
SCATTERED TO NUMEROUS THUNDERSTORMS ARE EXPECTED OVER ALL OF NORTH
AND CENTRAL TEXAS. SOME THUNDERSTORMS MAY BECOME
SEVERE...PRIMARILY THIS AFTERNOON AND EVENING. THE MAIN THREAT
WILL BE LARGE HAIL AND DAMAGING WINDS. THERE IS A POTENTIAL FOR
TORNADIC STORMS. IN ADDITION...LOCALLY HEAVY RAINFALL AND CLOUD
TO GROUND LIGHTNING IS POSSIBLE WITH ALL THUNDERSTORMS.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY.
A POTENT UPPER LEVEL SYSTEM WILL ARRIVE INTO TEXAS ON SUNDAY AND
SLOWLY CROSS THE REGION EARLY NEXT WEEK. THIS WILL RESULT IN A
CHANCE OF THUNDERSTORMS OVER THE ENTIRE REGION. SOME SEVERE
WEATHER IS LIKELY ON SUNDAY AND MONDAY DUE TO A VERY UNSTABLE
AIRMASS AND ORGANIZED WIND SHEAR OVER THE REGION.

.SPOTTER INFORMATION STATEMENT...
SPOTTER ACTIVATION WILL LIKELY BE REQUESTED THIS AFTERNOON AND
EVENING.

92