Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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000
FXUS64 KFWD 250918
AFDFWD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FORT WORTH TX
405 AM CDT WED MAR 25 2015

.DISCUSSION...
00Z UPPER AIR ANALYSIS HAD A SHORTWAVE TROUGH IN SOUTHERN ARIZONA
AND SINCE THAT TIME WATER VAPOR SATELLITE HAS TRACKED IT INTO
SOUTHERN NEW MEXICO. THE 00Z MODEL GUIDANCE HAS LATCHED ONTO THIS
FEATURE AND BRINGS THIS SHORTWAVE THROUGH NORTH TEXAS DURING THE
MIDDAY HOURS. THIS MEANS THE BEST LIFT OVER THE REGION WILL OCCUR
BEFORE PEAK HEATING...AND MORE IMPORTANTLY...SUBSIDENCE IN THE
WAKE OF THIS FEATURE WILL AFFECT THE REGION DURING THE MID-LATE
AFTERNOON HOURS. A STRONG CAP WILL BE IN PLACE OVER THE
REGION...AND IF THERE IS ANY TREND AMONG THE MODEL CONSENSUS OVER
THE LAST 24 HOURS...IT IS THAT THE CAP LOOKS STRONGER LIKELY DUE
TO THE LEAD SHORTWAVE TRACKING OVER TOO EARLY IN THE DAY.

A DRYLINE WILL DEVELOP AND REACH A LINE JUST WEST OF THE CWA
BORDER BY LATE AFTERNOON. OVER THE FAR WESTERN ZONES...DAYTIME
HEATING DOES ALLOW CIN TO ERODE TO LESS THAN 50 J/KG...BUT THIS IS
STILL A VALUE THAT IS NOT EASY TO OVERCOME. THE CAP STRENGTH
INCREASES EVEN MORE FARTHER EAST ALONG THE I-35 CORRIDOR TO THE
POINT WHERE SURFACE BASED CONVECTION WOULD BE IMPOSSIBLE THROUGH
THIS EVENING. WHILE RAP/NAM/WRF/GFS/ECMWF DO SHOW QPF AROUND 7 PM
NEAR THE DRY LINE...A CLOSER INTERROGATION OF THE MODEL MASS
FIELDS SUGGEST THIS PRECIPITATION IS ORIGINATING FROM HIGH BASED
CONVECTION ABOVE THE CAP. LAPSE RATES ARE VERY STEEP ABOVE THE
INVERSION...AND THAT IS A SITUATION THAT OFTEN LEADS TO
CONVECTION FORMING ABOVE THE CAP PROVIDED THERE IS ENOUGH MOISTURE
AVAILABLE. THIS BRINGS US TO THE FORECASTING DILEMMA OF THE DAY.

IT IS NICE TO TRY TO SIMPLIFY FORECASTING THUNDERSTORMS INTO A
BINARY EVENT SUCH AS WILL THE CAP BREAK OR WONT IT...BUT THE
ANSWER IS MORE COMPLEX THAN THAT. DRY FORECASTS THAT UNEXPECTEDLY
TURN INTO STORMY WEATHER ARENT USUALLY BECAUSE THE CAP
SPONTANEOUSLY BROKE...BUT OFTEN OCCUR WHEN STEEP LAPSE RATES AND
LIFT ARE AVAILABLE...BUT THE MODEL GUIDANCE UNDERESTIMATES THE
MOISTURE IN A SMALL LAYER SOMEWHERE ABOVE THE CAP. ONCE CONVECTION
STARTS ABOVE THE CAP AND BEGINS TO PRECIPITATE HEAVILY IT CAN SET
OFF A CHAIN REACTION AND SPARK NEW CONVECTION BY EITHER EVAPORATIVE
COOLING OF THE WARM DRY CAP LAYER...OR PROVIDING THE LOCALIZED
LIFT/FORCING WITH ITS PRECIPITATION INDUCED OUTFLOW BOUNDARIES TO
BREAK THE CAP MECHANICALLY NEARBY. OR IN THE SPECIAL CASE OF A
SUPERCELL THAT DEVELOPS ABOVE THE CAP...THE DYNAMIC LIFT GENERATED
BY THE STORM CAN REACH DOWN THROUGH THE INVERSION AND PULL UP HIGH
THETA-E SURFACE AIR PARCELS THAT ARE TOO THERMODYNAMICALLY STABLE
TO OVERCOME THE CAP ON THEIR OWN. OUT IN THE WILD WE CAN TELL WHEN
A SUPERCELL IS DOING THIS PROCESS BECAUSE IT WILL DISPLAY A SMOOTH
LAMINAR CLOUD BASE. BECAUSE I HAVE HAD A SHARE OF DRY FORECAST
BUST WHEN STEEP LAPSE RATES WERE PRESENT I HAVE LEARNED TO TREAD
CAUTIOUSLY ON DAYS LIKE TODAY. THE REALITY IS THAT MODEL GUIDANCE
IS JUST NOT GOOD ENOUGH TO FORECAST THE EXACT MOISTURE CONTENT
ACCURATELY THROUGH SEVERAL THOUSAND FEET...NOR IS IT GOOD ENOUGH
TO SIMULATE WHAT THE INDIVIDUAL CONVECTIVE CELLS WILL DO TO THE
ATMOSPHERE NEARBY ONCE THEY FORM AND PRECIPITATE. THAT IS NOT TO
SAY SETUPS LIKE TODAY ALWAYS RESULT IN CONVECTIVE EVENTS...BUT IT
IS ACCURATE TO SAY WE CANT TRUST A DRY MODEL FORECAST AS MUCH.

CONSIDERING THAT MODEL GUIDANCE IS ALREADY SHOWING SOME ELEVATED
CONVECTION EVEN THOUGH THE CAP APPEARS PRETTY TOUGH TO OVERCOME
FOR SURFACE BASED CONVECTION...WE WILL MAINTAIN 20-30 POPS OVER
THE WESTERN AND NORTHWESTERN ZONES THIS AFTERNOON AND EVENING. THE
INITIAL ELEVATED CONVECTION WILL POSE LITTLE TO NO SEVERE WEATHER
THREAT BECAUSE THERE IS NOT AS MUCH INSTABILITY TO WORK WITH. IT
IS JUST A QUESTION OF WHETHER THIS INITIAL ELEVATED CONVECTION
WILL EVOLVE TO USE THE MORE UNSTABLE AIR FROM BELOW THE CAP OR
NOT. SHOULD THIS OCCUR...CAPE OF 1000-2000 J/KG IS AVAILABLE FOR
MORE INTENSE UPDRAFTS AND A SEVERE HAIL AND WIND THREAT.
HOWEVER...THE OVERALL RISK OF SEVERE STORMS IS A BIT LOWER THAN IT
LOOKED LIKE IT WOULD BE YESTERDAY...AGAIN DUE TO THE MORE INTENSE
CAP THAT WILL EITHER SUPPRESS SEVERE CONVECTION ALTOGETHER OR
TAKE SOME OF THE JUICE OUT OF IT. WE WILL ADVERTISE THE SLIGHT
RISK OF SEVERE IN THE WESTERN AND NORTHWESTERN ZONES FOR THE LATE
AFTERNOON AND EARLY EVENING HOURS IN OUR OTHER PRODUCTS. SOME OF
THIS CONVECTION MAY STAY ELEVATED AND MOVE EAST AFFECTING THE I-35
CORRIDOR REGION THIS EVENING...BUT DUE TO CIN INCREASING TO OVER
100-150 J/KG...ANY SEVERE THREAT WOULD DROP OFF QUICKLY.

THE PRIMARY SHORTWAVE TROUGH WILL BE MOVING INTO THE REGION DURING
THE MID EVENING TO OVERNIGHT HOURS WHEN FORCING WILL FINALLY HELP
LIFT AND ERODE THE CAP. OBVIOUSLY WITH THE LOSS OF DAYTIME
HEATING...THE AIRMASS WILL NOT BE AS UNSTABLE AND CAPE WILL BE
1000 J/KG OR LESS. A COLD FRONT WILL BE BARRELING SOUTHWARD
THROUGH THE PLAINS TODAY AND WHEN IT ARRIVES IN THE NW ZONES
AROUND 10 PM...A THIN LINE OF SHOWERS AND STORMS SHOULD START TO
DEVELOP ALONG IT. THE FRONT WILL BE MOVING SO FAST...THAT IT WILL
UNDERCUT CONVECTION QUICKLY AND LIMIT ITS ABILITY TO BE SEVERE OR
PRODUCE MUCH RAINFALL. STRONG NW WINDS OF 20 TO 30 MPH IN THE
WAKE OF THE FRONT MAY NECESSITATE A WIND ADVISORY...BUT WILL
DEFER THE ISSUANCE OF SUCH TO THE NEXT SHIFT.

CLEARING SKIES WILL OCCUR DURING THE DAY THURSDAY WITH THE
NORTHERLY WINDS GRADUALLY SUBSIDING DURING THE AFTERNOON. HIGHS
WILL BE MUCH COOLER AND ONLY IN THE LOW 60S. LIGHT WINDS AND
CLEAR SKIES WILL ALLOW FOR GOOD RADIATIONAL COOLING THURSDAY NIGHT
AND LOWS FROM THE UPPER 30S TO MID 40S LOOK LIKELY. A STRING OF
FAIRLY PLEASANT DAYS ARE EXPECTED FRIDAY THROUGH SUNDAY WITH
TEMPERATURES WARMING AND SOUTH WINDS INCREASING A BIT EACH DAY.

A COLD FRONT IS FORECAST TO STALL OVER OKLAHOMA ON MONDAY AND
THEREFORE BELIEVE RAIN CHANCES WILL STAY TO THE NORTH OF THE AREA
UNTIL MONDAY NIGHT WHEN THE BOUNDARY MAY SLIP SOUTH OF THE RED
RIVER. GFS IS AT ODDS WITH THE ECMWF/CANADIAN WITH THE NEXT
SYSTEM. THE GFS SHOWS THE TROUGH MOVING THROUGH TEXAS WITH A GOOD
ROUND OF RAIN TUESDAY AND WEDNESDAY. THE OTHER MODELS SHOW IT
REMAINING WEAK AND STAYING A BIT TOO FAR SOUTH OF THE AREA TO
PROVIDE MUCH RAIN. WILL KEEP POPS LOW AND TEMPS NEAR OR JUST ABOVE
CLIMO FOR NOW.

TR.92

&&

.AVIATION...
/ISSUED 1154 PM CDT TUE MAR 24 2015/
/06Z TAFS/

CONCERNS...MORNING STRATUS...CONVECTIVE POTENTIAL.

STORMS IN SOUTHEASTERN OKLAHOMA HAVE DISSIPATED...AND WITH A
STRENGTHENING CAP...NO ADDITIONAL ACTIVITY IS ANTICIPATED
OVERNIGHT. FWS VWP SHOWS A LLJ OF 50KTS AT 2-3KFT AGL BENEATH THE
CAP...SUGGESTING POTENTIAL FOR STRATUS AGAIN WEDNESDAY MORNING.
HOWEVER...WITH ABSOLUTELY NO STRATUS UPSTREAM...THINK TAF SITES
WILL BE FREE OF LOW CEILINGS UNTIL DAYBREAK. THIS WOULD GREATLY
REDUCE THE DURATION OF MVFR CONDITIONS AND LIKELY PRECLUDE IFR
CEILINGS ALTOGETHER. THE THIN LAYER OF CLOUDS WILL LIFT AND ERODE
BY MID-MORNING...ALLOWING THE GUSTY WINDS TO REACH THE SURFACE
ONCE AGAIN.

NORTH TEXAS WILL CONTEND WITH A LINGERING CAP AGAIN WEDNESDAY
AFTERNOON...WITH NO BOUNDARIES TO AID IN REALIZING THE INSTABILITY
IN PLACE. WITH A DRYLINE WEST OF ABILENE AND WICHITA FALLS AT THE
HEAT OF THE DAY...FEEL THE CONVECTIVE POTENTIAL IS TOO LOW TO
INCLUDE IN THE TAFS UNTIL AFTER NIGHTFALL WEDNESDAY EVENING. THE
ASSOCIATED COLD FRONT WILL MOVE THROUGH THE METROPLEX AFTER
MIDNIGHT WEDNESDAY NIGHT.

25


&&

.PRELIMINARY POINT TEMPS/POPS...
DALLAS-FT. WORTH, TX  82  51  62  43  73 /  10  50  10   0   0
WACO, TX              80  53  63  40  73 /   5  50  10   0   0
PARIS, TX             80  51  61  40  65 /   5  60  10   0   0
DENTON, TX            83  49  62  40  72 /  10  40  10   0   0
MCKINNEY, TX          81  50  62  38  71 /  10  60  10   0   0
DALLAS, TX            82  51  62  45  73 /  10  50  10   0   0
TERRELL, TX           80  52  62  41  70 /   5  50  10   0   0
CORSICANA, TX         79  54  62  42  70 /   5  50  20   0   0
TEMPLE, TX            80  54  63  41  74 /   5  50  10   0   0
MINERAL WELLS, TX     85  49  62  40  74 /  20  40   5   0   0

&&

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

&&

$$



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