820
FXUS64 KFWD 131121 AAA
AFDFWD
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE FORT WORTH TX
621 AM CDT TUE AUG 13 2013
.AVIATION...
PRIMARY CHALLENGES INCLUDE TIMING OF BOTH CONVECTIVE POTENTIAL
AND WINDSHIFT AT METRO TAF SITES.
SKIES WERE MOSTLY CLEAR ACROSS METRO AREA AND AT WACO THIS
MORNING...WITH THICKER MID AND HIGH LEVEL CLOUDS JUST TO THE
NORTH ALONG THE RED RIVER. WIDESPREAD CONVECTION CONTINUED ACROSS
OKLAHOMA THIS MORNING...GENERALLY IN A 100SM WIDE BAND EXTENDING
FROM KCSM TO KOKC TO KFSM. THE CONVECTIVE CELLS WERE MOVING
EAST...WITH A SLOW SOUTHWARD SHIFT TO THE OVERALL SYSTEM.
EXPECT A CONTINUED SOUTHWARD SHIFT TO THE CONVECTION TODAY...WITH
A FASTER PROPAGATION BY THIS AFTERNOON AS OUTFLOW SPREADS SOUTH OF
EXISTING CONVECTION. EXPECT VFR CONDITIONS AT ALL TAF SITES
THROUGH AT LEAST THIS AFTERNOON BEFORE CONVECTION ARRIVES AT METRO
SITE BY 06Z /1 AM CDT/ TONIGHT. THERE WILL BE A LOW CHANCE OF
THUNDER THIS AFTERNOON...BUT THOSE CHANCES APPEAR TOO LOW TO
MENTION AT THIS TIME.
FOR METRO SITES...WILL CARRY VCSH BEGINNING AT 23Z /6 PM CDT/ AND
THEN REFLECT WINDSHIFT AND TEMPO FOR THUNDERSTORMS BEGINNING AT
06Z. CONVECTION SHOULD BE SHIFTING SOUTH AND DIMINISHING BY 12 UTC
WEDNESDAY MORNING /7 AM CDT/ BUT SCATTERED SHOWERS WILL PERSIST.
FOR KACT...EXPECT GENERALLY VFR CONDITIONS THIS PERIOD. WILL
INSERT VCSH AFTER MIDNIGHT CDT TONIGHT AND VCTS AFTER 08Z /3 AM CDT/.
09
&&
.PREV DISCUSSION... /ISSUED 425 AM CDT TUE AUG 13 2013/
AS OF 3 AM CDT...REGIONAL RADARS SHOWED A BROKEN LINE OF
THUNDERSTORMS EXTENDING FROM THE NORTHERN TX PANHANDLE EAST ACROSS
ALL OF CENTRAL OK. THERE WAS ALSO ONE ADDITIONAL BRANCH OF
CONVECTION LOCATED FROM JUST NORTH OF GAINESVILLE TX TO NEAR FORT
SMITH AR. THE FAR SOUTHERN RADAR RETURN OF THIS SECONDARY BRANCH
OF PRECIPITATION HAD BEEN MOVING SLOWLY EAST...RIGHT ALONG OR JUST
NORTH OF THE RED RIVER FOR THE PREVIOUS TWO HOURS. TDFW RADAR WAS
SHOWING SOME VERY LIGHT RADAR RETURNS MOVING SOUTHEAST AWAY FROM
THIS SOUTHERN BRANCH OF PRECIPITATION. THE RADAR RETURNS HAD THE
DISTINCT APPEARANCE OF GRAVITY WAVES...HOWEVER THE GAINESVILLE
AWOS DID PICK UP A NORTH WIND ON ITS 3 AM OBSERVATION...SO THESE
WEAK RADAR RETURNS COULD BE OUTFLOW PUSHING SOUTH INTO THE CWA
EARLY THIS MORNING. WHETHER THIS IS TRUE OUTFLOW OR A TEMPORARY
WIND SHIFT DUE TO THE INFLUENCE OF GRAVITY WAVES WILL PLAY AN
IMPORTANT ROLE IN OUR THUNDERSTORM CHANCES ACROSS NORTH TX THIS
AFTERNOON.
TODAY AND TONIGHT...EARLY MORNING WATER VAPOR IMAGERY OBSERVED
WHAT MODELS HAD BEEN ADVERTISING FOR SEVERAL DAYS NOW...A LARGE
UPPER LEVEL TROUGH DIGGING SOUTHEAST OVER THE NORTHERN PLAINS AND
GREAT LAKES REGION. BY THIS AFTERNOON THIS TROUGH IS EXPECTED TO
BUILD AS FAR SOUTH AS TENNESSEE CAUSING THE BELT OF STRONGER
WESTERLIES ALOFT TO TILT SOUTH OVER NORTH TX. THIS NORTHWEST FLOW
ALOFT...ALONG WITH A SOUTHWARD BUILDING COLD FRONT...IS EXPECTED
TO SEND SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS INTO THE
REGION FROM LATE THIS AFTERNOON THROUGH WEDNESDAY.
AS OF 3 AM CDT...THE COLD FRONT WAS ANALYZED JUST NORTH OF
AMARILLO TO OKLAHOMA CITY TO BENTONVILLE BASED ON REGIONAL SURFACE
OBSERVATIONS. THE BROKEN LINE OF SHOWERS AND THUNDERSTORMS
APPEARED TO BE NORTH OF THE COLD FRONT...WHICH SHOULD HELP SEND
THE FRONT SOUTH TOWARDS NORTH TX THIS MORNING. WHENEVER THE FRONT
ARRIVES...FORCING FOR ASCENT ALONG AND JUST BEHIND THE COLD FRONT
SHOULD BE SUFFICIENT FOR THUNDERSTORM INITIATION. WENT AHEAD AND
KEPT POPS IN THE 40 TO 60 PERCENT RANGE FOR COUNTIES BORDERING THE
RED RIVER THIS AFTERNOON...EXPANDING SOUTH TO THE INTERSTATE 20
CORRIDOR FOR THIS EVENING. WITH MOST GUIDANCE SHOWING SCATTERED TO
NUMEROUS STORMS NEAR THE COLD FRONT...CONFIDENCE IN MOST LOCATIONS
ALONG AND NORTH OF INTERSTATE 20 SEEING SOME THUNDERSTORM ACTIVITY
IS FAIRLY HIGH AT THIS TIME. THE COVERAGE OF STORMS THIS EVENING
SHOULD BE SIMILAR TO WHAT WAS OBSERVED ALONG THE FRONT ACROSS THE
TX PANHANDLE AND THROUGH CENTRAL OK EARLY THIS MORNING.
/IF YOU ARE A USER OF HIGH RESOLUTION MODEL GUIDANCE AND WANT TO
KNOW WHY THIS FORECAST DOES NOT HAVE HIGH POPS AROUND THE I-20
CORRIDOR EARLY THIS AFTERNOON READ ON. OTHERWISE...SKIP THE NEXT 3
PARAGRAPHS./
WHILE THE FRONT SHOULD BE FAIRLY EASY TO TRACK THROUGHOUT THE DAY
TODAY...THERE ARE SOME COMPLICATING FACTORS WORTH MENTIONING. THE
BIGGEST IS THAT SEVERAL HIGH RESOLUTION MODELS INDICATE THAT
SCATTERED THUNDERSTORMS WILL INITIATE ALONG THE INTERSTATE 20
CORRIDOR AS EARLY AS 1 PM THIS AFTERNOON. IT APPEARS THAT FOR
MODELS THAT ARE INITIATING CONVECTION BY 18Z...THEY ARE DOING SO
ASSUMING THERE IS AN OUTFLOW BOUNDARY IN PLACE NEAR THE I-20
CORRIDOR...SEEMINGLY PUT THERE BY RED RIVER PRECIPITATION EARLY
THIS MORNING.
ASSUMING THIS IS THE DRIVING CAUSE FOR EARLY AFTERNOON STORMS
FORECAST BY THESE MODELS... 08Z/3AM CDT OBSERVATIONAL EVIDENCE
SUGGESTS THAT AN OUTFLOW BOUNDARY WILL NOT MAKE IT TO THE I-20
CORRIDOR THIS MORNING. WARMING CLOUD TOPS ON EARLY MORNING
INFRARED SATELLITE IMAGERY AND WEAKENING REFLECTIVITY ECHOES NEAR
THE RED RIVER SUGGEST THAT THIS ACTIVITY WILL DISSIPATE BY
SUNRISE. IF THAT HAPPENS...THERE MAY BE AN OUTFLOW BOUNDARY
LOCATED JUST SOUTH OF THE RED RIVER BY EARLY THIS AFTERNOON...BUT
MAKING IT TO THE I-20 CORRIDOR SEEMS VERY UNLIKELY AT THIS TIME.
AS A RESULT...DESPITE SEVERAL HIGH RESOLUTION MODELS SHOWING
STORMS DEVELOPING ALONG THE I-20 CORRIDOR...WILL ONLY MAINTAIN
THE 20 PERCENT CHANCE OF STORMS FROM THE PREVIOUS FORECAST ALONG
THE I-20 CORRIDOR THROUGH THIS AFTERNOON. THAT SAID...WE WILL HAVE
TO MONITOR SURFACE OBSERVATIONS AND THE COLD FRONT MOVING SOUTH
EARLY THIS MORNING FOR ANY SIGNS OF DEVIATION FROM THIS FORECAST.
/BACK TO THE NORMAL DISCUSSION BELOW.../
TONIGHT AND WEDNESDAY...ONCE THE FRONT IS IN OUR CWA AND NEAR THE
INTERSTATE 20 CORRIDOR AROUND MIDNIGHT...ALMOST ALL MODEL GUIDANCE
HAS NUMEROUS THUNDERSTORMS PORTRAYED IN THEIR RESPECTIVE
SOLUTIONS. AS A RESULT KEPT 60 POPS IN THE FORECAST NEAR THE
INTERSTATE 20 CORRIDOR FOR EARLY WEDNESDAY MORNING. AFTER
12Z...THE FRONT BECOMES MORE DIFFICULT TO TRACK DUE TO THE
INFLUENCE OF MODEL GENERATED CONVECTION SHOWING UP IN VIRTUALLY
ALL LOW-LEVEL MASS FIELDS.
FOR THIS FORECAST MORE OR LESS USED CONVERGENT WINDS AT THE 850 MB
LEVEL AS A ROUGH TRACER FOR THE LOCATION OF THE COLD FRONT DURING
THE DAY ON WEDNESDAY. KEPT SLIGHTLY HIGHER POPS IN PLACE ALONG
AND NORTH OF THE FRONT. HOWEVER DUE TO MODELS MAINTAINING BROAD
BRUSHED QPF OVER MUCH OF THE CWA ON WEDNESDAY...AND LITTLE
CONFIDENCE ON EXACTLY WHAT THE MESOSCALE ENVIRONMENT WILL LOOK
LIKE AFTER A FEW ROUNDS OF THUNDERSTORMS THROUGH THE REGION...KEPT
AT LEAST CHANCE POPS IN THE FORECAST ACROSS THE CWA. HAVE 40 TO 50
POPS OVER THE SOUTHERN HALF OF THE FORECAST AREA WITH 30 POPS TO
THE NORTH ASSUMING THAT THE FRONT IS SOUTH OF I-20 AFTER SUNRISE
WEDNESDAY. AGAIN THIS IS SIMPLY FOLLOWING A MODEL CONSENSUS 850 MB
CONVERGENCE LINE. THE FORECAST WILL HAVE TO BE ADJUSTED ONCE THE
FRONTAL POSITION BECOMES BETTER RESOLVED.
FOR WEDNESDAY NIGHT AND THURSDAY...OPERATIONAL MODELS BEGIN TO
DISAGREE ON THE POSITION OF THE COLD FRONT AND HOW MUCH DRY AIR
WILL BUILD INTO PLACE BEHIND IT. THE 00Z NAM AND CANADIAN ARE BOTH
VERY AGGRESSIVE WITH NORTHEAST FLOW BEHIND THE COLD FRONT
BEGINNING WEDNESDAY NIGHT. THESE MODELS INDICATE A 20 TO 30 DEGREE
DROP IN SURFACE THETA-E FROM WEDNESDAY NIGHT THROUGH THURSDAY
AFTERNOON WHILE THE GFS INDICATES A 10 DEGREE DROP OR LESS FOR
MOST LOCATIONS...WITH THE EXCEPTION OF THE FAR NORTHEASTERN CWA
WHERE ALL GUIDANCE IS DRY. THE DEGREE TO WHICH DRIER AND COOLER
AIR FILTERS INTO THE CWA DURING THIS PERIOD WILL PLAY A LARGE
ROLE ON PRECIPITATION CHANCES DURING THIS PERIOD. THE ECMWF OFFERS
UP A SOLUTION THAT IS MORE OR LESS IN COMPROMISE TO THESE TWO
SOLUTIONS...AND AS A RESULT IT HAS BEEN FOLLOWED CLOSELY IN THIS
FORECAST.
THE PRIMARY DIFFERENCE BETWEEN THESE SOLUTIONS APPEARS TO BE HOW
MODELS HANDLE THE STRENGTH OF A SURFACE RIDGE OF HIGH PRESSURE
THAT EXTENDS FROM THE OHIO VALLEY SOUTHWEST TOWARDS ARKLATEX BY
EARLY THURSDAY MORNING. THE NAM AND CANADIAN ARE MUCH STRONGER
WITH THE INFLUENCE OF THIS RIDGE AND THEREFORE SEND STRONGER LOW-
LEVEL NORTHEASTERLY FLOW THROUGH NORTH TX. THE GFS IS SOMEWHAT
WEAKER WITH THE UPPER RIDGE...BUT MORE IMPORTANTLY IS STRONGER IN
MAINTAINING LEE SIDE CYCLOGENESIS THROUGH THIS PERIOD...CAUSING
LOW-LEVEL WINDS TO VEER MORE INTERRUPTING THE INFLUX OF COOL DRY
AIR INTO THE CWA. AT THE TIME OF THIS DISCUSSION THIS RIDGE OF
HIGH PRESSURE IS IN CANADA...SO WE WILL HAVE TO WAIT AND SEE WHAT
IT LOOKS LIKE AS IT BUILDS OVER THE DENSER CONUS SURFACE
OBSERVATION NETWORK OVER THE NEXT 24 HRS. AT ANY RATE...WENT AHEAD
AND KEPT THE FORECAST DRY FOR THURSDAY EVENING AND OVERNIGHT AS
THE ECMWF IS FAIRLY DRY BY THAT TIME.
FRIDAY THROUGH TUESDAY...WITH LEE SIDE CYCLOGENESIS OVER THE HIGH
PLAINS BUT DEEP NORTHERLY FLOW ALOFT OVER THE SOUTHERN
PLAINS...THERE SHOULD BE A NICE NORTH-SOUTH ORIENTED BAROCLINIC
ZONE THAT SETS UP JUST WEST OF THE CWA FOR AT LEAST THURSDAY AND
FRIDAY...POSSIBLY HOLDING TOGETHER INTO SATURDAY. THIS SHOULD
PROVIDE SOME LIFT FOR ISOLATED TO SCATTERED STORMS JUST WEST OF THE
CWA DURING THIS TIME FRAME. HAVE SOME 20 TO 30 POPS OVER THE
WESTERN CWA ON THURSDAY TO ACCOUNT FOR THIS...BUT ONLY WENT WITH
10 POPS ON FRIDAY FOR NOW WITH LESS CONFIDENCE IN THE STRENGTH OF
THIS LIFT.
ODDLY ENOUGH...NORTH TX IS EXPECTED TO BE ON THE COOL SIDE OF
THIS BOUNDARY/BAROCLINIC ZONE WHICH IS QUITE RARE FOR MID-
AUGUST. COOLER WEATHER IS EXPECTED TO BUILD IN ON WEDNESDAY AND
LAST THROUGH AT LEAST FRIDAY WITH CLOUD COVER HELPING TO OFFSET
STRONG SOLAR INSOLATION DURING THAT TIME. BY THIS WEEKEND MOSTLY
SUNNY SKIES ARE EXPECTED AND TEMPERATURES SHOULD WARM THROUGH
EARLY NEXT WEEK AS A RESULT. THIS HEATING WILL CAUSE ANY LEFT OVER
BAROCLINICITY TO DAMPEN OUT REDUCING THUNDERSTORM CHANCES OVER THE
FAR WESTERN CWA FOR THE REMAINDER OF THE FORECAST. TEMPERATURES
SHOULD CLIMB BACK UP TO SEASONAL NORMALS BY MONDAY.
CAVANAUGH
&&
.PRELIMINARY POINT TEMPS/POPS...
DALLAS-FT. WORTH, TX 99 76 88 74 89 / 20 60 30 30 10
WACO, TX 99 77 94 74 93 / 10 30 40 30 10
PARIS, TX 90 72 84 67 87 / 50 60 30 20 10
DENTON, TX 98 73 86 71 88 / 30 60 30 30 10
MCKINNEY, TX 98 73 87 70 87 / 30 60 30 30 10
DALLAS, TX 100 78 90 74 89 / 20 60 30 30 10
TERRELL, TX 98 75 87 72 89 / 20 60 30 30 10
CORSICANA, TX 99 77 91 72 91 / 10 60 40 30 10
TEMPLE, TX 99 75 95 73 95 / 10 20 40 30 20
MINERAL WELLS, TX 99 74 88 71 90 / 20 60 30 30 20
&&
.FWD WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
09/