FXUS64 KFWD 241933
AFDFWD
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FORT WORTH TX
133 PM CST THU DEC 24 2009
.AVIATION...
FOR THE 18Z TAFS...MAIN FORECAST CONCERN CONTINUES TO BE THE
IMPACT OF THE WINTER STORM SYSTEM IMPACTING NORTH TX. HAVE
MAINTAINED GENERALLY IFR CONDITIONS AS LIGHT SNOW HAS MOVED INTO
METROPLEX TAF SITES AND DO NOT ANTICIPATE VSBYS TO IMPROVE ABOVE
IFR LEVELS UNTIL THIS EVENING. VERY HIGH CONFIDENCE IN MODERATE
SNOWFALL AFFECTING METROPLEX AIRPORTS HERE THRU THE AFTERNOON HRS.
WHEN THE SNOW ACTUALLY ENDS IS ANOTHER QUESTION. LATEST GUIDANCE
KEEPS SOME LIGHT SNOW THRU 03Z ATM...WHICH MAY CONTINUE TO KEEP
VSBYS DOWN AT IFR LVLS. ENVIRONMENTAL WINDS ARE INCREASING QUICKLY
WITH SUSTAINED WINDS INCREASING TO NEAR 30 MPH HERE EARLY THIS
AFTERNOON. WIND GUSTS COULD APPROACH 50 MPH FOR ALL TAF SITES
LATER THIS AFTERNOON. AFTER 03Z CONFIDENCE IN SNOW TAPERING OFF IS
MUCH HIGHER ALTHO WILL HAVE TO EVALUATE THE ENVIRONMENTAL
WINDS...THE SURFACE LOW IS EXPECTED TO MOVE NORTHEAST FAIRLY
QUICKLY AFTER SUNSET...BUT STRONG NORTHWESTERLY WINDS MAY TEND TO
HOLD OVER THE AREA THRU MIDNIGHT. AFTER MIDNIGHT SKIES SHOULD
BEGIN TO CLEAR OUT AND VFR CONDITIONS LOOK LIKELY FOR THE
REMAINDER OF THE FORECAST PERIOD.
TODAY...IFR. PERIODS OF LIFR WITH MOD SNOW SHOWERS. VERY STRONG
NORTHWEST WINDS.
TONIGHT...IFR BECOMING VFR AFT MIDNIGHT. WINDS DIMINISHING.
CHRISTMAS DAY...VFR.
CAVANAUGH
&&
.UPDATE...
BASED ON REPORTS FROM OKLAHOMA MESONET ACROSS SOUTHERN OKLAHOMA
THE POLAR LOW LEVEL JET IS CONTRIBUTING TO FREQUENT WIND GUSTS OF
55 TO 60 MPH. WITH RUC/NAM BOTH INDICATING H850 WINDS OF 55 TO 60
KTS AT 18Z OVER NORTHWESTERN PORTIONS OF THE CWA FELT THAT WIND
GUSTS TO NEAR 60 MPH WOULD LIKELY MAKE IT ALONG AND NORTHWEST OF A
NOCONA TO DECATUR TO BRECKENRIDGE LINE. MODELS CONTINUE TO
INDICATE ISENTROPIC LIFT WILL INCREASE ACROSS THESE SAME COUNTIES
DURING THE LATE MORNING HRS INCREASING COVERAGE OF SNOWFALL HEADED
INTO THE AFTERNOON HRS. AFTER 18Z MODELS DEPICT THE TROWAL REGION
OF THE UPPER LOW WILL PASS OVER NORTHERN TX PROVIDING INCREASED
FRONTOGENETIC FORCING AND ISENTROPIC LIFT IN THE 750 TO 650 MB
LAYER.
OUR MAIN CONCERN FOR HEAVY SNOWFALL PRODUCTION AT THIS TIME
CONTINUES TO BE STABILITY ABOVE THIS LAYER OF MAXIMUM FORCING FOR
ASCENT. NAM FORECAST SOUNDINGS INDICATE MEAGER LAPSE RATES IN THE
H500-H700 LAYER WITH ONLY 12-14 DEG C DECREASE IN TEMPERATURE.
VERTICAL CROSS SECTIONS THROUGH THE PRIMARY AREA OF CONCERN
CONTINUE TO INDICATE AN AREA OF EPV (A MEASURE OF STATIC
STABILITY) WHICH IS NOT INDICATIVE OF MUCH POTENTIAL
INSTABILITY...UPRIGHT OR SLANTWISE. FEELING IS THAT ONCE REMOVED
FROM THE BEST LOW LVL ISENTROPIC LIFT HEAVIER SNOW PRODUCTION WILL
TEND TO BE LIMITED AS SOMEWHAT HIGHER STATIC STABILITY ABOVE THE
MAXIMUM FRONTOGENETIC FORCING REGION WILL LIMIT OVERALL VERTICAL
RESPONSE. REGARDLESS...LOOKS LIKE NORTHWEST OF THE METROPLEX BROAD
AREA OF ISENTROPIC ASCENT WILL CREATE PERIODS OF MODERATE TO HEAVY
SNOW WHERE WE HAVE THE WINTER STORM WARNING IN EFFECT. WE MAY NEED
TO UPGRADE PORTIONS OF THE WINTER STORM WARNING TO A BLIZZARD
WARNING IF THE HEAVY TO MODERATE SNOW MOVES SOUTH INTO OUR CWA
HERE FROM NORTH CENTRAL TX WITHIN THE NEXT HOUR OR SO. IF NOTHING
ELSE...NEAR BLIZZARD LIKE CONDITIONS ARE EXPECTED FOR AREAS WHERE A
WINTER STORM WARNING IS IN EFFECT. AT THIS TIME DUE TO UNCERTAINTY
ASSOCIATED WITH AFOREMENTIONED STABILITY CONCERNS AS THE TROWAL
MOVES EAST THIS AFTERNOON WE ARE NOT PLANNING ON EXTENDING THE
WINTER STORM WARNING INTO THE METROPLEX. WE WILL CONTINUE TO
EVALUATE THE EVOLUTION OF THIS SYSTEM AND DETERMINE IF ANY
ADDITIONAL CHANGES TO WINTER WEATHER HEADLINES ARE NEEDED AS WE
HEAD INTO THE AFTERNOON HRS.
&&
.PREV DISCUSSION... /ISSUED 321 AM CST THU DEC 24 2009/
COLD FRONT IS ABOUT 2/3RDS OF THE WAY THROUGH NORTH TEXAS AND WILL
HAVE MOVED THROUGH ALL OF NORTH TEXAS BY DAYBREAK. TEMPERATURES
ARE DROPPING SIGNIFICANTLY BEHIND THE COLD FRONT AND ALL LOCATIONS
WILL SEE THEIR HIGH TEMPERATURES TODAY EARLY THIS MORNING WITH
TEMPERATURES FALLING THROUGHOUT THE DAY. STRONG NORTHWEST WINDS
WILL CONTINUE TODAY AND THE WIND ADVISORY WILL REMAIN IN EFFECT
THROUGH 03Z/9 PM BUT MAY HAVE TO BE EXTENDED A FEW MORE HOURS.
THE EXCITING AND CHALLENGING PART OF THE FORECAST IS THE LIKELIHOOD
OF SNOW TODAY. THE ONLY SIGNIFICANT CHANGES FROM LAST NIGHT ARE TO
INCREASE THE SNOW TOTAL AMOUNTS AND EXTEND THE WARNING AND
ADVISORY THROUGH 00Z/6 PM. THE UPPER LEVEL LOW IS CURRENTLY NEAR
MIDLAND AND WILL ARRIVE IN NORTH TEXAS MID-LATE MORNING. THE
LATEST MODELS PROGRESS THE UPPER LEVEL LOW RIGHT ALONG THE
INTERSTATE 20 CORRIDOR...SETTING UP A SNOW EVENT FOR THE NORTHERN
HALF OF NORTH TEXAS. SNOW ACCUMULATIONS HAVE BEEN RAISED TO NEARLY
5" IN THE NORTHWEST TAPERING TO 1" IN THE ADVISORY AREA. GIVEN THE
SLOWER SPEED OF THE SYSTEM...HAVE ALSO LINGERED LIGHT SNOW ACROSS
THE NORTHEAST THROUGH MIDNIGHT AS THE SYSTEM EXITS THE REGIONS.
WE CHOSE TO USE A 7:1 SNOW RATIO AND THE AVERAGE OF REASONABLE
SREF MEMBERS FOR SNOW TOTALS.
THE COMBINATION OF SNOW AND STRONG WINDS WILL CAUSE BLOWING SNOW
ACROSS MUCH OF THE WARNING AND ADVISORY AREA...BUT MOST LIKELY IN
THE NORTHWESTERN COUNTIES. THIS WILL RESULT IN REDUCED
VISIBILITIES...DOWN TO A QUARTER MILE AT TIMES IN SOME
LOCATIONS...RESEMBLING BLIZZARD-LIKE CONDITIONS. MINOR SNOW
DRIFTS OF A FEW FEET WILL LIKELY FORM DUE TO THE BLOWING SNOW.
THESE SNOW DRIFTS WILL FORM AGAINST BUILDINGS AND OTHER STANDING
OBJECTS SUCH AS FENCES.
AS THE RUC SUGGESTS...IT IS POSSIBLE THAT WE MAY GET DRY-SLOTTED
OR TOO MUCH WARM AIR NEAR 700 MB MAY GET WRAPPED AROUND THE
SYSTEM. THIS WOULD RESULT IN LOWER SNOW ACCUMULATIONS OR A MIX OF
FREEZING RAIN/SLEET/SNOW MAY OCCUR. HOWEVER...GIVEN THE OVERALL
PERSISTENCE OF THE MODEL RUNS WE WILL CONTINUE WITH SNOW MENTIONED
AS THE MAIN PRECIPITATION TYPE.
BEYOND TODAY...THE FORECAST REMAINS NEARLY UNCHANGED UNTIL
TUESDAY. COOL AND DRY WEATHER WILL PERSIST FRIDAY THROUGH MONDAY.
THE ECMWF AND GFS HAVE COME IN BETTER AGREEMENT REGARDING THE NEXT
STORM SYSTEM ON TUESDAY. HAVE MENTIONED A WINTER PRECIPITATION MIX
OF RAIN AND SLEET IN THE FORECAST FOR TUESDAY DUE TO UNCERTAINTIES
IN THE FORECAST THIS FAR OUT. IT APPEARS MORE LIKELY THAT SNOW IN THE
NORTH AND FREEZING RAIN IN THE SOUTH MAY OCCUR BUT WILL FINE TUNE
AS NEEDED AS THE EVENT APPROACHES. HAVE SIGNIFICANTLY UNDERCUT
MOS TEMPERATURES ON TUESDAY TO COINCIDE WITH THE WINTER WEATHER EVENT.
82/JLD
&&
.PRELIMINARY POINT TEMPS/POPS...
DALLAS-FT. WORTH, TX 23 43 27 47 27 / 20 5 5 5 5
WACO, TX 27 47 28 52 28 / 5 5 5 5 5
PARIS, TX 22 39 23 43 23 / 50 5 5 5 5
DENTON, TX 22 41 25 48 24 / 30 5 5 5 5
MCKINNEY, TX 21 41 25 48 24 / 40 5 5 5 5
DALLAS, TX 23 44 27 47 29 / 20 5 5 5 5
TERRELL, TX 25 44 27 50 25 / 30 5 5 5 5
CORSICANA, TX 26 46 29 53 29 / 10 5 5 5 5
TEMPLE, TX 26 49 28 52 28 / 5 5 5 5 5
&&
.FWD WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 6 PM CST THIS EVENING FOR TXZ094-
095-104>107-118>123-130>134-141.
WIND ADVISORY UNTIL 9 PM CST THIS EVENING FOR TXZ092>095-103>107-
116>123-129>135-141>148-156>162-174-175.
WINTER STORM WARNING UNTIL 6 PM CST THIS EVENING FOR TXZ091>093-
100>103-115>117-129.
HIGH WIND WARNING UNTIL 6 PM CST THIS EVENING FOR TXZ091-100>102-
115.
&&
$$
