FXUS64 KJAN 101710
AFDJAN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSON MS
1110 AM CST THU DEC 10 2009
.UPDATE...
EARLIER THIS MORNING...ADJUSTED THE WINDS UPWARD A BIT AND ISSUED A
LAKE WIND ADVISORY FOR THE LARGER LAKES BASED ON OBSERVATIONAL
TRENDS AND A REPORT OF ROUGH WATER CONDITIONS ON THE LOWER ROSS BARNETT
RESERVOIR. THE WINDS WILL SUBSIDE DURING THE MID AFTERNOON HOURS AS
HIGH PRESSURE CONTINUES TO BUILD OVER THE ARKALAMISS. ALL ELSE IS
GENERALLY ON TRACK AND MADE NO OTHER SIGNIFICANT ADJUSTMENTS. THE
UPDATED AVIATION DISCUSSION IS BELOW. /EC/
&&
.PREV DISCUSSION... /ISSUED 351 AM CST THU DEC 10 2009/
SHORT TERM...TODAY THROUGH SATURDAY...PRIMARY FORECAST CONCERNS ARE
THE BELOW NORMAL TEMPERATURES AND RAIN CHANCES FOR FRIDAY NIGHT INTO
SATURDAY.
AT THE SURFACE...COLD SURFACE HIGH PRESSURE (1024 MB) IS CENTERED
OVER THE SOUTHERN PLAINS WITH A RIDGE AXIS EXTENDING SOUTHEASTWARD
OVER THE LOWER MISSISSIPPI VALLEY. THE ASSOCIATED MODIFIED AIRMASS
IS OF A CONTINENTAL POLAR ORIGIN. WHAT MAKES THIS AIRMASS
PARTICULARLY COLD IS ITS INTERACTION WITH A WIDESPREAD FRESH SNOW
COVER OVER THE NORTHERN ROCKIES AND MIDWEST. THE CORE OF THIS
AIRMASS IS PROGGED TO TRACK EASTWARD ACROSS ARKANSAS AND TENNESSEE
THROUGH FRIDAY BEFORE RETREATING NORTHEASTWARD OVER THE EASTERN OHIO
VALLEY REGION THIS WEEKEND. THIS WILL LEAVE THE FORECAST AREA ON THE
SOUTHERN PERIPHERY OF THE COLDER AND DRIER AIRMASS. THE RESULT WILL
BE TEMPERATURES SEVERAL DEGREES BELOW SEASONAL AVERAGES FOR THE
REMAINDER OF THIS WEEK...ALONG WITH DEWPOINTS IN THE TEENS AND 20S
AND PWATS 25 TO 45 PERCENT OF NORMAL. HIGH TEMPS TODAY WERE REDUCED
A COUPLE DEGREES FROM MAV GUIDANCE TOWARD THE LOWER END OF ENSEMBLE
MOS GUIDANCE GIVEN THE COLD AND DRY NATURE OF THE AIRMASS. THIS
YIELDS HIGH TEMPS GENERALLY IN THE MID AND UPPER 40S ACROSS THE CWA
TODAY. FOR TONIGHT...WITH THE SURFACE HIGH IN ITS CLOSEST PROXIMITY
TO THE CWA...TEMPERATURES WILL ONCE AGAIN FALL OFF INTO THE UPPER
20S AND LOWER 30S ACROSS THE REGION...WITH MID 20S ACROSS THE
NORTHEAST. WHAT WILL PREVENT TEMPS FROM FALLING OFF EVEN MORE WILL
BE AN H5-H3 MOISTURE PLUME FROM THE PACIFIC THAT WILL EXTEND
EASTWARD ACROSS THE CWA. THIS WILL TRANSLATE TO MAINLY HIGH-LEVEL
CIRRUS...THOUGH NWP MODEL GUIDANCE SUGGESTS RH VALUES AT LEAST
90 PERCENT THROUGHOUT THIS LAYER INDICATING THE CLOUD DECK BEING
POTENTIALLY THICK/DENSE ENOUGH TO INHIBIT STRONGER RADIATIONAL
COOLING. THIS WILL PARTICULARLY BE THE CASE OVER SOUTHERN PORTIONS
OF THE FORECAST AREA. COOL CONDITIONS WILL PERSIST INTO THE DAY ON
FRIDAY...ESPECIALLY AS CLOUD COVERAGE INCREASES.
IN THE UPPER LEVELS...A LONGWAVE TROUGH ENCOMPASSES MUCH OF THE
CENTRAL AND NORTHERN CONUS. IN THE BASE OF THIS TROUGH...A VERY
TIGHT HEIGHT GRADIENT IN THE UPPER LEVELS IS SUPPORTING A ZONAL JET
STREAK CONTAINING H2 WIND SPEEDS REACHING 170 KT. THIS JET STREAK
MARKS THE CONFLUENCE OF THE STRONG SOUTHERN JET STREAM ORIGINATING
FROM THE SUBTROPICAL EAST PACIFIC REGION WITH AN ASSOCIATED UPPER
MOISTURE PLUME...AND THE A POLAR JET STREAM IN THE BASE OF THE
AFOREMENTIONED LONGWAVE TROUGH. THROUGH THIS EVENING...THE JET
STREAK WILL CONTINUE MOVING EASTWARD TOWARD THE TENNESSEE RIVER
VALLEY REGION. AS A RESULT...STRONG ASCENT IN THE RIGHT ENTRANCE
REGION OF THIS JET STREAK WILL BECOME JUXTAPOSED WITH A REMNANT
FRONTAL TROUGH OVER THE WESTERN GULF OF MEXICO. THE LOWER
TROPOSPHERIC MASS RESPONSE WILL BE THE DEVELOPMENT OF A SURFACE LOW
ALONG THE FRONT...WHICH WILL TRANSLATE NORTHEASTWARD ALONG THE FRONT
ACROSS THE NORTHERN GULF. GULF MOISTURE IN THE VICINITY OF THE
DEVELOPING SURFACE LOW WILL BE ADVECTED INTO THE FORECAST AREA BY
INCREASING WEST-SOUTHWESTERLY FLOW ABOVE H85. THIS MOIST AIR WILL
EXPERIENCE ISENTROPIC LIFT ATOP THE EXPANSIVE COLD DOME ASSOCIATED
WITH THE COLD HIGH PRESSURE. AS A RESULT...WIDESPREAD PRECIP IS
EXPECTED FOR FOR FRIDAY NIGHT INTO SATURDAY...ESPECIALLY AS A SERIES
OF SHORTWAVE DISTURBANCES EMBEDDED WITHIN THE LONG FETCH OF MOIST
WEST-SOUTHWESTERLY FLOW ALOFT MOVE ACROSS THE REGION. POPS GENERALLY
FOLLOWED MAV GUIDANCE...THOUGH POPS WERE INCREASED ABOVE GUIDANCE
ALONG AND SOUTH OF THE NATCHEZ TRACE PARKWAY CORRIDOR...WHERE AT
LEAST 70 PERCENT OF SREF MEMBERS ARE INDICATING MEASURABLE PRECIP.
RAINFALL TOTALS OF ONE-HALF TO ONE INCH WILL BE POSSIBLE THROUGH
LATE SATURDAY AFTERNOON FROM THE RAIN. GIVEN THAT THESE TOTALS WILL
ACCUMULATE OVER THE COURSE OF AT LEAST 12 HOURS...AND GIVEN THE
OPPORTUNITY FOR DRYING SOILS OVER A FEW DAYS SINCE THE RECENT
RAINFALL...FLOODING CONCERNS ARE NOT CURRENTLY ANTICIPATED.
AS FAR AS TIMING OF THE PRECIP...OVERALL TREND FOR THE LAST FEW DAYS
AMONGST MULTI-MODEL ENSEMBLE GUIDANCE MEMBERS HAS BEEN FOR PRECIP TO
START PROGRESSIVELY LATER IN THE DAY ON FRIDAY AND TOWARD FRIDAY
NIGHT. THIS COULD BE DUE TO BETTER RESOLVING THE STRENGTH OF THE
COLD AND DRY AIRMASS OVER THE REGION. AS A RESULT...FORECAST
SOUNDINGS SHOW PRECIP ONSET OCCURRING ONCE BOUNDARY LAYER HAS WARMED
ABOVE FREEZING AND THE CORE OF THE COLDER AIRMASS HAS FURTHER
RETREATED AWAY FROM THE REGION. THUS...PRECIP IS EXPECTED TO BE MORE
OF A COLD RAIN AS OPPOSED TO A MIX OF PRECIPITATION TYPES. ELEVATED
INSTABILITY DRIVEN BY STRONG WARM AIR ADVECTION ALOFT COULD YIELD
MUCAPES UP TO 500 J/KG ACROSS CENTRAL AND SOUTHERN AREAS FOR
SATURDAY PER LATEST GFS/EC...WHERE SREF IS ALSO INDICATING A SLIGHT
CHANCE FOR THUNDER. WITH RAINFALL AND CLOUDS ON SATURDAY...COOL
TEMPS ARE ONCE AGAIN EXPECTED...WITH HIGH RANGING FROM THE UPPER 40S
TO THE LOWER 50S./COHEN/
LONG TERM...SATURDAY NIGHT THROUGH WEDNESDAY...LONGWAVE TRENDS FAVOR
A CONTRACTION/WEAKENING OF THE POLAR VORTEX IN ERN CANADA WHICH
RELAXES THE BROAD MEAN TROUGH AXIS IN THE ERN HALF OF THE CONUS AS WE
TRANSITION TO A FLATTER ZONAL FLOW PATTERN. THE GFS ENSEMBLE MEAN IS
A LITTLE QUICKER TO DRY US OUT SATURDAY NIGHT THAN EITHER THE OPS RUN
OR ECMWF WHICH KEEP SOME POPS THROUGH AT LEAST SUNDAY MORNING. WILL
HOLD ON TO SOME CHANCE OF LIGHT RAINS INCLUDING TSRA OVER SERN MS
SATURDAY NIGHT AND EARLY SUNDAY. DRIER WEATHER IS INDICATED FOR
SUNDAY NIGHT AND EARLY MONDAY BUT ANOTHER SHORTWAVE IS EXPECTED TO
RETURN ISENTROPIC RAINS BY MONDAY AFTERNOON. EXPECT THE WARMEST TEMPS
TO ARRIVE MONDAY WITH 850MB TEMPS OF 13-14C AND SURFACE HIGHS FROM
M60S TO L70S.
THEN...A STRONGER SHORTWAVE IS EXPECTED TO MOVE OVER THE SOUTHERN
PLAINS MONDAY NIGHT AND HELP DEVELOP A SURFACE LOW IN THE NWRN GULF
THAT IS PROGGED TO MOVE ALONG THE GULF COAST LAATE MONDAY NIGHT/EARLY
TUESDAY. RAIN CHANCES WILL INCREASE MONDAY NIGHT AHEAD OF THE THIS
SYSTEM AND TSRA SHOULD BREAK OUT OVER AT LEAST SRN HALF OF OUR CWFA.
PWATS RISE TO 1.6 INCHES MAINLY S OF I-20 WHERE MLCAPES BEING PROGGED
TO REACH 500-600 J/KG...SURFACE DWPTS 60-65F AND 7-500MB LAPSE RATES
NEAR 6C. WIND SHEAR EXPECTED TO BE MORE MARGINAL AND WILL NEED TO
MONITOR THIS TIME FRAME FOR TRENDS IN THE MODEL INSTABILITY AND
SHEAR. INITIAL QPF SHOULD AVERAGE 1 TO 2 INCHES BUT COULD BE HIGHER
IN SERN MS. THEN...GFS AND ECMWF SHOW A STRONG COLD FRONT DROPPING
INTO OUR REGION TUESDAY MORNING WITH COLDER AND DRIER WEATHER DIVING
IN TUESDAY NIGHT AND WEDNESDAY. THE COLDER AIR MASS FROM THE CENTRAL
PLAINS REMAINS QUITE STOUT AS IT CROSSES OVER A LARGE SNOWPACK WITH A
1040MB HIGH CENTER DROPPING DOWN THE MO VALLEY FOR WEDNESDAY. HAVE
LEANED TOWARD THE COLDER ENSEMBLE MEMBERS IN THE WAKE OF THE FRONT
BUT OTHERWISE HAVE STAYED CLOSE TO MEX MOS THROUGH THE LONG TERM./40/
&&
.AVIATION...
WIDESPREAD VFR CONDITIONS ARE EXPECTED THROUGH FRIDAY
MORNING. A SCATTERED TO BROKEN DECK OF HIGH-LEVEL CLOUDS WITH BASES
BETWEEN 15 AND 25 KFT WILL AFFECT THE REGION (ESPECIALLY SOUTH OF
I-20) INTO FRIDAY. NORTHERLY WINDS AT 8 TO 14 KT (AND OCCASIONALLY
GUSTY) ARE EXPECTED TODAY BEFORE BECOMING NORTHEASTERLY TONIGHT AND
DECREASING TO 4 TO 8 KT. RAIN WILL BECOME WIDESPREAD FOR FRIDAY
NIGHT INTO SATURDAY...WITH MVFR CEILINGS/VISIBILITIES EXPECTED IN
ADDITION TO A SLIGHT CHANCE FOR THUNDERSTORMS ALONG AND SOUTH OF THE
INTERSTATE 20 CORRIDOR. /BB/COHEN/
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...LAKE WIND ADVISORY UNTIL 4 PM CST THIS AFTERNOON FOR MSZ026-030-
031-043-049-052.
LA...LAKE WIND ADVISORY UNTIL 4 PM CST THIS AFTERNOON FOR LAZ009.
AR...LAKE WIND ADVISORY UNTIL 4 PM CST THIS AFTERNOON FOR ARZ075.
&&
$$
