FXUS64 KJAN 071038
AFDJAN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSON MS
438 AM CST MON DEC 7 2009
.SHORT TERM...TODAY THROUGH TUESDAY NIGHT...STILL EXPECTING A RATHER
WET SHORT TERM PERIOD WITH SEVERE STORM POTENTIAL TUESDAY. LATEST
WATER VAPOR IMAGERY/RUC ANALYSIS SHOWS A POTENT SHORTWAVE TROUGH
DROPPING INTO NORTHERN CALIFORNIA WITH A FAST ZONAL FLOW ALOFT
ACROSS THE SOUTHERN HALF OF THE CONUS. A SUBTLE SHORTWAVE IN THE
ZONAL FLOW WAS HELPING TO ENHANCE ISENTROPIC UPGLIDE OVER OUR REGION
THIS MORNING RESULTING IN LIGHT RAIN DEVELOPMENT. REGIONAL RADARS
STILL SHOW THE MEASURABLE RAIN ALONG AND WEST OF THE NATCHEZ TRACE
BUT THIS AREA WILL SHIFT EAST AS THE SHORTWAVE SHIFTS EAST.
INSTABILITY WILL REMAIN LIMITED AND WL NOT CARRY MENTION OF THUNDER
TODAY. MODELS AGREE THAT THE RAIN WILL TAPER OFF FROM THE WEST THIS
AFTERNOON BUT ANY LULL IN THE RAIN WILL BE SHORT LIVED AS RAIN
CHANCES INCREASE TONIGHT AHEAD OF ANOTHER APPROACHING SHORTWAVE.
DESPITE THE CLOUDS AND RAIN TODAY TEMPERATURES WILL TOP OUT SEVERAL
DEGREES WARMER THAN ON SUNDAY. THE GFS TEMP GUIDANCE WAS SEVERAL
DEGREES WARMER THAN THE NAM OVER MOST OF OUR AREA TODAY AND WAS
LEANED TOWARD. AS ANOTHER SHORTWAVE MOVES OVER THE REGION TONIGHT A
WEAK WAVE IS EXPECTED TO DEVELOP OVER SOUTHERN TX AND MOVE TO THE
NORTHEAST TOWARD OUR AREA. THIS WILL INCREASE MOISTURE CONVERGENCE
AT 850MB WITH +330K THETA E AIR NOSING INTO OUR SOUTHWEST BY
MIDNIGHT. A LITTLE MORE INSTABILITY IS EXPECTED WITH THIS SYSTEM AND
ELEVATED THUNDERSTORMS WILL BECOME POSSIBLE BY 12Z TUESDAY. OUR PW
WILL BE BACK TO AN INCH AND A HALF CONTRIBUTING TO LOCALLY HEAVY
RAINFALL POTENTIAL. WL CONTINUE TO MONITOR THIS TIME FRAME FOR
FLOODING POTENTIAL TODAY. WAA WILL INCREASE SIGNIFICANTLY OVER OUR
REGION AS WELL TUESDAY AHEAD OF THE POTENT MID LEVEL SHORTWAVE
TROUGH THAT IS DROPPING INTO CALIFORNIA THIS MORNING. THIS POTENT
SHORTWAVE WILL HELP DEEPEN A SURFACE LOW AS THAT IS EXPECTED TO MOVE
FROM THE FOUR CORNERS REGION EARLY TUESDAY MORNING TO THE MID
MISSISSIPPI RIVER VALLEY TUESDAY NIGHT. BY TUESDAY AFTERNOON DEEP
LAYER SHEAR IS EXPECTED TO HAVE INCREASED TO 45-60KTS WITH 0-1KM
SHEAR AROUND 40KTS OVER OUR AREA. PWS SHOULD BE BACK ABOVE AN INCH
AND A HALF WITH MID 60 DEW POINTS RISING THROUGH THE 60S. AFTERNOON
TEMPERATURES MAY CLIMB INTO THE LOWER 70S ACROSS THE SOUTHERN HALF
OF THE AREA. DESPITE THE INCREASE IN DEEP LAYER MOISTURE AND DEEP
LAYER SHEAR...INSTABILITY LOOKS TO REMAIN SOMEWHAT LIMITED WITH WEAK
LAPSE RATES AND MLCAPES AROUND 500J/KG BUT SUFFICIENT FOR SEVERE
THUNDERSTORM DEVELOPMENT. SPC HAS OUTLOOKED OUR AREA WITH A SLIGHT
RISK AND WL CONTINUE MENTION IN THE HWO. DAMAGING WINDS AND A FEW
TORNADOES LOOK TO BE THE MAIN THREATS. LATEST MODEL RUNS APPEAR
FASTER IN ENDING THE RAIN FROM THE WEST TUESDAY NIGHT WITH THE
STRONG COLD FRONT RACING THROUGH OUR AREA WEDNESDAY. STRONG GUSTY
WINDS ARE EXPECTED AHEAD AND BEHIND THIS SYSTEM WARRANTING A LAKE
WIND ADVISORY AT SOME POINT. /22/
.LONG TERM...WEDNESDAY THROUGH NEXT MONDAY...
PRIMARY FORECAST CONCERN IS PRECIPITATION FOR EARLY FRIDAY INTO
SATURDAY...INCLUDING TIMING...COVERAGE...AND TYPE.
BY WEDNESDAY MORNING...THE COLD FRONT...THAT IS EXPECTED TO HAVE
BROUGHT ACTIVE WEATHER TO THE ARKLAMISS ON TUESDAY...IS PROGGED TO
HAVE MOVED EAST/SOUTHEAST OF THE CWA...LEAVING THE REGION WITH
MOSTLY CLOUDY SKIES AND A FEW LINGERING SHOWERS IN THE EAST.
MEANWHILE...A LONGWAVE UPPER TROUGH WILL BE TAKING SHAPE OVER THE
NORTHERN CONUS...WHILE A BROAD UPPER RIDGE BECOMES ESTABLISHED OVER
THE SOUTHERN CONUS...NORTHERN MEXICO...AND THE GULF OF MEXICO. AS
THESE TWO FEATURES PHASE TOGETHER...A VERY TIGHT HEIGHT GRADIENT IN
THE UPPER LEVELS WILL SUPPORT A ZONAL JET STREAK CONTAINING H2 WIND
SPEEDS REACHING 180 KT BY EARLY THURSDAY OVER THE PLAINS AND NOSING
INTO THE OZARKS. THIS JET STREAK WILL ALSO MARK THE CONFLUENCE OF A
STRONG SOUTHERN JET STREAM ORIGINATING FROM THE SUBTROPICAL PACIFIC
OCEAN...THUS CONTAINING AN ABUNDANCE OF UPPER MOISTURE...AS WELL AS
A POLAR JET STREAM IN THE BASE OF THE AFOREMENTIONED LONGWAVE
TROUGH. BY THURSDAY EVENING...NWP GUIDANCE HAS A GOOD HANDLE ON
PROPAGATING THE JET STREAK EASTWARD OVER THE TENNESSEE VALLEY REGION
AND MID-ATLANTIC STATES. THIS WILL PLACE THE ASCENDING BRANCH OF THE
AGEOSTROPHIC CIRCULATION IN THE ENTRANCE REGION OF THE JET STREAK
OVER THE WESTERN GULF OF MEXICO...WHERE...CO-INCIDENTALLY...THE
REMNANT BAROCLINIC TROUGH FROM THE AFOREMENTIONED FRONT WILL HAVE
BECOME STATIONARY. THE LOWER TROPOSPHERIC MASS RESPONSE WILL BE TO
INDUCE A SURFACE WAVE ALONG THE FRONT OVER THE WEST CENTRAL GULF
FRIDAY MORNING...WHICH WILL DEEPEN INTO A FRONTAL LOW BY SATURDAY
MORNING OVER THE NORTHWEST 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...WHICH WILL
EXPERIENCE ISENTROPIC LIFT ATOP AN EXPANSIVE COLD DOME THAT WILL
BECOME ESTABLISHED OVER MUCH OF THE EASTERN UNITED STATES LATE IN
THE WEEK. ISENTROPIC UPGLIDE ON ISENTROPIC SURFACES FROM 290K TO
305K IS FORECAST TO BE INCREASING FROM FRIDAY MORNING THROUGH
SATURDAY...WHILE A SERIES OF SHORTWAVE DISTURBANCES EMBEDDED WITHIN
THE LONG FETCH OF MOIST WEST-SOUTHWESTERLY FLOW ALOFT MOVE ACROSS THE
REGION. THIS SET-UP WILL BE CONDUCIVE FOR WIDESPREAD PRECIP ACROSS
THE AREA FOR FRIDAY INTO SATURDAY...AND POPS WERE RAISED TO LIKELY
ACROSS MUCH OF THE CWA CONSISTENT WITH LATEST MEX GUIDANCE...WHICH
IS AT THE HIGHEST END OF THE GFS ENSEMBLE GUIDANCE. WAVES OF
PRECIPITATION ARE EXPECTED TO MOVE FROM SOUTHWEST TO NORTHEAST
ACROSS THE CWA BEGINNING EARLY FRIDAY AND CONTINUING THROUGH
SATURDAY CONSISTENT WITH SEVERAL PERTURBATIONS OF THE GFS
ENSEMBLES...CMC...AND THE ECMWF. THE TIMING OF THE PRECIP IS
UNCERTAIN...AND WILL BE HIGHLY TIED TO THE EVENTUAL TRACK OF THE
SURFACE LOW. GFS/CMC/GFS ENSEMBLE INDICATE PRECIP STARTING LATE
THURSDAY NIGHT AND CONTINUING INTO SATURDAY...WHEREAS EC STARTS
PRECIP LATE IN THE DAY FRIDAY AND CONTINUES IT INTO SATURDAY.
HOWEVER...THE EC HAS BEEN TENDING MORE MOIST IN RECENT RUNS...AND
THUS PRECIP CHANCES HAVE BEEN INCLUDED FOR LATE THURSDAY NIGHT ACROSS
THE WEST. SIMILAR TRENDING WAS NOTED WITH THE EC FOR THE RECENT SNOW
EVENT ACROSS THE AREA.
WHAT MAKES FOR THE TRICKY PART OF THE FORECAST WILL BE THE PRESENCE
OF COLD...AND POTENTIALLY SUBFREEZING...BOUNDARY LAYER AIR ACROSS
THE ARKLAMISS DURING PORTIONS OF THE PRECIPITATION EVENT. VERY
STRONG AND COLD SURFACE HIGH PRESSURE CONTINUES TO BUILD OVER THE
NORTHERN ROCKIES...WITH A 1042 MB HIGH OVER CENTRAL ALBERTA. ONCE
SURFACE LOW PRESSURE DEEPENS IN THE LEE OF THE ROCKIES...DRAGS THE
EARLY-WEEK COLD FRONT ACROSS THE ARKLAMISS...AND RACES TOWARD THE
GREAT LAKES...THE AFOREMENTIONED SURFACE HIGH WILL BEGIN SPRAWLING
SOUTHEASTWARD OVER THE EASTERN UNITED STATES. THE AIRMASS ASSOCIATED
WITH THIS HIGH WILL BE VERY COLD INITIALLY AS ITS SOURCE REGION IS
CURRENTLY OVER A RELATIVELY DEEP SNOW PACK...ALTHOUGH IT WILL MODIFY
AS IT SPREADS SOUTHEASTWARD AND CORE PRESSURES DECREASE.
NEVERTHELESS...A SHALLOW COLD LAYER WILL BE ENTRENCHED OVER THE
REGION BY EARLY FRIDAY...WITH THE CORE OF THE COLDER AIR OVER THE
OHIO VALLEY. LATEST GUIDANCE INDICATES THAT COLD AIR WILL BE SLOW TO
RETREAT EAST OF THE REGION...BRINGING A LONG DURATION OF SHALLOW
COLD AIR TO THE AREA LATE THIS WEEK.
FORECAST SOUNDINGS FOR LOCATIONS ALONG AND NORTH OF INTERSTATE 20
INDICATE AN ELEVATED WARM LAYER OF ABOVE-FREEZING TEMPERATURES FROM
950 MB TO 750 MB. ONLY A SMALL LAYER OF NEARLY SATURATED AIR ABOVE
THE -10C LEVEL WILL SUPPORT INITIAL ICE GROWTH ALOFT...AND IT IS
POSSIBLE THAT MUCH OF THIS ICE COULD MELT AS IT ENCOUNTERS THE
ELEVATED WARM LAYER. HOWEVER...BELOW 950 MB...SUBFREEZING
TEMPERATURES ARE FOUND TO EXIST NEARLY TO THE SURFACE...ESPECIALLY
IF PRECIPITATION INTENSITY IS SUFFICIENT TO ENHANCE WET-BULBING
EFFECTS IN THIS LAYER. GIVEN MAXIMUM TEMPERATURES IN THE ELEVATED
WARM LAYER AROUND 4C...SUCH A PROFILE COULD YIELD FREEZING RAIN
POTENTIAL EARLY IN THE PRECIPITATION EVENT. SLEET/SNOW ARE NOT OF
PARTICULAR CONCERN AT THIS TIME DUE TO THE SHALLOW NATURE OF THE
SURFACE-BASED COLD POOL...AS WELL AS STRENGTH OF WARM AIR ADVECTION
ALOFT AND LACK OF ICE INTRODUCTION ALOFT. HOWEVER...AS WITH MANY
PRECIPITATION TYPE FORECASTS...THERE IS HIGH UNCERTAINTY THIS FAR IN
ADVANCE. AT THIS TIME...THE CHANCES FOR FREEZING RAIN LOOK HIGHEST
IN THE NORTHERN THIRD OF THE CWA EARLY FRIDAY...PARTICULARLY IN THE
DELTA REGION...WHERE WEAK COLD AIR DRAINAGE COULD ENHANCE THE COLD
POOL. INTERESTINGLY...DESPITE THE ABSENCE OF PRECIP FROM THE
EC...SIMILARLY-STRUCTURED FORECAST SOUNDINGS ARE NOTED FOR NORTHERN
LOCATIONS LATE IN THE WEEK BETWEEN THE GFS AND EC FURTHER INDICATING
THE FREEZING RAIN POTENTIAL. DURING THE DAY FRIDAY...THE COMBINATION
OF A WARMING ELEVATED WARM LAYER...ALONG WITH INCREASING 1000-850 MB
THICKNESS VALUES WITH THE APPROACHING GULF SURFACE LOW...
PRECIPITATION IS EXPECTED TO BECOME ALL RAIN BY AFTERNOON. IN
FACT...ELEVATED INSTABILITY DRIVEN BY STRONG WARM AIR ADVECTION
ALOFT COULD YIELD MUCAPES IN EXCESS OF 400 J/KG ACROSS SOUTHEASTERN
AREAS PER LATEST GFS...WHERE A SLIGHT CHANCE FOR THUNDER HAS BEEN
ADDED FOR FRIDAY AFTERNOON. THE PRECIPITATION IS EXPECTED TO REMAIN
IN THE FORM OF RAIN INTO FRIDAY EVENING. HOWEVER...LATE FRIDAY NIGHT
INTO EARLY SATURDAY MORNING...FORECAST SOUNDINGS INDICATE SOME
POTENTIAL CHANGEOVER BACK TO FREEZING RAIN ACROSS THE NORTHERN THIRD
OF THE AREA...AS THE BOUNDARY LAYER COOLS BELOW FREEZING WHILE
PRECIPITATION CONTINUES. AGAIN...CONFIDENCE IN PRECIP TYPE THIS
EARLY IS QUITE LOW...AND RAIN IS THE WEATHER TYPE MENTIONED IN THE
GOING FORECAST. HOWEVER...IF FUTURE MODEL RUNS CONTINUE TO INDICATE
THE POTENTIAL FOR MIXED PRECIPITATION...MENTION WILL BE MADE IN
UPCOMING FORECASTS AND OUTLOOKS. MOST OF THE PRECIPITATION IS
EXPECTED TO HAVE ENDED LATE SATURDAY AS THE GULF LOW MOVES AWAY FROM
THE REGION...THOUGH LINGERING CLOUDS AND WEAKENING COLD AIR ADVECTION
WILL AFFECT THE AREA THROUGH THE REMAINDER OF THE WEEKEND.
AS FOR TEMPERATURES...MEX GUIDANCE CAME IN AT THE COLDER END OF MOS
ENSEMBLE GUIDANCE FOR THURSDAY INTO THE WEEKEND...WHICH IS
CONSISTENT WITH THE COLD AIRMASS EXPECTED TO BE IN PLACE. TEMPS WERE
EVEN INCREASED IN A FEW INSTANCES WHERE MEX CAME IN COLDER THAN
COLDEST OF MOS ENSEMBLE NUMBERS. FOR SUNDAY INTO MONDAY...THE SURFACE
RIDGE IS EXPECTED TO BE SLIDING EASTWARD TO THE NORTH OF THE
CWA...WITH SLOWLY MODERATING TEMPERATURES AND WINDS BECOMING EAST-
SOUTHEASTERLY WITH LOW CHANCES FOR RAIN. /COHEN/
&&
.AVIATION...LIGHT RAIN WAS NOTED ACROSS MUCH OF THE AREA BUT NOT A
SOLID AREA OF RAIN AND CEILINGS HAVE BEEN ABOVE AND BELOW MVFR AT
MOST LOCATIONS. A PASSING UPPER LEVEL DISTURBANCE AND A WEAK COLD
FRONT SHIFTING EAST AND CLOSER TO OUR AREA WL KEEP RAIN GOING INTO
THE AFTERNOON. RAIN AND LOWER CEILINGS RESULTING IN MVFR/IFR
CONDITIONS WILL CONTINUE THROUGH MUCH OF THE AFTERNOON AT ALL TAF
SITES...BUT RAIN WILL DIMINISH BY LATE AFTERNOON. INSTABILITY WILL
REMAIN LIMITED TODAY AND NO THUNDER IS EXPECTED. RAIN WILL INCREASE
TONIGHT FROM THE SOUTHWEST WITH LOWERING CEILINGS AND VISIBILITIES.
THUNDERSTORMS WILL ALSO BE POSSIBLE IN THE WEST BY 12Z. /22/
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 53 52 72 53 / 60 87 100 88
MERIDIAN 53 49 69 54 / 59 74 100 89
VICKSBURG 54 52 71 53 / 58 97 100 75
HATTIESBURG 57 53 72 59 / 65 60 90 87
NATCHEZ 56 55 75 54 / 63 98 100 79
GREENVILLE 51 46 63 48 / 55 98 100 63
GREENWOOD 51 46 63 51 / 56 91 100 78
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...NONE.
LA...NONE.
AR...NONE.
&&
$$
22/COHEN
