FXUS64 KJAN 022039
AFDJAN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSON MS
240 PM CST WED DEC 2 2009
.SHORT TERM...TONIGHT THROUGH FRIDAY...WATER VAPOR IMAGERY/RUC
ANALYSIS SHOWS THE CLOSED LOW SPINNING OVER THE MISSISSIPPI RIVER
NEAR THE MISSISSIPPI...TENNESSEE...AND ARKANSAS STATE LINES. SURFACE
ANALYSIS HAS THE LOW CENTERED OVER CENTRAL TENNESSEE. THE CLOSED LOW
WILL WEAKEN INTO AN OPEN TROUGH AS IT MOVES NORTHEAST TONIGHT ALONG
WITH THE SFC LOW ACROSS THE OHIO RIVER VALLEY. IN THE MEANTIME...THE
GRADIENT WINDS ASSOCIATED WITH THE LOW PRESSURE SYSTEM WAS RESULTING
IN GUSTY WINDS ACROSS THE CWA. A DRY SLOT FORMED OVER SOUTHEAST
MISSISSIPPI EARLIER BUT HAS SHIFTED OVER ALABAMA. THIS WAS THOUGHT
TO ENHANCE MIXING OF HIGHER WINDS ALOFT DOWN TO THE SURFACE OVER
EAST MISSISSIPPI AND THE PREVIOUS LAKE WIND ADVISORY AREA WAS
EXPANDED. THE LAKE WIND ADVISORY WILL REMAIN IN EFFECT UNTIL 6 PM
FOR EAST MISSISSIPPI AND ALL AREA LAKES. LOCAL RADARS WERE SHOWING
SOME LIGHT RAIN OVER THE HIGHWAY 82 CORRIDOR WITH ONLY PATCHY
DRIZZLE ELSEWHERE OVER THE ARKLAMISS. THERE WILL REMAIN A CHANCE OF
LIGHT RAIN THROUGH THE AFTERNOON IN THE NORTH BUT BY THIS EVENING
ONLY PATCHY LIGHT DRIZZLE IS EXPECTED AS THE LOW PRESSURE SYSTEM
MOVES FARTHER NORTH OF OUR CWA. IN THE WAKE OF THE LOWS TONIGHT
SURFACE HIGH PRESSURE WILL BEGIN BUILDING IN FROM THE NORTHWEST
WHILE OUR WINDS ALOFT BACK TO SOUTHWESTERLY AHEAD OF A DEEPENING
UPPER LEVEL TROUGH OVER NW CONUS. COOLER AND DRIER AIR WILL MOVE
INTO THE REGION FROM THE NORTHWEST BUT SW FLOW ALOFT WILL BRING SOME
MID TO HIGH LEVEL CLOUDS. OVERNIGHT LOWS WILL BE CLOSER TO NORMAL IN
THE MID TO UPPER 30S AT MOST SITES. THURSDAY AND THURSDAY NIGHT WILL
BE DRY AS HIGH PRESSURE AT THE SURFACE DROPS SOUTH TO THE SOUTHERN
PLAINS WHILE THE LARGE UPPER LEVEL TROUGH SHIFTS TO THE CENTRAL
CONUS. ALTHOUGH THE WINDS WILL BE MUCH LIGHTER THURSDAY THE CAA WILL
CONTINUE TO HOLD TEMPERATURES WELL BELOW NORMAL. MOST SITES WILL
STRUGGLE TO REACH 50 DEGREES IN THE NORTH WITH LOWER 50S EXPECTED
SOUTH. THURSDAY NIGHT WILL BE COLD AND A GOOD PORTION OF THE AREA
WILL DROP TO OR BELOW FREEZING. THE FORECAST GETS MORE INTERESTING
FRIDAY INTO FRIDAY NIGHT. A SHORTWAVE WILL ROUND THE BASE OF THE
LONG WAVE TROUGH AND HELP DEVELOP A SURFACE LOW OVER THE WESTERN
GULF. THE ASSOCIATED RAIN WITH THIS SYSTEM WILL APPROACH OUR CWA
FROM THE SW FRIDAY. THE GFS AND NAM BRING MOISTURE BACK INTO OUR
AREA EARLIER FRIDAY THAN PREVIOUS RUNS SUGGESTED. OUR AIRMASS WILL
BE QUITE DRY AND TAKE SOME TIME TO GET MEASURABLE PRECIPITATION BUT
HAVE LEANED TOWARD THE HIGHER GFS POPS IN THE SOUTHWEST WHILE
TRIMMING BACK IN THE NORTHEAST. AFTERNOON HIGHS WILL BE COOLER THAN
NORMAL BUT WELL ABOVE FREEZING FROM THE UPPER 40S TO LOWER 50S. THE
PRECIPITATION WILL STILL BE FALLING AS WE MOVE INTO FRIDAY NIGHT.
THE NAM AND GFS TEMPERATURE GUIDANCE WERE REMARKABLY CLOSE THROUGH
THE SHORT TERM AND THE GFS WAS GENERALLY ACCEPTED. THE GFS/SREF/NAM
AND TO A LESSER EXTENT THE ECMWF STILL AGREE ON SOME POTENTIAL FOR
SNOW FRIDAY NIGHT IN OUR CWA. LITTLE CHANGE FRIDAY NIGHT AND BEYOND
WAS MADE TO THE PREVIOUS FORECAST AND THE DISCUSSION FOLLOWS. /22/
.LONG TERM...FRIDAY NIGHT THROUGH WEDNESDAY...WHAT I HAD FEARED LAST
NIGHT IS BECOMING MORE OF A POSSIBILITY AS THE OUTLYING
MODEL...WHICH WAS DEVELOPING SNOW ACROSS THE AREA...IS NOT THE
OUTLIER ANYMORE. THE GFS...IS NOW LEADING THE WAY AND HAS STRONG
SUPPORT FROM THE SREF AND MOST OF ITS ENSEMBLE MEMBERS AND SOME
SUPPORT FROM THE NAM AND EURO. 8 OF 12 ENSEMBLE MEMBERS GENERATE
MEASURABLE SNOW FRI NIGHT ACROSS THE CWA AND SNOW PROBABILITIES FROM
THE SREF ARE RANGING FROM 40-60%. SUCH SUPPORT HAS TO BE TAKEN
SERIOUSLY AND THE RESULT WILL BE SIGNIFICANT CHANGES FOR THE FRI
NIGHT FORECAST WRT POPS AND PRECIP TYPE BECOMING AT LEAST LIGHT SNOW.
ONE MAY BE ASKING...WHAT HAS CHANGED TO NOW INCLUDE THIS INTO THE
FORECAST? MAINLY DEEPER MOISTURE AND STRONGER SUPPORT FROM MORE
MODELS. FORECAST LEADING INTO FRI NIGHT HAS GENERALLY ALWAYS
PORTRAYED SOME SORT OF S/W MOVING THROUGH THE BASE OF THE LONG WAVE
TROUGH. HOWEVER...THE TREND LAST NIGHT WAS FOR THE S/W TO BE
STRONGER. WITH THAT...THE LARGE MAJORITY OF THE MODEL GUID WAS
GENERALLY DRY AND ONLY HAD CLOUDS WITH THAT TROUGH PASSAGE. THE GFS
WAS REALLY THE ONLY MODEL SHOWING SNOW ALONG WITH A COUPLE OF ITS
ENSEMBLE MEMBERS. SINCE LAST NIGHT...MUCH MORE SUPPORT HAS COME IN
FROM VARIOUS GUID AND BASICALLY THERE HAS BEEN A 180 SWITCH AND NOT
THE OUTLYING SOLUTION IS THE DRIER VERSION.
AS FOR SOME OF THE DETAILS...THE S/W IN QUESTION WILL BE RATHER
POTENT AND PROVIDE STRONG ASCENT ACROSS THE REGION ALONG WITH
HELPING TO DEVELOP A SFC LOW ACROSS THE N GULF. IT WILL BE THIS SFC
LOW DEVELOPMENT THAT WILL GIVE THE S/W THE MOISTURE TAP TO DEVELOP
PRECIP ABOVE THE COLD AIRMASS. I WILL SAY...THAT UNCERTAINTY IS
STILL FAIRLY HIGH IN THIS SOLUTION AS IT IS SOME 60-72 HRS OUT AND
ANYTIME WE ARE DEALING WITH WINTRY PRECIP IN OUR AREA...THINGS OFTEN
CHANGE. THE GENERAL SETUP FOR THIS WINTRY POTENTIAL FRI NIGHT IS
QUITE FAVORABLE AS WE WILL SEE CAA ONGOING FRI INTO FRI NIGHT WITH
SFC PRESSURES RISING AS HIGH PRESSURE BUILDS IN FROM THE N. THE
COMBINATION OF THE DEVELOPING GULF LOW IS ALSO SUPPORTIVE B/C IT
PROVIDES MOISTURE. THE FLY IN THE OINTMENT IS WILL THE COLDER AIR
MOVING SOUTHWARD BE TOO DRY AND EVAPORATE ALL OF THE FALLING PRECIP.
THAT TYPICALLY HAPPENS. ADDITIONALLY...THE AIRMASS THAT WILL BE IN
PLACE WILL NOT BE VERY COLD AND HIGH TEMPS FRI SHOULD BE IN THE
MID/UPPER 40S. IF CLOUD COVER THICKENS FASTER AND VIRGA DEVELOPS
EARLIER THE EVAP COOLING PROCESS WILL KEEP TEMPS COOLER AND MORE
NEAR 40 DEGREES. WITH FREEZING CONDITIONS NOT ALREADY IN PLACE...WE
WILL NEED OTHER MEANS TO GENERATE SNOW. THAT DOES NOT LOOK TO BE A
PROBLEM AS DYNAMIC COOLING FROM THE STRONG S/W WILL PROVIDE THAT AS
STRONG ASCENT WILL GENERATE PRECIP ALOFT WHICH WILL EVAPORATE AND
COOL ANY LAYER THAT IS ABOVE FREEZING. ALSO...THIS ALL LOOKS TO COME
TOGETHER FRI NIGHT AND WILL BE AIDED BY DIURNAL COOLING WHICH WILL
MAKE IT EASIER TO DROP TEMPS TO CRITICAL LEVELS FOR PRECIP TO BECOME
LIGHT SNOW. IT APPEARS AS MOISTURE VALUES...VERTICAL MOTION AND
DURATION WILL ONLY SUPPORT LIGHT ACCUMULATIONS FROM 1/4 INCH TO
POSSIBLY 1 INCH. THIS WILL ALSO OCCUR WITH TEMPS 31-34
DEGREES...SIMILAR TO PREV SNOW EVENTS OVER THE PAST TWO WINTER
SEASONS. HOWEVER...THOSE EVENTS HAD MUCH HIGHER MOISTURE TO WORK
WITH WHICH RESULTED IN MORE ACCUMULATIONS OF WET SNOW. THIS LOOKS TO
BE MUCH LIGHTER AT THIS POINT. ANY ACCUMS SHOULD BE ON GRASSY SFC'S
OR ELEVATED SFC'S AND BE 1 INCH OR LESS.
IN THE WAKE OF THIS S/W FEATURE...SAT-SUN LOOK TO BE QUIET.
HOWEVER...THAT MAY NOT LAST TOO LONG AS THE PATTERN WILL BE MORE OF
FAST SW ALOFT WHICH OFTEN MAKES FOR QUICK TRANSITIONS AND A OVERALL
WETTER PATTERN. WHILE DETAILS WILL BE VERY TOUGH TO GET
DOWN...FOLLOWING THE GENERALITIES OF THE GUID SEEMS THE BEST WAY TO
GO FOR SUN NIGHT THROUGH WED. DECENT PRECIP CHANCES AND A WARMING
TREND LOOK TO BE IN STORE AND GUID VALUES WERE ACCEPTED.
AS FOR TEMPS/POPS AND WEATHER FOR FRI-SAT...I DID LOWER HIGHS ON FRI
DUE TO THE CLOUD COVER AND EVAP COOLING POTENTIAL. ENSEMBLE MOS
SUPPORTS THIS AS WELL. I DID LOWER POPS ON FRI AS I FEEL THE ON SET
OF ANY LIGHT PRECIP WILL BE LATE AND MAINLY IN THE FRI NIGHT PERIOD.
FOR FRI NIGHT...I LOWERED LOWS TO GET INTO THAT 31-34 RANGE WHICH
COVERS THE WET BULB EFFECT AND FOLLOWS THE ENSEMBLE MEAN WHICH HAS
BEEN LOWER THAN THE OPS GUID FOR THE PAST FEW RUNS. POPS WERE TOUGH
AS MY INTUITION WOULD BE TO LOWER POPS DUE TO THE UNCERTAINTY.
HOWEVER...ALL THE DATA I HAVE LOOKED SAYS TO JUST STAY WITH THE
LATEST GUID POPS AS MEASURING JUST A COUPLE HUNDREDTHS OF LIQUID
EQUIVALENT IS VERY POSSIBLE. FOR NOW...LIGHT SNOW WORDING FITS THE
SITUATION AS ANY SNOW ACCUM SHOULD BE AROUND 1 INCH OR LESS. /CME/
&&
.AVIATION...SURFACE LOW PRESSURE WILL CONTINUE TO MOVE NORTHEAST OF
THE REGION THROUGH THURSDAY...WHILE AN UPPER LEVEL TROUGH MOVES
NORTHEASTWARD ACROSS THE AREA. MOISTURE WRAPPING AROUND THE LOW WILL
CONTINUE TO RESULT IN AN OVERCAST DECK OF LOW CLOUDS THROUGH
THURSDAY...WITH MVFR CEILINGS ACROSS MUCH OF THE REGION. A FEW
BREAKS IN THE CLOUDS WILL BE FOUND ACROSS EASTERN MISSISSIPPI LATE
THIS AFTERNOON... THOUGH THESE BREAKS SHOULD BE FAIRLY SHORT-LIVED
BEFORE ADDITIONAL LOW CLOUDS MOVE IN. THROUGH LATE THIS
AFTERNOON...SOUTHWEST TO WEST WINDS AT 10 TO 15 KT WITH GUSTS
BETWEEN 17 AND 23 KT ARE EXPECTED...WITH THE STRONGEST WINDS ACROSS
CENTRAL AND EASTERN MISSISSIPPI. FOR TONIGHT...MVFR CEILINGS WILL
CONTINUE...WITH THE POTENTIAL FOR IFR CEILINGS...AND MVFR
VISIBILITIES DUE TO FOG...AROUND KMEI LATE TONIGHT AND EARLY
THURSDAY MORNING. FOR THIS EVENING AND CONTINUING INTO
THURSDAY...WINDS WILL SUBSIDE TO THE 4 TO 7 KT RANGE WHILE BECOMING
NORTHWESTERLY. MVFR CEILINGS ARE EXPECTED TO PERSIST INTO THURSDAY
MORNING. CEILINGS COULD BEGIN TO IMPROVE TO VFR ACROSS THE REGION
THURSDAY AFTERNOON...WITH SCATTERED CLOUDS WITH BASES AROUND 4000 FT
PERSISTING. /COHEN/
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 38 50 34 50 / 9 0 5 19
MERIDIAN 38 52 33 50 / 9 0 5 9
VICKSBURG 38 50 33 49 / 9 0 5 30
HATTIESBURG 41 53 35 52 / 7 0 5 18
NATCHEZ 37 51 34 48 / 6 0 5 34
GREENVILLE 36 48 33 46 / 15 0 5 21
GREENWOOD 37 49 32 47 / 15 0 5 21
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...LAKE WIND ADVISORY UNTIL 6 PM CST THIS EVENING FOR MSZ026-
028>033-037>039-043>046-049>052-055>058-063>066-072>074.
LA...LAKE WIND ADVISORY UNTIL 6 PM CST THIS EVENING FOR LAZ009.
AR...LAKE WIND ADVISORY UNTIL 6 PM CST THIS EVENING FOR ARZ075.
&&
$$
22/CME/COHEN
