FXUS64 KJAN 040301 AAA
AFDJAN
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE JACKSON MS
901 PM CST THU DEC 3 2009
.UPDATE...COUPLE SITES ACROSS THE ARKLAMISS ARE COOLING SLIGHTLY
FASTER THAN ORIGINALLY THOUGHT THIS EVENING AND BUMPED MIN TEMPS FOR
TONIGHT AT HBG/TVR/GLH DOWN A DEGREE OR TWO AS A RESULT. INCREASING
HIGH LEVEL CLOUD COVER FROM THE SW WILL HELP KEEP TEMPS FROM REALLY
BOTTOMING OUT BY MORNING. MADE SLIGHT ADJUSTMENTS TO SKY COVER BASED
ON LATEST SATELLITE TRENDS WITH LOW LEVEL CLOUD COVER ERODING...
GIVING WAY TO INCREASING HIGH LEVEL CLOUD COVER.
SNOWFALL FORECAST FOR FRIDAY EVENING STILL ON TRACK BASED ON LATEST
MODEL GUIDANCE AVAILABLE. DID ADD A SLIGHT CHANCE OF SNOW TO FAR
SOUTHWESTERN ZONES TOMORROW AFTERNOON AS LATEST MODEL SOUNDINGS SHOW
A DEEP SUBFREEZING LAYER DEVELOPING IN THIS AREA...WITH WET-BULB
ZERO HEIGHTS DROPPING TO NEAR 1400 FT ALLOWING FOR THE POSSIBILITY OF
SOME SNOW MIXED WITH RAIN REACHING THE SURFACE. NOT EXPECTING ANY
SNOW ACCUMULATION THROUGH THE AFTERNOON HOURS TOMORROW. LOCATIONS FOR
POSSIBLE SNOWFALL GREATER THAN 2 INCHES THROUGH 12Z SAT LOOKS TO BE
HANDLED WELL BY THE CURRENT WINTER STORM WATCH AND WILL NOT MAKE ANY
CHANGES. UPDATES ARE OUT. /BK/EC/
&&
.PREV DISCUSSION... /ISSUED 400 PM CST THU DEC 3 2009/
..EARLY SEASON SNOW EXPECTED FRIDAY NIGHT WITH A WINTER STORM WATCH
ACROSS SOUTHERN AREAS...
AT PRESENT...A BROAD UPPER-LEVEL TROUGH CURRENTLY ENCOMPASSES MUCH
OF THE WESTERN TWO-THIRDS OF THE CONUS. THE POLAR STREAM HAS DUG AS
FAR SOUTH AS THE FOUR-CORNERS REGION...WITH A CYCLONICALLY-CURVED
SOUTHERN STREAM EXTENDING FROM THE EAST PACIFIC OVER NORTHERN
MEXICO. THE TWO STREAMS MERGE OVER THE SOUTHEAST CONUS...WHERE THE
TIGHTENED UPPER HEIGHT GRADIENT IS SUPPORTING A BROAD SOUTHWESTERLY
STREAM OF 130 KT AT H2. MEANWHILE...A VIGOROUS UPPER SHORTWAVE
TROUGH OVER THE NORTHERN ROCKIES WILL CONTINUE QUICKLY DIVING
SOUTHEASTWARD THROUGH FRIDAY NIGHT WHILE RELOADING THE MEAN TROUGH.
BY FRIDAY EVENING...THE CYCLONICALLY-CURVED JET STREAK ASSOCIATED
WITH THE SHORTWAVE WILL HAVE REACHED THE BASE OF THE UPPER TROUGH
OVER THE NORTHERN GULF. IN RESPONSE...SIGNIFICANT AMOUNTS OF COLD AIR
WILL POOL INTO THE TROUGH WHILE THE TROUGH DEEPENS...WITH DYNAMIC
TROPOPAUSE LEVELS EXPECTED TO FALL TO AROUND THE 550 MB OVER
SOUTHERN MISSISSIPPI...AS H5 TEMPS FALL TO AROUND -25C BY FRIDAY
NIGHT ACROSS SOUTHERN PORTIONS OF THE CWA.
VERY STRONG MID/UPPER-LEVEL Q-VECTOR CONVERGENCE EAST OF THE
MID/UPPER-LEVEL SHORTWAVE AXIS WILL BECOME JUXTAPOSED WITH A
DIFFUSE WESTERN GULF BAROCLINIC TROUGH...ON WHICH NWP GUIDANCE IS
SUGGESTING THAT A SURFACE WAVE WILL SPIN UP WHILE TRACKING
NORTHEASTWARD ACROSS THE NORTHERN GULF AND EVENTUALLY DEEPEN INTO A
LOW. WHILE THE SURFACE LOW WILL REMAIN WELL SOUTH OF THE CWA...THE
PRESSURE GRADIENT NORTH OF THE LOW WILL TIGHTEN ACROSS THE
CWA...INCREASING LOW-LEVEL COLD AIR ADVECTION FOR FRIDAY NIGHT ACROSS
THE ENTIRE AREA. AT THE SAME TIME...SOUTHWESTERLY FLOW ABOVE H7 WILL
INCREASE IN ADVANCE OF THE SHORTWAVE...WHICH WILL EXPERIENCE
ISENTROPIC UPGLIDE FOR MUCH OF FRIDAY NIGHT AND SATURDAY. THE
STRONGEST OF THIS UPGLIDE WILL BE FOCUSED SOUTH OF INTERSTATE 20.
CORRESPONDING DYNAMIC COOLING ABOVE H7 WILL COMMENCE EARLY FRIDAY AND
CONTINUE UNTIL THE TROUGH AXIS PASSES BY EARLY SATURDAY. FORECAST
SOUNDINGS INDICATE MOST OF THE ATMOSPHERIC COLUMN REMAINING AT OR
BELOW 0C ACROSS THE CWA BY FRIDAY NIGHT. SOUTHWESTERLY FLOW ATOP THE
COLD LAYER WILL TRANSPORT LOW LEVEL MOISTURE ASSOCIATED WITH THE GULF
LOW INTO THE REGION...WHILE ADDITIONAL MID-LEVEL MOISTURE STREAMS
INTO THE REGION FROM THE WEST. PRECIPITATION IS EXPECTED TO BEGIN
OVER SOUTHWEST MISSISSIPPI AND NORTHEAST LOUISIANA BY FRIDAY
EVENING...THEN SPREADING NORTHEASTWARD ACROSS CENTRAL AND EASTERN
MISSISSIPPI THROUGH THE NIGHT. SNOW IS EXPECTED TO HAVE ENDED BY
SATURDAY MORNING.
THE BIGGEST FORECAST CONCERNS ARE WITH SNOWFALL TOTALS AND NORTHWARD
EXTENT OF SNOW. THE AREA OF PRECIPITATION WILL BE HIGHLY TIED TO THE
EVENTUAL TRACK OF THE SURFACE LOW...FOR WHICH NWP GUIDANCE IS STILL
EXPERIENCING MODEST SPREAD. THE LATEST NAM/EC TAKE THE LOW ON A
NORTHEASTWARD TRAJECTORY ACROSS THE CENTRAL AND NORTHEAST GULF WHILE
DEEPENING THE LOW AT ABOUT 1 MB PER 6 HOURS...WHILE THE OPERATIONAL
GFS TAKES A MORE SOUTHERN TRACK TOWARD WEST CENTRAL FLORIDA WITH
LITTLE CYCLOGENESIS. THE UKMET AND CMC FOLLOW SIMILAR TRACKS AS THE
NAM/EC COMBINATION. GIVEN BETTER CONSENSUS THAT THE GFS COULD BE
SUFFERING FROM GRID-SCALE FEEDBACK ISSUES OVER THE CENTRAL
GULF...THUS FORCING STRONGER ASCENT AND ITS OWN TRACK FURTHER SOUTH
AS WHAT WAS SEEN WITH THE PREVIOUS GULF LOW...THE MORE NORTHERLY LOW
TRACK WITH SLOW DEEPENING IS THE PREFERRED SOLUTION. THE IMPLICATION
FOR THE FORECAST IS FOR GREATER AMOUNTS OF LOW/MID-LEVEL MOISTURE TO
BE ADVECTED FURTHER NORTHWARD THAN WHAT THE GFS DEPICTS...ALONG WITH
HIGHER CONFIDENCE IN PRECIPITATION MOSTLY SNOW.
AT THIS POINT...IT LOOKS LIKE LOCATIONS SOUTH OF INTERSTATE 20 WILL
RECEIVE TOTAL SNOW ACCUMULATIONS OF 1 TO 3 INCHES...CONSISTENT WITH
THE 03/1500 UTC SREF RUN...WHICH IS IN EXCELLENT AGREEMENT WITH THE
RUN-TO-RUN CONSISTENT EC. A BAND OF MODERATE 850-700 MB FRONTOGENESIS
WILL DEVELOP OVER THIS REGION AS LOW-LEVEL CONVERGENCE ASSOCIATED
WITH THE DEEPENING LOW TIGHTENS A PRE-EXISTING BAROCLINIC ZONE
NORTHWEST OF THE SURFACE LOW. THIS WILL PROVIDE AN ADDITIONAL FOCUS
LOW-LEVEL ASCENT IN THE PRESENCE OF DEEP MOISTURE. ALSO IN THIS
REGION...SATURATED EQUIVALENT POTENTIAL VORTICITY VALUES WITHIN THE
H7-H5 LAYER ARE FOUND TO BECOME NEGATIVE OVERNIGHT FRIDAY...WHICH
SUGGESTS THAT SLANTWISE INSTABILITY WILL FURTHER ENHANCE BANDS OF
SNOW. FORECAST SOUNDINGS SOUTH OF INTERSTATE 20 INDICATE THE
DENDRITIC GROWTH ZONE BETWEEN THE -10 AND -20 LAYER OF THE ATMOSPHERE
WILL BE NEARLY SATURATED WITH LAPSE RATES OF 7 TO 8 C/KM ENHANCED BY
MID-LEVEL COLD AIR POOLING INTO THE REGION. THESE FACTORS WILL
COMBINE WITH STRONG MID/UPPER-LEVEL ASCENT TO PROMOTE EFFICIENT SNOW
GROWTH...ESPECIALLY AS THE DENDRITIC LAYER DESCENDS CLOSER TO H7
THROUGH THE NIGHT WITH A DECENT DEPTH OF AT LEAST 50-75 MB. ONE OR
MORE BANDS OF MODERATE SNOW WILL BE POSSIBLE ACROSS SOUTHERN
MISSISSIPPI AND SOUTHERN PORTIONS OF NORTHEAST LOUISIANA FRIDAY
NIGHT...WHERE A WINTER STORM WATCH HAS BEEN POSTED. THE SNOW IS
EXPECTED TO BE OF THE WET VARIETY...WHICH WILL REDUCE SNOW RATIOS TO
5 TO 10 INCHES OF SNOW FOR 1 INCH OF LIQUID. HOWEVER...SHOULD DYNAMIC
COOLING BE STRONGER OR MORE COLD AIR FILTER INTO THE AREA THAN
ANTICIPATED...THEN EVEN HIGHER SNOWFALL TOTALS WILL BE POSSIBLE. THE
WATCH INCLUDES HATTIESBURG AND NATCHEZ...BUT DOES NOT INCLUDE JACKSON
OR MERIDIAN.
A SHARP GRADIENT IN SNOWFALL TOTALS IS EXPECTED ACROSS THE REGION.
HOWEVER...NORTH OF THE WATCH AREA...INCLUDING JACKSON AND
MERIDIAN...LIGHT SNOW IS STILL EXPECTED...WITH ACCUMULATIONS UP TO 1
INCH. THE 03/1200 UTC NAM INDICATES THE AXIS OF HIGHER SNOWFALL
TOTALS SHIFTED FURTHER NORTH THAN MULTI-MODEL ENSEMBLE GUIDANCE.
HOWEVER...THE NAM IS AN OUTLIER IN THIS REGARD DESPITE ITS
REASONABLE GULF LOW TRACK. ALSO...DESPITE THE NAM/S INFLUENCE IN THE
SREF...THE LATEST SREF RUN STILL HAS NOT PUSHED THE HEAVIER SNOW
BAND NORTHWARD...AND THUS THE NAM QPF DEPICTION HAS BEEN DISCOUNTED
FOR THIS FORECAST PACKAGE...CONSISTENT WITH COLLABORATION WITH HPC.
HOWEVER...THE LATEST NAM INDICATES THAT THERE STILL IS UNCERTAINTY
IN THE AREA OF HIGHEST ACCUMULATIONS...AND THUS AN SPS/WINTER
WEATHER OUTLOOK FOR A FEW TIERS OF COUNTIES NORTH OF THE WATCH AREA.
IT SHOULD BE NOTED THAT MOST OF THE SNOW ACCUMULATION WILL BE ON
GRASSY AND ELEVATED SURFACES...AS SURFACE TEMPERATURES WILL NOT BE
TOO MUCH BELOW FREEZING. NEVERTHELESS...SLUSH WILL LIKELY ACCUMULATE
ON ROADWAYS ESPECIALLY IN THE WATCH AREA ESPECIALLY AS ANTECEDENT
CONDITIONS ARE NOT PARTICULARLY WARM...AND HAZARDOUS DRIVING
CONDITIONS WILL BE POSSIBLE PARTICULARLY IN THE WATCH AREA FOR
FRIDAY NIGHT.
AS FOR OTHER FORECAST MATTERS THROUGH FRIDAY NIGHT...CLOUDS WILL
PERSIST THIS EVENING ACROSS EASTERN AREAS...WITH ADDITIONAL CLOUDS
MOVING INTO SOUTHERN AREAS TONIGHT. MAV GUIDANCE CAME IN A FEW
DEGREES HIGHER THAN EARLIER ACROSS SOUTHERN AREAS...WHERE CLOUDS WILL
LIMIT DEGREE OF RADIATIONAL COOLING. FOR FRIDAY...CLOUDS WILL
INCREASE AREAWIDE.
SATURDAY THROUGH NEXT THURSDAY...SOME LIGHT SNOW COULD
LINGER ACROSS FAR EASTERN PORTIONS OF THE FORECAST AREA EARLY
SATURDAY MORNING...BUT WILL QUICKLY TAPER OFF AND SHIFT EAST AS
TEMPERATURES RISE AND HIGH PRESSURE BUILDS EAST INTO THE REGION.
CLOUDS WILL ALSO GRADUALLY CLEAR THROUGH THE DAY FROM WEST TO EAST
AS TEMPERATURES REMAIN ON THE COOL SIDE AND ONLY CLIMB INTO THE MID
40S. CLOUDS WILL LIKELY HANG ON A LITTLE LATER INTO THE AFTERNOON
IN THE EAST...THUS TEMPERATURES WILL BE A LITTLE COOLER IN THIS
AREA...ESPECIALLY IN SOUTHERN AREAS WHERE SOME SNOW PACK COULD BE
PRESENT. THEN WITH MOSTLY CLEAR SKIES EXPECTED SATURDAY NIGHT WITH A
DOWN RIGHT COLD AND DRY AIRMASS OVER THE REGION...LOWS TEMPERATURES
WILL BOTTOM OUT IN THE MID 20S AREA WIDE.
THE ACTIVE/WET PATTERN LOOKS TO CONTINUE FROM SUNDAY EVENING RIGHT
ON THROUGH INTO THE MIDDLE OF THE UPCOMING WORK WEEK. CLOUDS WILL
BUILD BACK INTO THE FORECAST AREA THROUGH THE DAY SUNDAY AND RAIN
CHANCES INCREASE SUNDAY NIGHT AS A WEAK COLD FRONT APPROACHES THE
REGION FROM THE WEST. TEMPERATURES ALSO MODERATE UP DURING THIS
TIME...AS WELL AS THROUGH THE REST OF THE PERIOD...WITH HIGHS SUNDAY
AFTERNOON GENERALLY CLIMBING BACK UP INTO THE LOW TO MID 50S AND
LOWS SUNDAY NIGHT RANGING FROM THE MID 30S TO THE MID 40S.
THE FRONT LOOKS TO BRIEFLY MEANDER ACROSS THE SOUTHEAST HALF OF THE
CWA MONDAY NIGHT. AS THIS HAPPENS...AN UPPER LEVEL SHORTWAVE WILL
SWING EAST ACROSS THE NATION'S MID-SECTION ON TUESDAY. THIS WILL
CAUSE THE FRONT TO LIFT BACK TO THE NORTH WHILE A SURFACE LOW
DEVELOPS WEST OF THE REGION AND APPROACHES THE FORECAST ON
WEDNESDAY. DECENT RAIN CHANCES WILL EXIST ACROSS THE ENTIRE CWA
DURING THIS TIME...BEFORE FINALLY COMING TO AN END THURSDAY. AS THE
LOW PRESSURE SYSTEM EXITS THE REGION TO THE EAST...STRONG HIGH
PRESSURE BUILDS SOUTHEAST INTO THE LOWER MISSISSIPPI RIVER VALLEY
THURSDAY FILTERING IN YET ANOTHER COLDER DRIER AIRMASS INTO THE
FORECAST AREA. /19/COHEN/
&&
.AVIATION...MVFR CIGS HANGING ON SPORADICALLY TONIGHT AS SFC HIGH
PRES BUILDS INTO THE ARKLAMISS. SHOULD SEE VFR CONDS DEVELOP
AREAWIDE EARLY MORNING AND PERSIST THROUGH THE AFTN...WITH MID AND
UPPER LVL CLOUD COVER INCREASING THROUGH THE DAY. A STORM SYSTEM
WILL PASS OVER THE NRN GULF TOMORROW...AND WILL BRING A GOOD CHANCE
OF SNOW TO CNTRL MS AFTER 20Z. HEAVIEST SNOWFALL IS EXPECTED SOUTH
OF I-20 THROUGH THE EVENING INTO THE NIGHTTIME HOURS. VFR CONDS
SHOULD HANG ON THROUGH THE AFTN...HOWEVER WILL LIKELY SEE A
DETERIORATION TO IFR/PSBL LIFR CIGS AFTER 00Z AS PRECIPITATION
COMMENCES. CAUTION DUE TO LOW LEVEL ICING IS ADVISED DURING THIS
TIME. WINDS WILL REMAIN NRLY THIS PERIOD WITH A CHANCE OF GUSTY
CONDS DEVELOPING SOUTH OF I-20 AFTER 21Z. /BK/
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 36 47 30 44 / 0 14 88 9
MERIDIAN 36 48 30 46 / 0 10 90 15
VICKSBURG 35 46 28 46 / 0 25 73 6
HATTIESBURG 37 51 32 45 / 0 16 100 15
NATCHEZ 35 46 29 44 / 0 38 100 7
GREENVILLE 32 44 28 44 / 0 10 36 5
GREENWOOD 33 45 27 44 / 0 10 49 7
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...WINTER STORM WATCH FROM FRIDAY EVENING THROUGH LATE FRIDAY NIGHT
FOR MSZ053>066-072>074.
LA...WINTER STORM WATCH FROM FRIDAY EVENING THROUGH LATE FRIDAY NIGHT
FOR LAZ024-026.
AR...NONE.
&&
$$
BK/EC
