FXUS64 KJAN 042155
AFDJAN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSON MS
355 PM CST FRI DEC 4 2009
...EARLY SEASON SNOW EXPECTED ALONG AND SOUTH OF THE INTERSTATE 20
CORRIDOR WHERE A WINTER WEATHER ADVISORY IS IN EFFECT...
SYNOPTIC OVERVIEW...
CURRENTLY...A BROAD LONGWAVE UPPER TROUGH ENCOMPASSES MUCH OF THE
EASTERN TWO-THIRDS OF THE CONUS. TWO SEPARATE JET STREAKS ARE FOUND
IN THE UPPER FLOW...WITH A 140-KT H2 JET STREAK ROUNDING THE BASE OF
THE MEAN TROUGH OVER THE SIERRA MADRE OCCIDENTAL REGION OF NORTHERN
MEXICO...AND A SECONDARY JET STREAK OVER THE EASTERN CONUS. THE
FIRST OF THESE JET STREAKS IS ASSOCIATED WITH A VIGOROUS SHORTWAVE
TROUGH EMBEDDED WITHIN THE MEAN TROUGH. SOUTHWESTERLY FLOW ABOVE H7
CONTINUES TO INCREASE ACROSS THE CWA IN ADVANCE OF THE APPROACHING
SHORTWAVE...WITH GRADUALLY INCREASING Q-VECTOR CONVERGENCE AND
LARGE-SCALE ASCENT HAVING OCCURRED THROUGH THE DAY TODAY. BY
TONIGHT...THE SHORTWAVE WILL BE SWINGING THROUGH THE AREA...WITH THE
LEFT EXIT REGION OF THE ASSOCIATED JET STREAK BECOMING COUPLED WITH
THE RIGHT ENTRANCE REGION OF THE EASTERN CONUS JET STREAK. THIS
COUPLING WILL INVIGORATE ASCENDING BRANCHES OF THE AGEOSTROPHIC
CIRCULATIONS OF THE RESPECTIVE JET STREAKS...MOST NOTABLY OVER THE
SOUTHERN PORTIONS OF THE FORECAST AREA. COLD AIR POOLING INTO THE
MEAN TROUGH IN RESPONSE TO THE RE-LOADING SHORTWAVE WILL RESULT IN
LOWERING DYNAMIC TROPOPAUSE HEIGHTS TONIGHT...THOUGH LATEST GUIDANCE
IS AGREEING THAT THEY WILL NOT FALL BELOW H3...WHICH IS ABOUT 200 MB
HIGHER THAN YESTERDAY/S GUIDANCE. AS SUCH...THE UPPER SHORTWAVE MAY
NOT BE AS STRONG AS WAS INDICATED 24 HOURS AGO. THIS COULD EXPLAIN
THE OVERALL DOWNWARD TREND IN PRECIP TOTALS DERIVED FROM GUIDANCE
DURING THE LAST DAY.
NEVERTHELESS...SUFFICIENTLY STRONG SYNOPTIC ASCENT EAST OF THE
AFOREMENTIONED MID/UPPER-LEVEL SHORTWAVE AXIS CONTINUES TO SPIN UP A
SURFACE WAVE ALONG A DIFFUSE WESTERN GULF BAROCLINIC TROUGH. THIS
WAVE IS EXPECTED TO UNDERGO CYCLOGENESIS AS THE ASCENT
INCREASES...AND WILL TRACK EASTWARD WHILE DEEPENING. WHILE THE
SURFACE LOW WILL REMAIN WELL SOUTH OF THE CWA...THE PRESSURE
GRADIENT NORTH OF THE LOW WILL TIGHTEN ACROSS THE AREA AS SURFACE
HIGH PRESSURE BUILDS OVER THE SOUTHERN PLAINS IN THE WAKE OF THE
DEPARTING SHORTWAVE. THIS WILL SERVE TO INCREASE LOW-LEVEL COLD AIR
ADVECTION FOR TONIGHT AND STRENGTHEN THE LOW-LEVEL COLD POOL
UNDERCUTTING THE MOIST SOUTHWESTERLY FLOW ABOVE H7.
MESOSCALE OVERVIEW/FORECAST CONCERNS...
THE TWO MAIN FORECAST ARE SNOWFALL TOTALS INCLUDING POTENTIAL FOR
RAIN/SNOW MIX IN THE SOUTHEAST...AS WELL AS NORTHWARD EXTENT OF SNOW.
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 WORKING INTO SOUTHWEST MISSISSIPPI AND NORTHEAST
LOUISIANA THIS AFTERNOON WILL REACH THE I-55 CORRIDOR BY EARLY
EVENING AND THEN EASTERN MISSISSIPPI BY LATE EVENING. THE
PRECIPITATION WILL INITIALLY START A RAIN/SNOW MIX OVER THE SOUTHERN
AREAS THIS EVENING. HOWEVER...AS THE PRECIPITATION INITIALLY FALLS
INTO THE SUB-CLOUD DRY LAYER...FORECAST SOUNDINGS INDICATE THAT
LOW-LEVEL COOLING DUE TO WET-BULBING PROCESSES WILL ALLOW WET-BULB
ZERO HEIGHTS TO QUICKLY FALL BELOW 750 FEET AGL. THIS WILL ALLOW
PRECIPITATION TO CHANGE OVER TO A WET SNOW...ESPECIALLY AS COLDER
AIR INFILTRATES THE REGION FROM THE NORTHWEST. THIS IS
WELL-HIGHLIGHTED IN RECENT FORT POLK LOUISIANA OBS...WHERE THE
TEMPERATURE/DEWPOINT HAS CHANGED FROM 40/30 TO 33/32 OVER THE COURSE
OF AN HOUR WHILE A MODERATE SNOW BAND WAS PASSING THROUGH. BY 1000 PM
CST...MOST OF THE PRECIPITATION IS EXPECTED TO BE SNOW ACROSS THE
CWA. SREF GUIDANCE IS INDICATING SNOW:LIQUID RATIOS AROUND 5:1...BUT
COOLING THERMAL PROFILES THROUGH THE NIGHT COULD RAISE THESE VALUES
MORE TOWARD 8:1.
AS FOR TOTAL SNOWFALL AMOUNTS...THE AREA OF PRECIPITATION WILL BE
HIGHLY TIED TO THE EVENTUAL TRACK OF THE SURFACE LOW...FOR WHICH NWP
GUIDANCE IS EXPERIENCING A LITTLE SPREAD. THE PREFERRED SOLUTION IS
A MULTI-MODEL BLEND OF THE EC/SREF/GFS...WHICH TAKES THE LOW ON AN
EASTWARD HEADING ACROSS THE CENTRAL GULF WITH LITTLE DEEPENING. THE
IMPLICATION IS FOR GREATER AMOUNTS OF PRECIPITATION TO REMAIN ALONG
AND SOUTH OF INTERSTATE 20. THE NAM TAKES THE LOW ON A MORE NORTHERN
TRACK...BRINGING HIGHER QPF TOTALS FURTHER NORTH. THIS SOLUTION IS
BEING DISCOUNTED...AS IT REMAINS AT THE FAR NORTHERN EDGE OF THE
GUIDANCE ENVELOPE ESPECIALLY WHEN COMPARED TO RECENT GFS ENSEMBLE
PERTURBATION LOW TRACKS. ALSO...GIVEN THAT THE GENESIS AREA OF THE
LOW IS SO FAR DISCONNECTED FROM THE PARENT UPPER TROUGH...THE
CHANCES FOR THE TWO BECOMING COMPLETELY COUPLED FORCING THE SURFACE
LOW FURTHER NORTH IS LOW.
AT THIS POINT...IT LOOKS AS THOUGH LOCATIONS ALONG AND SOUTH OF
INTERSTATE 20 WILL RECEIVE TOTAL SNOW ACCUMULATIONS OF AROUND
1 INCH...CONSISTENT WITH CONSENSUS AMONGST AFOREMENTIONED MODEL
BLEND. WHILE SOME WEAK 850-700 MB FRONTOGENESIS WILL BRUSH
SOUTHEASTERN PORTIONS OF THE AREA...THE STRONGER FRONTOGENESIS
FIELDS WILL BE FOUND SOUTH OF THE CWA...WHERE LOW-LEVEL CONVERGENCE
AROUND THE SURFACE LOW WILL TIGHTEN THE PRE-EXISTING BAROCLINIC ZONE
OVER THE NORTHERN GULF. ALSO...SATURATED EQUIVALENT POTENTIAL
VORTICITY VALUES WITHIN THE H7-H5 LAYER ARE FOUND TO BECOME NEGATIVE
THIS EVENING OVER MUCH OF THE SOUTHERN HALF OF THE CWA...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 AND COINCIDENT WITH STRONG
ASCENT...WHILE DESCENDING TOWARD 700 MB. THESE FACTORS WILL RESULT IN
ONE OR MORE BANDS OF LIGHT TO BRIEFLY MODERATE SNOW ACROSS CENTRAL
AND SOUTHERN MISSISSIPPI...AS WELL AS SOUTHERN PORTIONS OF NORTHEAST
LOUISIANA.
ALMOST ALL OF THE 04/1200 UTC GFS ENSEMBLE PERTURBATIONS INDICATE
THE SWATH OF 1 INCH ACCUMULATIONS SHIFTED FURTHER TO THE NORTH OF
THE DETERMINISTIC GFS RUN...COINCIDENT WITH THE SWATH OF 1 INCH
ACCUMULATIONS DEPICTED BY THE LATEST SREF MEAN. THUS...THE ADVISORY
AREA HAS BEEN EXPANDED NORTHWARD BY ONE TO TWO TIERS OF
COUNTIES/PARISHES AND NOW INCLUDES THE JACKSON AND MERIDIAN AREAS.
THE ADVISORY WORDING HAS BEEN UPDATED TO MENTION THE POTENTIAL FOR
LOCAL ACCUMULATIONS UP TO 2 INCHES ALONG THE HIGHWAY 84
CORRIDOR...WHERE POTENTIAL MESOSCALE SNOW BANDING WILL ENHANCE
SNOWFALL TOTALS. HOWEVER...THESE HIGHER TOTALS WILL BE VERY
LOCALIZED...ESPECIALLY SINCE SURFACE TEMPERATURES WILL INITIALLY
ONLY BE AROUND OR JUST ABOVE FREEZING WITH MELTING PROCESSES
RESTRICTING TOTALS. THE SHORTWAVE TROUGH IS EXPECTED TO BE
FAST-MOVING...WITH PRECIPITATION ENDING ACROSS SOUTHWEST MISSISSIPPI
SHORTLY AFTER MIDNIGHT...AND THEN CLEARING EASTERN AREAS BY DAWN.
THE LIMITED SNOWFALL DURATION WILL RESTRICT TOTAL SNOW ACCUMULATIONS
SOMEWHAT. THUS...NO WINTER STORM WARNINGS ARE PLANNED AT THIS TIME.
A SHARP GRADIENT IN SNOWFALL TOTALS IS EXPECTED NORTH OF THE
ADVISORY AREA...WHERE ONLY A FEW FLURRIES OR SOME BRIEF LIGHT SNOW
WILL BE POSSIBLE WITH NO ACCUMULATIONS EXPECTED.
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 BEFORE MIDNIGHT. NEVERTHELESS...SLUSH WILL
LIKELY ACCUMULATE ON ROADWAYS IN THE ADVISORY AREA ESPECIALLY AS
ANTECEDENT CONDITIONS ARE NOT PARTICULARLY WARM. LATE IN THE
NIGHT...AS COLD AIR ADVECTION FORCES TEMPERATURES FALL INTO THE
UPPER 20S...ANY WATER ON ROADWAYS COULD REFREEZE RESULTING IN ICY
PATCHES.
AS FOR OTHER FORECAST MATTERS THROUGH SATURDAY NIGHT...TEMPERATURES
ON SATURDAY WILL STRUGGLE TO RISE ABOVE THE MID/UPPER 40S WITH THE
CORE OF THE COLDER AIRMASS IN PLACE. GOOD RADIATIONAL COOLING
SATURDAY NIGHT WILL ALLOW TEMPERATURES TO FALL INTO THE 20S.
GUIDANCE CAME IN TOO COLD FOR MERIDIAN FOR SATURDAY NIGHT...WHERE
TEMPERATURES WERE RAISED A FEW DEGREES IN THE ABSENCE OF ANY
SNOW PACK. THE NEXT UPPER SHORTWAVE AND AN ASSOCIATED INVERTED
SURFACE TROUGH WILL BRING WIDESPREAD RAIN TO MUCH OF THE REGION FOR
MONDAY NIGHT. THE REGION WILL BE ON THE COOL SIDE OF A WEAK NORTH
GULF BAROCLINIC ZONE...WITH INSTABILITY TOO LIMITED FOR THUNDER.
/COHEN/
MONDAY THROUGH NEXT FRIDAY...A FRONT IS PROGGED TO MOVE INTO AND
THROUGH THE FORECAST AREA MONDAY MORNING...AND EVENTUALLY STALL
ACROSS THE NORTHERN GULF OF MEXICO MONDAY AFTERNOON...ALLOWING THE
CWA TO BRIEFLY DRY OUT.
ON TUESDAY...RAIN CHANCES REMAIN FAIRLY HIGH ACROSS THE FORECAST
AREA AS THE ONCE STALLED FRONT BEGINS TO LIFT NORTHWARD. THIS IS
IN RESPONSE TO A FAST MOVING UPPER LEVEL LOW SWINGING EAST ACROSS
THE CENTRAL PLAINS INTO THE MID MISSISSIPPI RIVER VALLEY...AND THE
DEEPENING SURFACE LOW ASSOCIATED WITH IT DOING THE SAME. AS THE
WARM FRONT LIFTS NORTHWARD...MOISTURE WILL ALSO SURGE NORTHWARD INTO
THE FORECAST AHEAD OF A STRONG WESTWARD MOVING COLD FRONT THAT'S
EXPECTED TO TRAVERSE THE ARKLAMISS REGION LATE TUESDAY NIGHT INTO
WEDNESDAY MORNING.
ALTHOUGH THIS IS SOME FIVE DAYS OUT...MODELS ARE CURRENTLY IN DECENT
AGREEMENT WITH THIS SYSTEM. SEVERAL PARAMETERS WITHIN THE MODELS
HAVE ALSO BEEN INDICATING FOR A FEW RUNS NOW THAT THERE IS AT LEAST
SOME STRONG/SEVERE THUNDERSTORM POTENTIAL OVER PORTIONS OF THE CWA
TUESDAY AFTERNOON INTO TUESDAY NIGHT AS THE SYSTEM APPROACHES AND
MOVES THROUGH. BECAUSE OF THIS...I'LL INTRODUCE SUCH WORDING INTO
THE HWO FOR THIS TIME FRAME.
AS THE COLD FRONT EXITS THE FORECAST AREA WEDNESDAY MORNING...HIGH
PRESSURE WILL QUICKLY BUILD INTO THE LOWER MISSISSIPPI RIVER FROM
THE WEST THROUGH THE AFTERNOON AND OVERNIGHT HOURS. SKIES WILL
CLEAR THROUGH THE DAY WEDNESDAY AS A MUCH COOLER DRIER AIRMASS
ADVECTS INTO THE FORECAST AREA DURING THIS TIME. THESE CLEAR AND
COOLER CONDITIONS ARE EXPECTED TO REMAIN OVER THE REGION THURSDAY
INTO FRIDAY. /19/
&&
.AVIATION...VFR CLOUD LAYERS OF 7-8K FEET WILL PREVAIL FOR SEVERAL
MORE HOURS WHILE CEILINGS WILL BE LOWERING IN OUR SWRN CWFA BY 00Z
TO AROUND 4-6K FEET. BY 03-04Z...CEILINGS SHOULD DROP TO AROUND 2K
FEET SRN PORTIONS TO ONLY 5-7K FEET N. FOCUS THEN QUICKLY SHIFTS TO
THE LIGHT MIXED RAIN/SNOW CHANGING TO MOSTLY -SN 02-08Z TONIGHT. FOR
THE MOST PART...CEILINGS SHOULD HOLD MVFR OR BETTER BUT THERE SHOULD
BE A TIME FRAME WHERE THAT LOWERS TO IFR LEVELS(VISIBILITIES) 04-08Z
ACROSS OUR SRN PORTIONS...KHBG-KHEZ AREAS. CLEARING IS EXPECTED TO
STEADILY PUSH FROM W TO E AFTER 08-10Z WITH ALL OF THE REGION
CLEARING BY 13-15Z. VFR WILL THEN PREVAIL INTO EARLY SUNDAY./40/
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 28 47 27 57 / 100 1 4 10
MERIDIAN 28 49 23 56 / 96 1 4 10
VICKSBURG 27 47 28 58 / 85 1 4 14
HATTIESBURG 30 49 27 58 / 100 2 5 9
NATCHEZ 28 47 28 59 / 100 1 4 15
GREENVILLE 28 46 28 52 / 25 1 4 12
GREENWOOD 29 46 27 55 / 32 1 4 10
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...WINTER WEATHER ADVISORY UNTIL 6 AM CST SATURDAY FOR MSZ043>066-
072>074.
LA...WINTER WEATHER ADVISORY UNTIL 6 AM CST SATURDAY FOR LAZ016-
023>026.
AR...NONE.
&&
$$
COHEN/19/40
