FXUS64 KJAN 111329 AAA
AFDJAN
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE JACKSON MS
729 AM CST FRI DEC 11 2009
.UPDATE...
FORECAST HAS BEEN UPDATED TO INCLUDE A SLIGHT CHANCE FOR SPRINKLES
AND LIGHT SLEET ALONG AND SOUTH OF THE HIGHWAY 84 CORRIDOR...WHERE
LATEST RUC/GFS ARE INDICATING SOME LIGHT PRECIP DEVELOPING THIS
MORNING. RADAR RETURNS IN THIS AREA ARE FILLING IN A BIT...INDICATING
MOISTENING AROUND 5500 FT. BELOW THIS LEVEL...11/1200 UTC KJAN
SOUNDING SHOWS AN ABUNDANCE OF DRY AIR...THUS COVERAGE OF PRECIP WILL
BE TOO SMALL TO WARRANT MEASURABLE PRECIP CHANCES. FORECAST SOUNDINGS
SHOW MAXIMUM WET BULB TEMP IN THE ELEVATED WARM LAYER GENERALLY
REMAINING BELOW 3C THIS MORNING...WITH A SUB-FREEZING LAYER ADJACENT
TO THE SURFACE EXTENDING UP TO AROUND 3000 FT AGL. THUS...PRIMARY
PRECIP TYPE THIS MORNING WILL BE SLEET IN THE FAR SOUTH...WITH A FEW
SPRINKLES OF RAIN MIXING IN AS LOWER ATMOSPHERE WARMS TOWARD NOON. NO
SLEET ACCUMULATION IS EXPECTED. /COHEN/
&&
.PREV DISCUSSION... /ISSUED 433 AM CST FRI DEC 11 2009/
SHORT TERM...TODAY THROUGH SATURDAY NIGHT...
A 1030 MB SURFACE HIGH IS CURRENTLY CENTERED OVER WESTERN
TENNESSEE...MARKING THE CORE OF A LARGE COLD DOME CENTERED OVER THE
EASTERN CONUS. THIS COLD AIRMASS IS CONTINENTAL POLAR IN NATURE...
AND WAS ENHANCED BY ITS INTERACTION WITH THE FRESH SNOW PACK ACROSS
THE NORTHERN ROCKIES AND MIDWEST. THE ARKLAMISS IS AT THE SOUTHERN
PERIPHERY OF THIS MODIFIED COLD AIRMASS...WHICH HAS BECOME RELATIVELY
SHALLOW THIS FAR SOUTH AND MOSTLY EVIDENT BELOW H85 BASED ON THE
11/0000 UTC KJAN SOUNDING. THE UPPER EXTENT OF THE COLD AIR WILL
GRADUALLY ERODE DURING THE NEXT DAY...AS THE SURFACE HIGH RETREATS
EWD. AT THE SAME TIME THERE WILL BE AN INCREASE IN SOUTHWESTERLY FLOW
ALOFT IN RESPONSE TO THE GRADUAL EASTWARD SHIFT IN THE LARGE POLAR
VORTEX NOW OVER THE NORTHEAST CONUS AND IN THE SUBTROPICAL RIDGE OVER
THE SOUTHWEST NORTH ATLANTIC...ALONG WITH THE EASTWARD SHIFT IN THE
ASSOCIATED UPPER JET STREAK BETWEEN THE TWO FEATURES. THE ASSOCIATED
170-KT H2 JET STREAK WILL BE RESPONSIBLE FOR THE DEVELOPMENT OF A
SURFACE LOW ALONG A WESTERN GULF BAROCLINIC ZONE...AS THE STRONG
ASCENDING BRANCH OF THE AGEOSTROPHIC CIRCULATION IN THE RIGHT
ENTRANCE REGION OF THE JET STREAK INTERACTS WITH THE PRE-EXISTING
LOW-LEVEL THERMAL GRADIENT.
AS THE GULF LOW DEEPENS...WHILE TRANSLATING NORTHEASTWARD WITH AN
ATTENDANT LARGE MASS OF CONVECTION...MOISTURE WILL BE TRANSPORTED
INTO THE FORECAST AREA ON SOUTHWESTERLY FLOW ALOFT...WHICH WILL BE
ISENTROPICALLY LIFTED ABOVE THE COLD AIRMASS. IN RESPONSE...
PRECIPITATION IS EXPECTED TO DEVELOP ACROSS THE CWA AS EARLY AS LATE
THIS AFTERNOON ACROSS SOUTHERN AREAS...WITH THE HIGHEST CHANCES FROM
TONIGHT AND INTO SATURDAY. GUIDANCE CONTINUES TO TREND DOWNWARD WITH
REGARD TO POPS FOR TODAY...AND A MULTI-MODEL ENSEMBLE CONSENSUS
POINTS TOWARD THE ONLY CHANCE...ALBEIT A VERY SMALL CHANCE...FOR
PRECIPITATION ACROSS THE CWA ALONG AND SOUTH OF HIGHWAY 84 IN THE
AFTERNOON. THIS IS CONSISTENT WITH THE 11/0000 UTC RUN OF THE LOCAL
HIGH-RES WRF. FARTHER NORTH...FORECAST SOUNDINGS SUGGEST SUBSTANTIAL
TEMPERATURE-DEWPOINT SPREADS THROUGH THE AFTERNOON. IN FACT BOUNDARY
LAYER RH VALUES (IN PERCENT) ARE EXPECTED TO BE IN THE 30S...AND EVEN
THE UPPER 20S ACROSS THE NORTH...WITH THE DRY AIR EXTENDING UP TO
AROUND 6 KFT AGL. THIS DRY AIR WILL LIMIT THE CHANCE FOR MEASURABLE
PRECIP DEVELOPMENT TODAY. GFS SOUNDINGS SHOW A SATURATED LAYER FROM
800-700 MB IN THE LATE AFTERNOON ACROSS THE NORTHERN TWO-THIRDS OF
THE FORECAST AREA...NOT EVEN EVIDENT IN MOST NAM SOUNDINGS. THIS
SCENARIO WOULD TRANSLATE TO A LOW/MID-LEVEL CLOUD DECK WITH SOME
VIRGA. GIVEN THE DELAYED START TIME FOR THE PRECIP...AND THE ABOVE-
FREEZING WET BULB TEMPERATURE PROFILES THIS AFTERNOON WHERE THERE IS
A SLIGHT CHANCE FOR RAIN...NO MENTION OF MIXED PRECIPITATION IS
INCLUDED IN THE FORECAST FOR TODAY.
THINGS CHANGE TONIGHT...AS COPIOUS AMOUNTS OF MOISTURE ARE TRANSPORTED
INTO THE REGION WITH PWATS BEGINNING TO CLIMB TOWARD 1.5 INCHES
TONIGHT. WIDESPREAD RAIN IS EXPECTED TO PUSH FROM SOUTH TO NORTH
ACROSS THE CWA...SPREADING FROM THE SOUTH IN THE EARLY EVENING AND
THEN REACHING THE NORTHERNMOST PART OF THE AREA BY DAWN. POPS WERE
RAISED ABOVE MAV GUIDANCE TO 100 PERCENT ACROSS THE SOUTH BASED ON
MOST MEMBERS OF THE SREF...ALONG WITH EC/NAM...GENERATING MEASURABLE
PRECIP TONIGHT. THE RAIN WILL INITIALLY BE LIGHT TO MODERATE...WITH
ACCUMULATIONS AROUND ONE-HALF INCH IN THE SOUTHERN TIER OF COUNTIES.
THE MAIN FORECAST PROBLEM FOR TONIGHT WILL BE ACROSS THE NORTH WHERE
FORECAST SOUNDING WET-BULB TEMPERATURE PROFILES ARE INDICATING A
NEAR-SURFACE SUBFREEZING LAYER...MAINLY BELOW 950 MB. THIS
SUB-FREEZING LAYER COULD BE FURTHER ENHANCED BY EVAPORATIVE COOLING
CAUSED BY ADDITIONAL DRY AIR ADVECTED INTO THE REGION FROM THE EAST.
THE ELEVATED WARM LAYER FROM 925 MB TO 800 MB CONTAINS MAXIMUM WET
BULB TEMPERATURES IN THE 3C TO 4C RANGE. THIS SUGGESTS THE POTENTIAL
FOR ICE GENERATED EVEN FURTHER ALOFT TO EITHER AT LEAST PARTIALLY
MELT AS IT DESCENDS INTO THE THE ELEVATED WARM LAYER. PROVIDED
RE-FREEZING OCCURS IN THE NEAR-SURFACE LAYER...COMPLETE MELTING IN
THE WARM LAYER WOULD YIELD A HIGHER POTENTIAL FOR FREEZING
RAIN...WHILE INCOMPLETE MELTING WOULD YIELD A HIGHER POTENTIAL FOR
SLEET. GIVEN SLIGHT VARIATIONS IN THE TEMPERATURES WITHIN THE WARM
LAYER AND UNCERTAINTY AMONGST NWP GUIDANCE MEMBERS...A SLIGHT CHANCE
FOR BOTH FREEZING RAIN AND SLEET HAS BEEN MENTIONED IN THE FORECAST
FOR TONIGHT AND EARLY SATURDAY MORNING AT THE ONSET OF PRECIPITATION
ACROSS THE NORTHERN THIRD OF THE CWA. ONCE PRECIPITATION BEGINS...AND
LATENT HEATING DUE TO CONDENSATION AND WARM AIR ADVECTION OVERCOME
THE NEAR-FREEZING LAYER...PRECIPITATION WILL CHANGE OVER TO ALL RAIN
BY LATE SATURDAY MORNING. ALSO...GIVEN THE TRANSIENT NATURE OF THE
RETREATING COLD AIRMASS...IT APPEARS THAT THE CONCERN FOR WINTRY
PRECIPITATION WILL BE SHORT-LIVED AND RESTRICTED TO THE NORTHERNMOST
PORTION OF THE CWA ONLY. NO ACCUMULATIONS OF ICE OR SLEET ARE EXPECTED.
FOR SATURDAY INTO SATURDAY NIGHT...THE ENTIRE AREA WILL RECEIVE
RAIN...WITH POPS RAISED ABOVE MAV GUIDANCE TO 100 PERCENT AREAWIDE
ON SATURDAY BASED ON ALL MEMBERS OF MULTI-MODEL ENSEMBLE GUIDANCE
GENERATING MEASURABLE PRECIP. THE RAIN WILL BECOME HEAVY ACROSS THE
SOUTHERN PORTIONS OF THE AREA. IN THIS AREA...MUCAPES ARE FORECAST
TO EXCEED 500 J/KG WITH SATURATED EQUIVALENT POTENTIAL VORTICITY
VALUES BECOMING INCREASINGLY NEGATIVE IN THE H7-H5 LAYER...
SUPPORTING BOTH SLANTWISE AND UPRIGHT CONVECTION...AND ENHANCING
QPF. TOTAL RAINFALL AMOUNTS...ESPECIALLY WHERE PERSISTENT BANDS OF
PRECIP SET UP...COULD REACH THE 1.25 TO 1.75 INCH RANGE ALONG AND
SOUTH OF THE INTERSTATE 20 CORRIDOR THROUGH SATURDAY NIGHT...WITH AT
LEAST 50 PERCENT OF SREF MEMBERS EXCEEDING 1 INCH QPF THROUGH THE
EVENT. THESE RAINFALL TOTALS COULD RESULT IN PONDING OF WATER IN
LOW-LYING AND POOR-DRAINAGE AREAS...ESPECIALLY GIVEN ENHANCED
RUN-OFF DUE TO COLDER SOIL TEMPS. HOWEVER...FLASH FLOOD GUIDANCE
VALUES ARE HIGH ENOUGH IN THIS AREA TO PRECLUDE FLASH FLOOD
CONCERN...ESPECIALLY WITH LONGER DURATION OF RAINFALL. MENTION OF
THE HEAVY RAIN WILL BE MADE IN THE HAZARDOUS WEATHER OUTLOOK. ALSO
OF CONCERN WILL BE THE RISK FOR THUNDERSTORMS FOR LATE SATURDAY INTO
SATURDAY NIGHT...TOWARD THE LATTER HALF OF THE RAIN EVENT. AT THIS
TIME...THE CUMULATIVE EFFECT OF WARM ADVECTION ATOP THE COLD LAYER
WILL HAVE WARMED 925-850 MB TEMPS THUS ENHANCING MID-LEVEL LAPSE RATES
WITH THE APPROACH OF A WEAK PERTURBATION IN WESTERLY FLOW ALOFT.
FORECAST SOUNDINGS INDICATE THAT A PARCEL LIFTED FROM THE 925MB
LEVEL AT KJAN COULD GENERATE OVER 500 J/KG OF CAPE LATE SATURDAY.
GIVEN SIMILAR CONDITIONS ACROSS THE FORECAST AREA...MAXIMUM MUCAPES
UP TO 700 J/KG ACROSS THE SOUTH...AND WITH EQUILIBRIUM LEVELS ABOVE
THE -20C ISOTHERM...CHANCE OF THUNDER IS INCLUDED AREAWIDE. ALL OF
THE CONVECTION WILL BE ELEVATED AS SURFACE BAROCLINIC ZONE REMAINS
SOUTH OF THE CWA. LONG STRAIGHT-LINE HODOGRAPHS THROUGHOUT THE
POSITIVELY-BUOYANT LAYER SUGGEST THE POTENTIAL FOR ELEVATED UPDRAFTS
TO ROTATE WEAKLY...WHICH COULD SUPPORT A RISK FOR SMALL HAIL...
PARTICULARLY ALONG AND SOUTH OF THE INTERSTATE 20 CORRIDOR... WHERE
BETTER LOW-LEVEL MOISTURE WILL BE. A THUNDERSTORM WITH MARGINALLY
SEVERE HAIL CANNOT BE ENTIRELY RULED OUT BASED ON LATEST TRENDS IN
SREF SEVERE HAZARDS GUIDANCE. /COHEN/
LONG TERM...SUNDAY THROUGH THURSDAY...OVERALL PATTERN TENDS TO FAVOR
A WEAKENING OF THE UPPER POLAR VORTEX IN ERN CANADA WHICH RELAXES
THE BROAD MEAN TROUGH AXIS IN THE ERN HALF OF THE CONUS. THIS TRENDS
TAKES US THROUGH A TRANSITION TO A FLATTER ZONAL FLOW PATTERN AND
THEN EVENTUALLY A BUILDING RIDGE IN THE WRN STATES WHICH MAINTAINS
NWLY FLOW ALOFT FOR THE LOWER MS VALLEY BY THE END OF THE PERIOD. THE
GFS ENSEMBLE MEAN DRIES US OUT SHORTLY AFTER DAYBREAK SUNDAY MORNING
FROM W TO E AND AGREES RATHER NICELY WITH THE 00Z GFS WHILE THE ECMWF
KEEPS SOME POPS A LITTLE LONGER INTO SUNDAY MORNING. WILL HOLD ON TO
A CHANCE OF LIGHT RAINS OVER ERN MS EARLY SUNDAY. DRIER WEATHER IS
INDICATED FOR SUNDAY AFTERNOON/NIGHT AND EARLY MONDAY BUT ANOTHER
SHORTWAVE IS EXPECTED TO RETURN INCREASING UPGLIDE RAINS BY MONDAY
AFTERNOON. THE WARMEST TEMPS FOR NEXT WEEK SHOULD ALSO ARRIVE MONDAY
WITH 850MB TEMPS OF 13-14C AND HIGH TEMPS FROM U60S N TO M70S FAR S.
THEN...A STRONGER SHORTWAVE IS EXPECTED TO CROSS THE SRN PLAINS
MONDAY NIGHT EJECTING A MINOR IMPULSE TO HELP DEVELOP A SURFACE LOW
IN THE NWRN GULF THAT IS PROGGED TO MOVE ALONG THE GULF COAST LATE
MONDAY NIGHT/EARLY TUESDAY. RAIN CHANCES WILL INCREASE MONDAY
AFTERNOON THROUGH MONDAY NIGHT AND SCATTERED TSRA SHOULD BREAK OUT
OVER MUCH OF OUR CWFA. INITIAL QPF SHOULD AVERAGE 1 TO 2 INCHES BUT
COULD BE HIGHER IN SERN MS AND WILL MENTION LOCALLY HEAVY RAINS. HAVE
INCLUDED A FLOODING HAZARD IN THE GRIDS OUTLOOK WITH MORE IMPRESSIVE
RUNOFF ISSUES POSSIBLE. PWATS RISE TO 1.6 INCHES MAINLY S OF I-20
WHERE MLCAPES BEING PROGGED TO REACH 600-1000 J/KG...SURFACE DWPTS
60-65F...SHOWALTERS OF -2C AND 7-500MB LAPSE RATES NEAR 6C. THE RIGHT
ENTRANCE REGION OF A 130+KT 200MB JET CORE WILL ASSIST IN THE RAINFALL
PRODUCTION AS VERTICAL WIND PROFILES ARE EXPECTED TO BE A BIT MORE
UNIDIRECTIONAL. WILL NEED TO MONITOR THIS TIME FRAME FOR TRENDS IN
THE MODEL INSTABILITY AND SHEAR FOR THE POTENTIAL OF STRONG/SVR TSRA.
THEN...GFS AND ECMWF SHOW A STRONG COLD FRONT DROPPING THROUGH OUR
REGION TUESDAY MORNING WITH COLDER AND DRIER WEATHER DOMINATING FROM
TUESDAY NIGHT THROUGH THURSDAY. 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 ON WEDNESDAY AND INTO
THE MID SOUTH FOR THURSDAY MORNING. HAVE LEANED TOWARD THE COLDER
ENSEMBLE MEMBERS IN THE WAKE OF THE FRONT BUT MAY HAVE TO ADJUST
THESE EVEN FURTHER DOWN IN FUTURE PACKAGES. OTHERWISE HAVE STAYED
CLOSE TO MEX MOS THROUGH THE LONGER TERM AS THEY ARE ACTUALLY NEAR
THE ENSEMBLE MEANS AND BELOW CLIMATOLOGY./40/
&&
.AVIATION...
FOR TODAY...WIDESPREAD VFR CONDITIONS ARE EXPECTED WITH
EAST WINDS AT 4 TO 9 KT. MOISTURE BEING TRANSPORTED INTO THE REGION
WILL RESULT IN LOWERING CEILINGS FROM SOUTH TO NORTH ACROSS THE
AREA THROUGH THE DAY...WITH CEILINGS FALLING TO AROUND 5 KFT BY
EVENING ACROSS SOUTHERN MISSISSIPPI. ISOLATED SHOWERS COULD RESULT
IN LOCAL MVFR CEILINGS THIS AFTERNOON ALONG AND SOUTH OF THE
INTERSTATE 20 CORRIDOR. THEN FOR TONIGHT...LIGHT TO OCCASIONALLY
MODERATE RAIN WILL SPREAD FROM SOUTH TO NORTH ACROSS MUCH OF THE
AREA...POTENTIALLY REACHING KGTR/KGWO/KGLH BY DAWN...WHERE THERE
WILL BE A SLIGHT CHANCE FOR LIGHT SLEET AND LIGHT FREEZING RAIN TO
INITIALLY MIX IN WITH THE PRECIPITATION BEFORE CHANGING OVER TO ALL
RAIN. NO ICE ACCUMULATION IS EXPECTED...AND THE RISK FOR WINTRY
PRECIPITATION IS TOO SMALL FOR MENTION IN THE TAFS AT THIS TIME.
MVFR CONDITIONS DUE TO REDUCED CEILINGS AND OCCASIONALLY REDUCED
VISIBILITIES WILL ACCOMPANY THE RAIN TONIGHT. ON SATURDAY...
WIDESPREAD RAIN WILL AFFECT THE ENTIRE AREA...WITH A CHANCE FOR
THUNDERSTORMS LATE IN THE DAY. MVFR TO IFR CEILINGS/VISIBILITIES
WILL ACCOMPANY THIS ACTIVITY...WITH SOME THUNDERSTORMS PRODUCING
SMALL HAIL ALONG AND SOUTH OF THE INTERSTATE 20 CORRIDOR. /COHEN/
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 49 37 48 48 / 13 81 100 86
MERIDIAN 47 35 47 47 / 12 71 100 87
VICKSBURG 50 38 48 47 / 18 80 100 74
HATTIESBURG 47 39 52 52 / 21 100 100 77
NATCHEZ 50 40 51 50 / 22 100 100 68
GREENVILLE 46 33 45 41 / 5 33 100 69
GREENWOOD 47 32 46 44 / 5 31 100 82
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...NONE.
LA...NONE.
AR...NONE.
&&
$$
COHEN/40
