FXUS64 KJAN 072218 AAA
AFDJAN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSON MS
418 PM CST MON DEC 7 2009
...STRONG STORM SYSTEM TO BRING WINDS/RAIN AND POSSIBLE SEVERE STORMS...
.DISCUSSION...FORECAST COMPLEXITY GROWING LARGER AS TIME PASSES.
FIRST STALLING FRONTAL BOUNDARY ALONG A KAEX-KMEI LINE STARTING TO
LIFT SLOWLY NWWD AND SHOULD PRODUCE WIDESPREAD FOG AS ONLY LIGHT RAINS
DIMINISH ALONG IT. EXPECT LINGERING DRIZZLE/LIGHT RAINS TO EXPAND
ACROSS THE CWFA THROUGH THE EVENING HOURS AS TEMPERATURES WILL BE
HOLDING IN THE UPPER 40S TO MID 50S. STRONGER WAA DEVELOPS MIDN-3AM
WHEN READINGS SHOULD BE CREEPING UPWARD ACROSS THE BOARD.
WE'RE ALREADY GETTING NEW CONVECTION OFF THE SW LA COAST AND THE NEXT
ROUND OF RAINS TO BREAK OUT OVER OUR WRN AND NRN PORTIONS WILL BECOME
LOCALLY HEAVY MIDN-6AM. HAVE DECIDED TO HOLD OFF ON A FLASH FLOOD WATCH
IN THE DELTA AREA FOR TUESDAY BUT WE STILL MAY NEED TO HOIST ONE IN
NEXT FORECAST PACKAGE AS THE RAIN AXIS BECOMES MORE ESTABLISHED. WARM
FRONTOGENESIS DEVELOPING ALONG THE GULF COAST(68F DWPTS IN KHUM) WILL BE
MOVINGE NWD UP THE LOWER MS VALLEY TUESDAY WHILE A SURGING ARCTIC
FRONT CROSSING THE OZARKS SETTLES SEWD. THE CLASH OF THESE TWO AIR
MASSES RESULTS IN HEAVY RAINS OVER OUR DELTA AND POTENTIAL SEVERE
STORMS BREAKING OUT FOR MAINLY SERN TWO THIRDS OF OUR CWFA. LOCAL
CHECKLISTS YIELDING STRONG MODERATE RISK INCLUDING TORNADIC SUPERCELLS.
PWATS RAMP UP TO 1.7 INCHES OR BETTER TUESDAY MORNING AND LARGE POOL
OF TROPICAL AIR IN THE WRN GULF JUST ADDS FUEL TO THE FIRE FOR THIS
ENTIRE STORM. SINCE LATEST NAM12 HANDLING THE SHALLOW ARCTIC AIR
BETTER AND DROPPING THIS AIR INTO OUR DELTA REGION TUESDAY MORNING...
HAVE USED IT AS A BASELINE FOR MOST PARAMETERS WITH NWD ADVANCEMENT
OF VERY MOIST AND WARM AIR FROM THE GULF. BY TUESDAY AFTERNOON...925MB
S WINDS OF 50-60KTS SHOULD PUSH THE WARM FRONT TO AT LEAST I-20 AND
THEN PROBABLY NEAR A LAKE PROVIDENCE TO COLUMBUS LINE BY LATE AFTERNOON.
DECIDED TO START WITH A LAKE WIND ADVISORY FOR THE CENTRAL AND S TUESDAY
AFTERNOON AND EVENING WHEN S WINDS COULD GUST 30-35MPH. THIS MAY NEED
TO BE EXTENDED INTO THE DELTA REGION IF COLDER AIR HOLDS BACK IN AR.
BY TUESDAY AFTERNOON DEEP LAYER SHEAR WILL LIKELY HAVE INCREASED
TO 60KTS WITH 0-1KM SHEAR AROUND 40KTS OVER OUR AREA. MLCAPES NOW
BEING ADVERTISED TO REACH 500-700J/KG WITH 7-500MB LAPSE RATES OF
5.5C. DESPITE THE INCREASE IN DEEP LAYER MOISTURE AND DEEP LAYER
SHEAR...THE WARM SECTOR MAY STILL BE UNDERDONE IN THE MODEL DATA AS
850MB THETA E VALUES PUSH 335K ON 60KT FLOW. STRONG TOR PARAMETERS ON
THE 18Z GFS SHOWING A 5-7 UNIT MAXIMUM PROGRESSING ALONG A WINNSBORO
TO STARKVILLE LINE DURING THE AFTERNOON AND EVENING WHICH MAY BE A
GOOD MARKER FOR THE RICHEST AIR AND TORNADIC POTENTIAL. HAVE PLACED
HIGHEST RISK FOR TORNADOES ALONG THIS AXIS IN THE HAZARD GRIDS.
COLD FRONT THEN SWEEPS SEWD ACROSS THE AREA TUESDAY EVENING WITH A
DEVELOPING SQUALL LINE BUT BRINGING THE END TO SEVERE RISK FROM NW TO
SE BEFORE MIDNIGHT TUESDAY NIGHT. WEDNESDAY WILL FEATURE MUCH COOLER
AND DRIER ADVECTION AS WE HEAD INTO THE LATTER HALF OF THE WEEK. A
1026MB SURFACE HIGH MOVES THROUGH THE MID SOUTH THURSDAY AND HAVE
ADJUSTED TEMPERATURES WEDNESDAY AND THURSDAY TOWARD THE COLDEST
ENSEMBLE MEMBERS OF THE MOS FOR NOW./40/
&&
REMAINDER OF THE LONG TERM HANDLED BY PREVIOUS DISCUSSION...
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...IFR CONDITIONS AND OCCASIONAL LIGHT RAIN WILL BE
WIDESPREAD THROUGH MIDDAY TUESDAY. INSTABILITY WILL REMAIN LIMITED
EARLY TONIGHT WITH RAIN AND SCATTERED ELEVATED TSRA EXPECTED TO FORM
FROM LATE EVENING THROUGH TUESDAY MORNING...MAINLY NW OF THE NATCHEZ
TRACE PARKWAY. SRN PORTIONS OF THE AREA SHOULD BECOME MVFR CEILINGS
TUESDAY AFTERNOON AS A WARM FRONT LIFTS INTO THE CENTRAL PORTIONS OF
THE STATE. SOUTH WINDS WILL ALSO INCREASE TUESDAY 15-25KTS BY MIDDAY.
/15/
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 48 72 50 55 / 87 100 90 14
MERIDIAN 47 67 55 59 / 74 100 92 15
VICKSBURG 49 71 47 53 / 97 100 78 13
HATTIESBURG 51 73 60 62 / 60 90 92 15
NATCHEZ 52 75 50 56 / 98 100 81 12
GREENVILLE 44 57 42 47 / 98 100 67 13
GREENWOOD 46 61 44 51 / 91 100 80 14
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...LAKE WIND ADVISORY FROM 11 AM TO 9 PM CST TUESDAY FOR MSZ040>066-
072>074.
LA...LAKE WIND ADVISORY FROM 11 AM TO 9 PM CST TUESDAY FOR LAZ007>009-
015-016-023>026.
AR...NONE.
&&
$$
