FXUS64 KJAN 240413 CCA
AFDJAN
AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE JACKSON MS
1006 PM CST WED DEC 23 2009
...POWERFUL STORM SYSTEM TO CROSS THE REGION CHRISTMAS EVE BRINGING
SEVERE THUNDERSTORMS TO THE ARKLAMISS...
.UPDATE...
ONLY MINOR CHANGES HAVE BEEN MADE TO THE GOING FORECAST. FORECAST WIND
SPEEDS WERE REDUCED A BIT PRIOR TO MIDNIGHT...BASED ON CURRENT
OBS...THOUGH THE INCREASING PRESSURE GRADIENT THROUGH THE NIGHT WILL
RESULT IN A CORRESPONDING INCREASE IN WIND SPEEDS. A FEW MINOR TWEAKS
WERE MADE TO HOURLY TEMP GRIDS...SPECIFICALLY TO RAISE THEM A COUPLE
OF DEGREES DURING THE NEXT COUPLE HOURS...AS THEY HAVE BEEN TRENDING
A LITTLE WARMER GIVEN EXTENSIVE CLOUD COVERAGE. INCREASING ISENTROPIC
LIFT IS ALREADY RESULTING IN THE DEVELOPMENT OF RAIN TONIGHT ACROSS
EASTERN AREAS. WHILE THIS ACTIVITY LIFTS NORTHEASTWARD...SHOWERS AND
THUNDERSTORMS WEST OF THE REGION ARE STILL EXPECTED TO APPROACH
WESTERN AREAS BY DAWN...WITH A RISK FOR SEVERE WIND GUSTS AND FLASH
FLOODING THIS THIS ACTIVITY AROUND DAWN. THEN FOR THURSDAY...A
SEVERE WEATHER EVENT IS LIKELY ACROSS THE ARKLAMISS...AS DISCUSSED
BELOW. THE NAM HAS BEEN TRENDING STRONGER WITH THE 24/0000 UTC
RUN...WITH 500-M MIXED-LAYER CAPE VALUES ACROSS SOUTHERN AREAS
PROGGED TO EXCEED 700 J/KG...AND SFC-3 KM CAPES EXCEEDING 150 J/KG.
THESE FACTORS CONTINUE TO SUPPORT TORNADOGENESIS POTENTIAL ON
THURSDAY. ALSO...STRONG PRESSURE FALLS...OVER 10 MB IN 12 HRS...ARE
DEPICTED FROM THE NAM OVER EASTERN MISSISSIPPI...WITH STRONG
ISALLOBARIC FORCING SUGGESTING THAT THE RISK FOR DAMAGING WINDS...EVEN
WITH LIMITED INSTABILITY...COULD EXTEND WELL NORTH OF INTERSTATE 20.
THIS IS SUPPORTED BY SIMULATED REFLECTIVITY DISTRIBUTIONS ON THE
LATEST HIGH-RES LOCAL WRF RUN...AS WELL.
ADDITIONALLY...THE FLASH FLOOD WATCH HAS BEEN EXPANDED IN COVERAGE TO
INCLUDE THE ENTIRE CWA. HIGH RAINFALL RATES WITH CONVECTIVE ACTIVITY
ON THURSDAY WILL LIKELY EXCEED FLASH FLOOD GUIDANCE VALUES ACROSS
PORTIONS OF THE REGION...WITH INCREASING CONFIDENCE IN THIS
OCCURRENCE ACROSS THE SOUTH. THE START TIME FOR THE WATCH ACROSS THE
NORTHEAST HAS BEEN ADJUSTED TO BEGIN AT NOON BASED ON LATEST LOCAL
WRF GUIDANCE.
/COHEN/
&&
.PREV DISCUSSION... /ISSUED 431 PM CST WED DEC 23 2009/
SYNOPSIS...WARM FRONTAL ZONE LIES ROUGHLY ALONG KSHV-KGPT LINE WHILE
THE FIRST BAND OF RAINFALL LIFTING AWAY TO THE NE. RAIN AMOUNTS OF 2
TO 2.5 INCHES HAVE OCCURRED OVER PORTIONS OF THE DELTA ALREADY. EXPECT
THE BOUNDARY TO CONTINUE CREEPING NWD TONIGHT AS MORE UPPER LEVEL
ENERGY MOVES THROUGH CENTRAL AND SRN TX AND A SURFACE LOW(1000MB)
FORMING IN TX BEGINS TO DEEPEN OVER NE TX BY MORNING. THE DEEPENING
SURFACE LOW(995MB) THURSDAY MORNING OVER SERN AR MOVES DROPS TO
990MB BY NOON OVER N AR AND 987MB BY 6PM OVER SRN MO. AS PREVIOUSLY
NOTED...THIS TRACK HAS BEEN NOTORIOUS FOR PRODUCING SVR WX INCLUDING
TORNADOES IN MS AS THE TRAILING FRONT/SQUALL LINE SWEEPS EWD.
IN THE SHORT TERM...EXPECT SCATTERED TO NUMEROUS SHRA/ISOLATED
ELEVATED TSRA TO CONTINUE MAINLY N AND W PORTIONS OF THE CWFA THIS
EVENING BUT BECOMING MORE CONCENTRATED AFTER MIDNIGHT. MOSTLY
ISOLATED ACTIVITY ELSEWHERE BEFORE MIDNIGHT. AS THE WARM FRONT SLIDES
NWD WITH INCREASING SLY WINDS AREAWIDE. WE HAVE CONTINUED LAKE WIND
ADVISORIES FOR THE WRN HALF THIS EVENING AND THEN EXPANDED THESE TO
INCLUDE THE ENTIRE CWFA AFTER MIDNIGHT LASTING THROUGH THE FROPA ON
THURSDAY. WINDS SHOULD GET EVEN STRONGER ALONG THE MS RIVER CHANNEL
FROM 06-18Z AND HAVE UPGRADED THESE LOCATIONS TO WIND ADVISORY FOR
GUSTS OF 40-45 MPH POSSIBLE. MEANWHILE...THE FLASH FLOOD WATCH FOR
LOCATIONS WEST OF A GRENADA TO NATCHEZ LINE TONIGHT WILL BE EXPANDED
TO INCLUDE THE REST OF THE HIGHWAY 82 CORRIDOR ON THURSDAY AS PWATS
OF 1.5-1.7 INCHES SURGE N AND INCREASE RAINFALL PRODUCTION.
AS A SQUALL LINE EVOLVES JUST AHEAD OF THE FRONT OVER AR/LA LATER
TONIGHT...IT SHOULD BEGIN ACCELERATING EWD TOWARD OUR AREA BY DAYBREAK.
THE PRIMARY TIMING FOR THE STRONGEST STORMS ON THURSDAY SHOULD BE FROM
AROUND 4-8AM IN NE LA/SE AR SWEEPING EWD THROUGH THE MS RIVER
COUNTIES/PARISHES DURING THE MORNING HOURS TO AROUND I-55 BY 11AM-2PM
AND THEN 3-6 PM FOR ERN MS. SEVERE WEATHER PARAMETERS HAVE HELD QUITE
CONSISTENT WITH THIS STORM SYSTEM AND PREFER THE OVERALL GFS TIMING
AS THE NAM TOO SLOW AND THE LOCAL WRFS APPEARING A BIT FAST.
AS SURFACE PRESSURE GRADIENTS TIGHTEN DURING THE OVERNIGHT AND
THURSDAY MORNING...THE ONLY QUESTIONABLE FACTOR REMAINS THE DEGREE OF
QUALITY BOUNDARY LAYER MOISTURE AND ULTIMATE INSTABILITY FOR SEVERE
STORMS. MLCAPE VALUES 400J/KG EDGING INTO THE SW CORNER OF THE CWFA BY
4-6AM WHICH EXPANDS RAPIDLY TO 7-900J/KG BY MID MORNING OVER SRN HALF
ACCELERATING EWD ACROSS THE AREA THROUGH MID AFTERNOON. PRE-SQUALL
DEEP LAYER SHEAR MORE THAN SUFFICIENT WITH 0-3KM HELICITY VALUES
4-600 IN PROGGED SOUNDINGS AND 0-1KM HELICITIES 3-400. THEN...A MID
LEVEL SPEED MAX OF 85+KTS AT 700MB BLASTS DOWN THE HIGHWAY 84
CORRIDOR ABOUT MIDDAY AND REALLY KICKS THE SQUALL LINE EAST.
ALTHOUGH DAMAGING WIND AND TORNADOES LOOK TO BE THE PREDOMINANT
SEVERE RISK WITH THIS EPISODE...LARGE HAIL WILL ALSO BE POSSIBLE.
RAINFALL AMOUNTS FROM 2 TO 3 INCHES ARE EXPECTED LATE TONIGHT AND
THURSDAY. THESE AMOUNTS CURRENTLY FALL BELOW FLOOD GUIDANCE NUMBERS
FOR E AND S PORTIONS...BUT THAT COULD CHANGE OVERNIGHT. PERIODS OF
HEAVY RAINFALL WILL BE POSSIBLE ACROSS THE ENTIRE FORECAST AREA
DURING THE DAY THURSDAY AND LOCALIZED FLOODING PROBLEMS WILL BE
POSSIBLE BUT HAVE CONCENTRATED ON NRN AND WRN ZONES FOR NOW DUE TO
THEIR PROXIMITY TO THE UPPER TROUGH AND BEST UVVS.
ALL ACTIVITY QUICKLY EXITS EARLY THURSDAY EVENING IN TIME FOR A MUCH
QUIETER AND COLDER CHRISTMAS WITH TEMPS BACK BELOW NORMALS FOR LATE
DECEMBER GOING INTO THE WEEKEND./40/
LONG TERM...THURSDAY NIGHT THROUGH WEDNESDAY...A STRETCH OF BELOW
NORMAL TEMPERATURES THAT SHOULD LAST THROUGH THE WEEKEND. STRONG CAA
IS EXPECTED THURSDAY NIGHT BEHIND THE COLD FRONT AND FRIDAY MORNING
LOWS WILL BOTTOM OUR NEAR FREEZING IN THE WESTERN MOST ZONES.
VIRTUALLY ALL PRECIPITATION WILL HAVE ENDED LONG BEFORE TEMPERATURES
GET DOWN INTO THE LOWER 30S BUT A FEW FLURRIES WILL NOT BE TOTALLY
OUT OF THE QUESTION IN PORTIONS OF THE DELTA EARLY FRIDAY MORNING. WL
NOT INCLUDE MENTION OF FLURRIES THIS PACKAGE THOUGH. MODELS SUGGEST
LOW CLOUDS WILL LINGER THROUGH MOST OF FRIDAY MORNING BUT HIGH
PRESSURE AT THE SURFACE IS EXPECTED TO SHIFT EAST OVER OUR AREA BY
AFTERNOON AND BRING SOME SUNSHINE. STILL MOST SITES WILL STRUGGLE TO
REACH THE UPPER 40S. THE LARGE UPPER VORTEX WILL BE CENTERED OVER THE
MID MISSISSIPPI VALLEY FRIDAY NIGHT WITH A SURFACE RIDGE AXIS ALONG
THE GULF COAST. LESS THAN IDEAL RADIATIVE COOLING WILL OCCUR BUT
TEMPERATURES SHOULD DROP BELOW FREEZING ACROSS MOST OF THE AREA BY
SATURDAY MORNING. THIS PATTERN WILL CHANGE VERY LITTLE THROUGH SUNDAY
AND OVERALL THIS RUN OF THE GFS IS COOLER AND DRIER THROUGH THE
WEEKEND. HIGHS WILL GENERALLY BE IN THE UPPER 40S AND OVERNIGHT LOWS
AROUND 30 SATURDAY...SUNDAY AND MONDAY. SUNDAY INTO MONDAY THE LARGE
UPPER VORTEX WILL FINALLY LIFT OUT TO THE NORTHEAST BUT ANOTHER LARGE
CLOSED LOW WILL BE DROPPING SOUTH DOWN THE WESTERN CONUS. SHORTWAVE
RIDGING SURFACE AND ALOFT IS EXPECTED OVER OUR REGION MONDAY INTO
TUESDAY. THIS SHOULD BRING US SOME WARMER TEMPS TUESDAY. THE GFS AND
ECMWF DIFFER SIGNIFICANTLY ON THE EVOLUTION OF THIS LOW THOUGH. THE
ECMWF IS MORE PROGRESSIVE THAN THE GFS THIS RUN AND MOVES RAIN BACK
INTO OUR CWA FROM THE SW BY TUESDAY AFTERNOON WHILE THE GFS HANGS THE
CLOSED LOW BACK OVER THE DESERT SOUTHWEST THROUGH TUESDAY NIGHT. BY
WEDNESDAY THOUGH THIS SYSTEM IS EXPECTED TO BRING THE NEXT BEST
CHANCE FOR WIDESPREAD RAIN TO OUR REGION. MAV GUIDANCE TEMPS WERE
STUCK CLOSE TO BUT THE SLIGHT CHANCE POPS WERE CUT FRIDAY THROUGH
MONDAY. POPS WERE INCREASED TUESDAY INTO WEDNESDAY TO ACCOUNT FOR A
LITTLE FASTER EASTWARD PROGRESSION OF THE NEXT LOW. /22/
&&
.AVIATION...RAIN/TS CHANCES WILL INCREASE TONIGHT...ACCOMPANIED
BY INCREASING COVERAGE OF IFR CEILINGS AS A COLD FRONT APPROACHES THE
AREA. THE FRONT IS RATHER STRONG AND A SQUALL LINE IS XPCTD TO BE
APPROACHING THE MS RIVER BY AROUND NOON...WITH DAMAGING WINDS AND
ISOLD TORNADOES ACCOMPANYING THE LINE AS IT MOVES ACROSS THE
ARKLAMISS...EXITING ERN MS BY AROUND 00Z. SFC WINDS WILL BE STRONG
AND SERLY AHEAD OF THE FRONT SHIFTING TO STRONG AND SWRLY RIGHT
BEHIND THE FRONT. /BK/EC/
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 58 64 35 47 / 100 100 24 14
MERIDIAN 55 65 35 51 / 72 100 63 14
VICKSBURG 58 61 34 48 / 100 100 15 14
HATTIESBURG 58 67 35 50 / 79 100 35 14
NATCHEZ 59 63 30 48 / 100 100 12 14
GREENVILLE 56 59 34 44 / 100 100 22 14
GREENWOOD 56 61 35 47 / 100 100 25 14
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...LAKE WIND ADVISORY UNTIL 2 PM CST THURSDAY FOR MSZ029>033-
037>039-044>046-050>052-055>058-063>066-072>074.
FLASH FLOOD WATCH THURSDAY AFTERNOON FOR MSZ028>033-037>039-
043>046-048>052-054>058-061>066-072>074.
LAKE WIND ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR MSZ018-034-040-
041-047-053-059-060.
FLASH FLOOD WATCH THROUGH THURSDAY AFTERNOON FOR MSZ018-019-
025>027-034>036-040>042-047-053-059-060.
WIND ADVISORY UNTIL NOON CST THURSDAY FOR MSZ018-034-040-041-047-
053-059-060.
LAKE WIND ADVISORY UNTIL 2 PM CST THURSDAY FOR MSZ019-025>028-
035-036-042-043-048-049-054-061-062.
LA...LAKE WIND ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR LAZ007>009-015-
016-023>026.
FLASH FLOOD WATCH THROUGH THURSDAY AFTERNOON FOR LAZ007>009-015-
016-023>026.
WIND ADVISORY UNTIL NOON CST THURSDAY FOR LAZ007>009-015-016-
023>026.
AR...LAKE WIND ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR ARZ074-075.
FLASH FLOOD WATCH THROUGH THURSDAY AFTERNOON FOR ARZ074-075.
WIND ADVISORY UNTIL NOON CST THURSDAY FOR ARZ074-075.
&&
$$
