FXUS64 KJAN 231716 AAA
AFDJAN
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE JACKSON MS
1116 AM CST WED DEC 23 2009
...POWERFUL STORM SYSTEM TO CROSS THE REGION CHRISTMAS EVE...
.UPDATE...WARM FRONTAL ZONE LIES ROUGHLY ALONG THE RED RIVER IN LA
WHILE THE FIRST MID-UPPER LEVEL SHORTWAVE HAS BYPASSED THE BOUNDARY
WITH RAIN SHIELD LIFTING NEWD THROUGH THE NWRN HALF OF MS. EXPECT
THE BOUNDARY TO CONTINUE CREEPING NWD THROUGH TONIGHT AS MORE UPPER
LEVEL ENERGY CROSSES TX FROM THE SRN ROCKIES AND A SURFACE LOW BEGINS
TO DEEPEN OVER NERN TX.
RAINFALL TOTALS IN THE DELTA REGION NOW REACHING 1.5 TO 2.2 INCHES IN
A FEW SPOTS. WE SHOULD SEE MOSTLY SCATTERED SHRA/ISOLATED TSRA THIS
AFTERNOON AND EVENING FOR MAINLY W AND N PORTIONS OF OUR AREA.
HOWEVER...EXPECT RAINS TO INCREASE AGAIN LATE TONIGHT OVER THE NWRN
PORTIONS AND HAVE STARTED THE FLASH FLOOD WATCH EARLIER AT 06Z FOR
THE SAME LOCATIONS AS THE PREVIOUS ISSUANCE. NO CHANGES TO THE LAKE
WIND ADVISORIES FOR NOW BUT SOME PORTIONS MAY BE UPGRADED TO WIND
ADVISORIES ALONG THE MS RIVER CHANNEL FOR TONIGHT.
THE DEEPENING SURFACE LOW(995MB) THURSDAY MORNING OVER SERN AR MOVES
ALMOST NWD TO 990MB BY NOON AND 987MB BY 6PM OVER SRN MO. THIS TRACK
HAS BEEN NOTORIOUS FOR PRODUCING SVR WX INCLUDING TORNADOES IN MS AS
THE TRAILING FRONT/SQUALL LINE SWEEPS EWD. THE PRIMARY TIMING FOR THE
STRONGEST STORMS ON THURSDAY REMAINS FROM AROUND 3-5 AM IN NE LA/SE AR
SWEEPING EWD THROUGH THE RIVER COUNTIES/PARISHES DURING THE MORNING
HOURS TO AROUND I-55 BY LATE MORNING AND THEN 3-6 PM FOR ERN MS.
MORNING UPDATES HAVE BEEN SENT WITH MORE TO COME THIS AFTERNOON.../40/
&&
MOST OF THE PREV DISCUSSION... /ISSUED 407 AM CST WED DEC 23 2009/
SHORT TERM...THE PRESSURE GRADIENT WILL GRADUALLY INCREASE TODAY
AHEAD OF THE DEEP LOW PRESSURE SYSTEM DEVELOPING TO OUR WEST. WIND
SPEEDS WILL SHOULD MARGINALLY MEET LAKE WIND ADVISORY CRITERIA AT
MANY AREAS AND THE LAKE WIND ADVISORY WILL REMAIN IN EFFECT. THE
GRADIENT WILL CONTINUE TO INCREASE TONIGHT SO THE ADVISORY HAS BEEN
EXTENDED THROUGH 12Z THURSDAY.
MUCH OF THE FORECAST AREA WILL RESIDE IN THE WARM SECTOR TONIGHT
AND SEVERAL PARAMETERS INDICATE A CHANCE OF STRONG TO SEVERE
CONVECTION OVER SWRN ZONES TOWARD SUNRISE AS WIND FIELDS INCREASE.
THE LEADING EDGE OF DEEPER AND STRONGER ASCENT WILL EDGE INTO
EXTREME SWRN AND WCNTRAL ZONES LATE TONIGHT. INSTABILITY WILL BE THE
LIMITING FACTOR FOR SEVERE STORMS TONIGHT WITH CAPE VALUES AROUND
200 EDGING INTO THE SWRN CORNER OF THE FORECAST AREA BY 6 AM
THURSDAY. ALTHOUGH THE CHANCE OF CONVECTION WILL BE LOWER FOR THE
PREDAWN HOURS LATE TONIGHT...MODELS AGREES THAT ANY CONVECTION THAT
CAN GET GOING OVER SWRN ZONES WILL HAVE THE POTENTIAL TO BECOME
SEVERE. DEEP LAYER SHEAR WILL BE MORE THAN SUFFICIENT WITH 0-3KM
HELICITY VALUES AT OR ABOVE 300 AND 0-1KM SHEAR ABOVE 30 KTS BY
SUNRISE.
THE BRUNT OF THIS SYSTEM WILL BE FELT THURSDAY WHEN THE STRONG COLD
FRONT MOVES ACROSS THE AREA. MODEL AGREEMENT LENDS CONFIDENCE TO
INTENSE WIND FIELDS SETTING UP OVER THE AREA WITH LOW LEVEL JET
SPEEDS IN EXCESS OF 50 KTS AND A STRONG UPPER JET DIGGING INTO A
DEEP UPPER TROUGH THAT WILL PIVOT INTO THE ARKLAMISS. AS HAS BEEN
THE CASE RECENTLY, LOW LEVEL MOISTURE AND INSTABILITY ARE AMONG THE
WEAKEST PARAMETERS FOR THE SEVERE WEATHER EPISODE SETTING UP FOR
THURSDAY BUT WITH HELICITY VALUES FROM 350 TO WELL ABOVE 500 AND
0-1KM SHEAR VALUES FROM 30 TO 40 KTS IT WON'T TAKE TOO MUCH
INSTABILITY TO RESULT IN SEVERE STORMS. WITH CAPE VALUES AROUND
1000...WIDESPREAD STRONG ASCENT PER Q VECTOR DIV FIELDS AND FORCING
IN THE VICINITY OF THE FRONT WE WILL SEE MORE THAN SUFFICIENT
FORCING FOR SEVERE CONVECTION. ALTHOUGH DAMAGING WIND AND TORNADOES
LOOK TO BE THE PREDOMINANT SEVERE RISK WITH THIS EPISODE LARGE HAIL
WILL ALSO BE POSSIBLE.
RAINFALL AMOUNTS FROM 1.5 TO 3 INCHES ARE EXPECTED LATE TONIGHT AND
THURSDAY. ALTHOUGH THESE AMOUNTS CURRENTLY FALL BELOW FLOOD GUIDANCE
NUMBERS FOR E AND S PORTIONS...RAINFALL OCCURRING TODAY MAY SATURATE
THE GROUND AND RESULT IN RUN OFF PROBLEMS WITH PERIODS OF HEAVIER
RAINFALL THAT WILL OCCUR IN THUNDERSTORMS THURSDAY. 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 WRN ZONES FOR NOW DUE TO THEIR PROXIMITY TO THE
UPPER TROUGH AND THE LONGER PERIODS OF UPPER DIVERGENCE INDICATED IN
THESE AREAS BY THE MODELS. INCREASED MOS POPS TO NEAR 100 PERCENT
ACROSS THE REGION FOR THURSDAY.
LONG TERM...THURSDAY NIGHT THROUGH WEDNESDAY...COME THURSDAY
EVENING THE SQUALL LINE SHOULD BE EXITING OUR CWA TO THE EAST
FOLLOWED BY THE COLD FRONT. THE COLD FRONT WILL SHIFT EAST OF OUR
AREA AS THE INITIAL CLOSED LOW AND ASSOCIATED SURFACE LOW MOVE NORTH
ACROSS THE MID MISSISSIPPI VALLEY. AT THE SAME TIME A SECOND CLOSED
LOW WILL DROP INTO THE CENTRAL PLAINS AND ROTATE UP TO THE NE OVER
THE MID MISSISSIPPI VALLEY FRIDAY. THIS WILL FURTHER AMPLIFY THE
LARGE UPPER LEVEL TROUGH OVER MOST OF THE CENTRAL CONUS AND PROVIDE
US WITH 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...STRONG SFC WINDS AND ASSOC TURBULENCE WILL BE THE FOCUS
FOR THIS AFTN...ACCOMPANIED BY PRIMARILY VFR CONDS ALTHOUGH
INTERMITTENT MVFR WILL BE PSBL OVR ERN AREAS. EARLY THIS EVENING
XPCT GRADUAL DETERIORATION TO IFR CONDS IN THE E AS LOW LEVELS
CONTINUING MOISTENING IN SRLY FLOW. RAIN/TS CHANCES WILL RETURN LATE
TONIGHT...ACCOMPANIED BY INCREASING COVERAGE OF IFR CONDS AS A COLD
FRONT APPROACHES THE AREA. THE FRONT IS RATHER STRONG AND A SQUALL
LINE IS XPCTD TO BE SWEEPING THE AREA IN THE MORNING...WITH DAMAGING
WINDS AND ISOLD TORNADOES BEING THE PRIMARY RISK. SFC WINDS WILL BE
STRONG AND SERLY AHEAD OF THE FRONT SHIFTING TO STRONG AND SWRLY
RIGHT BEHIND THE FRONT. /BK/
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 65 58 60 35 / 68 88 100 24
MERIDIAN 65 55 62 37 / 45 70 100 63
VICKSBURG 68 58 59 34 / 95 98 100 15
HATTIESBURG 67 58 64 37 / 35 73 100 35
NATCHEZ 68 59 58 34 / 69 99 100 12
GREENVILLE 62 56 58 33 / 100 100 100 22
GREENWOOD 60 56 58 34 / 93 91 100 25
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...LAKE WIND ADVISORY UNTIL 6 AM CST THURSDAY FOR MSZ018-019-
025>028-034>036-040>043-047>049-053-054-059>062.
FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH THURSDAY AFTERNOON
FOR MSZ018-019-025>027-034>036-040>042-047-053-059-060.
LA...LAKE WIND ADVISORY UNTIL 6 AM CST THURSDAY FOR LAZ007>009-015-
016-023>026.
FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH THURSDAY AFTERNOON
FOR LAZ007>009-015-016-023>026.
AR...LAKE WIND ADVISORY UNTIL 6 AM CST THURSDAY FOR ARZ074-075.
FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH THURSDAY AFTERNOON
FOR ARZ074-075.
&&
$$
