FXUS64 KJAN 231120
AFDJAN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSON MS
407 AM CST WED DEC 23 2009
.SHORT TERM...AREAS OF RAIN DEVELOPED OVER WRN ZONES OVERNIGHT AS
ISENTROPIC LIFT DEVELOPED AND THE FIRST IN A SERIES OF WAVES LIFTED
INTO THE REGION. THE BETTER DYNAMICS ARE EXPECTED EARLY
TODAY...DECREASING A BIT WHEN THE WAVE LIFTS OUT AND ISENTROPIC LIFT
SUBSIDES DURING THE AFTERNOON. SATELLITE PIX SHOWING A BREAK IN THE
WAVE TRACK AFTER THIS FIRST IMPULSE. 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 THROUGH THURSDAY
EVENING. ALTHOUGH THESE AMOUNTS CURRENTLY FALL BELOW FLOOD GUIDANCE
NUMBERS THIS MORNING...RAINFALL OCCURRING TODAY MAY SATURATE THE
GROUND AND RESULT IN RUN OFF PROBLEMS WITH PERIODS OF HEAVIER
RAINFALL THAT WILL OCCUR IN THUNDERSTORMS THURSDAY. HAVE ISSUED A
FLASH FLOOD WATCH FOR THURSDAY FOR AREAS ALONG AND WEST OF I-55.
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...WIDESPREAD RAINFALL OVER AREAS WEST OF THE NATCHEZ TRACE
WILL RESULT IN AREAS OF MVFR CIGS/VSBYS THIS MORNING. VFR CONDS WILL
CONTINUE OVER OTHER AREAS WITH LIGHTER SHOWERS POSSIBLE. CONDITIONS
WILL IMPROVE TO GENERALLY VFR AFTER NOON TODAY AS RAINFALL
DIMINISHES. GUSTY SLY WIND WILL DEVELOP OVER THE REGION WITH
SUSTAINED SPEEDS FROM 10 TO 20 MPH AND GUSTS TO 30 MPG POSSIBLE.
RAINFALL WILL INCREASE FROM THE WEST TONIGHT AND GUSTY S WINDS WILL
CONTINUE. A FEW TSTMS...POSSIBLY SEVERE...WILL BE POSSIBLE OVER SWRN
ZONES TOWARD SUNRISE THURSDAY MORNING.
&&
.PRELIMINARY POINT TEMPS/POPS...
JACKSON 64 58 60 35 / 68 88 100 24
MERIDIAN 63 55 62 37 / 45 70 100 63
VICKSBURG 64 58 59 34 / 95 98 100 15
HATTIESBURG 66 58 64 37 / 35 73 100 35
NATCHEZ 67 59 58 34 / 69 99 100 12
GREENVILLE 59 56 58 33 / 100 100 100 22
GREENWOOD 59 56 58 34 / 93 91 100 25
&&
.JAN WATCHES/WARNINGS/ADVISORIES...
MS...LAKE WIND ADVISORY FROM 11 AM THIS MORNING TO 6 AM CST THURSDAY
FOR MSZ018-019-025>028-034>036-040>043-047>049-053-054-
059>062.
FLASH FLOOD WATCH FROM THURSDAY MORNING THROUGH THURSDAY
AFTERNOON FOR MSZ018-019-025>027-034>036-040>042-047-053-
059-060.
LA...LAKE WIND ADVISORY FROM 11 AM THIS MORNING TO 6 AM CST THURSDAY
FOR LAZ007>009-015-016-023>026.
FLASH FLOOD WATCH FROM THURSDAY MORNING THROUGH THURSDAY
AFTERNOON FOR LAZ007>009-015-016-023>026.
AR...LAKE WIND ADVISORY FROM 11 AM THIS MORNING TO 6 AM CST THURSDAY
FOR ARZ074-075.
FLASH FLOOD WATCH FROM THURSDAY MORNING THROUGH THURSDAY
AFTERNOON FOR ARZ074-075.
&&
$$
03/MME/03
