Severe Thunderstorm Warning 512, NWS Jackson, MS

AREA FORECAST DISCUSSION...UPDATE
NATIONAL WEATHER SERVICE JACKSON MS
405 PM CDT THU AUG 20 2009

.DISCUSSION...

REMAINDER OF THE AFTERNOON...

THE SWD PROPAGATING OUTFLOW BOUNDARY NOW EXTENDS FROM NEAR WINNSBORO
TO JACKSON TO MACON AND IS CONTINUING TO PUSH SOUTH. THIS BOUNDARY
IS OUT-RUNNING BETTER UPPER LEVEL SUPPORT AND DEEP LAYER FLOW ACROSS
AR AND NRN MS...BUT SFC DESTABILIZATION WITH DIURNAL HEATING AHEAD
OF THE BOUNDARY (MU CAPE NEAR 4000 J/KG)HAS LED TO AN ABUNDANCE OF
TSTM ACTIVITY...ESPECIALLY SINCE THE BOUNDARY BEGAN INTERACTING WITH
OTHER BOUNDARIES INDUCED BY EARLIER TSTM ACTIVITY ALONG AND SOUTH OF
THE I-20 CORRIDOR. IT APPEARS THE TSTM ACTIVITY HAS REACHED A
CRESCENDO AND A LESSENING OF TSTM INTENSITY SHOULD OCCUR AS WE
APPROACH SUNSET.

FOR TONIGHT...

THE BIG QUESTIONS CONCERN HOW MUCH CONVECTION WILL REDEVELOP IN THE
WAKE OF THE OUTFLOW BOUNDARY GIVEN THE STRONG MESO-HIGH IN PLACE.
MODELS ARE NOT HELPING MUCH AS THEY HAVE UNDERESTIMATED THE STRENGTH
OF OUTFLOW AND PROPAGATION...SO INCORPORATING THEIR SOLUTIONS
OVERNIGHT IS DIFFICULT. BUT CANNOT DISCOUNT THE POTENTIAL FOR
REDEVELOPMENT GIVEN THE INFLUENCE OF STRONG SHORTWAVE ENERGY TO THE
NORTH AND PLENTY OF TROPICAL MOISTURE AVAILABLE...SO WILL KEEP
HIGHER POPS GOING TONIGHT ACROSS THE NRN HALF OF THE AREA. ALL LOCAL
WRF MEMBERS LOCAL WRF SHOW A LINEAR CONVECTIVE SYSTEM MOVING IN
AFTER MIDNIGHT WITH A COUPLE OF MEMBERS SHOWING THE SYSTEM COLD POOL
REACHING AS FAR SOUTH AS THE I-20 CORRIDOR. WOULD TEND TO BELIEVE
THE FARTHER SOUTH SOLUTIONS BASED ON RECENT EXPERIENCES. GIVEN MID
LEVEL FLOW OF 20-30KTS HELPING TO ORGANIZE STORMS...STRONG TO
POSSIBLY SVR WIND GUSTS AND SMALL HAIL WILL BE A POSSIBILITY.

FOR FRIDAY AND FRIDAY NIGHT...IF THE FASTER OUTFLOW BOUNDARY
SOLUTION PANS OUT TONIGHT...THEN TSTMS WILL BE LIMITED MORE TO
SOUTHEAST PORTIONS OF THE FORECAST AREA (SOUTH OF I-20) FRIDAY
BECAUSE UPPER LEVEL SUPPORT WILL BE WANING AND LOW LEVEL FLOW WILL
BECOME MORE NWRLY BY 18Z FRIDAY. HAVE CONTINUED THE TREND OF CUTTING
POPS FOR NRN PORTIONS OF THE AREA. EXPECT MOSTLY DRY AND COOLER
CONDITIONS TO TAKE OVER FRIDAY NIGHT WITH THE ONLY LINGERING POPS
EXPECTED OVER SE PORTIONS OF THE FORECAST AREA. /EC/

SATURDAY THROUGH WEDNESDAY...GENERALLY DRY CONDITIONS WILL CONTINUE
IN THE LONG TERM INTO NEXT WEEK. WILL SEE HEALTHY COLD AIR ADVECTION
DURING THE DAY SATURDAY IN THE WAKE OF THE FRONT. MODELS INDICATE A
RELATIVELY DEEP 850 TEMP TROUGH OVER THE REGION AND MODEL SOUNDINGS
SUGGEST A WEAK INVERSION SETTING UP AROUND 850 MB SATURDAY
AFTERNOON. THIS SUGGESTS THE POSSIBILITY OF PERSISTENT CLOUD COVER
SATURDAY AS MOISTURE GETS TRAPPED BENEATH THE INVERSION WITH SCT-BKN
STRATOCU POSSIBLE THROUGH THE DAY. WITH SUCH VIGOROUS COLD AIR
ADVECTION AND THE POSSIBILITY OF A LOW LEVEL MOISTURE CAP...HAVE CUT
HIGHS A COUPLE OF DEGREES FOR SATURDAY AFTERNOON. OTHERWISE
GENERALLY FOLLOWED MOS TEMP NUMBERS... PARTICULARLY THE SLIGHTLY
COOLER TREND ON LOWS.

THE SRN PORTIONS OF THE LONG WAVE TROUGH THAT MOVES INTO THE
ARKLAMISS REGION THIS WEEKEND WILL STALL JUST TO OUR EAST...GETTING
CUT OFF FROM THE WESTERLIES AND CAUGHT BETWEEN UPPER RIDGING OVER
THE PLAINS AND ATLANTIC RIDGING THAT DEVELOPS IN THE WAKE OF
HURRICANE BILL. THIS UPPER TROUGH EVENTUALLY DIGS TO THE SW INTO THE
LOWER MS VALLEY...ITS PRIMARY EFFECT BEING TO REINFORCE MID/UPPER
COOLING OVER THE AREA. A WEAK LOW LEVEL TROUGH SHIFTS WWD ACROSS THE
LOWER MS VALLEY IN RESPONSE TO THE DIGGING UPPER FEATURE. MOISTURE
REMAINS LIMITED BUT VERY SLOWLY BEGINS TO INCREASE IN LOW LEVEL
S/SELY FLOW THAT DEVELOPS IN THE WAKE OF THE LOW LEVEL TROUGH EARLY
NEXT WEEK. WILL SEE INCREASING 850 MB THETAE ADVECTION MONDAY AND
WEAK LOW LEVEL WARM AIR ADVECTION TUE INTO WED. DEEPER MOISTURE
RETURN IS HELD OFF INTO WED AND MOS LOOKS REASONABLE KEEPING
THE FORECAST DRY AND INTRODUCING SLIGHT CHANCES OF SHOWERS IN THE
LATE TUE/WED TIME FRAME. WARM AIR ADVECTION WILL RESULT IN WARMER
TEMPERATURES...ESPECIALLY FOR AFTERNOON HIGHS...EARLY NEXT WEEK. THE
AI MASS WILL BE MODERATED SLOWLY ENOUGH TO ALLOW OVERNIGHT LOWS TO
LINGER IN THE 60S INTO MID WEEK AT MANY LOCATIONS. /03/

&&

.AVIATION...SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WILL
MOVE NORTHEASTWARD AROUND 15 KT ACROSS THE ARKLAMISS THIS
AFTERNOON...AND WILL BE CAPABLE OF PRODUCING WIND GUSTS TO 35
KT...FREQUENT LIGHTNING...AND VERY HEAVY RAINFALL. ALSO...A NEARLY
SOLID LINE OF STORMS EXTENDS FROM NEAR COLUMBUS TO TALLULAH...WITH
THE MAIN LINE DRIFTING SOUTHEASTWARD AROUND 5 KT. MOST OF THIS
ACTIVITY WILL DISSIPATE BY 21/0100 UTC. A COMPLEX OF SHOWERS AND
THUNDERSTORMS WILL THEN MOVE SOUTHEASTWARD INTO NORTHWESTERN AREAS
21/0300 UTC. THESE STORMS COULD PRODUCE WIND GUSTS TO 50 KT...ALONG
WITH VERY HEAVY RAINFALL...WHILE WEAKENING THROUGH THE NIGHT.
EXPECT LOCALIZED MVFR TO IFR CONDITIONS WITH THE STORMS. OUTSIDE OF
STORMS...GENERALLY VFR AND MVFR CONDITIONS WILL PERSIST. MORE
SHOWERS AND THUNDERSTORMS WILL DEVELOP OVER THE SOUTHEAST THIRD OF
MISSISSIPPI ON FRIDAY AHEAD OF A WEAK COLD FRONT DRIFTING SOUTHWARD.
/COHEN/

&&

.JAN WATCHES/WARNINGS/ADVISORIES...
MS...NONE.
LA...NONE.
AR...NONE.

&&

EC/COHEN/03