Severe Thunderstorm Warning 378, NWS Sioux Falls, SD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
350 AM CDT WED SEP 18 2002

COMPLEX UPPER TROUGH TAKING SHAPE OVER NATIONS MIDSECTION...WITH
MISH MASH OF VORTICITY CENTERS...ONE WRN SD...ONE IN SWRN NE...AND
THE MAJOR JET PUSH THRU NV INTO SRN UT ATTM. LEADING EDGE OF THIS
JET PUSHING INTO CNTRL PLAINS. AHEAD OF THIS...BAND OF ELEVATED
THUNDERSTORMS ONGOING FM KBKX TO KSUX...ALG LEADING CONVERGENCE OF
NLG PROF 40-50KT LLJ...TAIL OF WRN SD PV CENTER... LINES UP WELL
WITH MAX THETA E ADVECTION IN 850-700 HPA LAYER IN ETA...AND DRIER
STREAM ALFT ON WV REPRESENTING ELEVATED INSTABILITY MAX.  EVEN A
LITTLE LARGE PEA SIZE HAIL AT THE OFFICE.  THIS WX WL LIKELY FALL
BETWEEN THE CRACKS AND WL BE ADDED TO ZNS IN PRE FIRST PD WORDING.
MAIN ISSUE TODAY IS ADDITIONAL CONVECTIVE DEVELOPMENT IN THE SHORT
TERM...DEALING WITH THE WRAPAROUND BAND OF PCPN...AND THEN CLEARING
PCPN FROM REGION ON THU.

PRETTY SIG DIFFERENCES IN AVN/ETA IN DEALING WITH EVOLUTION. AVN
MUCH MORE SLOW AND DIGGY WITH MAIN ENERGY...ALLOWING SIG CLOSED
LOW TO DEVELOP IN CNTRL PLAINS THU AND MOVE INTO IA THU NT. THIS
SLOWS PROGRESSION OF ALL FEATURES...CLEARING...PCPN ESPECIALLY.
STRENGTH OF UPSTREAM SYSTEM FAVORS A MORE PROGRESSIVE PUSH...AND
AVN SEEMINGLY ALONE ON SLOWNESS. FAVOR TREND TO MORE ETA TIMING AND
STRENGTH...SUPPORTED BY CANADIAN.

FOR TDA...AFTER INITIAL BAND OF PCPN...DIVERGENCE AHEAD OF UPR
JET AND INDICATION OF GOOD QG FORCING WITH AREA IN SCNTRL NE MOVING
TOWARD NW IA EARLY THIS MORNING. WL CONTINUE SCT TSRA THREAT IN S.
THEN WITH UPR CIRC TO NW...WL GET INTO A DIRTY DRY SLOT FOR MUCH OF
DAY...WITH BEST LIFT FORCING LEADING THIS FEATURE TO N AND E. WL
HAVE TO SEE IF CAN DESTABILIZE ENOUGH FOR SCT TSRA TO POP ALG FRONT
AS IT WORKS TO JMS VLY LTR AFTN...AND THEN MOVES QUICKLY INTO MN AND
IA BEFORE MIDNIGHT.  CAP FORMIDIBLE TO START...BUT SHOWS SIGNS OF
WKNG A BIT IF ENOUGH HEATING BY MID AFTN AND WITH STRENGTH OF LOW LVL
FRONTOGENESIS...AND PROXIMITY OF JET EXIT.

BEST OF FRONTOGENETIC DEFORMATION BAND LIFTS NWRD AND SKIMS NRN CWA
AT BEST TONIGHT INTO THU MORNING. WL NOT GO AS SLOW ON PCPN AS AVN
EXPECTS...AND WL END ALONG MID LVL FRONTOGENESIS. AS PART OF ENERGY
ROUNDS BASE OF TROUGH AND EJECTS NE TOWARD IA THU AFTN AND EARLY
EVNG...MAY SEE A FEW SHOWERS SKIRT SE CRNR...AND KEPT LOW POP IN
EARLY EVENING THERE.

FRIDAY STILL APPEARS TO BE A FINE BREEZY AND DRY PREFRONTAL
DAY...WITH FRONT GETTING TO NWRN CWA BY ARND 00Z. WL SEE A FEW SHRA
AND DIURNALLY FAVORED TSRA ALG FNT...WHICH WL MOVE RAPIDLY SE.
A STEP DOWN IN COOLING FRI NIGHT...WITH BETTER CD PUSH WL COME ON
SAT...AFTER PASSAGE OF JET ENTRANCE...WHEN AGEO SUPPORT IS BETTER.

IN EXTENDED...MADE A FEW CHANGES.  AVN STILL SHOWING A FAIRLY SIG
COOLDOWN...AND SUPPORTED BY CANADIAN AND ECMWF...ALTHOUGH NOT AS
BULLISH ON COOLING.  ACTUALLY MODIFIED TMPS A WEE BIT FOR SAT PD
BASED ON MIXING POTENTIAL.  HIGHS LOWERED ON DAYS 6-7 TO FIT COOLER
MIXING POTENTIAL...BUT 3-7F WARMER THAN AVN WOULD SUGGEST.  FOR
THIRD DAY...STILL SUGGESTS POTENTIAL FOR A LITTLE SNOW OVER NRN HALF
OF CWA IN AND AROUND TUESDAY.  HAVE SEEN THIS BEFORE /LAST YEAR/ AND
HAVE MY DOUBTS...ESPECIALLY THAT EXTREME OF COOLING.

.FSD...NONE
CHAPMAN