Severe Thunderstorm Warning 74, NWS La Crosse, WI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
307 PM CDT SUN JUN 17 2001

MAIN FORECAST PROBLEM IS CONVECTIVE CHANCE TONIGHT THROUGH MONDAY
NIGHT WITH EVOLVING WARM SECTOR AND COLD FRONTAL PASSAGE.

INSTABILITY CONTINUES TO BUILD ACROSS IOWA WITH SBCAPES APPROACHING
2500J/KG.  RUC2 ALSO PLACES MOST OF IA UNDER A STRONG 100J/KG
CIN CAP. CAP WEAKENS NORTHEAST BUT CAPES ARE 500 OR LESS OVER WI.
SURFACE ANALYSIS SHOWS TWO BOUNDARIES. FIRST ALONG KMLI-KAEL-KFSD
LINE WITH INSTABILITY SOUTH OF IT AND SOUTHERLY WIND. SECOND CAPPED
AIR MASS OVER KS/NEB/WRN MO.  BOTH ARE EVOLVING NORTHWARD. RADAR AND
SATELLITE INDICATE ELEVATED SHOWERS OVER MN MOVING EAST. LIGHTNING
STRIKES NEAR KOMA AND WRN IA WITH ELEVATED CONVECTION IN AREA OF LOW-
MID LEVEL CONVERGENCE. SURFACE-BASED STORMS NOW ERUPTING NEAR KABR
WITH SEWRD MOTION.

I AM QUITE PLEASED WITH AVN/ETA SOLUTIONS TODAY...WHICH ARE QUITE
SIMILAR AS THEY HAVE BEEN LAST FEW DAYS. HOWEVER...THE ETA
TOTALLY MISSED THE SD CONVECTION THIS MORNING...OLDER RUNS HAD IT.
MSAS SURFACE ANALYSIS CONTINUES TO SHOW 1MB/HR PRESSURE FALLS
CENTERED ON KOMA. AS PRESSURE FALL IN PLAINS...SOUTHERLY FLOW AND
LOW-LEVEL JET WILL ENHANCE OVERNIGHT AND FIRE INTO NORTHERN SURFACE
BOUNDARY AND UPSLOPE OVER FRONT.  850 MB MOISTURE TRANSPORT IS
CONVERGING RIGHT INTO SOUTHERN MN AT 00Z AND THIS THEN SHIFTS
EASTWARD INTO FORECAST AREA. WITH 2500-300J/KG CAPES RUNNING OVER
FRONTAL BOUNDARY ACROSS AREA...MCS WILL EVOLVE FROM A KFSD ORIGIN
POINT. CORFIDI VECTORS SUGGEST SOUTHEAST MOTION BUT FACTORING IN
EVOLVING LLJ TO NORTH...IT MAY BE MORE EAST. WITH PRECIPITABLE
WATER NEAR 1.5 IN SOURCE AIR MASS...WILL ISSUE A FLASH FLOOD
WATCH FOR SERN MN KRST-KAUM COUNTIES THAT HAVE HAD PROBLEMS LATELY.
SOME SUPPORT IN ETA FOR A POSSIBLE SECOND CLUSTER FURTHER NORTH OF
THIS WHICH IS NOW GROWING NEAR KABR. MUST ALSO WATCH SURFACE BOUNDARY
FOR SURFACE BASED CONVECTION NERN IA/SWRN WI/SERN MN. 0-6KM BULK
SHEAR IS 50KTS AND WEAKENS SLIGHTLY THROUGH THE NIGHT...THUS MAIN
THREAT FOR SEVERE IS WEST OF MISS RIVER WITH MCS AS IT MOVES IN.
APPEARS IT IS MAINLY HAIL/WIND BUT 0-3KM HELICITY IS AROUND 200 IN
SOUTHERLY FLOW. WILL RAIN MOST AREAS TONIGHT WITH ALL THESE FACTORS.

WARM FRONT SHOULD SURGE NORTHWARD MONDAY AFTER MORNING CONVECTION
PULLS EAST AND AHEAD OF STRONG SHORTWAVE TROUGH AND COLD FRONT.
ETA/AVN PULL IN A STRONG CAP AND WARM LOW-LEVEL TEMPERATURES TO 18C.
WILL BRING AN AFTERNOON BREAK INTO MONDAY PERIOD WITH STRONG SWRLY
FLOW...WARM SURFACE HIGHS VIA MIXING...AND LOWERING OF TSRA CHANCES
FURTHER EAST OF COLD FRONT. COLD FRONT SHOULD LIE ALONG A KMSP-KSUX
LINE AT 18Z. THEN...INSTABILITY SHOULD BE QUITE LARGE BY 00Z WHEN
FRONT MOVES THROUGH. SPEED MAX ALOFT MOVES IN WITH 60+ KTS AT 500 MB.
0-6KM BULK SHEAR SUPPORT SUPERCELLS WITH CAPES OF 2500-3500J/KG IN
PLACE OVER CWA. VGP/BRN SHEAR LOOK GOOD FOR ISOLATED TORNADOES...BUT
CURVATURE IS NOT STRONG IN UNIDIRECTIONAL HODOGRAPH. SPC HAS A MOD
RISK FOR THIS PERIOD AND MAIN THREAT IS VERY LARGE HAIL AND WIND.
STORMS WILL BE SUPERCELL SPLITERS AND ISOLATED TORNADOES COULD FORM
IF DEVIANT MOTION OCCURS OFF HODOGRAPH...ESPECIALLY IN LARGE CAPE
ENVIRONMENT. CAP IS HUGE IN ETA AND IT DOESNT CONVECT THE AIR MASS
DUE TO WEAK SURFACE CONVERGENCE AND FN VECTOR CONVERGENCE OVER THE
FORECAST AREA. I AM IGNORING THIS FOR NOW BUT THESE FACTORS ALONG
WITH LARGE SCALE UPPER JET TO THE NORTH COULD POSSIBLY LIMIT OUTCOME
MONDAY EVENING. THIS WEATHER SCENARIO TRANSLATES SOUTHEAST ACROSS
FORECAST AREA OVERNIGHT INTO TUESDAY MORNING.

TUESDAY IS A MUCH DIFFERENT AIR MASS OVER THE REGION WITH THE
SOUTHERN PORTIONS STILL NEAR FRONT FOR A CHANCE OF RAIN.  IT WILL
BE A COLD AIR/DRY ADVECTION DAY. COLD POOL AND UPPER LOW MOVES ACROSS
WI FORECAST AREA ON WEDNESDAY FOR INSTABILITY SHOWERS.

EXTENDED...MODELS AGREE ON DRY PERIOD WITH GRADUAL WARMING.

COORDINATED WITH MKX/GRB/MPX/DMX.

.LSE...FLASH FLOOD WATCH SERN MN TONIGHT...MNZ079-086-087-094-095.

DB