Flash Flood Watch 1, NWS La Crosse, WI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
320 PM CDT TUE JUN 22 2010

.SHORT TERM...THROUGH FRIDAY

SHORT TERM SEVERE WEATHER CHANCES FOR TONIGHT AND WEDNESDAY ARE THE
MAIN FOCUS FOR THE FORECAST SHIFT TODAY. NEXT WEATHER CHANCE IS IN
THE LONGER TERM FORECAST BELOW.

WATER VAPOR AT 1830Z SHOWING A NUMBER OF SHORTWAVE TROUGHS UPSTREAM
OF THE FORECAST AREA THAT WILL PROVIDE STRONG SUPPORT TO THUNDERSTORM
DEVELOPMENT OVERNIGHT IN THE MIDWEST. THE MOST SIGNIFICANT ARE THE
ONE EJECTING OUT OF NERN UT AND ANOTHER IN SRN ID AT THIS TIME.
22.12Z NAM/GFS BOTH HAD A GOOD HANDLE ON THESE AT 18Z TODAY PER
POTENTIAL VORTICITY MAPPING OF 1.5 PVU SURFACE. THESE CONSOLIDATE
OVER NEXT 12 HOURS TO FOR A DEEPER TROPOPAUSE FOLD TO 450 MB
/CURRENTLY DOWN TO ABOUT 370 MB/.

CURRENTLY MONITORING THE DESTABILIZATION ACROSS THE MIDWEST. THERE
SEEM TO BE TWO HOT SPOTS...THE MARITIME TROPICAL PLUS AIR MASS
BOUNDARY ACROSS SRN IA AND THE COLD FRONT ALIGNED FROM KINL-KABR-
SWWD. CURRENTLY THE SPC ANALYSIS SHOWS ABOUT 4000 J/KG OF SBCAPE IN
THE VICINITY OF THE SRN IA AND ERN SD. OVER THE PAST HOUR THE 0-6KM
SHEAR HAS INCREASED FROM 32 TO 55 KT AT SLATER IA PER
PROFILER...INDICATING THE INCREASING DEEP SHEAR MOVING IN FROM THE
WEST...SUPERCELL SHEAR WITH A STRAIGHT LINE HODOGRAPH /WOOD LAKE HAS
50 KTS OF 0-6KM BULK SHEAR/. CLOSER TO THE FORECAST AREA...FAIRLY
UNREMARKABLE WITH LITTLE CONVERGENCE SEEN IN THE SURFACE LAYER...
ALTHOUGH A WIND SHIFT FROM S TO SE DOES EXIST FROM CENTRAL IA SEWRD.
IT APPEARS WHILE THE AREA IS UNSTABLE WITH A MLCAPE OF 2-3K J/KG...
CONVECTIVE INITIATION IS NOT IMMINENT.

IN ASSESSING THE MODEL GUIDANCE FOR THE OVERNIGHT OUTCOME...HAVE
NOT TAKEN THE 22.12Z GFS AS A VALID SOLUTION WITH THE VERY DEEP AND
CONVECTIVE ROOTED DEEPENING OVER WY OF THE SHORTWAVE TROUGH. THE
SOLUTION APPEARS AS AN OUTLIER COMPARED TO THE 22.12Z ECMWF AND NAM
WITH A SOLUTION TOO FAR NORTH. OVERALL...THE MODELS ENHANCE THE LOW-
LEVEL JET THIS EVENING AS HEIGHTS FALL IN THE LOW LEVELS IN RESPONSE
TO THE SHORTWAVE TROUGH FORCING MOVING OUT OF THE ROCKIES. BY 00Z...A
CONVERGENT LOW-LEVEL JET IS POSITIONED INTO NWRN IA/SWRN MN WITH AN
EASTERN EXTENT TO NEAR I-35 INTO NERN IA. BY 06Z THIS INTENSIFIES AND
SHIFTS EAST. THUS...A CONVECTIVE SYSTEM IS EXPECTED TO INITIATE NEAR
KFSD-KABR AND THEN CONSOLIDATE AND MOVE ALONG THE CAPE TOWARD THE
FORECAST AREA. MOST LIKELY TOWARD KDBQ. THIS IS A LATE EVENING-
OVERNIGHT ARRIVAL OF A MATURE SQUALL LINE. THERE IS A CHANCE THAT
WEST-CENTRAL MN COULD INITIATE IN NEXT SEVERAL HOURS TOO WITH A
SLIGHTLY FURTHER NORTH MCS. 22.18Z RUC SUGGESTS MOST OF THE FORECAST
AREA WILL BE IN 3000 J/KG SBCAPE AT 06Z /PLUS DYNAMICAL LIFT/ TO
PRODUCE A MORE WIDESPREAD DAMAGING WIND THREAT. PRIOR...THE EASTERN
EXTENT OF THE LOW-LEVEL JET CONVERGENCE MAY INITIATE MORE CELLULAR
CONVECTION IN THE FORM OF SOME SUPERCELLS THIS EVENING. WITH A S-SE
WIND FLOW POSSIBLE AT THE SURFACE...TORNADOES COULD NOT BE RULED OUT
FROM THIS CONVECTION...MAINLY WEST OF THE MISSISSIPPI RIVER THIS
EVENING. HAIL/WIND AND HEAVY RAIN ARE POSSIBLE ALL NIGHT /SEE
HYDROLOGY DISCUSSION/. THIS OVERNIGHT EVOLUTION IS SIMILAR TO THE 12Z
SPC WRF 4KM RUN AT NCEP.

STORMS/RAIN WILL BE ONGOING IN THE MORNING WEDNESDAY WITH THE FRONT
AND SURFACE LOW IN THE AREA. THROUGH THE DAY...THIS WILL SLOWLY
SHIFT SOUTHEAST AS WILL MOISTURE AXIS. IT APPEARS THE SEVERE
THREAT WILL QUICKLY SHIFT SOUTHEAST WEDNESDAY WITH ONLY GRANT COUNTY
WI HAVING A CHANCE. HAVE INCREASED THE RAIN CHANCES WEDNESDAY IN
THE FORECAST WITH SLOWER TIMING TO SYSTEM.

FEW CHANGES MADE TO NICE WEATHER DAYS ON THU-FRI.

.LONG TERM...FRIDAY NIGHT THROUGH TUESDAY

LARGE SCALE MODELS IN GOOD AGREEMENT ON THE BIG PICTURE OF THE LARGER
SCALE RIDGE BUILDING NORTHWARD ACROSS THE PLAINS EARLY IN THE
PERIOD. THIS WILL SURGE MOISTURE AND INSTABILITY NORTHWARD THROUGH
WRN MN AND THE DAKOTAS FRIDAY. A WARM FRONT WILL SHIFT NORTH AND LAY
ACROSS THE CENTRAL MN/WI FOR CONVECTIVELY INITIATED EVENTS TO
ROLL EAST OUT OF MN DOWN I-94 AND EAST TOWARD KGRB. BELIEVE THIS WILL
TRANSITION TO THE WARM AND MORE CAPPED WARM SECTOR ACROSS OUR
SOUTHERN FORECAST AREA. HAVE HIGHER CONFIDENCE ON THIS WARM FRONT
SHIFTING NORTH AS THE SYNOPTIC LOW APPEARS TO BE EJECTING OUT OF THE
INTERMOUNTAIN WEST INTO THE DAKOTAS AND THE CONVECTIVE INIT REGION
WILL BE FAR WEST...ALLOWING A CLEAN PROGRESSION NORTHWARD. THIS WILL
LIKELY BRING BENEFICIAL RAINS TO THE NORTHERN FORECAST AREA AND NRN
WI VIA LATE NIGHT MCS ACTIVITY INTO SAT AND SUN MORN. HAVE
INCREASED THE RAIN CHANCES IN ALL AREAS TO TREND THESE SYSTEMS
AFFECTING THE NORTHERN FORECAST AREA. SOME TIMING DIFFERENCES BUT
IT APPEARS A COLD FRONT WILL THEN SHIFT THROUGH THE AREA LATE SUNDAY
FOR A DRY AND COOLER MON/TUE.

SEVERE CHANCES ARE TOUGH TO DETERMINE DUE TO NOCTURNAL NATURE OF
THESE SYSTEMS AFFECTING THE AREA...IN THE VERY EARLY MORNING.
WILL NEED TO WAIT UNTIL WE GET CLOSER TO BETTER ASSESS.

&&

.HYDROLOGY...RECENT PERIODS OF RAIN HAVE ALLOWED THE SOILS OVER
SOME OF THE FORECAST AREA TO BECOME SATURATED. RAINFALL
EXPECTED TONIGHT WILL FALL AND RUNOFF INTO AREA STREAMS AND RIVERS
CAUSING RISES. SOME OF THESE MAY BE RAPID DEPENDING ON THE RAINFALL
RATE. THIS AFTERNOON...INSTABILITY WAS GROWING OVER IOWA AND
SPREADING RAPIDLY NORTH. VERY WARM AND HUMID AIR WITH WARM CLOUD
DEPTHS OVER 4KM AND PRECIPITABLE WATERS NEARING 200 PERCENT OF NORMAL
WILL CAUSE RAINFALL RATES OF 1-2 INCHES PER HOUR WITH STRONGER
THUNDERSTORMS OVERNIGHT. THUNDERSTORMS ARE EXPECTED TO FORM IN THE
EVENING...AND SOME AREAS SOUTH OF I-90 COULD SEE REPEAT THUNDERSTORMS
TONIGHT INTO EARLY WEDNESDAY MORNING. THE MOST LIKELY AREA FOR THE
HEAVIEST RAINFALL AND REPEAT THUNDERSTORMS WOULD BE OVER SOUTHEAST
MINNESOTA AND NORTHEAST IOWA AND INTO SOUTHWEST WISCONSIN. THE WORST
CASE SCENARIO WOULD BE EARLY THUNDERSTORM DEVELOPMENT THIS EVENING IN
AN E-W BAND NEAR THE IA-MN BORDER. THE STORMS WOULD LIKELY MOVE EAST
ALONG THE SAME AREAS. CONFIDENCE IS MEDIUM ON THAT OCCURRING...
POSSIBLE BUT NOT PROBABLE. WE COULD SEE TOTALS OVER 5 INCHES IF THIS
SETS UP. THEREFORE...A FLASH FLOOD WATCH WILL BE ISSUED FROM I-90
SOUTH TO RAISE AWARENESS OF THIS NIGHT TIME HEAVY RAIN POTENTIAL...
FROM TONIGHT THROUGH WEDNESDAY AT NOON...WHEN THE INSTABILITY WILL
SHIFT SOUTH OF THE AREA. HOPEFULLY...THOSE TRAVELING ABOUT AFTER DARK
WOULD HAVE THIS IN THEIR MIND IF HEAVY RAIN AND A FLOOD THREAT SETS
UP.
&&

.AVIATION...LATE THIS AFTERNOON THROUGH WEDNESDAY

A WARM FRONT WILL CONTINUE TO MOVE NORTHWARD THROUGH IOWA THROUGH
THIS EVENING. AS THE CAP ERODES THIS AFTERNOON AND THE MOISTURE
TRANSPORT INCREASES INTO THE REGION...ISOLATED TO SCATTERED
THUNDERSTORMS ARE EXPECTED TO DEVELOP ACROSS SOUTHEAST
MINNESOTA...NORTHEAST IOWA...AND SOUTHWEST WISCONSIN BETWEEN 22.21Z
AND 23.03Z. THESE THUNDERSTORMS WILL BE CAPABLE OF PRODUCING LARGE
HAIL AND DAMAGING WINDS. IN ADDITION...THERE WILL BE THE POSSIBILITY
OF A FEW TORNADOES IN THE VICINITY OF THE WARM FRONT.

A MORE WIDESPREAD AREA OF THUNDERSTORMS IS EXPECTED TO MOVE THROUGH
THE REGION BETWEEN 23.06Z AND 23.12Z. THESE THUNDERSTORMS WILL HAVE
THE POTENTIAL OF PRODUCING DAMAGING WINDS AND LARGE HAIL GENERALLY
ALONG AND SOUTH OF INTERSTATE 90 CORRIDOR.

AT THIS TIME...THE GREATEST THUNDERSTORM CHANCES LOOK TO BE FROM
23.07Z AND 23.10Z AT KLSE...AND 22.23Z AND 23.09Z.

AS FAR AS WEDNESDAY...THERE WILL BE THE POSSIBILITY OF ADDITIONAL
THUNDERSTORMS AHEAD OF A COLD FRONT...BUT THE TIMING AND STRENGTH IS
RATHER UNCERTAIN.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...FLASH FLOOD WATCH FROM 7 PM CDT THIS EVENING THROUGH WEDNESDAY
     AFTERNOON FOR WIZ041-WIZ042-WIZ053-WIZ054-WIZ055-WIZ061.

MN...FLASH FLOOD WATCH FROM 7 PM CDT THIS EVENING THROUGH WEDNESDAY
     AFTERNOON FOR MNZ086-MNZ087-MNZ088-MNZ094-MNZ095-MNZ096.

IA...FLASH FLOOD WATCH FROM 7 PM CDT THIS EVENING THROUGH WEDNESDAY
     AFTERNOON FOR IAZ008-IAZ009-IAZ010-IAZ011-IAZ018-IAZ019-
     IAZ029-IAZ030.

&&

SHORT/LONG TERM...BAUMGARDT
SHORT TERM/AVIATION...BOYNE

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