Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 051730
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1230 PM CDT TUE MAY 5 2015

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 232 AM CDT TUE MAY 5 2015

MAIN FOCUS IS ON RAIN CHANCES AND RESULTANT TEMPERATURES. WEAK
SHORT WAVE TROUGH ENHANCED BY WHAT APPEARS TO BE AN EMBEDDED
MESOSCALE CONVECTIVE VORTICITY WAS SPINNING THROUGH IA OVERNIGHT
RESULTING IN WIDESPREAD SHOWERS AND EMBEDDED THUNDERSTORMS. THUS
FAR THE RAIN HAS BEEN CONFINED TO IA/IL AND EXTREME SOUTHERN WI.
VARIOUS SHORT TERM MODELS SUGGEST SOME OF THE FRONTOGENETIC
FORCING WILL TRY AND SET UP FURTHER NORTH NEAR THE I-90 CORRIDOR
BUT THUS FAR NOTHING HAS BEEN DEVELOPING. NORTHWARD DEVELOPMENT
MAY BE HAMPERED BY ENTRAINMENT OF DRY AIR ON EAST/NORTHEAST WINDS
WHERE LOW LEVEL DEWPOINTS ARE IN THE 30S.

OTHERWISE ANTICIPATE THAT THE RAIN AREA WILL SHIFT EAST AND ANY
LINGERING RAIN CHANCE WILL REALLY DIMINISH AS THE DAY WEARS ON.
WILL MAINTAIN A SLIGHT CHANCE OVER THE SOUTH WHERE BETTER MOISTURE
AND RESIDUAL BOUNDARIES MAY WORK WITH DAYTIME HEATING TO GENERATE
SOME SHOWERS LATER TODAY. ATTENTION THEN SHIFTS TO CONVECTIVE
POTENTIAL TONIGHT AS THE FRONTAL BOUNDARY ACROSS IA MOVES BACK
NORTH. TREND HAS BEEN FOR LESS AND LESS ACTION ALONG THE NORTHWARD
MOVING AND WEAKENING WARM FRONT. STILL...MAINTAINED RAIN RISK LATE
TONIGHT INTO EARLY WEDNESDAY AS MOISTURE CONVERGENCE AND WEAK
FRONTOGENETIC SIGNAL TRANSLATE THROUGH.

CLOUDS AND RAIN SHOULD KEEP TEMPERATURES SOMEWHAT IN CHECK. THIS
ALREADY ACCOUNTED FOR IN PREVIOUS FORECAST SO MINIMAL CHANGES.

.LONG TERM...(WEDNESDAY THROUGH MONDAY)
ISSUED AT 232 AM CDT TUE MAY 5 2015

ACTIVE WEATHER PATTERN ON TAP WITH SEVERAL DISTURBANCES MOVING
THROUGH THE REGION OUT OF THE MEAN LONG WAVE TROUGH OVER THE
WESTERN CONUS. CHALLENGE FOR THE LONGER TERM IS TRYING TO IDENTIFY
THE BEST CHANCE FOR NO RAIN AND HOW TO DEPICT THAT IN THE
FORECAST. SIGNIFICANT SHORT WAVE TROUGH EJECTS NORTHEAST ACROSS
THE PLAINS AND INTO THE DAKOTAS BY THURSDAY...INDUCING A SURFACE
LOW AND TRACKING IT INTO WESTERN ONTARIO BY THURSDAY NIGHT. BEST
RAIN CHANCES WILL REMAIN WEST OF THE GENERAL FORECAST AREA...BUT
THERE IS A SUGGESTION BY SEVERAL OF THE MEDIUM RANGE MODELS THAT
SOME ACTIVITY COULD DEVELOP IN THE WARM/MOIST ADVECTION REGIME OUT
AHEAD OF THE SYSTEM. SO AM FORCED TO KEEP SOME CHANCES IN THE
FORECAST. COLD FRONT TRAILING THIS SYSTEM WILL EDGE THROUGH THE
FORECAST AREA ON FRIDAY AND MAY LINGER AROUND THE AREA INTO
SATURDAY. MODELS SOMEWHAT AT ODDS AS TO HOW FAR SOUTH IT WILL
PUSH...WITH THE GFS MOST AGGRESSIVE WITH THE COOLER AND DRIER AIR.
IF THIS SCENARIO PANS OUT...SATURDAY COULD BE QUITE PLEASANT AND
DRY. ECMWF LESS OPTIMISTIC WITH THE BOUNDARY NEARBY. MESSY
PATTERN THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK AS THE CLOSED
CIRCULATION IN THE BASE OF THE LONG WAVE TROUGH FINALLY EJECTS
NORTHEAST THROUGH THE UPPER MIDWEST. MODELS DIFFER IN THE
DETAILS...AND THERE ARE NOW INDICATIONS THAT THIS GENERAL
CIRCULATION WILL LINGER AROUND THE REGION THROUGH NEARLY ALL OF
NEXT WEEK.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY MORNING)
ISSUED AT 1230 PM CDT TUE MAY 5 2015

A WARM FRONT WAS LOCATED ACROSS CENTRAL IA AND NORTHERN IL AT MID=-
DAY...ROUGHLY NEAR I-80. NORTH OF THIS FRONT WAS A BROAD BAND OF
MVFR/IFR CIGS/VSBYS. A BAND OF LIGHT RAIN WAS EXITING THE TAF SITES
EARLY THIS AFTERNOON...BUT THIS RAIN GENERALLY HAVING LITTLE IMPACT
ON VSBYS. SFC-850MB FLOW WAS EAST TO SOUTHEAST ACROSS THE AREA. THIS
FLOW WILL BECOME MORE SOUTHERLY LATE THIS AFTERNOON/TONIGHT...
LIFTING THE FRONT NORTHWARD ACROSS THE AREA...AND SPREADING THE
LOWER CIGS/VSBYS ACROSS THE TAF SITES THIS AFTERNOON/TONIGHT.
TRENDED KRST/KLSE TAFS TOWARD MORE MVFR/IFR CIGS THIS AFTERNOON AND
ESPECIALLY TONIGHT AS THE FRONT LIFTS TOWARD/INTO THE AREA. ADDED
MVFR/IFR VSBYS IN BR LATER TONIGHT INTO EARLY WED MORNING AS WELL.
FRONT IS LOOKING TO LIFT NORTH OF I-90 BY 13Z WED...WITH GENERALLY
VFR CONDITIONS FOR WED MORNING ONCE THE FRONT LIFTS NORTH OF THE TAF
SITES. MAY BE SOME SCT -SHRA AROUND LATER TONIGHT/WED MORNING
WARRANTING A VCSH...BUT PROBABILITIES AND CONFIDENCE IN ANY OF THE
SHRA DETAILS AFTER 06Z ON THE LOWER SIDE AND LEFT THESE MENTIONS OUT
OF THE TAFS FOR NOW. WINDS MAINLY SOUTHEAST AROUND 10KTS THRU THE
PERIOD.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&

$$

SHORT TERM...MW
LONG TERM....MW
AVIATION.....RRS



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