Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 011130
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
629 AM CDT FRI MAY 1 2015

.SHORT TERM...(TODAY THROUGH SATURDAY NIGHT)
ISSUED AT 329 AM CDT FRI MAY 1 2015

EARLY THIS MORNING...HIGH PRESSURE AT THE SURFACE WAS CENTERED
OVER SOUTHERN WISCONSIN AND NORTHERN ILLINOIS. A WEAK COLD FRONT
EXTENDED FROM NORTHWEST MINNESOTA INTO CENTRAL ILLINOIS. REGIONAL
RADARS SHOW A NARROW LINE OF SHOWERS ALONG THE FRONT. WATER VAPOR
SATELLITE SHOWS THAT THESE SHOWERS ARE BEING AIDED BY THE LIFT
AHEAD OF SHORT WAVE TROUGH EXTENDING FROM MANITOBA INTO THE
DAKOTAS. THE 01.00Z MODELS ARE IN GOOD AGREEMENT IN SHOWING THIS
SHORT WAVE TROUGH TO SPLIT INTO TWO PIECES WITH THE NORTHERN AND
STRONGER PIECE PASSING ACROSS SOUTHERN CANADA WHILE THE SOUTHERN
AND WEAKER PIECE MOVES ACROSS THE AREA. THIS SOUTHERN WAVE WILL BE
MOVING INTO WHAT REMAINS OF THE UPPER LEVEL RIDGE AND WILL BE
WEAKENING AS IT DOES. THIS IS EXPECTED TO RESULT IN JUST SOME WEAK
PV ADVECTION IN THE 500-300 MB LAYER DURING THE AFTERNOON. THE
SURFACE RIDGE IS EXPECTED TO REMAIN NEARLY STATIONARY TODAY AND
THE WEAK COLD FRONT SHOULD DISSIPATE AS IT TRIES TO MOVE INTO
THIS. WHAT FRONTOGENESIS THERE IS ALONG THE FRONT PRETTY MUCH
DISSIPATES BY THE TIME THE FRONT GETS INTO THE LOCAL AREA. SOME
WEAK ISENTROPIC UP GLIDE OF 1 UBAR/S MAY SURVIVE AND GET INTO THE
AREA THIS AFTERNOON...BUT THIS THEN LOOKS TO DISSIPATE THIS
EVENING AS WELL. PLAN TO CARRY SOME 20 PERCENT RAIN CHANCES INTO
THE AREA STARTING LATE THIS MORNING ACROSS THE WEST AND OVER MOST
OF THE AREA BY MID AFTERNOON BEFORE DIMINISHING THEM AS THE
FORCING WEAKENS INTO THE EARLY EVENING.

ANOTHER WEAK SHORT WAVE TROUGH WILL APPROACH SATURDAY WITH THE
MODELS TAKING THIS ACROSS IOWA SATURDAY NIGHT. THIS RESULTS IN
JUST SOME VERY WEAK PV ADVECTION IN THE 500-300 MB LAYER ACROSS
THE SOUTHERN SECTIONS OF THE FORECAST AREA. THERE WILL AGAIN BE
SOME WEAK ISENTROPIC UP GLIDE OF 1 TO 2 UBAR/S ON THE 295K SURFACE
WITH THE AREA ON THE EASTERN...CONVERGENT SIDE OF THE MOISTURE
TRANSPORT ON THIS LAYER. FORECAST SOUNDINGS ACROSS THE SOUTH SHOW
ANY SATURATION THAT OCCURS WOULD BE FROM 750 MB ON UP WITH MOST OF
THE VERTICAL MOTION BELOW THIS SATURATED LAYER. THE FORCING MAY BE
JUST ENOUGH TO GENERATE SOME ACTIVITY ACROSS THE SOUTH AND WILL
SHOW A 20 PERCENT CHANCE FOR THIS.

.LONG TERM...(SUNDAY THROUGH THURSDAY)
ISSUED AT 329 AM CDT FRI MAY 1 2015

THE PATTERN LOOKS TO BECOME MORE ACTIVE TO CLOSE THE WEEKEND AND
THROUGH MUCH OF NEXT WEEK. THE ZONAL FLOW OVER THE NORTHERN CONUS
WILL BRING A RATHER STRONG SHORT WAVE TROUGH ACROSS SOUTHERN
CANADA SUNDAY AND SUNDAY NIGHT. THE MODELS INDICATE A PIECE OF
ENERGY WILL BREAK OFF FROM THIS AND TRACK ACROSS THE AREA DURING
THE SAME TIME FRAME. THE BEST PV ADVECTION WILL REMAIN TO THE
NORTH WITH SOME WEAK PV ADVECTION COMING ACROSS WITH THE SOUTHERN
WAVE SUNDAY NIGHT. THIS SYSTEM IS EXPECTED TO BE STRONG ENOUGH TO
PUSH THE SURFACE RIDGE OUT OF THE REGION ALLOWING A COLD FRONT TO
MOVE THROUGH. THE 05.01Z GFS INDICATES SOME WEAK TO MODERATE
FRONTOGENESIS IN THE 1000-700 MB LAYER SHOULD ACCOMPANY THE
PASSAGE OF THE FRONT SUNDAY NIGHT. THE ISENTROPIC UP GLIDE ON THE
300K SURFACE AHEAD OF THE FRONT SHOULD BE ON THE ORDER OF 2 TO 4
UBAR/S. THERE SHOULD BE GOOD MOISTURE TRANSPORT BUT SOMEWHAT
TROUBLING THAT THIS LOOKS TO BE DRIVEN BY THE STRONGER SHORT WAVE
TROUGH TO THE NORTH. THIS MAY CAUSE THE MOISTURE TRANSPORT TO GO
THROUGH THE AREA WITH THE NOSE OF THIS FARTHER TO THE NORTH. THE
FORCING SHOULD BE ENOUGH TO SUPPORT A LINE OF CONVECTION ALONG THE
FRONT AND WILL HAVE UP TO 70 PERCENT CHANCES SUNDAY NIGHT. DECENT
CAPE SHOULD BUILD OVER THE AREA SUNDAY AFTERNOON WITH THE GFS
SHOWING AROUND 1500 J/KG OF ML CAPE WHILE THE NAM GETS THIS UP TO
ABOUT 2000 J/KG. THE GFS KEEPS THE BEST SHEAR BEHIND THE FRONT
WHILE THE NAM HAS 25 TO 30 KNOTS OF 0-3 KM SHEAR AHEAD OF THE
FRONT SUNDAY AFTERNOON INTO THE EVENING. IF THIS OCCURS...IT WOULD
BE ENOUGH TO SUPPORT SOME SEVERE STORMS WITH DAMAGING WINDS AND
LARGE HAIL THE MAIN THREATS.

THE MODELS ARE IN GOOD AGREEMENT THAT HIGH PRESSURE WILL BUILD IN
BEHIND THE FRONT...BUT THE QUESTION BECOMES HOW FAR SOUTH THE
FRONT WILL MAKE IT BEFORE STALLING OUT. THE GENERAL CONSENSUS
BETWEEN THE 01.00Z ECMWF AND GFS IS THAT FRONT SHOULD GET TO ABOUT
THE IOWA/MISSOURI BORDER. THIS COULD BE CLOSE ENOUGH TO ALLOW SOME
OVERRUNNING ACTIVITY INTO THE SOUTHERN PARTS OF THE AREA MONDAY
AND MONDAY NIGHT. THE FLOW WILL THEN START TO TURN MORE SOUTHWEST
WITH THE REMAINS OF THE FRONT COMING BACK NORTH TUESDAY AND
TUESDAY NIGHT. A SHORT WAVE TROUGH IS THEN EXPECTED TO COME OUT OF
THE SOUTHWEST AND LIFT ACROSS THE REGION WEDNESDAY THROUGH
THURSDAY. THERE WILL BE RAIN CHANCES JUST ABOUT EVERYDAY NEXT
WEEK...WITH CURRENT INDICATIONS THAT THE BEST CHANCE MAY COME
THURSDAY AS A COLD FRONT MOVES IN WITH THE PASSAGE OF THE SHORT
WAVE TROUGH.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z SATURDAY MORNING)
ISSUED AT 629 AM CDT FRI MAY 1 2015

WAVE MOVING THROUGH CENTRAL CANADA HAS WEAK COLD FRONT ASSOCIATED
WITH IT...THAT IS PRODUCING THIN BAND OF SHOWERS AND CEILINGS ACROSS
CENTRAL MINNESOTA AT DAYBREAK FRIDAY. WITH SUCH WEAK FORCING...NOT
ENOUGH LIFT TO OVERCOME DRY AIRMASS SO COMBINATION OF FACTORS IS
LEADING TO DISSIPATION OF AND EXTENT OF VFR RAIN SHOWERS AROUND.
STILL COULD GET INTO KRST BEFORE COMPLETELY DISSIPATING SO
INTRODUCED VCSH FOR NOW WITH MINIMAL IMPACT ON AVIATION.

AS THIS FEATURE WASHES OUT...SO WILL THE CLOUDS AS DEBRIS FIELD
SETTLES TO THE SOUTH AND EAST. COULD SEE CLEARING TREND TONIGHT BUT
IN ANY CASE...VFR CONDITIONS SHOULD PREVAIL.

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION...SHEA



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