Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 021803
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
103 PM CDT SAT MAY 2 2015

.SHORT TERM...(TODAY THROUGH MONDAY)
ISSUED AT 314 AM CDT SAT MAY 2 2015

WATER VAPOR SATELLITE IMAGERY THIS MORNING MAKES SOME SUGGESTION
OF A WEAK SHORT WAVE TROUGH OVER EASTERN WYOMING. THIS IS THE
FEATURE THE 02.00Z MODELS INDICATE WILL PASS ACROSS IOWA THIS
AFTERNOON AND EVENING. THIS WAVE WILL ONLY BE ABLE TO PRODUCE SOME
VERY WEAK PV ADVECTION IN THE 500-300 MB LAYER. HOWEVER...AS IT
COMES ACROSS THE 02.00Z NAM SUGGEST THE LOW LEVEL RETURN FLOW WILL
INCREASE DURING THE AFTERNOON AHEAD OF IT WITH SOME WEAK MOISTURE
TRANSPORT INTO THE AREA AT 850 MB FOR LATE THIS AFTERNOON INTO THE
EVENING. SOME ISENTROPIC UP GLIDE ON THE ORDER OF 1 TO 2 UBAR/S
WILL DEVELOP AS WELL DURING THE LATE AFTERNOON AND THEN INCREASE
TO 2 TO 4 UBAR/S OVERNIGHT ON THE 300K SURFACE. THE MOISTURE
TRANSPORT ON THIS LAYER COULD ALSO INCREASE OVERNIGHT INTO THE
AREA. THESE FEATURES COULD BE ENOUGH TO GENERATE A FEW SHOWERS AND
STORMS SIMILAR TO WHAT THE NAM AND 02.00Z HI-RES NMM SHOW. WILL
CONTINUE TO HAVE SOME RAIN CHANCES UP TO ABOUT 20 PERCENT FOR MOST
OF THE AREA TONIGHT...BUT IF THE HI-RES MODELS ARE CORRECT...THE
ACTIVITY COULD BE CONFINED TO THE IOWA/MINNESOTA BORDER AND SOUTH.

THE FOCUS THEN TURNS TO THE SYSTEM COMING IN FOR SUNDAY AND SUNDAY
NIGHT. THE SHORT WAVE TROUGH CURRENTLY OVER SOUTHERN BRITISH
COLUMBIA WILL AMPLIFY AS IT MOVES EAST BUT FOR THE MOST PART
REMAINS SOMEWHAT POSITIVELY TILTED AS IT MOVES ACROSS SOUTHERN
CANADA AND THE EXTREME NORTHERN PORTIONS OF THE U.S. THE MODELS
HAVE ALSO BEEN CONSISTENT IN TRYING TO SHOW SOME SECONDARY SHORT
WAVE TROUGHS COMING ACROSS THE UPPER MIDWEST AHEAD OF THE MAIN
SHORT WAVE TROUGH...WHETHER THESE BREAK OFF FROM THE MAIN TROUGH
OR ARE GENERATED BY CONVECTION...IS DIFFERENT FROM MODEL TO MODEL.
NONE OF THE MODELS REALLY GENERATE ANY PV ADVECTION OVER THE AREA
WITH THE 02.00Z ECMWF THE FARTHEST SOUTH BRINGING SOME WEAK PV
ADVECTION ACROSS NORTHERN WISCONSIN SUNDAY NIGHT. IF THIS IS
TRUE...IS WILL LEAVE THE LOW LEVEL FORCING ALONG THE FRONT AS THE
MAIN DRIVER FOR THE DEVELOPMENT OF THE CONVECTION. THE
FRONTOGENESIS ALONG THE FRONT LOOKS TO MAINLY BE IN THE 1000-700
MB LAYER AND SHOULD BE WEAK TO MODERATE IN STRENGTH. THE
ISENTROPIC UP GLIDE ON THE 305K SURFACE LOOKS LIKE IT SHOULD BE IN
THE 3-5 UBAR/S RANGE. MOST OF THE MOISTURE TRANSPORT STILL LOOKS
TO BE THROUGH THE AREA TERMINATING NORTH OF THE AREA WITH THE MAIN
SHORT WAVE TROUGH. WITH THESE FORCING SIGNALS...HAVE SOME CONCERNS
ABOUT HOW MUCH CONVECTION WILL ACTUALLY FORM ALONG THE FRONT.
GRANTED...IT IS RIGHT AT THE END OF THE HI-RES MODEL RUNS...BUT
THE ARW AND NMM BOTH ONLY SHOW A BROKEN AND NARROW LINE OF
CONVECTION ALONG THE FRONT BY 00Z MONDAY. THESE MODELS ALSO SHOW
THIS ONLY GETTING INTO THE FAR NORTHWESTERN SECTIONS OF THE
FORECAST AREA LATE SUNDAY AFTERNOON. THE STORMS THAT DO DEVELOP
STILL LOOK TO HAVE SOME POTENTIAL TO PRODUCE SEVERE WEATHER WITH
DAMAGING WINDS AND LARGE HAIL THE MAIN THREATS. AN AXIS OF 1500
J/KG OF ML CAPE SHOULD BE IN PLACE AHEAD OF THE FRONT FOR SUNDAY
AFTERNOON INTO THE EARLY EVENING. THE BEST SHEAR WILL BE CONFINED
TO THE 0-3 KM LAYER BUT MAY BE ONLY ABOUT 25 KNOTS AHEAD OF THE
FRONT WITH THE NAM AND GFS SHOWING THE BEST SHEAR REMAINING BEHIND
THE FRONT.

THE FRONT AND CONVECTION SHOULD MOVE THROUGH PRETTY QUICKLY SUNDAY
NIGHT WITH THE ACTIVITY CONFINED TO THE SOUTHEAST HALF OR SO OF
THE AREA AFTER 06Z MONDAY. THE FRONT THEN LOOKS TO BECOME
STATIONARY SOUTH OF THE AREA FOR MONDAY. THERE WILL BE ACTIVITY
ALONG AND NORTH OF THE FRONT AND THE CONCERN IS JUST HOW FAR NORTH
THIS WILL EXTENT. THE NAM AND ECMWF WOULD SUGGEST IT WOULD STAY
SOUTH OF THE AREA WHILE THE GFS BRINGS IT INTO THE SOUTHERN
SECTIONS OF THE AREA. IF THE FRONT DOES MAKE IT INTO NORTHERN
MISSOURI BEFORE STALLING OUT...WHICH IS THE MODEL CONSENSUS...FEEL
THE NAM AND ECMWF HAVE A BETTER CHANCE OF BEING CORRECT AND WILL
ONLY SHOW A SMALL CHANCE ACROSS THE SOUTH TO HONOR THE GFS.

.LONG TERM...(MONDAY NIGHT THROUGH FRIDAY)
ISSUED AT 314 AM CDT SAT MAY 2 2015

ONCE THE SHORT WAVE TROUGH FROM SUNDAY NIGHT MOVES PAST THE
AREA...THE FLOW IS STILL EXPECTED TO BECOME SOUTHWEST WITH A LARGE
UPPER LEVEL LOW OFF THE PACIFIC NORTHWEST COAST AND ANOTHER LOW
OVER THE SOUTHWEST. AS THE LOW OFF THE NORTHWEST COAST MOVES
INLAND AND THE UPPER LEVEL RIDGE AMPLIFIES OVER THE SOUTHEAST
CONUS...IT WILL FORCE THE SOUTHWEST SYSTEM TO MOVE NORTHEAST
TOWARD THE REGION THROUGH THE PERIOD. IT DOES NOT LOOK LIKE THIS
WILL COME THROUGH AS ONE PIECE OF ENERGY BUT ACTUALLY GET STRUNG
OUT AND ACT AS SEVERAL SHORT WAVE TROUGHS. THE FRONT THAT STALLS
SOUTH OF THE AREA WILL GET DRAWN BACK NORTH TUESDAY AND TUESDAY
NIGHT AND SHOULD BE NORTH OF THE AREA BY WEDNESDAY NIGHT. A COLD
FRONT WILL THEN START TO APPROACH FROM THE WEST THURSDAY AND MOVE
INTO THE AREA FRIDAY. WITH THIS PATTERN...WILL HAVE TO CARRY RAIN
CHANCES FOR EACH PERIOD BUT DOUBTFUL THERE WILL ACTUALLY BE THAT
MUCH RAIN AROUND. COULD SEE SOME POTENTIAL FOR A DRY PERIOD OR TWO
ONCE THE FRONT MOVES NORTH OF THE AREA...BUT CONFIDENCE IS NOT
HIGH ENOUGH TO TRY AND INCORPORATE THAT INTO THE FORECAST AT THIS
TIME.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 102 PM CDT SAT MAY 2 2015

A SHORT WAVE TROUGH MOVING THROUGH THE MID MISSISSIPPI RIVER
VALLEY IS PRODUCING A 7-12K BROKEN DECK OF CLOUDS SOUTH OF
INTERSTATE 90. THE HRRR SUGGESTS THAT SCATTERED SHOWERS AND
THUNDERSTORMS WILL DEVELOP IN THIS AREA AROUND 03.20Z. MEANWHILE
THE RAP KEEPS THE AREA DRY. AT THIS TIME...LEANING MORE TOWARD THE
RAP. THESE CLOUDS WILL MOVE EAST OF THE AREA BY 03.06Z. BOTH TAF
SITES ARE ON THE NORTHERN EDGE...SO JUST MENTIONED A SCATTERED
7 TO 8K DECK. IN ADDITION TO THE CLOUDS...THE SOUTH WINDS WILL
GUST INTO THE 20 TO 25 KNOT RANGE THROUGH 03.01Z.

ON SUNDAY MORNING...A BROKEN 25K DECK OF CLOUDS WILL MOVE INTO THE
AREA AHEAD OF AN APPROACHING COLD FRONT. WITH DIURNAL MIXING...
THE WINDS WILL START TO GUST INTO THE 20 TO 25 KNOT RANGE AFTER
03.14Z.

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION...BOYNE



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