Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 311733
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1133 AM CST SAT JAN 31 2015

.UPDATE...
ISSUED AT 951 AM CST SAT JAN 31 2015

VERY TRICKY FORECAST FOR SNOWFALL AMOUNTS ACROSS OUR FORECAST
AREA TONIGHT AND SUNDAY. END RESULT IS THAT QPF... SNOWFALL AND
HAZARDS WILL ALL REMAIN THE SAME FOR NOW.

OUR 2 MAIN AREAS FOR POSSIBLE HAZARD ADJUSTMENTS THROUGH THE DAY
AS WE ANALYZE NEW DATA INCLUDE...

1. ADVISORY EXPANSION NORTHWARD INTO MOWER EASTWARD TO VERNON

2. WARNING ACROSS SOUTHERN FAYETTE...CLAYTON AND GRANT.

NOW WHAT WE ARE BATTLING / FORECAST CONCERNS...

1. 31.14Z RAP...31.09Z SREF...31.00Z ECMWF...31.12Z NAM/REGIONAL
CANADIAN ARE ALL PUTTING OUT MORE QPF FOR TONIGHT THAN WHAT WE
HAVE FORECAST. HOWEVER...THE ECMWF IS LOWER THAN ALL THE
AFOREMENTIONED MODELS ALONG/NORTH OF I-90.

2. LOOKING AT FRONTOGENESIS FIELDS...MOST OF OUR CWA TONIGHT INTO
SUNDAY IS DOMINATED BY FRONTOGENESIS BETWEEN 700-450MB...AND MUCH
OF THIS REGION IS ABOVE THE DENDRITIC GROWTH ZONE. AS YOU GO SOUTH
TOWARDS DUBUQUE...ANOTHER FRONTOGENESIS ZONE APPEARS BETWEEN
925-850MB. THIS LAYER BECOMES FAIRLY STRONG LATE THIS EVENING AND
PERSIST THROUGH THE NIGHT...REFLECTING SOME OF THE HIGHER MODEL
QPF. FORCING DOES DIMINISH THROUGH THE DAY SUNDAY.

3. A NORTHEASTERLY FLOW DEVELOPING THIS AFTERNOON AND PERSISTING
THROUGH SUNDAY WILL HELP ADVECT DRIER AIR PRESENT NORTH OF
WISCONSIN HIGHWAY 29 SOUTHWARD THROUGH THE FORECAST AREA. THE
NORTHERN SIDE OF THE FRONTOGENESIS CIRCULATION WILL ALSO HELP PULL
THAT DRY AIR SOUTHWARD. EXPECT THE DRIER AIR TO CUT INTO THE QPF
AS SNOW FALLING OUT OF THE 700-450MB LAYER GETS ABSORBED.
THEREFORE...A SHARP GRADIENT IN QPF WILL EXIST ACROSS OUR AREA.

4. WARM SECTOR CONVECTION WELL TO OUR SOUTH...AS WELL AS THE
DOUBLE LAYER OF FRONTOGENESIS JUST SOUTH OF OUR FORECAST
AREA...MAY INTERCEPT A LOT OF THE MOISTURE HEADING TOWARDS US.
THESE TWO FACTORS COULD SUGGEST THE MODEL QPF IS TOO HIGH IN OUR
AREA.

5. SNOW RATIOS. WE NEVER SEE ANYWHERE IN OUR FORECAST AREA A GOOD
INTERSECTION OF STRONG LIFT WITHIN THE DENDRITIC GROWTH ZONE. AT
MOST WE MIGHT HAVE ABOUT 75MB OF EFFECTIVE DENDRITIC GROWTH.
THEREFORE...ANTICIPATING RATIOS WILL HAVE A HARD TIME EXCEEDING 15
TO 1...WHICH IN TURN KEEPS SNOW AMOUNTS DOWN.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 345 AM CST SAT JAN 31 2015

CHALLENGES ABOUND TONIGHT THROUGH SUNDAY WITH IMPENDING WINTER
STORM...ESPECIALLY AS IT RELATES TO SNOW AMOUNTS AND THE EXPECTED
SHARP GRADIENT FROM NORTH TO SOUTH. OVERALL...31.00Z MODEL SUITE IS
IN GOOD AGREEMENT EXCEPT FOR THE AMOUNT OF AVAILABLE MOISTURE AND
WILL ADDRESS THAT BELOW. OTHERWISE...WILL USE A BLENDED MODEL
APPROACH.

COLD FRONT ACROSS ND INTO NORTHWEST MN EARLY THIS MORNING WILL DROP
SOUTH ACROSS THE UPPER MS RIVER VALLEY LATER TODAY. TEMPERATURES TO
RISE INTO THE MID 20S TO MID 30S BEFORE BEGINNING TO DROP MID-
AFTERNOON WITH INCREASING NORTH-NORTHEAST WIND. IN ADDITION...EXPECT
INCREASING CLOUDS THROUGH THE DAY.

FOR TONIGHT...850 HPA LOW EJECTS NORTHEASTWARD FROM FOUR CORNERS
TROUGH AND INTO SOUTHEAST NE/NORTHEAST KS BY 01.06Z TONIGHT. STRONG
925 TO 850 HPA FRONTOGENESIS DEVELOPS ON NORTH SIDE OF THIS CYCLONE
MAINLY ACROSS NORTHERN IA. UPPER LEVEL (500 TO 300 HPA) Q-VECTOR
CONVERGENCE INDICATIVE OF LARGE SCALE LIFT A BIT DELAYED WITH SLOWER
WAVE DROPPING SOUTH FROM CANADA...ALTHOUGH RIGHT ENTRANCE REGION
FROM DEPARTING 300 HPA JET STREAK WILL STILL PROVIDE SOME ADDITIONAL
LIFT. EXPECT AREA OF LIGHT SNOW TO LIFT FROM SOUTHWEST TO NORTHEAST
THIS EVENING ACROSS NORTHEAST IA AND SOUTHWEST WI AS MOISTURE
INCREASES WITH PWATS FROM 0.3 TO 0.4 INCHES. AS WEST TO EAST
ORIENTED LOW-LEVEL FRONTOGENESIS SLIDES EASTWARD INTO IL...EXPECT
PROLONGED PERIOD OF LIGHT SNOW TO CONTINUE OVERNIGHT AND INTO
SUNDAY. SNOW AMOUNTS OVERNIGHT ALONG AND SOUTH OF I-90 FROM 1 TO 5
INCHES POSSIBLE WITH A VERY TIGHT GRADIENT LIKELY...ESPECIALLY
ALONG NORTHERN FLANK OF PRECIPITATION SHIELD GIVEN WEAKER OVERALL
FORCING AND DRIER AIR PUSHING SOUTH WITH A 1035 HPA SURFACE HIGH.

.LONG TERM...(SUNDAY THROUGH FRIDAY)
ISSUED AT 345 AM CST SAT JAN 31 2015

LIGHT SNOW TO CONTINUE THROUGH MUCH OF SUNDAY...ESPECIALLY ACROSS
NORTHEAST IA INTO SOUTHWEST WI. FAIRLY DEEP DENDRITIC GROWTH LAYER
DEVELOPS BY SUNDAY MORNING BUT BY THIS TIME...GREATEST LIFT WHICH IS
ONLY 2 TO 4 UBAR/SECOND...IS GENERALLY ABOVE IT IN THE 600 TO 300
HPA LAYER. SURFACE GRADIENT WILL ALSO BE QUITE STRONG SUNDAY MORNING
WITH ROUGHLY A SIX HOUR PERIOD WITH 30 TO 35 KTS AT THE TOP OF THE
BOUNDARY LAYER. THIS SHOULD TRANSLATE TO NORTH-NORTHEASTERLY WINDS
WITH OCCASIONAL GUSTS UP TO 35 MPH CREATING AREAS OF BLOWING AND
DRIFTING SNOW...ESPECIALLY ACROSS THE WIND PRONE AREAS OF SOUTHEAST
MN INTO NORTHEAST IA. AN ADDITIONAL 1 TO 3 INCHES OF SNOW ALONG
AND SOUTH OF I-90 IS POSSIBLE ON SUNDAY.

OVERALL...STILL SOMEWHAT UNCERTAIN WITH SNOW TOTALS...ESPECIALLY
NORTH OF THE MN/IA BORDER. OPERATIONAL MODELS GENERALLY PAINT A
BROAD SWATH OF 0.25 TO 0.3 LIQUID INCHES AS FAR NORTH AS THE I-90
CORRIDOR AND AS MUCH AS 0.6 LIQUID INCHES ACROSS THE SOUTHERNMOST
FORECAST AREA. WITH THE STRONGEST LOW-LEVEL MOISTURE TRANSPORT
EXPECTED ACROSS SOUTHERN MO INTO CENTRAL IL AND MODELS OVER
PREDICTION OF LOW-LEVEL MOISTURE THIS WINTER SEASON...KNOCKED OFF A
SOLID TENTH OF AN INCH FROM THESE VALUES. WITH SNOW RATIOS
INCREASING THROUGH THE EVENT FROM NEAR 10:1 TO 20:1 WITH COOLING
COLUMN...THIS GENERALLY TRANSLATES TO SNOW AMOUNTS FROM 1 TO 4
INCHES FROM THE MN/IA BORDER NORTH TO THE I-90 CORRIDOR...3 TO 6
INCHES ACROSS EXTREME NORTHEAST IA INTO ADJACENT PORTIONS OF
SOUTHWEST WI AND 5 TO 8 INCHES ACROSS THE SOUTHERNMOST COUNTIES.

NORMALLY...6 PLUS INCHES IN 12 HOURS IS CONSIDERED WARNING
CRITERIA...BUT CONSIDERING IT WILL TAKE 24 TO PERHAPS EVEN 36
HOURS FOR SNOW TOTALS TO REACH THESE HIGHER END AMOUNTS...WILL
COVER NORTHEAST IOWA INTO SOUTHWEST WI WITH A WINTER WEATHER
ADVISORY FROM 01Z TONIGHT THROUGH 03Z SUNDAY NIGHT. AS MENTIONED
ABOVE...SNOW TOTALS NORTH OF THE MN/IA BORDER NOT AS CERTAIN...SO
HELD OFF ON AN ADVISORY FOR NOW. ALL SAID...THE POSSIBILITY EXITS
FOR A WARNING ACROSS THE FAR SOUTH AND NORTHERN EXTENSION OF THE
ADVISORY INTO THE I-90 CORRIDOR. WILL NEED TO WATCH THIS CAREFULLY
AND ADJUST AS NECESSARY. UNTREATED ROADS ACROSS NORTHWEST IA INTO
SOUTHWEST WI FOR SURE TO BECOME SLICK...CREATING DIFFICULT TRAVEL
AT TIMES THROUGH SUNDAY.

LINGERING SNOW TO MOVE OUT OF THE REGION SUNDAY NIGHT. ADDITIONAL
LIGHT SNOW CHANCES POSSIBLE MONDAY NIGHT THROUGH MID-WEEK WITH A
SERIES OF WEAK SHORT-WAVES IN NORTHWEST FLOW ALOFT. THE LATTER HALF
OF THE WEEK LOOKS DRY. BIGGER STORY WILL BE MUCH COOLER TEMPERATURES
WITH DAILY HIGHS FROM THE MID TEENS TO MID 20S AND NIGHTTIME LOWS BY
THURSDAY/FRIDAY MORNINGS DROPPING BELOW ZER0.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1133 AM CST SAT JAN 31 2015

LOW STRATUS PERSISTS OVER THE TAF SITES...MAINLY OF THE MVFR
VARIETY...THOUGH RST JUST CLIMBED TO MVFR. A NORTHEASTERLY FLOW IS
SETTING UP...WHICH IS ONLY FORECAST TO INCREASE OVER THE NEXT 24
HOURS AS A STORM SYSTEM STRENGTHS TO THE SOUTH. GUSTS AT THE MORE
EXPOSED RST LOCATION COULD TOP OUT NEAR 25 KT SUNDAY MORNING. EXPECT
SNOW ASSOCIATED WITH THE SYSTEM TO SPREAD INTO THE TAF SITES
BETWEEN 04-05Z...WITH CEILINGS HOLDING STEADY OR SLOWLY FALLING
UNTIL THEN. FURTHER LOWERING SHOULD OCCUR TO BOTH CEILINGS AND
VISIBILITIES ONCE THE SNOW STARTS...WITH RST LIKELY DROPPING TO
IFR/LIFR BY 06Z. ANTICIPATING THE VISIBILITY AT LSE TO NOT DROP TO
LIFR BECAUSE OF THE SNOW NOT BEING HEAVY ENOUGH AND LESS WIND TO
BLOW IT AROUND. BOTH SITES SHOULD SEE IMPROVEMENTS IN CEILINGS AND
VISIBILITIES ON SUNDAY...DUE TO THE SYSTEM SLOWLY MOVING AWAY FROM
THE AREA AND DRIER AIR BEING BROUGHT IN FROM THE NORTHERN GREAT
LAKES.

TOTAL SNOW ACCUMULATIONS OF 3 TO 4 INCHES ARE POSSIBLE AT BOTH RST
AND LSE...MUCH OF WHICH FALLS BETWEEN MIDNIGHT AND NOON SUNDAY.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...WINTER WEATHER ADVISORY FROM 7 PM THIS EVENING TO 9 PM CST
     SUNDAY FOR WIZ054-055-061.

MN...NONE.
IA...WINTER WEATHER ADVISORY FROM 7 PM THIS EVENING TO 9 PM CST
     SUNDAY FOR IAZ008>011-018-019-029-030.

&&

$$

UPDATE...AJ
SHORT TERM...ROGERS
LONG TERM...ROGERS
AVIATION...AJ


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