Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 310512
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1115 PM CST FRI JAN 30 2015

.SHORT TERM...(THIS EVENING THROUGH SATURDAY)
ISSUED AT 340 PM CST FRI JAN 30 2015

AT 3 PM...A CANADIAN COLD FRONT WAS LOCATED OVER WESTERN ONTARIO
AND SOUTHERN MANITOBA. SOME LIGHT SNOW WAS LOCATED ALONG AND
AHEAD OF THIS FRONT. LIKE THE PAST COUPLE OF THE DAYS...THE BEST
FORCING AND MOISTURE TRANSPORT WILL REMAIN WELL NORTH OF THE
FORECAST AREA. AS A RESULT...KEPT THE FORECAST AREA DRY FOR
TONIGHT.

.LONG TERM...(SATURDAY NIGHT THROUGH FRIDAY)
ISSUED AT 340 PM CST FRI JAN 30 2015

SATURDAY NIGHT INTO SUNDAY CONTINUE TO BE THE MAIN CONCERN FOR
THIS TIME PERIOD. LIKE THE 30.00Z MODELS...THE 30.12Z MODELS
REMAIN MUCH FURTHER NORTH THAN THE LAST 2 DAYS. MUCH OF THE
DIFFERENCE HAS TO DEAL WITH HOW THE SOUTHERN STREAM SYSTEM...WHICH
WILL BE MOVING EAST THROUGH IOWA ON SATURDAY NIGHT AND THE MID
MISSISSIPPI RIVER VALLEY ON SUNDAY...AND A NORTHERN STREAM
SYSTEM...WHICH WILL BE MOVING SOUTHEAST FROM THE DAKOTAS INTO
SOUTHWEST IOWA...INTERACT. WITH THE MORE RECENT MODEL
RUNS...THERE WAS MORE PHASING WITH THE 2 SYSTEMS...THUS...A SHIFT
NORTHWARD AND AN INCREASE IN PRECIPITATION TOTALS. FOR EXAMPLE AT
LA CROSSE...THE NAM QPF WENT FROM 0.11 INCHES IN THE 30.00Z RUN TO
0.34 INCHES IN THE 30.12Z RUN. THE GFS CLIMBED FROM 0.22 INCHES
AT 30.00Z TO 0.25 INCHES AT 30.12Z. FINALLY DUE TO SEVERAL VERY
WET NMM MEMBERS...THE 30.15Z SREF HAS AN AVERAGE QPF OF
0.41 INCHES.

ONE CONCERN WITH THE NORTHERN SHIFT IS THE HOW WELL THE MODELS
ARE HANDLING THE PHASING OF THESE 2 WAVES. THE OLDER GFS HAD A
KNOWN BIAS OF PHASING THE NORTHERN AND SOUTHERN STREAM TOO MUCH.
AS A RESULT...WE HAVE NOTICED THAT IT WILL RAMP OF PRECIPITATION
BETWEEN 24 AND 48 HOURS AHEAD OF THE EVENT AND THEN LOWER THE
PRECIPITATION AMOUNTS AS IT TREATS THE 2 WAVES AS TWO DISTINCT
SYSTEMS CLOSER TO THE PRECIPITATION ONSET. WHETHER THIS WILL BE
THE SAME CASE WITH THE NEW GFS IS A BIT UNCERTAIN.

ANOTHER CONCERN WITH THIS SYSTEM IS THE NAM QPF WHICH HAS TENDED
TO BE TOO HIGH LATELY. THERE IS A GULF CONNECTION WITH THE
SOUTHERN WAVE. HOWEVER MUCH OF ITS MOISTURE TRANSPORT IS INTO THE
MID MISSISSIPPI RIVER AND OHIO RIVER VALLEYS. OVERALL...FELT THE
ECMWF QPF WAS A GOOD COMPROMISE...SO TRENDED THE PRECIPITATION
TOTALS TOWARD IT.

AS FAR AS SNOW TOTALS DURING THIS TIME FRAME...THE SOUNDINGS SHOW
THAT THE TEMPERATURE PROFILE IN THE DENDRITIC GROWTH ZONE WILL BE
LIMITED TO AROUND 50 MB ON SATURDAY NIGHT. THIS WILL HOLD THE SNOW
RATIOS TO AROUND 10 TO 1. AS THE COLDER AIR MOVES INTO THE REGION
ON SUNDAY...THE TEMPERATURES PROFILE IN THE DENDRITIC GROWTH ZONE
WILL INCREASE INTO THE 100 TO 200 MB RANGE. HOWEVER AT THE SAME
TIME...THE OMEGA WILL BE DECREASING AS THE RIGHT ENTRANCE REGION
OF THE 140 KNOT 250 MB JET MOVES EAST INTO EASTERN ONTARIO.
WHILE THE SNOW RATIOS WILL CLIMB TO AROUND 15 TO 1...THEY WILL BE
NOT AS HIGH AS THEY COULD POTENTIALLY BE IF THERE WAS A DEEPER
OMEGA CO-LOCATED WITH IT. WITH ALL THIS IN MIND...WE CURRENTLY
THINK THAT 3 TO 6 INCHES OF SNOW WILL FALL ACROSS NORTHEAST IOWA
AND SOUTHWEST WISCONSIN...AND SNOW AMOUNTS WILL DRAMATICALLY
DECREASE AS YOU HEAD NORTHWARD INTO INTERSTATE 90 AND 94
CORRIDORS. THIS WILL BE A RESULT OF DRIER AIR AND THE INTERACTION
WITH THE COLD CONVEYOR BELT. CONSIDERED ISSUING A WINTER WEATHER
ADVISORY FOR THIS SNOWFALL...BUT OPTED TO WAIT UNTIL THE NORTHERN
WAVE WAS BETTER SAMPLED BY THE NORTH AMERICAN RADIOSONDE NETWORK.

FROM MONDAY NIGHT INTO TUESDAY NIGHT...THERE WILL BE 2 ADDITIONAL
SYSTEMS MOVING THROUGH THE REGION. WHILE THE OPERATIONAL MODELS
ARE IN GOOD AGREEMENT. THE 12 GFS ENSEMBLE MEMBERS ARE STILL NOT
IN GOOD AGREEMENT. THE DIFFERENCES ARE A RESULT OF WHERE THE
BAROCLINIC ZONE IS LOCATED.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SATURDAY NIGHT)
ISSUED AT 1115 PM CST FRI JAN 30 2015

UPPER LEVEL SHORTWAVE PASSING NORTH OF THE REGION WILL SLIDE A SFC
FRONT ACROSS THE TAF SITES IN THE 12-15Z TIME FRAME SAT. EXPECTING
SOME MVFR/IFR CIGS WITH THE BOUNDARY...BUT HOW LOW/COVERAGE IS NOT
CERTAIN. A LOT OF SPREAD IN THE MODELS...BUT KRST IS MORE FAVORED
THAN KLSE. THAT SAID...CURRENT SATELLITE TRENDS FAVOR BRINGING MVFR
CIGS INTO KLSE...WHICH COULD THE LINGER AT BOTH TAF SITES THROUGH
THE REST OF THE DAY. SOME INDICATIONS THAT THE CIGS COULD BREAK OUT
INTO SCT AND/OR VFR CONDITIONS FOR THE AFTERNOON - BUT CONFIDENCE
SHAKY IN THIS STEAD.

SAT NIGHT...IF THE LOW CIGS BECOME SCT SAT AFTERNOON...THEY WILL
RETURN SAT NIGHT AS A LOW PRESSURE SYSTEM APPROACHES THE REGION.
AREAS OF SNOW WILL ALSO MOVE IN BY MID EVENING...PERSISTING THROUGH
SUN MORNING. LOOK FOR IFR/LIFR VSBYS AND CIGS AT TIMES...WITH
ACCUMULATIONS PERHAPS FROM 2 TO 4 INCHES. STILL SOME UNCERTAINTY TO
JUST HOW FAR NORTH THIS SNOW MAKER WILL ADVANCE - AND THUS
CONFIDENCE NOT HIGH IN AMOUNTS QUITE YET. INCREASING NORTHERLY WINDS
COULD BLOW SOME OF THAT SNOW AROUND AT KRST...AND SOME BLSN POSSIBLE
OVERNIGHT SAT/SUN MORNING DEPENDING ON HOW MUCH SNOW FALLS.

&&

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

$$

SHORT TERM...BOYNE
LONG TERM...BOYNE
AVIATION...RIECK


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