Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 281138
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
538 AM CST FRI NOV 28 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 221 AM CST FRI NOV 28 2014

CURRENTLY AS OF 08Z...WATER VAPOR LOOP AND RAP 500MB HEIGHT ANALYSIS
SHOWED DEEP...POTENT TROUGHS OVER BRITISH COLUMBIA AND OFF THE COAST
OF CALIFORNIA...RIDGING OVER THE WESTERN U.S. AND TROUGHING OVER THE
EASTERN U.S.. A SHORTWAVE WAS EVIDENT RIDING THROUGH THE NORTHWEST
FLOW FROM NORTHWEST MN INTO WESTERN WI. DPVA FROM THIS SHORTWAVE
COMBINED WITH WARM ADVECTION AND PRECIPITABLE WATER NEAR 0.5 INCHES
HAS HELPED SUPPORT AN AREA OF SNOW FROM CENTRAL MN INTO THE FORECAST
AREA. THE AFOREMENTIONED WARM ADVECTION WAS NICELY EVIDENT AT 850MB
FROM 00Z RAOBS WITH READINGS IN THE 8-12C RANGE IN WESTERN SOUTH
DAKOTA AND NEBRASKA...COMPARED TO -10 TO -14C OVER THE FORECAST
AREA. THIS TIGHT TEMPERATURE GRADIENT WAS ADVECTING EASTWARD BY A 30-
40 KT SOUTHWEST WIND PER VWP/PROFILER DATA. INTERESTINGLY TOO...
SURFACE DEWPOINTS OVER WESTERN SOUTH DAKOTA AND NEBRASKA WERE IN THE
20S...ABOUT 10F WARMER THAN CURRENT TEMPERATURES OVER THE FORECAST
AREA.

GOOD MODEL AGREEMENT EXISTS ON THE 500MB FLOW PATTERN THROUGH THE
SHORT TERM PERIOD...SHOWING THE WESTERN U.S. RIDGE FLATTENING OUT
AND GETTING PUSHED EASTWARD BY THE POTENT BRITISH COLUMBIA TROUGH
DROPPING INTO THE PACIFIC NORTHWEST. THIS PATTERN EVOLUTION RESULTS
IN CONTINUOUS WARM AND MOIST AIR ADVECTION FROM SOUTH DAKOTA AND
NEBRASKA TOWARDS THE FORECAST AREA.

REGARDING THE CURRENT SNOW...REPRESENTED WELL BY 800-700MB
FRONTOGENESIS...ANTICIPATING THE EASTWARD PUSH WITH THE UPPER FLOW
WILL HELP KEEP THE SNOWBAND MOVING EASTWARD THROUGH THE DAY. THERE
COULD BE A SECOND...CALL IT SURGE...OF SNOW AND FRONTOGENESIS
DROPPING THROUGH NORTHERN WI LATE THIS MORNING INTO THIS AFTERNOON
AS SUGGESTED BY HIGH RESOLUTION MODEL GUIDANCE. THEREFORE...HAVE
KEPT THE WINTER WEATHER ADVISORY IN TACT FOR TAYLOR COUNTY. ALSO
LATE THIS MORNING THROUGH THE AFTERNOON...WE MAY HAVE TO CONTEND
WITH ANOTHER BAND OF PRECIPITATION. THIS BAND CAN CURRENTLY BE SEEN
COMING INTO THE WESTERN DAKOTAS ASSOCIATED WITH A SHORTWAVE JUST
WEST OF IT...WHICH THE 28.00Z GFS/NAM/HIRES-ARW/NMM AND LATEST
RAP/HRRR WANT TO TRAVERSE IT ACROSS LOCATIONS SOUTHWEST OF I-94.
THERE COULD BE MID-LEVEL SATURATION ISSUES IN THE WAKE OF THE
CURRENT SNOW...BUT MODELS DO WANT TO SATURATE THINGS BACK UP.
THEREFORE...HAVE ADDED SOME PRECIPITATION CHANCES. TRACK WISE...
REFER THE MORE NORTHERN RAP/HRRR OVER THE FARTHER SOUTH NAM/HIRES-
ARW/NMM AS THE FARTHER SOUTH MODELS ARE WAY TOO INTENSE AND TOO FAR
SOUTH COMPARED TO CURRENT RADAR IMAGERY. PRECIPITATION TYPE BECOMES
AN ISSUE WITH THIS WAVE AS WELL WITH WARMER AIR COMING IN ALOFT.
COMPARING MODEL DERIVED REFLECTIVITY OUTPUT WITH THE MAXIMUM
TEMPERATURE ALOFT...THE SOUTHWESTERN EDGE MAY POSE A MIX WITH SLEET
OR EVEN FREEZING RAIN CONCERN. SINCE THERE IS VERY LOW CONFIDENCE ON
ANY SLEET/FREEZING RAIN OCCURRING...HAVE NOT ISSUED ANY WINTER
WEATHER ADVISORY FOR THIS WAVE OF PRECIPITATION.

AS WE HEAD INTO TONIGHT...CONCERN SHIFTS TO THE CONTINUOUS ADVECTION
OF LOW LEVEL MOISTURE. SURFACE DEWPOINTS LATE THIS AFTERNOON COULD
BE AS HIGH AS THE MID TO UPPER 30S ALONG I-29 ON THE WESTERN EDGE OF
THE SNOWPACK. AS THIS HIGHER MOISTURE TRIES TO ADVECT ACROSS THE
SNOWPACK INTO THE FORECAST AREA...WHICH MAY ALREADY HAVE DECENT LOW
LEVEL SATURATION FROM THE WAVES OF PRECIPITATION MOVING THROUGH
TODAY...ANTICIPATING VERY LOW STRATUS AND FOG TO DEVELOP. WARMER AIR
MOVING IN ALOFT NOTED BY 850MB TEMPS CLIMBING TO 6-10C SHOULD ONLY
FURTHER STRENGTHEN AN INVERSION...AIDING IN TRAPPING THE LOW LEVEL
MOISTURE. HIGHER TERRAIN LOCATIONS SHOULD BE MOST SUSCEPTIBLE TO
FOG. LATER SHIFTS MAY NEED TO CONSIDER A DENSE FOG ADVISORY...
PARTICULARLY ALONG AND SOUTH OF I-90. WITH THE STRATUS IN PLACE
AND IN A GENERAL WARM ADVECTION REGIME...ANTICIPATING VERY LITTLE
FALL IN TEMPERATURES TONIGHT FROM AFTERNOON HIGHS.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 221 AM CST FRI NOV 28 2014

FOCUS OF ATTENTION FOR THE FIRST PART OF THE LONG TERM PERIOD IS
SQUARELY ON THE POTENT TROUGH OVER THE PACIFIC NORTHWEST AT 12Z
SATURDAY. MODELS ARE IN GOOD AGREEMENT MARCHING THIS FEATURE
EASTWARD THROUGH THE WEEKEND WITH ITS AXIS OVER THE FORECAST AREA BY
12Z MONDAY. PRIOR TO ITS ARRIVAL...925MB TEMPS ARE PROGGED TO
CONTINUING WARMING THROUGH THE DAY SATURDAY...CLIMBING TO PERHAPS AS
MUCH AS 4-9C. DESPITE STRATUS AND MORNING FOG...THESE READINGS
SHOULD ALLOW FOR HIGHS IN THE UPPER 30S TO MID 40S...WITH EVEN A
SMALL CHANCE OF HITTING 50 IN VALLEY AREAS AS SUGGESTED BY THE
REGIONAL CANADIAN. A SERIES OF TWO COLD FRONTS ARE THEN PROGGED TO
MOVE THROUGH THE AREA DURING THE OVERNIGHT HOURS OF SATURDAY NIGHT
INTO SUNDAY AS THE TROUGH APPROACHES...RESULTING IN A FALLING
TEMPERATURE TREND. 925MB TEMPS PLUMMET TO -14C SOUTH TO -20C NORTH
BY 12Z MONDAY. A 25-35KT NORTHWEST WIND AT 925MB SUNDAY INTO SUNDAY
NIGHT ALSO ADDS IN A WIND CHILL FACTOR. LOWS SUNDAY NIGHT LOOK TO
DROP INTO THE SINGLE DIGITS ABOVE ZERO...A NEAR 40 DEGREE
TEMPERATURE DROP FROM SATURDAYS HIGHS. MAY HAVE TO WATCH FOR A
POSSIBLE WIND CHILL ADVISORY MONDAY MORNING. PRECIPITATION
CHANCES LOOK VERY LOW THROUGH THE WEEKEND DESPITE THE APPROACH OF
THE POTENT TROUGH...MOSTLY DUE TO A LACK OF MOISTURE.

MONDAY AND MONDAY NIGHT FEATURE CHILLY CONDITIONS AS AN ARCTIC HIGH
PRESSURE AREA BUILDS INTO THE REGION FROM THE DAKOTAS. DESPITE
ANOTHER SUNNY DAY LIKE YESTERDAY...HIGHS ON MONDAY SHOULD STRUGGLE
TO CLIMB INTO THE TEENS. SOME PLACES IN SOUTHEAST MN AND NORTH
CENTRAL WI MAY NOT EVEN GET TO 10 ABOVE. THIS SETS THE STAGE FOR
TEMPERATURES TO TANK MONDAY EVENING WITH LIGHT WINDS. HOWEVER...THE
HIGH DOES MOVE OFF TO THE EAST QUICKLY AFTER MIDNIGHT MONDAY
NIGHT...RESULTING IN WARM ADVECTION ACROSS THE FORECAST AREA AND
RISING TEMPERATURES. SO...THE BIG QUESTION IS HOW LOW CAN
TEMPERATURES DROP PRIOR. THE EC AND CANADIAN SHOW POTENTIAL FOR 10S
BELOW ZERO IN SOUTHEAST MN AND NORTH CENTRAL WI...WHEREAS MEX
GUIDANCE IS MORE IN THE SINGLE DIGITS ABOVE AND BELOW ZERO. SPLIT
THE DIFFERENCE FOR NOW FOR LOWS.

FOR THE REST OF NEXT WEEK...ANOTHER TROUGH IS PROGGED TO DROP DOWN
INTO THE REGION FOR TUESDAY INTO WEDNESDAY. MUCH OF THE COLD AIR
WITH THIS TROUGH...AS WELL AS POTENTIAL FOR PRECIPITATION...STAYS TO
THE NORTH OF THE FORECAST AREA. IN FACT...THERE SHOULD BE A GOOD
REBOUND IN TEMPERATURES FOR TUESDAY AS 925MB TEMPS CLIMB TO -4 TO -
6C. WARM ADVECTION RETURNS FOR WEDNESDAY NIGHT INTO THURSDAY AS THE
UPPER FLOW TURNS ZONAL AGAIN...ALLOWING FOR ANOTHER REBOUND FOR
TEMPERATURES. HOWEVER...THERE ARE SUGGESTIONS IN THE 28.00Z GFS AND
27.18Z DGEX FOR SOME LIGHT PRECIPITATION TO SPREAD INTO THE REGION
AS MOISTURE ALSO RETURNS. THE 28.00Z ECMWF/CANADIAN CONTINUE TO
POINT DRY...BUT FOR NOW IN COLLABORATION WITH NEIGHBORING OFFICES
HAVE INTRODUCED 20 PERCENT CHANCES FOR THURSDAY FROM A MODEL
CONSENSUS.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z SATURDAY MORNING)
ISSUED AT 538 AM CST FRI NOV 28 2014

THE BAND OF -SN HAS EXITED EAST OF THE TAF SITES AS OF 11Z...WITH
VFR CIGS/VSBYS EXPECTED THRU THE MORNING. SOUTHERLY FLOW AND WARMING
TEMPS EXPECTED TO PRODUCE BOUNDARY LAYER MOISTURE TO THE SOUTH FROM
MELTING SNOW. THIS MOISTURE IS THEN BE TRAPPED UNDER A STRONG
INVERSION NEAR 925MB THIS AFTERNOON AND TONIGHT. MVFR CLOUD DECKS
EXPECTED AT THE TAF SITES THIS AFTERNOON/TONIGHT. A SMALL CHANCE OF
MIXED PRECIP AT THE TAF SITES THIS AFTERNOON AS YET ANOTHER WEAK
SHORTWAVE PASSES...BUT MOISTURE APPEARS RATHER SHALLOW OR ANY PRECIP
FALLING FROM CLOUDS AT OR ABOVE 700MB. LEFT ANY MENTION OF PRECIP
THIS AFTERNOON OUT OF THE TAFS FOR NOW GIVEN THE LOWER CHANCES AND
SOME UNCERTAINTY IF IT WILL OCCUR OR NOT. BIGGER CONCERN IS
CONTINUED SATURATION OF THE BOUNDARY LAYER/LAYER BELOW THE INVERSION
TONIGHT AND POTENTIAL FOR BR/FG AFTER MIDNIGHT. INTRODUCED SOME 2-
4SM BR TO THE TAF SITES AFTER 06Z-08Z. ONCE ANY BR/FG WOULD
FORM...APPEARS IT COULD PERSIST WELL INTO THE DAY SAT.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...WINTER WEATHER ADVISORY UNTIL 3 PM CST THIS AFTERNOON FOR WIZ017.

MN...NONE.
IA...NONE.
&&

$$

SHORT TERM...AJ
LONG TERM....AJ
AVIATION.....04


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