Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 251732
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1132 AM CST THU DEC 25 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 342 AM CST THU DEC 25 2014

THE MAIN FOCUS FOR TODAY IS WITH THE CLOUD COVER/TEMPERATURES
FOLLOWED BY THE CHANCES FOR DRIZZLE/FREEZING DRIZZLE TONIGHT.

CURRENTLY...A SURFACE RIDGE OF HIGH PRESSURE IS ALIGNED ALONG THE
MISSISSIPPI RIVER AND DRIFTING EASTWARD AS A BROAD AREA OF LOW
PRESSURE DEVELOPS IN THE PLAINS. FURTHER ALOFT...THE MID LEVEL
FLOW IS FLATTENING OUT ACROSS THE NORTHERN PLAINS/UPPER
MISSISSIPPI RIVER VALLEY AS A TROUGH DIGS ACROSS THE ROCKIES. A
LOW LEVEL STRATUS DECK IS PRODUCING SOME SCATTERED FLURRIES DUE TO
THE TOP OF THE SATURATED LAYER/CLOUD DECK BEING AROUND -10C
ACCORDING TO IR SATELLITE. THIS IS COLDER THAN WHAT LOCAL SHORT
TERM SOUNDINGS SUGGEST IS OCCURRING. IR SATELLITE/OBS ACTUALLY
SHOW THE WESTERN EDGE OF IT RAPIDLY CLEARING IN ACROSS CENTRAL
IOWA/SOUTHWEST MINNESOTA AND IS TIMED TO BE MOVING THROUGH THE
REGION THIS MORNING INTO EARLY AFTERNOON WITH EROSION OCCURRING
FROM SOUTHWEST TO NORTHEAST. HOWEVER...HIGH LEVEL CLOUD COVER IS
QUICK ON ITS HEELS AND WILL LIMIT THE POTENTIAL OF SEEING ANY
SUNLIGHT UNLESS IT ENDS UP THINNING OUT AS IT APPROACHES. BASED
ON THE LOW CLOUDS CLEARING OUT...BUMPED UP FORECAST HIGH
TEMPERATURES A FEW DEGREES AS SURFACE WINDS COME AROUND TO THE
SOUTHWEST AND BRING SOME WARMER AIR INTO THE REGION.

GOING INTO TONIGHT THE QUESTION IS WITH WHETHER ANY
DRIZZLE/FREEZING DRIZZLE WILL OCCUR AS LOW LEVEL MOISTURE
TRANSPORT RAMPS UP INTO THE REGION. THE 25.00Z MODELS ARE ALL IN
AGREEMENT THAT A 30-40KT LOW LEVEL JET WILL DEVELOP ACROSS THE
SOUTHERN PLAINS THIS EVENING WITH IT NOSING NORTHEASTWARD AS THE
NIGHT PROGRESSES. FORECAST SOUNDINGS FROM THE NAM/GFS BOTH SHOW A
DEEPENING SATURATED LOW LEVEL LAYER WHICH IS NOT COLD ENOUGH TO
FORM ICE. THERE APPEARS TO BE SOME WEAK LIFT THANKS TO THE LLJ
NOSE/SURFACE BOUNDARY PROVIDING SOME CONVERGENCE. THE 25.00/06Z
NAM SURFACE REFLECTIVITY PANES SHOW THIS DRIZZLE DEVELOPING FROM
THE WEST OVERNIGHT...PARTICULARLY AFTER MIDNIGHT AND ALONG WHERE
THE CONVERGENCE IS IN THE SURFACE/LOW LEVELS. WITH THE SOUTHERLY
WINDS KEEPING THE WARMER AIR IN PLACE...THERE IS SOME QUESTION
ABOUT HOW COLD TEMPERATURES WILL BE AND WHETHER THE DRIZZLE WILL
BE FREEZING OR NOT. WILL HAVE TO MONITOR TRENDS...BUT WITH
TEMPERATURES AT OR JUST ABOVE FREEZING...EXPECT THAT THE IMPACT
WILL NOT BE THAT GREAT.

.LONG TERM...(FRIDAY THROUGH WEDNESDAY)
ISSUED AT 342 AM CST THU DEC 25 2014

GOING INTO THIS WEEKEND ON INTO THE MIDDLE OF NEXT WEEK...THE
FOCUS IS ON THE POTENTIAL FOR AN ACCUMULATING SNOW FRIDAY NIGHT
INTO SATURDAY FOLLOWED BY THE DRASTICALLY COLDER AIR MASS THAT
WILL SETTLE IN BY MIDWEEK.

EYES WILL BE ON THE MID LEVEL TROUGH DIGGING ACROSS THE
SOUTHWESTERN US AND HOW IT WILL EJECT INTO THE UPPER MIDWEST.
PREVIOUS MODEL RUNS HAD THIS WAVE COMING THROUGH AS MAINLY AN OPEN
TROUGH/SURFACE LOW BUT THE 25.00Z TRENDS WITH THE NAM/GFS/ECMWF
ARE TO GO DEEPER. THE ECMWF IS THE DEEPEST/SLOWEST WITH THIS
FEATURE AND BRINGS THE HEAVIEST SNOW TOTALS...POSSIBLY ADVISORY
WORTHY...ALONG WITH IT. ALL THREE MODELS ARE SHOWING A BAND OF
HEAVIER SNOW OCCURRING IN THE DEFORMATION AREA OF THE TROUGH WHICH
RUNS FROM SOUTHEAST MINNESOTA INTO NORTH CENTRAL WISCONSIN. MOST
OF THE AREA SHOULD JUST SEE SNOW FROM THIS SYSTEM...BUT SOME
PORTIONS OF EASTERN IOWA AND SOUTHWEST WISCONSIN COULD BE WARM
ENOUGH TO CHANGE IT OVER TO RAIN. THE ECMWF TRENDS WILL HAVE TO BE
MONITORED BECAUSE IT WOULD SET US UP FOR SOME HIGHER SNOWFALL
TOTALS THAN CURRENTLY BEING FORECAST. WITH IT BEING A NEW
DIRECTION BEING TAKEN WITH THIS ONE MODEL...DECIDED IT WOULD BE
BEST TO JUST BLEND THAT SOLUTION WITH THE NAM/GFS IDEA TO COME UP
WITH SNOW ACCUMULATIONS OF 1 TO 3 INCHES. THE MAIN PERIOD TO WATCH
WILL BE SATURDAY MORNING AS THE ECMWF HAS AROUND 0.25 INCHES OF
QPF IN THAT PERIOD WHEN THE NAM/GFS ONLY HAS 0.01 INCHES OR SO.

BEYOND THIS SYSTEM IT LOOKS LIKE WE WILL HAVE AN EXTENDED PERIOD
OF HAVING A COLDER AIR MASS WHERE HIGHS ONLY GET INTO THE
TEENS...PARTICULARLY GOING INTO TUESDAY AND WEDNESDAY. PERHAPS THE
SILVER LINING HERE IS THAT WITH THIS COLD AIR AND A BROAD RIDGE OF
HIGH PRESSURE DROPPING DOWN ACROSS MUCH OF THE CENTRAL US...SKIES
SHOULD BE CLEAR WHICH WOULD BE WELCOME AFTER THIS EXTENDED PERIOD
OF OVERCAST CONDITIONS.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)
ISSUED AT 1132 AM CST THU DEC 25 2014

THE SOUTH TO SOUTHWESTERLY FLOW ABOVE THE GROUND HAS BEEN STRONG
ENOUGH TO HELP BRING IN SOME DRIER AIR FROM IOWA AND SCATTER OUT
THE CLOUDS AT RST. RAP MODEL RUNS HAVE BEEN A BIT TOO
PESSIMISTIC...SO HAVE REVERTED AND MADE THE TAFS MORE OPTIMISTIC.
HAVE NOW BROUGHT SCATTERING TO VFR CONDITIONS AT LSE AT 21Z. A
CONDITIONS SHOULD REMAIN VFR THROUGH A GOOD DEAL OF TONIGHT. LATE
IN THE NIGHT...A TROUGH OVER SOUTHWEST MN LOOKS ON TRACK TO CROSS
RST...AND TOWARDS SUNRISE AT LSE. LOW CEILING STRATUS RETURNS IN
THE WAKE OF THE TROUGH...WITH RST LIKELY DROPPING TO IFR AND
LIFR. MEANWHILE...THE VALLEY SHOULD HELP PREVENT LSE FROM DROPPING
BELOW MVFR. ADDITIONALLY...SOME LOWER VISIBILITIES APPEAR LIKELY
TO ACCOMPANY THE LOW STRATUS...PARTICULARLY OVER THE HIGHER
TERRAIN SITE OF RST. SOME GUIDANCE WANTS TO BRING THEM DOWN TO
LIFR. CONFIDENCE ONLY HIGH ENOUGH TO BRING IT DOWN TO IFR. THERE
COULD ALSO BE SOME PATCHY FREEZING DRIZZLE WITH THESE LOW
VISIBILITIES. COVERAGE AT THIS TIME NOT WIDESPREAD ENOUGH TO
INCLUDE IN THE TAF.

&&

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

$$

SHORT TERM...HALBACH
LONG TERM...HALBACH
AVIATION...AJ


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