Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 180520
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1120 PM CST WED DEC 17 2014

.SHORT TERM...(TONIGHT THRU THU NIGHT)
ISSUED AT 240 PM CST WED DEC 17 2014

MAIN FCST CONCERNS THIS PERIOD...CLOUDS AND TEMPERATURES.

18Z DATA ANALYSIS HAD HIGH PRESSURE RIDGING FROM WESTERN ONT TO
EASTERN ND TO MO/AR. SKIES HAD GENERALLY CLEARED UNDER/NEAR THE
RIDGE AXIS ACROSS MUCH OF MN/IA/EASTERN DAKOTAS. FURTHER EAST...A
TROUGH/FRONT WAS SLOWLY DROPPING SOUTH ACROSS LK SUPERIOR. CLOUDS
LINGERED IN THE LOW LEVEL CONVERGENCE AHEAD OF THIS TROUGH ACROSS
THE SOUTHWEST 1/2 TO 2/3 OF WI...WITH THE NORTH-NORTHEAST FLOW
BEHIND THE FRONT/TROUGH SPREADING MORE CLOUDS SOUTHWESTWARD ACROSS
LK SUPERIOR...THE MI U.P. AND NORTHEAST WI. EVEN WITH THE SUNSHINE
ACROSS MN/IA...TEMPS SLOW TO WARM...EVEN WHERE THE GROUND IS SNOW-
FREE.

17.12Z MODELS GENERALLY INITIALIZED QUITE WELL BUT ALL WERE 1 TO 2MB
TOO WEAK/FAR NORTHWEST WITH THE MO TO ND TO WESTERN ONT RIDGE AXIS.
SOLUTIONS VERY SIMILAR AS WEAK SHORTWAVE RIDGING SLIDES ACROSS THE
REGION TONIGHT/THU. TIGHTER SIGNAL FOR A WEAKENING SHORTWAVE TO MOVE
INTO THE NORTH HALF OF THE REGION THU NIGHT...BEING SQUEEZED BY
RIDGING TRYING TO DEVELOP BETWEEN EAST COAST/WEST COAST TROUGHING.
TREND FAVORS FASTER/STRONGER OF THE EARLIER MODEL RUNS. SHORT-TERM
FCST CONFIDENCE IS GOOD THIS CYCLE.

FOR THE SHORT TERM...SFC-850MB TROUGHING/FRONT CONTINUES TO DROP
SOUTH ACROSS THE AREA TONIGHT...EAST OF THE WEAK MID LEVEL RIDGE
AXIS. PLENTY OF 925MB MOISTURE TRAPPED UNDER AN INVERSION PROGGED TO
PERSIST ACROSS THE NORTHEAST HALF OF THE FCST AREA TONIGHT IN/NEAR
THIS TROUGH AXIS. WOULD THINK WITH THE SFC-850MB RIDGE AXIS DRIFTING
ACROSS THE AREA THU THAT THE LOW CLOUDS WOULD BREAK UP/CLEAR BUT
925MB RH PROGS SHOW LOWER LEVEL MOISTURE FIRMLY STUCK OVER THE AREA
UNDER THE STRONG INVERSION NEAR 925MB. THEN FOR LATE TONIGHT THRU
THU NIGHT...MID/HIGH LEVEL MOISTURE AHEAD OF THE SHORTWAVE COMING
ACROSS THE NORTHERN PLAINS SPREADS ACROSS THE AREA...SO EVEN IF THE
LOWER CLOUDS BREAK/CLEAR...MORE MID/HIGH CLOUDS LOOKING TO SPREAD
ACROSS THE AREA LATER TONIGHT INTO THU NIGHT. TRENDED TOWARD MORE
CLOUD COVER THRU THE SHORT-TERM...ESPECIALLY OVER THE NORTHEAST 2/3
OF THE FCST AREA TONIGHT/THU. WITH MORE CLOUDS...TRENDED TOWARD
WARMER OF GUIDANCE LOWS TONIGHT/THU NIGHT AND COOLER OF GUIDANCE
HIGHS THU.

.LONG TERM...(FRIDAY THRU WEDNESDAY)
ISSUED AT 240 PM CST WED DEC 17 2014

MAIN FCST CONCERNS FOR FRIDAY THRU SAT NIGHT...CLOUDS/TEMPERATURES.

17.12Z MODELS REMAIN IN GOOD AGREEMENT FRI AS ONE SHORTWAVE TRACKS
ACROSS THE NORTH END OF THE REGION AND MORE ENERGY DIGS TOWARD
WESTERN IA. GOOD AGREEMENT CONTINUES FRI NIGHT SAT AS THESE FEATURES
PASS NORTH/SOUTH OF THE FCST AREA AND YET ANOTHER STRONGER WAVE
TRACKS ACROSS THE NORTHERN ROCKIES FRI NIGHT AND INTO THE NORTHERN
PLAINS SAT. MODELS THEN DIVERGE WITH THE EVOLUTION OF THIS SHORTWAVE
SAT NIGHT...ECMWF TAKES MOST OF THE SHORTWAVE ENERGY NORTH OF THE
US/CAN BORDER...CAN-GEM QUITE AMPLIFIED DIGGING MUCH OF THE ENERGY
INTO NEB/DAKOTAS...GFS IN THE MIDDLE. OVERALL TREND IS SLOWER/
STRONGER WITH CENTRAL/NORTHERN PLAINS TROUGHING SAT NIGHT. FCST
CONFIDENCE IS GOOD FRI THRU SAT THEN AVERAGE SAT NIGHT.

BULK OF THE FORCING/LIFT AND 850-500MB SATURATION WITH THE WAVE
APPROACHING THU NIGHT PASSES NORTH OF THE FCST AREA FRI. THAT WITH
THE WAVE MOVING INTO WESTERN IA SAT MORNING PASSES SOUTH OF THE FCST
AREA SAT. FCST AREA LOOKS TO REMAIN DRY FRI THRU SAT...BUT PLENTY OF
MOISTURE ABOVE 500MB SPREADS ACROSS THE AREA AND SOME MOISTURE BELOW
850MB LINGERS ACROSS THE AREA...FOR AT LEAST PARTLY TO MOSTLY CLOUDY
SKIES FRI THRU SAT. SOUTHERLY LOW LEVEL FLOW PROGGED TO PREVAIL
ACROSS THE FCST AREA FOR FRI THRU SAT NIGHT. A SLOW MODERATION OF
925MB TEMPS FRI THRU SAT NIGHT AS WEAK TO MDT LOW LEVEL WARM
ADVECTION PERSISTS ACROSS THE REGION. WITH A GENERAL LACK OF SNOW
COVER...TEMPS LOOK TO WARM A FEW DEGREES EACH DAY...EVEN WITH PLENTY
OF CLOUDS EXPECTED. THE SOUTHERLY FLOW EVENTUALLY PULLS INCREASING
SFC-850MB MOISTURE NORTHWARD INTO THE AREA SAT NIGHT...WITH
INCREASING/THICKENING LOW CLOUDS AND WARMER LOWS. USED A BLEND OF
THE GUIDANCE HIGHS/LOWS FOR FRI THRU SAT NIGHT.

FOR SUNDAY THRU WEDNESDAY...DAY 4 TO 7...

MAIN FCST CONCERNS THIS PERIOD...PRECIP CHANCES AND TEMPERATURES
THRU THE PERIOD.

MEDIUM RANGE MODEL RUNS OF 17.00Z/17.12Z IN REASONABLE AGREEMENT FOR
LONGWAVE TROUGHING TO BE  DEVELOPING OVER CENTRAL NOAM SUN/MON.
HOWEVER PLENTY OF DETAIL DIFFERENCES...AND IMPACTS ON THE SENSIBLE
WEATHER FOR THE REGION THESE PERIODS...DEPENDING ON THE INTERACTION
OF WAVE INTO THE NORTHERN PLAINS AT 12Z SUN AND THE NEXT FEATURE
DIGGING INTO THE CENTRAL/NORTHERN PLAINS BEHIND IT SUN NIGHT/MON.
GOOD CONSENSUS FOR A CENTRAL NOAM LONGWAVE TROUGH CONTINUES
TUE/WED...BUT PLENTY OF DETAIL DIFFERENCES CONTINUE WITH HOW THE
SHORTWAVE PIECES INTERACT AND IMPACT THE UPPER MIDWEST WEATHER
HEADING INTO THE CHRISTMAS HOLIDAY. OVERALL TREND IS STRONGER...
MORE AMPLIFIED WITH THE TROUGH/ENERGY FOR THE FIRST HALF OF NEXT
WEEK. DAY 4-7 FCST CONFIDENCE IS AVERAGE THIS CYCLE.

INCREASING LOW LEVEL WARM ADVECTION SPREADS IN SUNDAY AHEAD OF THE
DEEPENING TROUGHING INTO THE CENTRAL/NORTHERN PLAINS. QUESTION IS
HOW QUICKLY DEEPER MOISTURE RETURNS WITH THIS FLOW. SMALL MAINLY -SN
CHANCE SUN PER THE MODEL/ENSEMBLE CONSENSUS REASONABLE FOR NOW.
MOISTURE CONTINUES TO SLOWLY INCREASES SUN NIGHT THRU TUE IN
PERSISTENT SOUTHERLY FLOW AHEAD OF TROUGHING/LOW PRESSURE SLOWLY
APPROACHING AND MOVING INTO THE UPPER MIDWEST. MODELS DIFFER IN THE
LOWER LEVEL/MESO-SCALE DETAILS BY SUN NIGHT THRU TUE...AS FAR AS
925MB TEMPS AND POTENTIAL PRECIP TYPES. DIFFERENCES WITH THE
TIMING/STRENGTH/POSITION OF THE SFC-850MB LOW INTO/ACROSS THE AREA
TUE AS WELL. UNTIL THE DETAILS SORT THEMSELVES OUT...20-50 PERCENT
-RA/-SN CHANCES FOR MUCH OF THE SUN NIGHT THRU TUE NIGHT PERIOD
REASONABLE AT THIS POINT. PRECIP CHANCES SHOULD BE DECREASING AND
TEMPS COOLING WED AS THE SFC THRU MID LEVEL LOW/TROUGH WOULD BE
EXITING EAST OF THE REGION. GIVEN THE STRONGER TREND AND POTENTIAL
FOR SYSTEM TO SLOW EVEN MORE...LINGERING 20-40 PERCENT -SN CHANCES
ON WED OKAY FOR NOW. MODEL/ENSEMBLE CONSENSUS OF THE HIGHS/LOWS FOR
SUN-WED APPEARED GOOD.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)
ISSUED AT 1120 PM CST WED DEC 17 2014

STRATUS DECK MOVED INTO RST DURING THE EVENING...BUT ITS
EXTREMELY CLOSE TO CLEARING WITH CLEAR SKIES PRESENT OVER EASTERN
OLMSTED COUNTY. FIGURE ANOTHER 1-2 HOURS AND THE STRATUS SHOULD
LEAVE THE TAF SITES. THE CLEARING SHOULD LEAD TO SOME MVFR BR
DEVELOPMENT. MEANWHILE...LSE IS ALSO ON THE EDGE OF STRATUS...BUT
IN THIS CASE STRATUS IS JUST NORTHEAST OF THE TAF SITE OVER
NORTHERN LA CROSSE COUNTY. A NORTH-NORTHEAST FLOW SHOULD PUSH
THIS STRATUS INTO LSE BY 08Z. THAT SAME FLOW IS EXPECTED TO
CONTINUE MARCHING THE STRATUS DECK TOWARDS RST...MOVING IN
SOMEWHERE AROUND 12Z. ONCE THE STRATUS MOVES IN THIS MORNING...THE
COMBINATION OF LIGHT WINDS AND BEING NEAR THE MINIMUM OF DAYLIGHT
LENGTH SUPPORTS THE STRATUS HANGING AROUND INTO THE EVENING HOURS.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...AJ



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