Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 210458
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1058 PM CST SAT DEC 20 2014

.SHORT TERM...(TONIGHT THRU SUNDAY NIGHT)
ISSUED AT 245 PM CST SAT DEC 20 2014

THE MAIN FCST CONCERNS THIS PERIOD...PRECIP TYPES/CHANCES THRU THE
PERIOD...TEMPERATURES.

DATA ANALYSIS AT 18Z HAD BROAD HIGH PRESSURE RIDGING FROM QUEBEC TO
AR WITH LOW PRESSURE DEVELOPING IN THE LEE OF THE US/CAN ROCKIES.
VIS IMAGERY/SFC OBS SHOWING A BROAD LOW STRATUS SHIELD  ACROSS
THE UPPER MIDWEST AND NEARBY AREAS EARLY THIS AFTERNOON. PRESSURE
GRADIENT SLOWLY INCREASING ACROSS MN/IA/WI AS THE HIGH MOVES EAST
AND PRESSURES FALL IN THE NORTHERN PLAINS. SOUTHERLY FLOW SLOWLY
BRINGING WARMER AIR NORTHWARD...WITH TEMPS UNDER THE LOW STRATUS
DECK SLOWLY CREEPING TO NEAR/ABOVE 32F ACROSS MUCH OF IA/SOUTHERN
MN/WESTERN WI.

20.12Z MODELS INITIALIZED WELL AND SOLUTIONS QUITE SIMILAR FOR
TONIGHT THRU SUN NIGHT. STRONG ENERGY WITH THE 180-190KT 250-300MB
PAC JET COMES ONSHORE NEAR THE US/CAN BORDER LATE THIS AFTERNOON/
TONIGHT...KICKING A SHORTWAVE NEAR THE MT/ND BORDER EAST/NORTHEAST
TONIGHT/EARLY SUN. THIS PAC ENERGY THEN DEVELOPS STRONGER
TROUGHING IN THE CENTRAL/NORTHERN PLAINS SUN/SUN NIGHT AS THE
STRONG JET MAX DIGS THRU CENTRAL/NORTHERN ROCKIES. SHORT-TERM
FCST CONFIDENCE REMAINS GOOD THIS CYCLE.

FOR THE SHORT TERM...SOUTH TO SOUTHWEST SFC-700MB FLOW OVER THE AREA
INCREASES THRU THE PERIOD AS HGTS FALL AND SFC LOW PRESSURE DEEPENS
IN THE NORTHERN PLAINS. SFC-850MB TEMPS WARM IN THIS PERSISTENT
SOUTH/SOUTHWEST LOW LEVEL WARM ADVECTION FLOW. THRU LATE TONIGHT/
SUN MORNING THE SFC-925MB LAYER IS PROGGED TO WARM ABOVE 0C ACROSS
ALL BUT THE NORTH END OF THE FCST AREA. THIS ABOVE FREEZING LAYER
DEEPENS TO 900-850MB THRU SUN AFTERNOON/NIGHT OVER THE CENTRAL/
SOUTH PARTS OF THE FCST AREA. THIS LAYER DOES NOT APPEAR TO GET
OVERLY WARM...WITH MAX TEMPS IN THE LAYER BETWEEN +2C AND +4C.
THIS LAYER NOT A TRUE WARM LAYER ALOFT EITHER AS THERE IS NO
FREEZING LAYER BETWEEN IT AND THE SFC. THUS TOP-DOWN APPROACH
WOULD LIMIT PRECIP TYPES BY THE DEPTH OF THE WARM LAYER MELTING
PARTICLES AS THEY FALL THRU IT AND THE GROUND/SFC TEMPS. THIS
LIMITS PRECIP TYPES BY LATER SUN/SUN NIGHT TO -RA/-SN OR -FZRA IF
THE GROUND SFCS ARE COLD ENOUGH. ROAD TEMP SENSOR DATA THAT IS
AVAILABLE SHOWING READINGS IN THE UPPER 20S TO MID 30S THIS
AFTERNOON...AND ONLY LIKELY TO GO UP THE NEXT 36-48HRS. WITH A BIT
STRONGER SURGE OF SFC-850MB WARM ADVECTION TONIGHT AND INCREASING
WINDS FOR BOUNDARY LAYER MIXING...LOWS TONIGHT MAY WELL BE IN THE
EVENING WITH STEADY/SLOWLY RISING TEMPS OVERNIGHT. TOP OF MOISTURE
COLUMN ONLY AROUND 850MB TONIGHT WITH CLOUD-TOP TEMPS WARMER THAN
-5C. WILL CONTINUE THE PATCHY/AREAS -FZDZ LATER TONIGHT
TRANSITIONING TO -RA OR A -RA/-SN MIX FOR SUN/SUN NIGHT AS THE
DEPTH OF MOISTURE/WARM LAYER AND LIFT INCREASE. WILL CONTINUE
SOME -FZRA MENTION SUN MORNING WHEN ROAD SFCS MAY STILL BE COLD
ENOUGH TO FREEZE THE DROPS.

AGAIN TRENDED TOWARD WARMER OF GUIDANCE LOWS FOR TONIGHT/SUN NIGHT
WITH THICK LOW CLOUDS AND INCREASING SOUTH GRADIENT WINDS. USED A
BLEND OF THE GUIDANCE HIGHS FOR SUN.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 245 PM CST SAT DEC 20 2014

FCST CONCERNS FOR MON THRU TUE NIGHT...PRECIP CHANCES/TYPES AND
TEMPERATURES THRU THE PERIOD.

20.12Z MODELS IN GENERAL AGREEMENT FOR A DEEPENING TROUGH OVER THE
CENTRAL CONUS MON THRU TUE NIGHT. HOWEVER MODELS DIFFER QUITE A BIT
ON THE DETAILS IN THIS TROUGH AND RESULTING LOW LEVEL FEATURES/FIELDS
BY LATER MON NIGHT AND FOR TUE/TUE NIGHT. OVERALL TREND IS MORE OF
THE ENERGY IN THE SOUTHERN/BOTTOM PORTION OF THE TROUGH BY TUE/TUE
NIGHT WITH LESSER RUN-TO-RUN CONSISTENCY IN THESE PERIODS. SUSPECT
THIS TREND WILL CONTINUE IN FUTURE RUNS WITH A WEAKER LOOK/SLOWING
OF THE UPSTREAM SYSTEM BEHIND IT OVER THE EASTERN PAC. FCST
CONFIDENCE IS AVERAGE TO GOOD MON/MON NIGHT THEN AVERAGE FOR TUE/TUE
NIGHT.

DEPTH OF MOISTURE AND STRENGTH/DEPTH OF FORCING/LIFT INCREASE FOR
MON AS THE MAIN BAND OF LOWER LEVEL THERMODYNAMIC FORCING AND A
ROUND OF 500-300MB PV ADVECTION WITH SOME OF THE LEAD SHORTWAVE
ENERGY IN THE TROUGH ROTATES ACROSS THE REGION. RAISED PRECIP
CHANCES MON INTO THE 70-90 PERCENT RANGE. MODEL SOUNDINGS/X-SECTIONS
SHOW THE SFC-900MB LAYER SHOULD BE WARM ENOUGH FOR THE BULK OF THIS
TO FALL AS RAIN ACROSS THE FCST AREA...POSSIBLY MIXED WITH SOME SNOW
ACROSS THE NORTH END OF THE FCST AREA WHERE THE FREEZING LEVEL MAY
BE MORE BETWEEN 950-925MB. WITH THE DEVELOPING SPLIT OF THE ENERGY
IN THE TROUGH THE NORTHERN SFC-700MB CIRCULATION REMAINS WEST/NORTH
OF THE FCST AREA THRU MON NIGHT. A WEAK SFC-700MB FRONT/TROUGH SOUTH
OF THIS LOW SLIDES ACROSS THE AREA MON NIGHT...WITH SLOW COOLING OF
THE LOWER PORTIONS OF THE COLUMN. A 700-500MB DRY WEDGE SPREADS
NORTHEAST ACROSS THE FCST AREA AS WELL MON NIGHT...SO MAY LOSE ICE
IN THE CLOUDS. PRECIP TO TAPER OFF MON NIGHT AND GRADUALLY MIX
WITH/CHANGE TO -SN. WITH LOSS OF ICE WOULD TRANSITION TO -DZ. CARRIED
HIGHEST CHANCES OVER THE EAST HALF OF THE FCST AREA MON EVENING.

COLUMN CONTINUES TO COOL TUE/TUE NIGHT AS THE NORTHERN SFC-700MB LOW
WEAKENS AND DRIFTS ACROSS THE REGION. ENERGY TRANSFERRING TO THE
SOUTHERN PORTION OF THE TROUGH DEVELOPS ANOTHER LOW IN THE SOUTHERN
MS VALLEY TUE AND LIFTS IT NORTH/NORTHEAST TUE NIGHT. PLENTY OF
TIMING/DETAIL/TRACK DIFFERENCES WITH THIS LOW AND RESULTING
FORCING/LIFT/MOISTURE FIELDS FOR LATER TUE/TUE NIGHT. LEFT -RA/-SN
CHANCES TUE IN THE 30-50 PERCENT RANGE AND SNOW CHANCES TUE NIGHT IN
THE 20-30 PERCENT RANGE...PENDING WHICH MODEL OUTCOME MAY BE MORE
CORRECT. USED A BLEND OF THE GUIDANCE HIGHS FOR MON/TUE AND TRENDED
TOWARD WARMER OF GUIDANCE LOWS FOR MON/TUE NIGHT.

FOR WEDNESDAY THRU SATURDAY...DAYS 4 TO 7...

FCST CONCERNS THIS PERIOD...SNOW CHANCES WED AND FRI...
TEMPERATURES THRU THE PERIOD.

20.12Z MEDIUM RANGE MODELS IN REASONABLE AGREEMENT FOR WED THRU
SAT...BUT THIS LEAVES SOME OF THEM WITH RATHER POOR RUN-TO-RUN
CONSISTENCY. MODELS IN AGREEMENT TO SWEEP THE FULL LATITUDE
TROUGH/LOW OVER THE CENTRAL CONUS QUICKLY INTO THE EASTERN CONUS FOR
THU...WITH SHORTWAVE RIDGING BUILDING OVER THE UPPER MIDWEST FOR
THU. NEXT ENERGY THRU THE FLOW HEADS TOWARD THE REGION FROM THE
CENTRAL ROCKIES FRIDAY...THEN COLD TROUGHING/NORTHWEST FLOW OVER THE
REGION SAT. FCST CONFIDENCE AVERAGE FOR WED THEN AVERAGE TO BELOW
FOR THU-SAT UNTIL MODELS SETTLE ON A MORE RUN-TO-RUN CONSISTENT
SOLUTION.

MORE ENERGY IN THE SOUTHERN PORTION OF THE TROUGH WED DRAGS ANY MID
LEVEL LOW AND THE DEEPER FORCING/LIFT/MOISTURE EVEN FURTHER SOUTH/
EAST OF THE FCST AREA FOR WED. BULK OF SNOW ON WED NOW LOOKING TO BE
EAST OF THE FCST AREA AS THE LOW LIFTING OUT OF THE LOWER MS VALLEY
WOULD DEEPEN OVER MI OR THE OH VALLEY...LEAVING WED TO MAINLY BE A
BREEZY DAY WITH SOME -SN OR FLURRIES. NOW LOOKING LIKE ANY PRE-
CHRISTMAS SNOW FOR THE AREA LATER TUE INTO WED WILL BE AN INCH OR
LESS. TROUGH/MID LEVEL LOW BEING PULLED EAST MORE QUICKLY SPREADS
DRYING NORTHWEST FLOW ALOFT ACROSS THE AREA FOR WED NIGHT. WEAK
RIDGE OF SFC HIGH PRESSURE TO SLIDE INTO/ACROSS THE AREA LATER WED
NIGHT/THU. SYSTEM COMING FROM THE CENTRAL ROCKIES FRI WOULD SPREAD A
ROUND OF LIFT ACROSS THE AREA...BUT IN THE WAKE OF THE STRONGER
SYSTEM LIFTING THRU THE EASTERN CONUS WED/WED NIGHT APPEARS THIS ONE
WILL HAVE MINIMAL MOISTURE TO WORK WITH. 35-45 PERCENT -SN CHANCES
CENTERED ON FRI GOOD FOR NOW. CAN HIGH PRESSURE WOULD BUILD IN FOR
SAT UNDER DRIER/COLDER NORTHWEST FLOW ALOFT. MODEL/ENSEMBLE
CONSENSUS HIGHS/LOWS FOR WED THRU SAT APPEAR GOOD AT THIS TIME.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SUNDAY NIGHT)
ISSUED AT 1058 PM CST SAT DEC 20 2014

STRATUS REMAINS AT BOTH TAF SITES WITH MVFR CEILINGS AT LSE AND
IFR CEILINGS AT RST. SOUTHERLY FLOW IS EXPECTED TO CONTINUE
THROUGH THE NEXT 24 HOURS...ALLOWING A WARM FRONT OVER SOUTHERN
IOWA AND ITS ASSOCIATED MOISTURE TO LIFT NORTH INTO THE TAF
SITES. THEREFORE...THE STRATUS WILL PERSIST WITH THE INCOMING
MOISTURE ALLOWING CEILINGS AND VISIBILITIES TO FALL. RST STANDS TO
SEE THE WORST CONDITIONS BEING A HIGHER ELEVATION...AND LIFR TO
VLIFR CONDITIONS ARE POSSIBLE BY 12Z. ADDITIONALLY...WITH THE
APPROACH OF THE WARM FRONT...ANTICIPATING DZ/FZDZ TO FORM. THIS
TOO SHOULD BE MORE PREVALENT AT RST. ANY IMPROVEMENT ON SUNDAY
SHOULD BE VERY SLOW TO OCCUR...IF AT ALL...GIVEN THE PERSISTENT
MOISTURE FEED. IF THERE IS NOT ANY IMPROVEMENT ON SUNDAY...WILL
HAVE TO WATCH VISIBILITIES SUNDAY NIGHT CLOSELY AT RST. RIGHT NOW
BROUGHT RST BACK DOWN TO LIFR. CEILING SHOULD ALSO LOWER AT LSE TO
IFR WITH CONTINUED INFLUX OF MOISTURE AND NIGHTTIME COOLING.

REGARDING THE WINDS...EXPECT THEM TO INCREASE THROUGH THE NEXT 24
HOURS AS THE GRADIENT TIGHTENS. 15-20 KT WINDS SHOULD BE COMMON AT
RST IN THE AFTERNOON AND EVENING...AND AROUND 10 KT AT LSE.

&&

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

$$

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


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