Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 200509
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1109 PM CST FRI DEC 19 2014

.SHORT TERM...(TONIGHT THRU SATURDAY NIGHT)
ISSUED AT 300 PM CST FRI DEC 19 2014

MAIN FORECAST CONCERNS THIS PERIOD...LOW LEVEL MOISTURE/CLOUDS AND
ANY -FZDZ CHANCES...TEMPERATURES.

18Z DATA ANALYSIS HAD A BROAD AREA OF HIGH PRESSURE FROM EASTERN ONT
SOUTHWEST TO MO. 19.12Z RAOBS SHOWED A RATHER STRONG INVERSION NEAR
900MB ACROSS THE REGION...WITH PLENTY OF MOISTURE TRAPPED IN/UNDER
IT PRODUCING RATHER WIDESPREAD LOWER STRATUS CLOUDS ACROSS THE
REGION. SOME BREAKS IN THE CLOUDS OVER PARTS OF IL/EASTERN WI.
OTHERWISE DEVELOPING SOUTHEAST TO SOUTH FLOW ON THE WEST SIDE OF THE
RIDGE AXIS WAS ADVECTING MORE MOISTURE NORTHWARD INTO IA/MN. WITH
LACK OF SNOW COVER...TEMPS ACTUALLY WARMING A BIT TODAY UNDER THE
CLOUDS IN THE DEVELOPING SOUTHERLY WARM ADVECTION FLOW.

19.12Z MODELS LOOK TO HAVE INITIALIZED WELL...THOUGH ALL WERE ABOUT
1MB TOO HIGH WITH SFC PRESSURES IN THE BROAD HIGH FROM ONT TO THE
MID MS VALLEY. SOLUTIONS SIMILAR FOR TONIGHT THRU SAT NIGHT AND
CONVERGING TOWARD A TIGHTER COMPROMISE OF THEIR EARLIER RUNS AS
ONE SHORTWAVE PASSES SOUTH OF THE AREA TONIGHT AND ANOTHER MOVES
INTO MN SAT NIGHT. SHORT-TERM FCST CONFIDENCE IS GENERALLY GOOD
THIS CYCLE.

IN THE SHORT TERM...THE MAIN CONCERN TONIGHT THRU SAT NIGHT IS THE
DEPTH OF THE LOWER LEVEL MOISTURE/STRATUS AND EXTENT OF ANY LIFT
IN/JUST ABOVE THE MOISTURE LAYER. THIS MOISTURE REMAINS TRAPPED
UNDER AN INVERSION AROUND 850MB TONIGHT AND AROUND 900MB SAT NIGHT.
DEEPEST MOISTURE IS OVER THE FCST AREA LATE THIS AFTERNOON/EVENING
BUT MOST CLOUD BASES IN THE 1500-2500 FT RANGE WITH SFC TEMP-DEW
POINT SPREADS OF 5F TO 10F. THIS NOT OVERLY CONDUCIVE TO TO -DZ/
-FZDZ AND REMOVED ANY -FZDZ MENTION THRU THIS EVENING. WEAK/MDT
925MB WARM ADVECTION AND 280-285K PERSIST ACROSS THE AREA TONIGHT
THRU SAT NIGHT. SOME OF THE STRONGER OF THIS SAT NIGHT AS THE 925MB
FLOW INCREASE TO AROUND 20KTS. SFC-925MB SATURATION INCREASES THRU
TONIGHT...WITH LOWERING CIGS AND TEMP-DEW POINT SPREADS. WILL
CONTINUE THE PATCHY -FZDZ SPREADING ACROSS MUCH OF THE FCST AREA
DURING THE LATE EVENING/OVERNIGHT HOURS INTO EARLY SAT MORNING.
ADDED A PATCHY FOG MENTION OVERNIGHT INTO SAT MORNING AS WELL. LEFT
THE BULK OF SAT/SAT EVENING DRY BUT CANNOT RULE OUT ISOLATED -DZ/
-FZDZ AND PATCHY FOG. CONTINUED THE PATCHY -FZDZ MENTION LATER SAT
NIGHT WITH THE INCREASE OF LIFT LATE BUT DID REMOVE ANY -SN MENTION
AS TOP OF THE MOISTURE COLUMN LATER SAT NIGHT WOULD ONLY BE ABOUT
-5C AT THE COLDEST WITH NO ICE IN THE CLOUDS. ADDED SOME FOG FOR
THIS PERIOD AS AN INCREASINGLY MOIST SFC-925MB AIRMASS IS PUSHED
NORTHWARD OVER THE COLD LANDSCAPE. FAVORED WARMER OF GUIDANCE LOWS
TONIGHT/SAT NIGHT WITH THE THICKENING LOW CLOUDS AND A BLEND OF
GUIDANCE HIGHS FOR SAT.

.LONG TERM...(SUNDAY THROUGH FRIDAY)
ISSUED AT 300 PM CST FRI DEC 19 2014

MAIN FCST CONCERNS FOR SUN THRU MON NIGHT...PRECIP TYPES/CHANCES AS
THE TROUGH/LOW DEEPEN AND APPROACH...TEMPERATURES.

19.12Z MODELS SHOW A TIGHTENING CONSENSUS AS STRONG SHORTWAVE ENERGY
TOPS THE WESTERN NOAM RIDGING SUN THEN DROPS INTO THE CENTRAL/
NORTHERN PLAINS SUN NIGHT/MON. THIS CONSENSUS FAVORS SLOWER/
STRONGER OF THE EARLIER RUNS AT 00Z TUE. GOOD AGREEMENT AMONG GFS/
ECMWF/NAM FOR A 500MB LOW NEAR KFSD BY 00Z TUE...MIGRATING TOWARD
KDSM BY 12Z TUE. CAN-GEM MORE PROGRESSIVE/FURTHER NORTH AND
APPEARS AS AN OUTLIER BY MON NIGHT. GIVEN GOOD/TIGHT CONSENSUS
AMONG NAM/ECMWF/ GFS THRU 12Z TUE...FCST CONFIDENCE FOR SUN THRU
MON NIGHT REMAINS GENERALLY GOOD.

FALLING HGTS AND STRENGTHENING LOW PRESSURE IN THE NORTHERN PLAINS
CONTINUES THE SOUTH/SOUTHWEST LOW LEVEL FLOW INTO THE AREA SUN/SUN
NIGHT. SFC THRU 850MB LOW REMAINS NORTHWEST OF THE FCST AREA THRU
00Z TUE. THIS PERSISTENT SOUTH/SOUTHWEST WARM ADVECTION FLOW AHEAD
OF THE LOW WARMS THE SFC-925MB LAYER TO NEAR/ABOVE 0C BY MON/MON
NIGHT. DEPTH OF MOISTURE THE AREA STARTS TO INCREASE SUN ALONG WITH
THE STRENGTH/DEPTH OF THE FORCING/LIFT. MOISTURE/LIFT INCREASE EVEN
MORE FOR SUN NIGHT THRU MON NIGHT WITH PRECIP BECOMING LIKELY FOR
MUCH OF THE AREA MON/MON NIGHT. TRANSITIONED ANY PRECIP FROM -DZ
TO -RA SUN MORNING WITH DEEPER MOISTURE AND ONE STRONGER SURGE OF
850- 700MB WARM ADVECTION LIFTING ACROSS THE AREA. BIT OF A LULL
IN THE LIFT SUN NIGHT WITH THE INCREASE OF DYNAMIC LIFT/PV
ADVECTION AHEAD OF THE TROUGH/SHORTWAVE MOVING ACROSS MON/MON
NIGHT. RAISED PRECIP CHANCES MON INTO MON NIGHT. ADDED PATCHY FOG
FOR SUN NIGHT/MON MORNING AS WELL WITH INCREASING DEW POINTS OVER
THE COLD GROUND. TRENDED PRECIP SUN NIGHT THRU MON NIGHT TOWARD
MORE -RA AS MODEL SOUNDINGS AND X-SECTIONS FOR THIS PERIOD SHOWING
AT LEAST THE SFC- 900MB LAYER WARMING INTO THE 0C TO +3C RANGE...
ESPECIALLY OVER THE CENTRAL/SOUTH PARTS OF THE FCST AREA. LINGERED
A -SN CHANCE MAINLY DURING THE NIGHTTIME/EARLY MORNING HOURS WITH
A COOLER BOUNDARY LAYER AND ANY HEAVIER PRECIP RATES IF THEY OCCUR
CAPABLE OF COOLING THE COLUMN ENOUGH FOR SNOWFLAKES TO REACH THE
GROUND. WILL ALSO HAVE TO WATCH THE LATE NIGHT/EARLY MORNING HOURS
OF SUN/MON NIGHTS FOR -FZRA ON COLD GROUND SURFACES. USED A BLEND
OF THE GUIDANCE HIGHS/ LOWS FOR SUN THRU MON NIGHT...RAISING LOWS
A BIT SUN NIGHT WITH THE SOUTHERLY BOUNDARY LAYER FLOW/THICK LOW
CLOUDS OVER THE AREA.

FOR TUESDAY THRU FRIDAY...DAYS 4 TO 7...

MAIN FORECAST CONCERNS THIS PERIOD...PRECIP CHANCES/TYPES TUE/WED
AND AGAIN FRI...TEMPERATURES.

MEDIUM RANGE MODEL RUNS OF 19.00Z/19.12Z IN QUITE GOOD AGREEMENT TUE
WITH THE TROUGH DEEPENING OVER THE CENTRAL CONUS. TREND FAVORS
SLOWER/STRONGER OF THE EARLIER RUNS. CONSISTENCY REMAINS GOOD FOR
THE TROUGH TO BE PROGRESSIVE INTO THE EASTERN CONUS WED/THU AS
STRONG ENERGY ROTATES UP THE EAST SIDE OF THE TROUGH. IMPROVING
SIGNAL FOR SHORTWAVE RIDGING TO BUILD OVER THE MID/UPPER MS VALLEY
FOR CHRISTMAS DAY...AS ONE TROUGH LIFTS INTO THE CAN MARITIMES AND
MORE ENERGY DIGS INTO THE CENTRAL/SOUTHERN ROCKIES. POTENTIAL FOR
SOME PORTION OF THE ENERGY TO LIFT TOWARD/INTO THE MID/UPPER MS
VALLEY FRI. PLENTY OF SPREAD AMONG THE MODELS AND ENSEMBLE
MEMBERS WITH THE DETAILS BY DAY 7. CONFIDENCE IN THE DAY 4-7
PERIOD IS AVERAGE TO GOOD TUE-THU THEN AVERAGE TO BELOW FOR FRI.

IMPROVING MODEL CONSENSUS FOR THE SFC-700MB LOW TO BE OCCLUDING/
AS IT MOVES ACROSS THE UPPER MIDWEST TUE. FORCING/LIFT OVER THE
FCST AREA WEAKENS THRU TUE/TUE NIGHT WITH THE MOISTURE CONVEYOR
BEING PUSHED FURTHER FROM THE AREA BY THE OCCLUSION. COLUMN DOES
COOL TUE AS THE OCCLUDING LOW/MID LEVEL TROUGH AXIS PASS. HIGHER
CHANCES OF PRECIP TUE/TUE NIGHT AS A DEFORMATION BAND WOULD
ROTATE ACROSS THE AREA. MIX OF -RA/-SN TUE MORNING GIVING WAY TO
-SN FOR LATE TUE/TUE NIGHT. STRONG ENERGY IN THE SOUTHERN PORTION
OF THE LONGWAVE TROUGH ROTATING UP THE EASTERN SEABOARD WED DRAGS
THE MID LEVEL LOW INTO THE EASTERN GREAT LAKES BY WED NIGHT...
SPREADING DRYING NORTHWEST 850-500MB FLOW OVER THE REGION. ANY
LINGERING -SN LOOKING TO TAPER OFF FROM SOUTHWEST TO NORTHEAST
WED/WED EVENING. SFC THRU MID LEVEL RIDGING PROGGED TO SLIDE
ACROSS THE UPPER MIDWEST THU...WITH OUR BEST SHOT AT SOME SUNSHINE
NEXT WEEK ON CHRISTMAS DAY. CONFIDENCE AVERAGE AT BEST FRI BUT
LATEST DETERMINISTIC MODEL RUNS WOULD LIFT A PORTION OF THE
ROCKIES ENERGY INTO THE UPPER MIDWEST AT THE END OF THE WEEK.
PLENTY OF LOWER LEVEL DETAIL DIFFERENCES BY FRI WITH ANY
POTENTIAL SFC LOW TRACK/STRENGTH AND HOW MUCH WARM AIR IS DRAWN
NORTH AHEAD OF THE SYSTEM FOR PRECIP TYPE ISSUES. WILL STAY WITH
THE MODEL/ENSEMBLE CONSENSUS ON FRI FOR NOW. MODEL/ENSEMBLE
CONSENSUS OF HIGHS/LOWS FOR TUE-FRI SEEMS TO HAVE THEM WELL
TRENDED AS WELL.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SATURDAY NIGHT)
ISSUED AT 1109 PM CST FRI DEC 19 2014

CEILINGS HAVE STAYED UP A LITTLE HIGHER THAN EXPECTED THIS
EVENING...WHICH MIGHT BE A FUNCTION OF NOT BEING IN THE HIGHER
MOISTURE RETURN TAKING PLACE ALONG AND WEST OF I-35. A SOUTH TO
SOUTHEAST WIND TO THIS POINT HAS HELPED KEEP LOW CEILINGS OFF TO
THE WEST. HOWEVER...JUST ABOVE THE GROUND THE WINDS ARE FORECAST
TO TURN MORE TO THE SOUTHWEST OVER THE NEXT FEW HOURS. AS A
RESULT...ANTICIPATING RST TO LOWER FROM MVFR TO IFR BY 10Z...ALONG
WITH THE FORMATION OF SOME MVFR BR. AT LSE...CONDITIONS WILL STAY
MVFR...BUT CEILINGS SHOULD DROP SOME. TOWARDS THE MIDDLE OF THE
DAY...DAYTIME HEATING PLUS THE FLOW JUST ABOVE THE GROUND BACKING
MORE SOUTHERLY SHOULD ALLOW CEILINGS TO COME UP SOME...AT LEAST
ENOUGH TO PULL RST OUT OF IFR. THE FLOW STAYS CONSISTENT INTO THE
EVENING...THUS HELD CEILINGS STEADY. ITS POSSIBLE RST FALLS TO
IFR LATE IN THE EVENING...BUT CONFIDENCE IS TOO LOW TO INCLUDE AT
THIS TIME.

&&

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

$$

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


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