586
FXUS63 KARX 082051
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
251 PM CST SUN NOV 8 2009
SHORT TERM
TONIGHT THROUGH TUESDAY NIGHT
MAIN FORECAST CONCERNS ARE ON CLOUDS...PRECIPITATION CHANCES AND
TEMPERATURES FOR TONIGHT.
CURRENTLY AS OF 21Z...WATER VAPOR LOOP AND RUC 500MB HEIGHT ANALYSIS
SHOWED OVERALL BROAD TROUGHING FROM THE EAST PACIFIC INTO THE
NORTHERN ROCKIES...AND STRONG RIDGING FROM THE OHIO VALLEY INTO THE
SOUTHEAST U.S.. WITHIN THE BROAD TROUGH...THE MOST NOTABLE FEATURES
ARE TWO SHORTWAVES...ONE IN SOUTH DAKOTA AND THE OTHER IN SOUTHERN
SASKATCHEWAN. ASSOCIATED WITH THE SOUTH DAKOTA SHORTWAVE IS A 1009MB
SURFACE LOW NEAR SPENCER IOWA...WITH A WARM FRONT THAT EXTENDED EAST
ALONG THE MINNESOTA/IOWA BORDER. SOUTH OF THIS FRONT...TEMPERATURES
HAVE WARMED INTO THE 70S. EVEN NORTH OF THE FRONT ACROSS THE
NORTHERN PORTIONS OF THE FORECAST AREA...READINGS HAVE CLIMBED INTO
THE MID 60S THANKS TO MORE SUNSHINE THAN EXPECTED. RUC SOUNDINGS
STILL DEPICT PLENTY OF LOW LEVEL MOISTURE ALONG THE FRONT BELOW
900MB... WITH RELATIVE HUMIDITIES AT OR ABOVE 90 PERCENT...BUT LOW
STRATUS HAS YET TO MATERIALIZE. THERE ARE THREE POSSIBLE REASONS:
TIME OF DAY NOT FAVORABLE TO FORM LOW CLOUDS...RELATIVE HUMIDITIES
ARE ACTUALLY CORRECT BUT NOT REACHING FULL SATURATION...LACK OF
CLOUD CONDENSATION NUCLEI. WHATEVER THE CASE...MOSTLY SUNNY SKIES
EXIST WITH ONLY SOME PASSING CIRRUS. BEHIND THE SURFACE LOW IN AN
AREA OF FRONTOGENESIS AND DPVA AHEAD OF THE SOUTH DAKOTA
SHORTWAVE...A BAND OF HIGH BASED ALTOSTRATUS EXISTS. SOME LIGHT RAIN
HAS BEEN REPORTED IN WEST CENTRAL MINNESOTA...OTHERWISE THE BAND IS
DRY.
TONIGHT...SHORTWAVE IN SOUTH DAKOTA IS PROGGED TO RAPIDLY MOVE
NORTHEAST...MOVING ALMOST INTO NORTHEAST ONTARIO BY 12Z MONDAY. THE
ASSOCIATED LOW NEAR SPENCER IOWA THEREFORE ALSO MOVES RAPIDLY
NORTHEAST...AND ITS COLD FRONT BLOWS THROUGH ALL OF THE FORECAST
AREA BY 12Z MONDAY. GIVEN TRENDS UPSTREAM ACROSS NEBRASKA AND SOUTH
DAKOTA...AM NOT VERY IMPRESSED ON PRECIPITATION CHANCES...AND EVEN
CLOUDS BELOW AN ALTOSTRATUS LEVEL. HOWEVER...THERE IS A SIGNAL FOR
DEVELOPMENT THIS EVENING OF A VFR STRATUS DECK NORTH OF I-90 BY THE
07.12Z GFS...NAM...NMM AND 07.15Z SREF...IN ADDITION WITH ENOUGH
VERTICAL MOTION AROUND TO PRODUCE PRECIPITATION. THEREFORE...HAVE
MAINTAINED SOME 20 PERCENT CHANCES OF PRECIPITATION IN AN AREA FROM
WABASHA COUNTY EAST AND NORTHWARD...SPECIFICALLY BETWEEN 03Z AND 09Z
TO ALLOW FOR THE STRATUS DECK DEVELOPMENT AND WHEN THE COLD FRONT
MOVES THROUGH. UNTIL THE FRONT MOVES THROUGH...EXPECT TEMPERATURES
TO REMAIN WARM ALONG WITH INCREASING DEWPOINTS. SHOULD SEE TEMPS
FALL OFF QUICKLY BEHIND THE FRONT...THOUGH...AND HAVE KEPT LOWS
FAIRLY SIMILAR TO THE PREVIOUS FORECAST.
MONDAY AND MONDAY NIGHT...SHORTWAVE CURRENTLY OVER SOUTHERN
SASKATCHEWAN IS FORECAST TO HEAD EAST INTO NORTHERN ONTARIO ON
MONDAY WHILE A SECOND SHORTWAVE OVER BRITISH COLUMBIA MOVES INTO
MANITOBA. WITH THE EASTWARD MOVEMENT OF THESE FEATURES...IT SHOULD
HELP PUSH THE COLD FRONT FROM TONIGHT FARTHER TO THE EAST-SOUTHEAST.
THIS IS CRITICAL BECAUSE PREVIOUS MODEL RUNS SUGGESTED A
FRONTOGENESIS ZONE ALONG AND BEHIND THE COLD FRONT AFFECTING
NORTHEAST IOWA AND FAR SOUTHWEST WISCONSIN. THIS FRONTOGENESIS ZONE
IS NOW SOUTH AND EAST OF OUR AREA...ENOUGH SUCH THAT CHANCES OF
PRECIPITATION WERE REMOVED FROM THE FORECAST. IN ADDITION...CLOUD
COVER WAS REDUCED AS WELL...THOUGH IT DOES APPEAR SOME CIRRUS WILL
MOVE ACROSS THE AREA MONDAY NIGHT. THE HIGH CLOUDS ARE RELATED TO
THE SHORTWAVE MOVING INTO MANITOBA ON MONDAY DROPPING SOUTH INTO
WESTERN LAKE SUPERIOR. WITH MORE SUNSHINE ON MONDAY AND 850MB TEMPS
STILL AROUND 6C...EXPECT ANOTHER MILD DAY WITH HIGHS IN THE UPPER
50S AND LOW 60S. LOWERED DEWPOINTS AS WELL WITH BETTER MIXING AND
SEEING MID 20S TO LOW 30S READINGS UPSTREAM IN THE WESTERN DAKOTAS.
WITH ONLY SOME PASSING HIGH CLOUDS MONDAY NIGHT AND DRY AIR...EXPECT
COOLER READINGS. STAYED ON THE LOW END OF GUIDANCE...ESPECIALLY IF
THE CIRRUS DOES NOT MATERIALIZE AS MUCH AS EXPECTED.
TUESDAY AND TUESDAY NIGHT...A QUIET PERIOD IS ANTICIPATED AS HIGH
PRESSURE BUILDS IN AT THE SURFACE AND RIDGING IS PROGGED TO BUILD IN
ALOFT. IN FACT...MUCH OF THIS PERIOD SHOULD BE CLEAR WITH A DRY
CANADIAN AIRMASS BUILDING IN. DESPITE PLENTY OF SUN...850MB TEMPS
ARE A FEW DEGREES C COOLER...AND AS A RESULT HIGHS WILL BE A LITTLE
COOLER THAN MONDAY. GIVEN THE COOLER DAY...THE CLEAR SKIES AND DRY
AIR...TUESDAY NIGHT WILL BE CHILLY COMPARED TO THE PAST FEW NIGHTS.
STAYED AT OR BELOW GUIDANCE.
LONG TERM
WEDNESDAY THROUGH SUNDAY
DETERMINISTIC AND ENSEMBLE MODEL GUIDANCE SHOWS FAIRLY GOOD
AGREEMENT THROUGH FRIDAY MORNING...THEN DECREASES HEADING INTO THE
WEEKEND. THERE ARE A FEW PARTS OF THE PATTERN THAT ARE CERTAIN...
WHICH INCLUDE TROUGHING OVER THE WESTERN U.S. THURSDAY THROUGH
SATURDAY AND RIDGING THROUGH THE ENTIRE PERIOD FROM THE OHIO VALLEY
INTO THE SOUTHEAST U.S.. THERE IS ALSO SIGNAL FOR RIDGING ACROSS
WESTERN CANADA ON TOP OF THE WESTERN U.S. TROUGH. THEREFORE A SPLIT
FLOW PATTERN IS LIKELY...WHICH WITH THE FORECASTED LOCATIONS OF BOTH
NORTHERN AND SOUTHERN STREAMS...LOOKS TO FAVOR A DRIER FORECAST.
ANOTHER ITEM FAVORING THE DRIER FORECAST IS A LOW PRESSURE SYSTEM
PROGGED BY MOST MODELS OVER THE SOUTHERN PLAINS THIS WEEKEND...
TRAPPING THE GULF OF MEXICO MOISTURE TO OUR SOUTH.
NOW REGARDING THE DETAILS OF THE FORECAST. FOR WEDNESDAY AND
WEDNESDAY NIGHT...RETURN FLOW WILL BE THE RULE IN THE WAKE OF THE
HIGH PRESSURE AREA THAT CROSSED THE AREA TUESDAY NIGHT. THE 08.00Z
ECMWF REMAINS CONSISTENT SHOWING SOME LIGHT PRECIPITATION NORTH OF
I-90 ON WEDNESDAY...ASSOCIATED WITH ISENTROPIC LIFT AHEAD OF A
SHORTWAVE CROSSING MANITOBA AND NORTHERN ONTARIO. THE GFS SHOWS THIS
AS WELL...BUT KEEPS THE AREA DRY. PREFER TO STAY WITH THE DRY
FORECAST FOR NOW...BUT STILL NEEDS WATCHING. NICE TO SEE THE 08.12Z
ECMWF HAS BACKED OFF ON THE PRECIPITATION. THURSDAY AND THURSDAY
NIGHT LOOK DRY AS WELL...WITH A COLD FRONTAL BOUNDARY OUT OVER THE
NORTHERN PLAINS AND JUST BROAD WARM ADVECTION OVER THE FORECAST
AREA. IN FACT...850MB TEMPS COULD APPROACH 8-10C BY 00Z FRIDAY...SO
A MILD THURSDAY IS IN STORE. THINGS GET MORE UNCERTAIN FOR FRIDAY
INTO SATURDAY...BUT IT DOES APPEAR THAT THE COLD FRONT IN THE
DAKOTAS WILL MARCH EAST ACROSS THE AREA DURING THIS TIME FRAME...IN
RESPONSE TO A NORTHERN STREAM SHORTWAVE CROSSING SOUTHERN CANADA.
SPECIFIC TIMING VARIES...WHICH COULD HAVE SIGNIFICANT IMPLICATIONS
ON TEMPERATURES. STILL APPEARS MILD ON FRIDAY...AND IF THERE IS MORE
SUN THAN EXPECTED...WOULD NOT BE SURPRISED TO HAVE ANOTHER 70 DEGREE
DAY. WILL NOT GO THAT WARM GIVEN THE UNCERTAINTIES PRESENT. COOLER
CONDITIONS INVADE FOR SATURDAY PER 08.00Z ECMWF AND 08.06Z GFS AS
850MB TEMPS ARE PROGGED TO FALL TO AROUND 0C. WITH THE COLD FRONT...
MAINTAINED 20 PERCENT CHANCES FOR PRECIPITATION...THOUGH IT IS
CONCEIVABLE THAT DRY CONDITIONS COULD PREVAIL SINCE THE FRONT IS
MOISTURE STARVED. LOOKING AHEAD FOR SATURDAY NIGHT AND SUNDAY...HAVE
KEPT THAT PORTION DRY UNTIL CERTAINTY INCREASES. TEMPERATURES
EXPECTED NEAR NORMAL AS WELL FOR THE END OF THE WEEKEND.
AVIATION
TONIGHT AND MONDAY
AVIATION CONCERNS LIE IN THE POTENTIAL FOR MVFR/IFR CEILING
DEVELOPMENT FOR TONIGHT INTO MONDAY. MODELS HAVE BEEN CONSISTENT
WITH BLOSSOMING LOW LEVEL SATURATION ACROSS KRST/KLSE AFTER 21Z
TODAY...CONTINUING THROUGH 07-10Z MONDAY WHEN A COLD FRONT SWEEPS
EASTWARD ACROSS THE REGION...SCOURING OUT THIS CLOUD DECK. 08.12Z
NUMERICAL GUIDANCE NOT AS PESSIMISTIC AS PREVIOUS RUNS WHICH
DEPICTED A PROLONGED PERIOD OF IFR CONDITIONS. LATEST SATELLITE
IMAGERY SHOWS SOME LOWER CLOUDS OVER TX/SOUTHERN PLAINS...AND THE
08.12Z RUN/NAM12 ARE CAPTURING THIS. HOWEVER...THEY ALSO DEPICT 925
MB SATURATION OVER IA...WHICH IS MOSTLY CLOUD FREE AT THE MOMENT.
MOST OF THIS SUPPOSED LOW...NEAR SFC MOISTURE IS ALSO SOUTH OF THE
WARM FRONT...WHICH SHOULD STRETCH ACROSS NORTHERN WI BY 00Z TONIGHT.
WOULD EXPECT A BETTER CHANCE FOR LOW STRATUS NORTH OF THE FRONT
RATHER THAN SOUTH OF IT. BELIEVE THE LATEST RUC13 IS HANDLING THE
CLOUD SCENARIO THE BEST...KEEPING THE TAF SITES CLEAN OF LOW CLOUDS
UNTIL THE COLD FRONT MOVES IN...AFTER 00Z. EVEN THEN...CONDITIONS
ARE MORE FAVORABLE TO STAY VFR RATHER THAN DROP INTO THE MVFR/IFR
CATEGORIES. HAVE TRENDED THE 18Z TAFS THIS WAY.
ARX WATCHES/WARNINGS/ADVISORIES
WI...NONE.
MN...NONE.
IA...NONE.
SHORT/LONG TERM...AJ
AVIATION...REICK
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