Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 110906
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
406 AM CDT THU SEP 11 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 406 AM CDT THU SEP 11 2014

MAIN FORECAST CONCERNS CLOUD/TEMPERATURES TRENDS...SMALL -SHRA
CHANCE WEST OF THE MS RIVER LATE TONIGHT.

DATA ANALYSIS AT 06Z HAD A 992MB LOW OVER EASTERN ONT WITH THE COLD
FRONT SOUTHWEST THRU THE OH VALLEY TO CENTRAL TX. A LARGE 1039MB
CANADIAN HIGH CENTERED OVER SOUTHERN ALB EXTENDED TO HUDSONS BAY AND
TO NORTHERN TX. PRESSURE GRADIENT OVER THE REGION BETWEEN THE HIGH/
LOW WAS SLOWLY RELAXING...BUT NORTH WINDS OF 10-20 MPH REMAINED
COMMON ACROSS MUCH OF WI/IA/SOUTHERN MN. FOG-PRODUCT IMAGERY/SFC OBS
AND RA-OBS SHOWED AN EXPANSIVE AREA OF LOWER LEVEL MOISTURE/CLOUDS
ACROSS THE MID/UPPER MS VALLEY EARLY THIS MORNING...TRAPPED IN/UNDER
AN INVERSION BETWEEN 900-850MB.

11.00Z MODEL RUNS HAVE INITIALIZED WELL...WITH A TIGHT CONSENSUS
FAVORING A COMPROMISE OF THE EARLIER RUNS ON STRENGTH/LOCATION OF
FEATURES OVER CENTRAL NOAM. CONSENSUS THRU 00Z FRI FAVORS A
COMPROMISE OF THE EARLIER RUNS THEN SLOWER OF THE EARLIER RUNS
TONIGHT AS THE STRONG SHORTWAVE/TROUGH MOVES INTO THE NORTHERN
PLAINS. FASTER OUTLIER GFS HAS COME MORE IN LINE WITH THE OTHER
MODELS...WHICH IS MORE IMPORTANT TO THE FCST JUST BEYOND THE
SHORT- TERM. GIVEN THE TIGHT CONSENSUS...SHORT TERM FCST
CONFIDENCE IS GOOD THIS CYCLE.

CLOUDS WILL BE STUBBORN AND PROBLEMATIC TODAY. MODEL SOUNDINGS
SHOWING THE INVERSION LIFTING A BIT TODAY BUT REMAINING STRONG. 925-
850MB GRADIENT RELAXES WITH WINDS AT THESE LEVELS 10-15KTS BY THIS
AFTERNOON...AND ADVECTION OF THIS MOISTURE/CLOUDS OUT OF THE AREA
WILL BE SLOWER. WITH THE 850MB COLD POOL /0C TO +4C/ OVER THE AREA
STEEP SFC-850MB LAPSE RATES WILL ALLOW FOR GOOD MIXING...STRATO-CU
REDEVELOPMENT IF ANY CLEARING DOES OCCUR. ON THE PLUS SIDE FOR
DECREASING CLOUDS TODAY THE SFC-850MB FLOW DOES BECOME ANTI-
CYCLONIC...ESPECIALLY FOR THE AFTERNOON. WILL HOLD GENERALLY CLOUDY
SKIES OVER THE AREA THIS MORNING THEN MAINTAIN SOME OPTIMISM FOR
PARTLY SUNNY SKIES THIS AFTERNOON. EVEN WITH FULL SUNSHINE...925 AND
850MB TEMPS ONLY SUPPORT HIGHS IN THE MID 50S TO LOW 60S TODAY. WITH
THE CLOUDS...REALISTIC HIGHS TODAY ARE MORE IN THE 50S.

SOME LOWER LEVEL MOISTURE LINGERS TONIGHT BUT SFC-850MB ANTICYCLONIC
FLOW/DIVERGENCE SHOULD CONTINUE TO HELP BREAK UP THE LOWER CLOUDS AS
THE SFC-850MB RIDGE AXIS CONTINUES TO NOSE INTO THE REGION. ANY
CLEARING TONIGHT WILL BE SHORT LIVED AS 700-500MB MOISTURE AHEAD OF
THE NORTHERN PLAINS TROUGH ALREADY STARTS TO SPREAD EAST ACROSS THE
FCST AREA BY MIDNIGHT. LEAD 500-300MB PV ADVECTION AHEAD OF THIS
TROUGH ALONG WITH INCREASING 300-320K ISENTROPIC LIFT AND 700-500MB
FN CONVERGENCE SPREAD OVER THE FCST AREA LATE TONIGHT WITH THE
INCREASE OF MOISTURE ALOFT. CONCERN REMAINS ARE MOISTURE/LIFT ALOFT
ENOUGH TO OVERCOME THE DRIER AIR/DRYING BELOW 700MB WITH THE RIDGE
OF HIGH PRESSURE. CONSISTENT SIGNAL AMONG THE MODELS THAT IT WILL BE
INCREASING MOISTURE/LIFT ALOFT WILL ENOUGH TO OVERCOME THE DRIER AIR
BELOW 700MB AND RAISED -SHRA CHANCES WEST OF THE MS RIVER LATE
TONIGHT A BIT. MOSTLY CLOUDY TO CLOUDY SKIES AGAIN AFTER MIDNIGHT
WILL KEEP LOWS FROM BOTTOMING TOO FAR TONIGHT UNDER THE CHILLY 925-
850MB AIRMASS. USED A BLEND OF THE NUMERICAL GUIDANCE FOR THE LOWS.

.LONG TERM...(THURSDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 406 AM CDT THU SEP 11 2014

MAIN CONCERNS FOR FRIDAY THRU SATURDAY NIGHT...-SHRA CHANCES FRI...
FROST/VALLEY FOG POTENTIAL LATE FRI NIGHT/EARLY SAT... TEMPERATURES
THRU THE PERIOD.

11.00Z MODELS OFFER A BETTER CONSENSUS AS THE NORTHERN PLAINS TROUGH
MOVES ACROSS THE REGION FRI/FRI NIGHT. EARLIER FAST OUTLIER GFS IS
NOW MORE IN LINE WITH OTHER SLOWER MODELS WITH THIS TROUGH. GOOD
CONSENSUS FOR SHORTWAVE RIDGING TO BUILD ACROSS THE UPPER MIDWEST
LATER FRI NIGHT INTO SAT NIGHT. FCST CONFIDENCE IN THE FRI THRU SAT
NIGHT PERIOD IS ON THE GOOD SIDE THIS CYCLE.

LATEST CONSENSUS BRINGS THE MID LEVEL TROUGH AXIS INTO THE WEST END
OF THE FCST AREA BY 00Z SAT...THEN QUICKLY EAST OF THE AREA FRI
EVENING. RATHER IMPRESSIVE 500-300MB PV ADVECTION PROGGED TO MOVE
ACROSS THE FCST AREA FRI INTO EARLY FRI EVENING...ACCOMPANIED BY THE
700-500MB FN CONVERGENCE AND 300-320K ISENTROPIC UPGLIDE. MODEL
SOUNDINGS/X-SECTIONS SHOW THE DEEPEST MOISTURE/HIGHER PW VALUES OF
AROUND 3/4 INCH ARE MAINLY OVER THE SOUTHEAST HALF TO 2/3 OF THE
FCST AREA FRI. THIS ALONG WITH THE FN CONVERGENCE AND ISENTROPIC
LIFT MORE IN THE 700-600MB LAYER. MODELS REMAIN CONSISTENT THAT THE
FORCING/LIFT ALOFT WILL BE ENOUGH TO OVERCOME THE DRIER AIR BELOW
850 TO 700MB WITH SOME -SHRA OR -RA ACROSS AT LEAST THE SOUTHEAST
HALF OF THE FCST AREA. RAISED FRI RAIN CHANCES OVER THE SOUTHEAST
1/2 TO 2/3 OF THE FCST AREA TO 50-70 PERCENT. STILL APPEARS MOST
RAIN AMOUNTS FRI WILL BE ON THE LIGHT SIDE. RAIN CHANCES EXIT
QUICKLY EAST OF THE AREA FRI EVENING AS THE TROUGH AXIS PASSES.

QUESTION THEN BECOMES HOW QUICKLY WILL THE CLOUDS CLEAR OUT ALLOWING
FOR RADIATIONAL COOLING AND VALLEY FOG OR FROST POTENTIAL ACROSS THE
AREA. MDT/STRONG DEEP LAYERED SUBSIDENCE SPREADS ACROSS THE AREA
BEHIND THE TROUGH/SHORTWAVE...WITH THE SFC-700MB RIDGE AXIS/
ANTICYCLONIC FLOW BUILDING/MOVING MN/IA/WI. CLOUDS TO LINGER LONGEST
OVER THE EAST END OF THE FCST AREA...WHERE THE THREAT OF FROST IN
THE LOW LAYING AREAS ALONG/NORTHEAST OF I-94 IS ALSO THE GREATEST
WITH A FULL NIGHT OF RADIATIONAL COOLING. RAINS FROM FRI THEN THE
SFC RIDGE AXIS BUILDING IN WITH LIGHT WINDS UNDER THE CLEARING SKIES
ALSO SETS THE STAGE FOR VALLEY FOG FORMATION. PREVIOUS GRIDSET
ALREADY INCLUDED A VALLEY FOG MENTION. WITH 850MB TEMPS AROUND 0C AT
12Z SAT AND CLEARING EXPECTED TO WORK ACROSS THE AREA OVERNIGHT...
ADDED PATCHY TO AREAS OF FROST TO GRIDS WHERE LOWS ARE EXPECTED TO
DIP INTO THE LOW-MID 30S. MAY YET NEED A FROST ADVISORY FOR LATE FRI
NIGHT/EARLY SAT MORNING FOR LOCATIONS ALONG/ NORTHEAST OF I-94.
PLENTY OF TIME TO WORK OUT THE DETAILS FOR THAT.

LOW LEVEL WARM ADVECTION RETURNS FOR SAT/SAT NIGHT AS THE RIDGE AXIS
MOVES EAST AND THE NEXT TROUGH MOVES INTO THE NORTHERN PLAINS.
QUIET/MOSTLY SUNNY/MOSTLY CLEAR AND SLOWLY WARMING TEMPS FOR SAT/SAT
NIGHT. USED THE MODEL/ ENSEMBLE CONSENSUS FOR FRI THRU SAT NIGHT
HIGHS/LOWS.

FOR SUNDAY THROUGH WEDNESDAY...

MAIN CONCERNS THIS PERIOD...SHRA CHANCES WITH THE NEXT TROUGH/FRONT
SUN NIGHT/MON...TEMPERATURES.

MODEL RUNS OF 11.00Z IN RATHER GOOD AGREEMENT FOR FALLING HGTS/
TROUGHING TO SINK INTO THE NORTHERN PLAINS SUN/MON...WITH A STRONGER
SHORTWAVE TO ROTATE ACROSS THE UPPER MIDWEST SUN NIGHT/MON.
MODELS REMAIN CONSISTENT WITH NORTHWEST FLOW ALOFT TO PERSIST
ACROSS THE NORTHEAST CONUS TUE/WED...DOWNSTREAM OF RIDGING OVER
WESTERN NOAM. GIVEN THE GOOD BETWEEN MODEL AND RUN-TO-RUN
CONSISTENCY FCST CONFIDENCE FOR SUN THRU WED IS ABOVE AVERAGE.

MOISTURE STARTS TO RETURN SUNDAY BUT MUCH OF THIS AND THE SFC-700MB
TROUGH AXIS REMAIN WEST/NORTH OF THE FCST AREA. 925MB TEMPS WARM
INTO THE +11C TO +14C RANGE BY 00Z MON WITH SUNDAY HIGHS INTO THE
LOW-MID 60S. NEXT ROUND OF FORCING/LIFT TO SPREAD ACROSS THE AREA
SUN NIGHT/MON MORNING WITH PW VALUES PROGGED TO CLIMB TO AROUND AN
INCH SUN NIGHT. MODEL CONSENSUS 20-40 PERCENT -SHRA CHANCES SUN
NIGHT/MON LOOK GOOD AT THIS TIME. SOME LOW LEVEL COLD ADVECTION
FOLLOWS IN THE WAKE OF THE TROUGH/FRONT BUT 925MB TEMPS ONLY FALL TO
AROUND +8C BY 12Z TUE. WEAK LOW LEVEL WARM ADVECTION ALREADY RETURNS
FOR TUE THRU WED HIGH HIGH PRESSURE FROM THE CENTRAL/NORTHERN
PLAINS SLOWLY DRIFTING EAST ACROSS THE MID/UPPER MS VALLEY.
MODEL/ENSEMBLE CONSENSUS HIGHS IN THE 60S MON THRU WED APPEAR WELL
TRENDED...ALONG WITH SUN NIGHT THRU TUE NIGHT LOWS MAINLY IN THE
40S.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)
ISSUED AT 1100 PM CDT WED SEP 10 2014

CLOUD SHIELD OF MOSTLY MVFR CIGS CONTINUES TO EXTEND WESTWARD INTO
THE NORTHERN PLAINS...PER LATEST FOG/STRATUS SATELLITE IMAGERY AND
SFC OBS. BUFKIT AND RH TRENDS ARE TO KEEP THESE LOW CLOUDS ACROSS
THE TAF SITES THROUGH WED MORNING. AFTER THAT...MODELS START TO
DISAGREE WITH HOW QUICKLY THE CIGS WILL LIFT...WITH THE GFS BEING
THE QUICKER. VFR CIGS ARE EVIDENT OVER CENTRAL MN...AND 850 MB WINDS
ARE NEARLY DUE SOUTH OVERNIGHT THROUGH WED MORNING...BUT 700 MB
WINDS WESTERLY. THE BUMP IN CIGS COULD COME EARLIER THAN PREVIOUSLY
THOUGHT...AND HAVE SKETCHED THIS INTO THE FORECAST - ALTHOUGH STILL
PESSIMISTIC WITH A LATE AFTERNOON IMPROVEMENT. THIS WILL BE THE MAIN
STICKING POINT FOR THE FORECAST.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....RIECK



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