Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 172018
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
318 PM CDT WED SEP 17 2014

.SHORT TERM...(THIS EVENING THROUGH THURSDAY NIGHT)
ISSUED AT 318 PM CDT WED SEP 17 2014

AS OF MID AFTERNOON...WATER VAPOR IMAGERY COMBINED WITH 500 MB
RAP HEIGHTS SHOWED LONGWAVE TROUGHING OVER THE GREAT LAKES REGION
WITH SUBTLE SHORTWAVE MOVING THROUGH NORTHERN WISCONSIN. TO THE
WEST...THE REMNANTS OF OLDIE ARE LAID OUT ACROSS SOUTHWEST CONUS
IN THE MIDST OF A LARGE SCALE RIDGE OVER THE INTERMOUNTAIN WEST.

AT THE SURFACE...BACKDOOR COLD FRONT LIES FROM THE BOUNDARY
WATERS REGION OF NORTHERN MINNESOTA THROUGH THE U.P. OF
MICHIGAN...DENOTED BY NORTHEASTERLY FLOW AND LOW STRATUS IN ITS
WAKE. IT IS THIS FEATURE THAT WILL BE THE NEAR/SHORT TERM FORECAST
CONCERN.

17.12Z GUIDANCE IN REASONABLE AGREEMENT SHOWING THE FRONT
ADVANCING SOUTHWESTWARD TONIGHT...REACHING THE I-94 CORRIDOR OF
CENTRAL WISCONSIN AFTER MIDNIGHT. 925 TO 850 MB RH FIELDS SUPPORT
STRATUS DECK CONTINUING ITS MARCH...HOLDING UP JUST ALONG AND EAST
OF THE MS RIVER BY SUNRISE THURSDAY. INCREASED OVERNIGHT SKY COVER
QUITE A BIT ACROSS WEST CENTRAL WISCONSIN. TEMPERATURES DO FALL
OFF WITH ITS PASSAGE...SO STILL LOOKING AT LOWS IN THE UPPER 30S
FOR THOSE AREAS DESPITE CLOUD COVER.

THE REST OF THE FORECAST AREA WILL BE COMPLICATED BY FOG AND/OR
DEVELOPING STRATUS POTENTIAL OVERNIGHT. WIND PROFILES FROM
KLSE/KRST/KDEH AHEAD OF THIS FRONT ARE VERY LIGHT THROUGH
09Z...LESS THAN 10 KTS UP TO 700 MB...THOUGH BEGIN TO INCREASE
BETWEEN 09-12Z. ENVISION INITIALLY PATCHY TO MAYBE AREAS OF FOG
CONCENTRATED IN THE MS AND WISCONSIN RIVER VALLEYS/TRIBS BUT ALSO
GUIDANCE SUGGESTS PATCHY FOG IS POSSIBLE ACROSS NORTHEAST IOWA AND
SOUTHEAST MINNESOTA. AS WINDS AT BLUFF TOP INCREASE...FOG SHOULD
DISSIPATE AND BECOME BROKEN STRATUS. 17.12Z HI-RES MODELS AND SREF
PROBABILITIES SUGGEST THIS THINKING AS WELL.

.LONG TERM...(FRIDAY THROUGH WEDNESDAY)
ISSUED AT 318 PM CDT WED SEP 17 2014

FOCUS IN THIS PERIOD IS ON SHOWER/STORM CHANCES FRIDAY AND
SATURDAY AS WEATHER SYSTEM PASSES THROUGH THE UPPER MIDWEST. BRIEF
UPPER LEVEL RIDGING FRIDAY BECOMES ZONAL FRIDAY AFTERNOON AS
SHORTWAVE TRAVERSES NORTHERN PLAINS. THERE COULD BE SOME FRIDAY
MORNING WARM AIR ADVECTION SHOWERS WEST OF THE MS RIVER BUT IS
LIKELY TO BE WEST OF THE FORECAST AREA. AS THE CAP
STRENGTHENS...THIS WILL SHUT OFF AND/OR MOVE PRECIP TO THE
NORTHEAST. THE BULK OF THE AREA WILL BE CAPPED OFF FRIDAY AS PLUME
OF WARM 700 MB AIR /+12C/ OFF THE HIGH PLAINS NOSES INTO THE UPPER
MIDWEST. THE GFS/NAM ARE AGGRESSIVE WITH THIS CAP...SHUTTING OFF
ANY PRECIP CHANCE DURING THE DAY UP TO KMSP/KEAU AREAS. STRONG
SOUTHERLY FLOW WILL MAKE FOR A BREEZY DAY...ESPECIALLY ACROSS OPEN
AREAS OF SOUTHERN MN. WIND GUSTS 25 TO 35 MPH LIKELY LATE MORNING
THROUGH AFTERNOON.

FINALLY BY FRIDAY EVENING SOME COOLING AND FALLING HEIGHTS WITH
SHORTWAVE TROUGH MOVING THROUGH SHOULD HELP TO BREAK CAP AND ALLOW
CONVECTION TO DEVELOP ACROSS CENTRAL MINNESOTA TO NORTHWEST
WISCONSIN. SHEAR PROFILES ARE IMPRESSIVE NOTED BY 0-6 KM BULK
VALUES 30+ KTS WITH SOME CURVATURE IN THE HODOGRAPHS IN THE
LOWEST 3 KM. AT THE SAME TIME...WARM CLOUD DEPTHS ARE 3.5 TO 4.0
KM...WHICH WILL LIMIT HIGHER HAIL THREAT BUT AID IN THE HEAVY RAIN
PRODUCTION. INITIATION LOOKS TO BE CENTRAL MINNESOTA TO NORTHWEST
WISCONSIN...THEN TRACK EAST/SOUTHEAST THROUGH THE NORTHERN HALF OF
THE FORECAST AREA LATE FRIDAY EVENING. THE BULK OF THE ACTIVITY
LOOKS TO BE IN THE 03-12Z TIME FRAME. SOME STRONGER TO SEVERE
STORMS ARE POSSIBLE INITIALLY...MAINLY DAMAGING WINDS AND HEAVY
RAIN. MARGINAL SEVERE HAIL THREAT. THE GREATEST THREAT IS NORTH
OF I-90.

STILL SOME TIMING DIFFERENCES BETWEEN 17.12Z GUIDANCE BUT COLD FRONT
SHOULD SLIP THROUGH 12-18Z ACROSS MUCH OF THE AREA. AHEAD OF
IT...WEAK CAP REMAINS IN THE PLACE WHICH COULD LIMIT COVERAGE OF
SHOWERS/STORMS WITH THE HIGHER CHANCES LOOKING TO BE NORTHEAST
IOWA INTO SOUTHWEST WISCONSIN.

COOL CANADIAN HIGH PRESSURE THEN LOOKS TO BUILD IN SUNDAY THROUGH
WEDNESDAY PROVIDING THE UPPER MIDWEST WITH SEVERAL
PLEASANT...SEASONABLY COOL DAYS. HIGHS IN THE MIDDLE/UPPER 60S AND
LOWS IN THE 40S.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 105 PM CDT WED SEP 17 2014

PLAN ON VFR CONDITIONS PREVAILING THIS AFTERNOON INTO THIS
EVENING. ATTENTION THEN TURNS TO CLEAR SKIES AND LIGHT WINDS LATER
TONIGHT AND THE POTENTIAL FOR SOME RIVER VALLEY FOG VCNTY KLSE.
NORMALLY WOULD GO WITH THE THINKING THAT FOG/STRATUS WOULD BE A
SLAM DUNK GIVEN LIGHT WINDS/CLEAR SKIES...BUT THERE WILL BE A BIT
OF A CONUNDRUM WITH PASSAGE OF A COLD FRONT AND EXPECTED MVFR CLOUD
COVER IN ITS WAKE. MEDIUM LEVEL OF CONFIDENCE THAT THERE WILL BE
SOME INCOMING MVFR CLOUD COVER WITH THE FRONTAL PASSAGE AT KLSE AND
KRST AS WELL PER LATEST MODEL OUTPUT. DUE TO THIS INCREASING
CONFIDENCE IN MVFR CLOUD COVER...HAVE BACKED OFF ON IFR CONDITIONS
THAT WAS PREVIOUSLY FORECASTED AT KLSE AFTER 09Z. THINKING THAT
THE COMBINATION OF THIS MVFR CLOUD COVER //WHICH WOULD ACT TO
MITIGATE RADIATIONAL COOLING/RIVER VALLEY FOG// AND LIGHT
NORTHEASTERLY FLOW AT BLUFFTOP LEVEL WILL KEEP THE VALLEY MIXED A
BIT. THINKING THERE WILL BE SOME LOWER STRATUS IN THE MISSISSIPPI
RIVER CHANNEL...BUT NOT EXTENSIVE ENOUGH FOR PREVAILING
CONDITIONS AT KLSE. KEPT KRST VFR TIL 09Z...THEN BROUGHT IN 4SM
BR SCT025 IN THE 09-14Z TIME FRAME. LOOKS LIKE MVFR CLOUDS WILL
THEN BECOME PREVALENT 14-18Z.

&&

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

$$

SHORT TERM...ZT
LONG TERM....ZT
AVIATION.....DAS



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