Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 211200
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
700 AM CDT SUN SEP 21 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 203 AM CDT SUN SEP 21 2014

MAIN CONCERNS ARE RAIN CHANCES AND CLOUD TRENDS TODAY...FOLLOWED BY
VALLEY FOG POTENTIAL TONIGHT

CURRENTLY AS OF 07Z...WATER VAPOR LOOP AND RAP 500MB HEIGHT ANALYSIS
SHOWED AN UPPER LOW OVER CENTRAL CALIFORNIA...STRONG RIDGING OVER
MUCH OF WESTERN NORTH AMERICA...AND A POTENT UPPER TROUGH MOVING
THROUGH THE WESTERN GREAT LAKES REGION. IT WAS THIS UPPER TROUGH...
WHICH MOVED IN A LITTLE FASTER THAN PREVIOUSLY ANTICIPATED...THAT
BROUGHT THE BAND OF SHOWERS AND STORMS THROUGH LAST EVENING. THE
FASTER ARRIVAL TIME ALLOWED FOR A BETTER JUXTAPOSITION WITH DAYTIME
HEATING...RESULTING IN THE ISOLATED SEVERE WEATHER IN SOUTHEAST
MN/NORTHEAST IA. STRONG SUBSIDENCE AND DRYING WITHIN THE UPPER
TROUGH...AS SEEN ON THE 00Z ABR AND MPX SOUNDINGS...HAS LED TO
MAINLY CLEAR SKIES SOUTHWEST OF I-94. NORTH OF I-94 THROUGH NORTHERN
MN AND NORTHERN WI...EXTENSIVE CLOUDS ALONG WITH A BAND OF SHOWERS
EXISTS. THIS BAND OF SHOWERS IS A REFLECTION OF MID-LEVEL
FRONTOGENESIS TAKING PLACE ON THE NORTH SIDE OF THE POTENT UPPER
TROUGH. ADDITIONALLY...THE SHOWERS RESIDE ALONG A SECONDARY COLD
FRONT. REGARDING COLD AIR...RAP SHOWS 925MB TEMPS HAVE FALLEN
ALREADY TO 11-14C...DOWN FROM 18-20C AT 00Z. A BRISK 30-45KT WIND AT
925MB HAS HELPED TO ADVECT THIS COLD AIR INTO THE REGION.

THE POTENT UPPER TROUGH OVER OUR REGION IS PROGGED TO CONTINUE
MARCHING EAST-SOUTHEAST AT A FAST CLIP...DIGGING DOWN INTO THE OHIO
VALLEY BY 00Z MONDAY AND APPALACHIA BY 12Z MONDAY. AS THE UPPER
TROUGH MOVES AWAY FROM THE REGION...IT HELPS PULL THE MID-LEVEL
FRONTOGENESIS BAND OF SHOWERS AND ASSOCIATED SECONDARY COLD FRONT
SOUTHWARD THROUGH THE FORECAST AREA TODAY. HOWEVER...MAJORITY OF
MODELS SUGGEST THAT THE FRONTOGENESIS BAND OF SHOWERS FALLS APART
THROUGH THE MORNING AS IT DROPS SOUTH. THUS...PRECIPITATION CHANCES
GENERALLY REMAIN IN THE 20-40 RANGE. CLOUDS SHOULD SLOWLY BREAK UP
THROUGH THE DAY...DUE TO THE UPPER TROUGH MOVING AWAY FROM THE
REGION AND ASSISTANCE FROM DAYTIME HEATING. ANY CELLULAR CLOUDS LATE
IN THE AFTERNOON WILL THEN DISSIPATE WITH LOSS OF HEATING THIS
EVENING. A MUCH COOLER DAY IS ON TAP WITH 925MB TEMPS PROGGED TO
HOLD STEADY BETWEEN 8-10C BETWEEN 12-18Z...A BREEZY NORTH WIND AND
CLOUDS. HIGHS IN THE 60S SEEM REASONABLE...THOUGH SOME CENTRAL WI
LOCATIONS MAY STRUGGLE TO REACH 60.

THE SURFACE RIDGE OF HIGH PRESSURE CURRENTLY SITUATED ALONG THE LEE
OF THE ROCKIES IS PROGGED TO BUILD FULLY OVER THE REGION BY 12Z
MONDAY. THIS RIDGE WILL ALLOW WINDS TO DECOUPLE...AND GIVEN TODAYS
GRADUALLY CLEARING OF CLOUDS AND RECENT PRECIPITATION SHOULD LEAD TO
A DECENT SETUP FOR VALLEY FOG. TRIBUTARIES ARE EXPECTED TO BE MOST
SUSCEPTIBLE...AND EVEN THE MAIN MS CHANNEL MAY ENTIRELY FOG UP TOO.
THE ONLY THING THAT COULD DISRUPT THE MS CHANNEL FROM FOGGING UP IS
IF THERE ARE SOME SCATTERED LINGERING CLOUDS FROM TODAY. A COOL
NIGHT IS LIKELY AS WELL...WITH SOME OF THE TYPICAL COLD SPOTS LIKELY
FALLING INTO THE MID-UPPER 30S. NO FROST ANTICIPATED AT THIS
TIME...SINCE THESE SITES SHOULD SEE FOG INSTEAD.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 203 AM CDT SUN SEP 21 2014

A SERIES OF STRONG TROUGHS MARCHING ACROSS THE EAST PACIFIC OVER THE
NEXT FEW DAYS WILL HELP TO PUSH THE UPPER RIDGE THATS OVER WESTERN
NORTH AMERICA INTO THE FORECAST AREA BY TUESDAY. SUBSIDENCE UNDER
THIS UPPER RIDGE...COMBINED WITH LOW PRECIPITABLE WATER VALUES OF
0.5-0.7 INCHES AND HIGH PRESSURE AT THE SURFACE...WILL YIELD DRY
CONDITIONS AND PLENTY OF SUNSHINE. THIS SUN SHOULD HELP MODIFY THE
COOL AIRMASS THAT CAME INTO THE AREA TODAY...REFLECTED BY MODEL
925MB TEMPS CLIMBING TO 12-14C ON MONDAY AND AROUND 14C FOR TUESDAY.
THE SETUP CONTINUES TO BE FAVORABLE FOR VALLEY FOG AT NIGHT...AND IN
FACT SOUNDINGS DEPICT A VERY DEEP LIGHT WIND FIELD FOR 12Z TUESDAY.

FOR TUESDAY NIGHT THROUGH WEDNESDAY NIGHT...THOSE SAME EAST PACIFIC
TROUGHS WILL HELP EJECT THE UPPER LOW OVER CENTRAL CALIFORNIA AND
LIFT IT INTO OUR REGION. HOWEVER...THE UPPER LOW LOOKS TO SHEAR OUT
AS IT RUNS INTO THE BUILT UP RIDGING AHEAD OF IT. INCREASING
MOISTURE RETURN AHEAD OF THE SHEARING OUT UPPER LOW...REFLECTED BY
PRECIPITABLE WATER CLIMBING TO 1-1.5 INCHES WEST OF I-35 AT 12Z
WEDNESDAY...AND DPVA SHOULD HELP TO INDUCE SOME SHOWER ACTIVITY.
HOWEVER...21.00Z GUIDANCE CONTINUES TO TREND DRIER. IN FACT...THERE
ARE INDICATIONS NOW THAT ANY SHOWER ACTIVITY THAT DOES FORM TO OUR
WEST TUESDAY NIGHT MAY NOT EVEN HOLD TOGETHER BEFORE MOVING INTO THE
FORECAST AREA FOR WEDNESDAY AND WEDNESDAY NIGHT. THIS IS DESPITE
THOSE HIGHER PRECIPITABLE WATER VALUES MOVING INTO THE FORECAST
AREA. HAVE THEREFORE CUT PRECIPITATION CHANCES MORE. ALSO REMOVED
THUNDER MENTION AS ANY INSTABILITY...EVEN IN THE WETTER 21.00Z
ECMWF RUN...STAYS ACROSS THE PLAINS. WITH LESS PRECIPITATION
FORECAST...925MB TEMPS DONT COOL AS MUCH AS PREVIOUSLY
EXPECTED...RESULTING IN WARMER HIGHS FOR WEDNESDAY.

FOR THURSDAY INTO NEXT WEEKEND...MODELS REMAIN CONSISTENT IN A VERY
DEEP TROUGH OVER THE EAST PACIFIC AT 12Z WEDNESDAY MOVING ONSHORE BY
SATURDAY. A LITTLE BIT OF UNCERTAINTY EXISTS BEYOND THEN WITH THE
PROGRESSION OF THE TROUGH...OR IF IT CUTS OFF. THE DOWNSTREAM
RESPONSE IS TO CONTINUE TO HOLD AND EVEN AMPLIFY RIDGING ACROSS THE
EASTERN TWO THIRDS OF THE COUNTRY...WITH 500MB HEIGHTS APPROACHING
590 DAM OVER OUR AREA. A FUN LITTLE TIDBIT...LOOKING AT THE CIPS
ANALOG PAGE...TO FIND A SIMILAR 500MB PROG ONE HAS TO GO BACK TO
EARLY OCT 2010. IN ANY EVENT...THE STRONG UPPER RIDGING COMBINED
WITH HIGH PRESSURE AT THE SURFACE WILL KEEP THE AREA DRY. EXPECT
AIRMASS MODIFICATION TO GRADUALLY WARM TEMPERATURES EACH DAY...WITH
READINGS LIKELY ABOVE NORMAL MUCH OF THE PERIOD. THE WARMEST AIR
STILL LOOKS TO REMAIN WELL WEST OF THE AREA OVER THE WESTERN
PLAINS...WHERE STRONGER RETURN FLOW EXISTS.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z MONDAY MORNING)
ISSUED AT 652 AM CDT SUN SEP 21 2014

A TROUGH OF LOW PRESSURE WILL PRODUCE SHOWERS AS IT MOVES SOUTH
THROUGH THE AREA THIS MORNING. THESE SHOWERS WILL LIGHT ENOUGH
THAT THERE SHOULD NOT BE MUCH REDUCTION OF VISIBILITIES FROM THEM.
CEILINGS WILL BE MVFR AT BOTH TAF SITES THROUGH 21.18Z...AND THEN
THESE CLOUDS WILL SCATTER OUT. NORTHWEST WINDS WILL GUST UP TO
25 KNOTS THIS MORNING...AND UP TO 20 KNOTS THIS AFTERNOON.

WHILE THE MODEL SOUNDINGS ARE CONTINUING TO SHOW LIGHT WINDS IN
THE LOWEST HALF KILOMETER...THEY ALSO SHOW QUITE A BIT OF DRY AIR
IN THIS LAYER TOO. HOWEVER WITH COOLER AIR OVER THE RELATIVELY
WARM MISSISSIPPI RIVER CHANNEL...COULD NOT RULE OUT PATCHY TO
AREAS OF VALLEY FOG. WITH COVERAGE A QUESTION...JUST WENT WITH
BCFG FOR NOW.

&&

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

$$

SHORT TERM...AJ
LONG TERM...AJ
AVIATION...BOYNE



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