Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 211725
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1225 PM 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 18Z TAFS THROUGH 18Z MONDAY AFTERNOON)
ISSUED AT 1225 PM CDT SUN SEP 21 2014

HIGH PRESSURE WILL BUILD INTO THE REGION TONIGHT. MODELS INDICATING
THE N-S RIDGE AXIS WILL HANG UP JUST WEST OF THE MISSISSIPPI RIVER.
BUKIT MODEL SOUNDING SHOWING THE LOWER BOUNDARY LAYER IN THE
MISSISSIPPI RIVER CHANNEL DECOUPLING WITH SURFACE WINDS BECOMING
LIGHT AND VARIABLE AFTER MIDNIGHT. THE COMBINATION OF LINGERING
CLOUD COVER TODAY WITH CLEARING THIS EVENING...AND GIVEN RELATIVELY
MOIST LOWER BOUNDARY LAYER FROM RECENT RAINFALL...ALONG WITH LACK OF
GOOD MIXING WITH DECOUPLING WINDS...IS PRODUCING INCREASING
CONFIDENCE FOR SOME FOG FORMATION IN THE MISSISSIPPI RIVER CHANNEL
LATE TONIGHT/EARLY MONDAY MORNING. THINKING NORTHWEST WINDS AT 5-
10KT AT BLUFFTOP MAY HELP TO STIR FOG MORE INTO A STRATUS LAYER.
HOWEVER...NOT CONFIDENT THAT THIS FOG/STRATUS WILL SET UP DIRECTLY
OVER THE KLSE AIRPORT. PREVIOUS FORECAST HAD P6SM BCFG/SCT002
MENTION FROM 09-12Z...AND THIS SEEMS REASONABLE GIVEN AFOREMENTIONED
SIGNALS. PLAN ON STAYING WITH THIS WITH 18Z ISSUANCE AND HAVE THE
EVENING SHIFT FORECASTER MONITOR CLOSELY FOR POSSIBLE DETERIORATING
CONDITIONS. THINKING KRST WILL STAY VFR GIVEN MORE STIRRING/DRYING
WITH WEAK WEST-NORTHWEST WINDS.

&&

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

$$

SHORT TERM...AJ
LONG TERM...AJ
AVIATION...DAS


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