Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 010449
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1149 PM CDT THU JUL 31 2014

.UPDATE...
ISSUED AT 1149 PM CDT THU JUL 31 2014

DESPITE LOSS OF DAYTIME HEATING WHICH ALLOWED PRECIPITATION TO
DISSIPATE ACROSS THE FORECAST AREA...SHOWERS AND STORMS HAVE
REGENERATED ACROSS CLARK COUNTY AND NEIGHBORING EAU CLAIRE COUNTY.
RAP SOUNDINGS DEPICT PLENTY OF ELEVATED INSTABILITY LIFTED FROM
930MB...AROUND 500-1000 J/KG THROUGH MUCH OF THE NIGHT. THERE HAS
BEEN SOME DISSIPATION TO THE SHOWERS THAT POPPED UP NORTHWEST OF
EAU CLAIRE...SO BELIEVE THE SAME WILL EVENTUALLY TRANSPIRE IN A
FEW HOURS OVER CLARK COUNTY. HAVE ALIGNED THE FORECAST
PRECIPITATION CHANCES TO FOLLOW THIS TREND. FOR WHAT ITS
WORTH...MESOSCALE AND DETERMINISTIC MODELS ARE REALLY STRUGGLING
WITH THIS RECENT ACTIVITY. ALSO TO NOTE IS THAT SOME OF THE CORES
HAVE INTENSIFIED ENOUGH TO PRODUCE AT LEAST SMALL HAIL...FLIRTING
EVEN WITH SEVERE LIMITS. HAVE THUS ISSUED AN SPS FOR CLARK
COUNTY.

&&

.SHORT TERM...(THIS EVENING THROUGH FRIDAY)
ISSUED AT 321 PM CDT THU JUL 31 2014

FORECAST CONCERNS ARE FOG POTENTIAL OVERNIGHT AND PRECIPITATION
CHANCES FRIDAY AFTERNOON/EVENING.

AS OF 3 PM...WEAK SHORTWAVE ROTATING ALONG THE PERIPHERY OF THE
MAIN 500 MB LOW THIS AFTERNOON WILL CONTINUE SCATTERED SHOWERS
AND STORMS MAINLY ALONG AND NORTHEAST OF I-94 IN WISCONSIN. THIS
ACTIVITY IS DIURNALLY DRIVEN SO EXPECTING IT TO WANE TOWARD
SUNSET. OTHERWISE...ANOTHER SEASONABLE DAY FOR THE UPPER MS RIVER
VALLEY WITH TEMPERATURES IN THE UPPER 70S TO LOWER 80S.

BAGGY SURFACE WIND FIELD TONIGHT IN CONJUNCTION WITH MODEST DEWPOINT
RECOVERY COULD RESULT IN PATCHY RIVER VALLEY FOG OVERNIGHT.
COMPARED TO THE PAST FEW NIGHTS...WINDS THROUGH 500 MB ARE NOTABLY
LIGHT /LESS THAN 10KTS/ WHICH WILL AID IN FOG FORMATION.
HOWEVER...UNCERTAINTY REMAINS ON HOW MUCH BOUNDARY LAYER
SATURATION IS EXPECTED TO TAKE PLACE...AND LATEST GUIDANCE IS NOT
SUGGESTING MUCH. AS SUCH...WILL LIMIT FOG COVERAGE TO ONLY PATCHY
ALONG THE RIVER VALLEYS...AND SOME OF THE LOW-LYING BOG AREAS OF
CENTRAL WISCONSIN.

AFTER ANY MORNING FOG BURNS OFF...ANOTHER PIECE OF UPPER LEVEL
ENERGY WILL PIVOT THROUGH THE UPPER MIDWEST...AS SEEN FROM 500-300
MB PV ADVECTION WHICH PEAKS DURING THE AFTERNOON AND EVENING
HOURS. SUFFICIENT SURFACE HEATING /LOW 80S/ COUPLED WITH COOLER
500 MB TEMPERATURES /-16C/ AND SOME MID LEVEL DRYING WILL LEAD TO
RATHER IMPRESSIVE 850-500 MB LAPSE RATES ON THE ORDER OF 7 C/KM BY
THE AFTERNOON. AT THE SAME TIME...MAX SURFACE BASED CAPE VALUES
ACROSS MUCH OF THE AREA LOOK TO BE AROUND 1500 TO 1700 J/KG.
COMPARED TO PAST FEW DAYS...THE CAPE ENVIRONMENT IS MORE NOTABLE
WITH NCAPE VALUES AROUND 0.2 FROM RST TO LSE TO NEC.
FINALLY...FREEZING LEVELS WILL BE LOWER...ON THE ORDER OF 10 TO 12
KFT WITH A FAIR AMOUNT OF THE CAPE PROFILE IN THE HAIL GROWTH
ZONE. HOWEVER...THE SHEAR ENVIRONMENT LEAVES A LOT TO BE
DESIRED...WITH PRACTICALLY NO DIRECTIONAL OR SPEED SHEAR NOTED.

IN SHORT...THE COMBINATION OF THE UPPER LEVEL ENERGY...STEEP
LAPSE RATES...AND AN UNCAPPED ENVIRONMENT LOOKS TO BE SUFFICIENT
FOR VIGOROUS UPDRAFTS TO DEVELOP BY MID AFTERNOON...LASTING INTO
THE EARLY/MID EVENING HOURS. THE 31.12Z DETERMINISTIC AND HI-RES
MODELS SHOW A FAIR AMOUNT OF COVERAGE OF SHOWERS/STORMS THROUGH
EVENING...SUPPORTING THE ABOVE ENVIRONMENTAL PARAMETERS. THE MAIN
THREATS WOULD SEVERE HAIL ALONG WITH GUSTY WINDS. THE SUB CLOUD
LAYER IS RATHER DRY SEEN BY THE INVERTED-V TYPE SOUNDINGS OFF THE
NAM/GFS.

HAVE INCREASED PRECIPITATION CHANCES FOR THE LATE AFTERNOON HOURS
ACROSS THE AREA...ESPECIALLY EAST OF THE MISSISSIPPI RIVER WHERE
THE BETTER ENVIRONMENTAL SETUP RESIDES. WHILE MOST OF THE ACTIVITY
WILL BE DIURNALLY DRIVEN AND BEGIN TO WANE TOWARD SUNSET...THERE
IS STILL CONSIDERABLE 500-300 MB PV ADVECTION THROUGH 06Z SO HAVE
CARRIED HIGHER CHANCE POPS ACROSS SOUTHWEST WISCONSIN THROUGH
03Z...THEN TAPERED TO SLIGHT CHANCES BY MIDNIGHT.

.LONG TERM...(FRIDAY NIGHT THROUGH THURSDAY)
ISSUED AT 321 PM CDT THU JUL 31 2014

FORECAST IN THE EXTENDED FOCUSED ON WEAK FRONTAL PASSAGE
SUNDAY/SUNDAY NIGHT...FOLLOWED BY A MORE ACTIVE WEATHER PATTERN BY
THE MIDDLE OF NEXT WEEK.

SEASONABLE TEMPERATURES WITH WEAK RIDGING WILL BE IN PLACE
SATURDAY...BUT THIS BEGINS TO BREAK DOWN BY SUNDAY AS WEAK UPPER
LEVEL SHORTWAVES MOVE THROUGH THE FLOW. ONE SUCH SHORTWAVE TROUGH
ATTEMPTS TO PUSH A FRONT THROUGH SUNDAY AFTERNOON/NIGHT...WITH THE
GREATEST SHOWER/STORM CHANCES ALONG AND NORTH OF I-94.

AN UNSETTLED WEATHER PATTERN IS BECOMING MORE CONSISTENT BY
MONDAY THROUGH MUCH OF NEXT WEEK. MONDAY AND TUESDAY COULD FEATURE
A COUPLE DIFFERENT SHORTWAVES...BUT BY TUESDAY NIGHT THE 31.12Z
MODEL CYCLE CONTINUES TO SHOW CYCLOGENESIS TAKING PLACE ACROSS THE
CENTRAL PLAINS. THIS WOULD DRAPE A WARM FRONT SOMEWHERE THROUGH
THE FORECAST AREA IN CONJUNCTION WITH A RATHER VIGOROUS UPPER
LEVEL SHORTWAVE. COMBINED WITH INCREASING 850 MB MOISTURE
TRANSPORT AND SURFACE DEWPOINTS 65 TO 70 F...AND THE THREAT FOR
SHOWERS/STORMS WILL BE ON THE RISE. THE PAST FEW MODEL CYCLES HAVE
SHOWN THIS SIGNAL...SO INCREASED PRECIPITATION CHANCES TO THE
60-70 PERCENT RANGE. IT WILL ALSO BE THIS PERIOD THAT WILL HAVE
TO BE WATCHED FOR POSSIBLE SEVERE WEATHER ACROSS THE UPPER
MISSISSIPPI RIVER VALLEY...WITH THE STRONG DYNAMICS IN PLACE
COMBINED WITH WARM AND MOIST ENVIRONMENT.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z FRIDAY NIGHT)
ISSUED AT 1149 PM CDT THU JUL 31 2014

LIGHT WINDS AND CLEAR SKIES EARLY THIS MORNING BRINGS UP CONCERN
FOR VALLEY FOG AT LSE. TEMPERATURE/DEWPOINT SPREADS HAVE REALLY
LOWERED TO A POINT WHERE THEY ARE FAVORABLE FOR VALLEY FOG. THE
ONLY ISSUE IS IT HAS BEEN A FEW DAYS SINCE THE LAST MEASURABLE
RAIN. WITH THE FAVORABLE TEMPERATURE/DEWPOINT SPREAD...HAVE
LOWERED THE CEILING AND VISIBILITY IN THE 10-12Z TEMPO GROUP TO
LIFR. ANY FOG AND STRATUS SHOULD QUICKLY DISSIPATE AFTER 13Z.

VFR CONDITIONS ARE GENERALLY EXPECTED OTHERWISE THROUGH THE
EVENING. HOWEVER...THERE IS POTENTIAL FOR SOME SHOWERS AND STORMS
TO PASS THROUGH THE TAF SITES LATE IN THE AFTERNOON / EARLY
EVENING. CONTINUED TO MENTION A VCTS FOR THIS POTENTIAL.

LOOKING AHEAD...ANOTHER VALLEY FOG NIGHT IS POSSIBLE FOR LSE FOR
LATE FRIDAY NIGHT.

&&

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

$$

UPDATE...AJ
SHORT TERM...ZT
LONG TERM....ZT
AVIATION...AJ


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