Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 312325
AFDARX

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

.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 00Z TAFS THROUGH 00Z FRIDAY EVENING)
ISSUED AT 625 PM CDT THU JUL 31 2014

A RIDGE OF HIGH PRESSURE BUILDING INTO THE TAF SITES TONIGHT WILL
ALLOW VFR CONDITIONS TO CONTINUE. HOWEVER...LIGHT WINDS ASSOCIATED
WITH THIS RIDGE COULD ALLOW VALLEY FOG/STRATUS TO FORM AT LSE. THE
BIG ISSUE IS THE TEMPERATURE/DEWPOINT SPREAD BEING AT 23F AS OF
23Z...AND IF THERE IS ENOUGH TIME TO ALLOW FOG TO FORM PRIOR TO
SUNRISE. FOR NOW HAVE KEPT THE BCFG BUT ALSO INCLUDED A TEMPO FOR
IFR BR BETWEEN 10-12Z. CONDITIONS IN GENERAL ARE MORE FAVORABLE
FOR FOG COMPARED TO LAST NIGHT. ANY FOG/STRATUS SHOULD MIX OUT
QUICKLY AFTER SUNRISE.

HEADING INTO FRIDAY AFTERNOON...A LOW PRESSURE TROUGH WILL
APPROACH THE TAF SITES. CONFIDENCE IS FAIRLY HIGH THAT THIS TROUGH
WILL PRODUCE AT LEAST SCATTERED SHOWERS AND
THUNDERSTORMS...THEREFORE...BOTH TAFS HAVE A VCTS INCLUDED.

&&

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

$$

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


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