Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 210144
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
844 PM CDT WED AUG 20 2014

.UPDATE...
ISSUED AT 844 PM CDT WED AUG 20 2014

THE MAIN FOCUS CONTINUES TO BE ON THE CONVECTIVE TRENDS FOR THE
OVERNIGHT PERIOD AS THE LOW LEVEL JET STRENGTHENS INTO THE UPPER
MISSISSIPPI RIVER VALLEY REGION. AN INITIAL BAND OF LIGHT RAIN
SHOWERS HAS DEVELOPED ACROSS SOUTHWEST MINNESOTA AND NORTH CENTRAL
IOWA ALONG THE LEADING/EASTERN EDGE OF THE LOW LEVEL MOISTURE
TRANSPORT/WARM AIR ADVECTION WING. THESE SHOWERS HAVE BEEN VERY
LIGHT AND ARE NOT EXPECTED TO STRENGTHEN IN INTENSITY THROUGH THE
LATE EVENING HOURS.

THE MAIN FOCUS IS FURTHER WEST UNDER THE MAIN CORRIDOR OF BETTER
LOW LEVEL MOISTURE TRANSPORT THAT IS PUSHING UP INTO EASTERN SOUTH
DAKOTA AND FAR SOUTHWESTERN MINNESOTA. CONVECTION IS STARTING TO
DEVELOP ACROSS THIS CORRIDOR AND WILL INCREASE IN COVERAGE AND
TRACK EAST OVERNIGHT. THE PAST FEW RUNS OF THE HRRR SEEM TO BE
CATCHING UP TO THE CURRENT CONDITIONS AND SHOWING THIS TREND
BETTER WITH THE MAIN BATCH OF RAIN/CONVECTION COMING THROUGH
AROUND DAYBREAK TOMORROW MORNING. THEREFORE...HAVE BACKED OFF ON
PRECIPITATION CHANCES THROUGH THIS EVENING AND FOCUSED THEM MORE
ON THE EARLY MORNING HOURS ON THURSDAY. THE 21.00Z RAP SHOWS THE
MAIN BELT OF ELEVATED INSTABILITY RUNNING FROM EASTERN IOWA INTO
SOUTH CENTRAL SOUTH DAKOTA WITH A GRADUAL SHIFT NORTHWARD OF IT
OVERNIGHT. THE TREND IS FOR THIS INSTABILITY TO CONTINUALLY WEAKEN
AS THE NIGHT PROGRESSES WITH AROUND 1000 J/KG OF 0-3KM MUCAPE LEFT
BY DAYBREAK. WITH THE INCREASING LIFT THANKS TO THE LLJ...STILL
CAN NOT RULE OUT THAT A FEW OF THE STORMS COULD BECOME
STRONGER...BUT THE THREAT OF WIDESPREAD STRONG TO SEVERE WEATHER
APPEARS LOW.

&&

.SHORT TERM...(THIS EVENING THROUGH THURSDAY)
ISSUED AT 218 PM CDT WED AUG 20 2014

PRIMARY FORECAST CHALLENGE WILL BE SHOWER AND THUNDERSTORM TIMING
AND COVERAGE TONIGHT THROUGH THURSDAY AND THE POTENTIAL FOR
LOCALIZED FLOODING. CONVECTIVE CHANCES WILL CONTINUE TO BE A
CONCERN FRIDAY THROUGH THE WEEKEND AS WELL. HIGH RESOLUTION MODELS
BEGINNING TO CONVERGE ON A SOLUTION TONIGHT WHICH MATCHES WELL
WITH SYNOPTIC/MESOSCALE SIGNALS IN GLOBAL MODEL SUITE. WILL USE A
BLENDED MODEL SOLUTION WITH A SLIGHT PREFERENCE TOWARD THE ECMWF/
NAM AS THE GFS IS A BIT FASTER/FARTHER NORTH WITH FRONTAL PASSAGE
TOMORROW.

20.20Z SURFACE ANALYSIS SHOWS A LOW PRESSURE CENTER ACROSS
SOUTHWEST NE WITH ITS WARM FRONT DRAPED TO THE EAST ACROSS
SOUTHERN NE INTO NORTHWEST MO. ACTUAL CONVECTION HAS BEEN FURTHER
NORTH AND EAST ACROSS CENTRAL IA COINCIDENT WITH BEST MID-LEVEL
WARM AIR ADVECTION...MOISTURE CONVERGENCE AND ISENTROPIC LIFT.
SURFACE LOW IS PART OF A MUCH BROADER TROUGH THAT WILL SLOWLY LIFT
NORTH AND EAST TONIGHT THROUGH THURSDAY PLACING THE SURFACE WARM
FRONT ACROSS THE SOUTHERN FORECAST AREA BY 21.12Z. EXPECT A
NORTHWEST TO SOUTHEAST ORIENTED LINE OF CONVECTION TO DEVELOP LATE
THIS EVENING AHEAD OF THE FRONT AND MOVE NORTHEASTWARD ALIGNED
ALONG OR JUST NORTH OF THE MISSISSIPPI RIVER VALLEY BY DAYBREAK
THURSDAY.

ELEVATED CAPE OVERNIGHT GENERALLY REMAINS LESS THAN 1000
J/KG...BUT 0 TO 6 KM BULK SHEAR INCREASES TO NEAR 40 KTS BY 21.12Z
WITH IMPINGING 300 HPA JET STREAK. NOT EXPECTING WIDESPREAD
SEVERE...BUT COULD SEE SOME ISOLATED LARGE HAIL OR DAMAGING WIND
FROM THE STRONGEST STORMS. BIGGER CONCERN WILL BE HEAVY RAIN.
EXPECT STRONG MID-LEVEL MOISTURE TRANSPORT WITH PRECIPITABLE WATER
VALUES INCREASING TO NEAR 2.0 INCHES. SEE HYDROLOGY DISCUSSION FOR
MORE DETAILS.

20.12Z ECMWF/NAM SUGGEST WARM FRONT MAY STALL THURSDAY AFTERNOON
NEAR THE MISSISSIPPI RIVER VALLEY...RESULTING IN A PROLONGED
PERIOD OF SHOWERS AND THUNDERSTORMS ACROSS WESTERN/CENTRAL WI.
INCREASED POPS IN THESE AREAS THROUGH THE AFTERNOON AND LOWERED
TEMPERATURES A HANDFUL OF DEGREES. SOUTH OF THE FRONT WILL WARM
NICELY WITH TEMPERATURES RISING INTO THE UPPER 80S TO NEAR 90
DEGREES.

.LONG TERM...(THURSDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 218 PM CDT WED AUG 20 2014

MAIN SURFACE TROUGH WILL SWEEP ACROSS THE FORECAST AREA THURSDAY
NIGHT...RESULTING IN DECREASING CONVECTION AND DRYING FROM WEST
TO EAST. HOWEVER...REGION WILL REMAIN IN WARM MOIST AIR MASS
RESULTING IN HIGH TEMPERATURES FRIDAY CLIMBING INTO THE MID TO
UPPER 80S WITH DEW POINT TEMPERATURES INTO THE LOWER 70S.

HARD TO PINPOINT A PROLONGED DRY PERIOD SATURDAY THROUGH WEDNESDAY.
500 HPA RIDGE BUILDS ACROSS THE UPPER MISSISSIPPI RIVER VALLEY
WITH ITS AXIS SLIDING SLOWLY TO THE EAST THROUGH THE WEEK
RESULTING IN INCREASED SOUTHWEST FLOW ALOFT. A SERIES OF ILL-
RESOLVED EMBEDDED SHORT WAVES COMBINED WITH A WAVERING SURFACE
BOUNDARY WILL PROVIDE THE FOCUSING MECHANISMS FOR CONVECTION...
BUT TIMING AND COVERAGE UNCERTAINTY ABOUNDS. THIS ALSO WRECKS
HAVOC ON SEVERE POTENTIAL AND WILL NEED TO MONITOR SUBSEQUENT
MODEL RUNS. WILL FOLLOW ALL BLEND POPS FOR NOW HIGHLIGHTING 20 TO
50 PERCENT CHANCES FOR MEASURABLE RAIN EACH PERIOD.

TEMPERATURES THROUGH SUNDAY WILL REMAIN ON THE HIGH SIDE OF NORMAL
WITH DAILY HIGHS IN THE MID TO UPPER 80S AND POSSIBLY EVEN BREACHING
90 DEGREES GIVEN 925 HPA TEMPERATURES RANGING FROM 22 TO 27 C.
COULD SEE SOME RELIEF MONDAY/TUESDAY AS PRIMARY EASTERN PACIFIC
TROUGH ATTEMPTS TO MAKE SOME HEADWAY INTO STUBBORN UPPER LEVEL
RIDGE ACROSS THE GREAT LAKES.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING)
ISSUED AT 652 PM CDT WED AUG 20 2014

COMPLICATED FORECAST SCENARIO FOR TONIGHT...WITH LOW PRESSURE AND WARM
FRONT STRETCHED SOUTHWEST OF KRST/KLSE. AS NIGHTTIME LOW LEVEL
MOISTURE TRANSPORT INCREASES...EXPECTING AN ARC OF SHOWERS/STORMS
TO ERUPT ACROSS SOUTHWEST MINNESOTA INTO NORTH CENTRAL IOWA. THESE
SHOULD TRACK EAST ACROSS THE AREA...REACHING KRST AFTER
MIDNIGHT...AND KLSE BY 08Z. MVFR CEILINGS/VISIBILITIES
LIKELY...AND IFR IS POSSIBLE THOUGH CONFIDENCE IS NOT HIGH ENOUGH
TO INCLUDE AT THIS POINT. PRECIPITATION SHOULD LINGER THROUGH THE
MID MORNING HOURS...BEFORE A WARM AIR MASS SURGE WILL CAP OFF THE
ENVIRONMENT AND GIVE WAY TO SCATTERED/BROKEN MID LEVEL CLOUDS.


&&

.HYDROLOGY...
ISSUED AT 218 PM CDT WED AUG 20 2014

URBAN AND SMALL STREAM FLOODING AND/OR LOCALIZED FLASH FLOODING IS
POSSIBLE TONIGHT THROUGH THURSDAY GIVEN MODEST FORCING IMPINGING
ON A VERY MOIST AIR MASS. PRECIPITABLE WATER VALUES NEAR 2.0
INCHES ARE NEAR THE 99TH PERCENTILE OF THE CLIMATOLOGICAL NORMAL
AT KMPX/KDVN. WARM CLOUD DEPTHS ARE ALSO VERY DEEP...WHICH SHOULD
RESULT IN EFFICIENT WARM RAIN PROCESSES. EVEN WITH RECENT RAINFALL...
REGIONAL SOILS ARE STILL RELATIVELY DRY AND SHOULD BE ABLE TO
ABSORB HEAVY RAINFALL UNLESS RATES BEGIN TO EXCEED 2 INCHES/HOUR
PER LATEST FLASH FLOOD GUIDANCE. CERTAINTY IN WIDESPREAD FLOODING
RAINS NOT HIGH ENOUGH AT THIS TIME TO WARRANT A FLASH FLOOD
WATCH...BUT FUTURE SHIFTS WILL NEED TO MONITOR THIS POTENTIAL.

&&

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

$$

UPDATE...HALBACH
SHORT TERM...ROGERS
LONG TERM....ROGERS
AVIATION.....ZT
HYDROLOGY....ROGERS


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