Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 310804
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
304 AM CDT SUN AUG 31 2014

.SHORT TERM...(TODAY THROUGH TUESDAY)
ISSUED AT 304 AM CDT SUN AUG 31 2014

THE MAIN FORECAST CONCERN IS WITH THE RAIN CHANCES TODAY THROUGH
MONDAY.

THERE LOOKS TO BE A SMALL CHANCE FOR SOME SHOWERS TO DEVELOP OVER
THE WESTERN SECTIONS OF THE FORECAST AREA THIS MORNING. THE 31.00Z
NAM SHOWS THE LOW LEVEL JET FEEDING THE ONGOING CONVECTION OVER
NORTH DAKOTA SHOULD SLIDE RAPIDLY EAST WITH THE EASTERN AND
CONVERGENT SIDE OF THIS JET/MOISTURE TRANSPORT AXIS MOVING OVER
THE AREA BETWEEN 12 AND 18Z. SEVERAL OF THE MESO SCALE MODELS
DEVELOP SOME ACTIVITY ALONG THE EASTERN EDGE OF THIS MOISTURE
TRANSPORT AND HAVE ADDED IN A SMALL RAIN CHANCE OVER THE WESTERN
SECTIONS THIS MORNING TO ACKNOWLEDGE THIS CHANCE.

AFTER THIS MORNING...THE ATTENTION THEN TURNS TO THE MEAN TROUGH
AXIS CROSSING THE NORTHERN ROCKIES AND THE EMBEDDED SHORT WAVE
TROUGHS. THE 31.00Z MODELS HAVE REMAIN FAIRLY CONSISTENT WITH THE
TIMING OF THIS FEATURE TAKING THE LONG WAVE TROUGH ACROSS THE
UPPER MIDWEST LATE TONIGHT THROUGH MONDAY. THE MAIN SHORT WAVE
TROUGH THAT WILL IMPACT THE AREA WILL ALSO SWING ACROSS THE AREA
LATE TONIGHT AND MONDAY MORNING. THIS WAVE LOOKS TO BE ABLE TO
PRODUCE MODERATE TO STRONG PV ADVECTION IN THE 500-300 MB LAYER
AND IF THE NAM IS CORRECT...THE FORCING WILL BE AIDED BY SOME JET
DYNAMICS OF THE RIGHT ENTRANCE REGION OF A JET STREAK ROTATING
THROUGH THE LONG WAVE TROUGH AROUND 12Z MONDAY. AS THE COLD FRONT
ASSOCIATED WITH THIS SYSTEM COMES THROUGH...THERE SHOULD BE A BAND
OF WEAK FRONTOGENESIS IN THE 1000-500 MB LAYER THAT COMES THROUGH.
GOOD ISENTROPIC UP GLIDE WILL OCCUR AHEAD OF THE COLD FRONT AND
SHORT WAVE TROUGH WITH UP TO 6 UBAR/S SHOWN ON THE 305K SURFACE.
WITH ALL THIS FORCING...EXPECT TO SEE WIDESPREAD SHOWERS AND
THUNDERSTORMS DEVELOP ALONG THE FRONT AND MOVE ACROSS THE AREA
STARTING THIS EVENING WITH THE BULK OF THE ACTIVITY MOVING EAST OF
THE AREA MONDAY MORNING.

THE BEST INSTABILITY WILL BUILD WEST OF THE FORECAST AREA TODAY
AHEAD OF THE COLD FRONT. THERE SHOULD BE AROUND 2000 J/KG OF ML
CAPE IN PLACE ALONG THE INTERSTATE 35 CORRIDOR AROUND 00Z THIS
EVENING...BUT THIS THEN QUICKLY STARTS TO ERODE WITH THE LOSS OF
HEATING. THE NAM QUICKLY TAKES THIS DOWN TO 1000 J/KG OR LESS
DURING THE EARLY EVENING...BUT THEN MAINTAINS THAT THROUGH THE
NIGHT AHEAD OF THE FRONT. THE MAJORITY OF THE SHEAR LOOKS TO BE IN
THE 0-3KM LAYER WITH THIS IN THE 30 TO 40 KNOT RANGE. THE TIMING
OF THE SYSTEM IS NOT IDEAL BUT STILL LOOKS LIKE THERE WILL BE
ENOUGH INSTABILITY AND SHEAR FOR THE POSSIBILITY OF SOME LARGE
HAIL AND DAMAGING WINDS WITH THE ACTIVITY TONIGHT. THE GREATEST
CHANCES WILL BE ACROSS THE WESTERN SECTIONS LATE THIS AFTERNOON
INTO THE EARLY EVENING BEFORE THE CAPE DROP OFFS...BUT COULD
STILL SEE SOME SCATTERED STRONG TO SEVERE STORMS THROUGH THE
ENTIRE NIGHT IF THE CAPE CAN MAINTAIN LIKE THE MODELS ARE
SUGGESTING.

BY TUESDAY ANOTHER WEAK SHORT WAVE TROUGH COULD BE PASSING OVER
THE UPPER MIDWEST IN THE BACK EDGE OF THE LONG WAVE TROUGH. THIS
COULD BE ENOUGH TO GENERATE SOME WIDELY SCATTERED ACTIVITY ACROSS
THE NORTHERN SECTIONS OF THE FORECAST AREA AND PLAN TO CONTINUE
WITH A SMALL RAIN CHANCE FOR THAT REGION.

.LONG TERM...(TUESDAY NIGHT THROUGH SATURDAY)
ISSUED AT 304 AM CDT SUN AUG 31 2014

ONCE THE LONG WAVE TROUGH MOVES PAST THE REGION...NEARLY ZONAL
FLOW IS EXPECTED TO SET UP ACROSS THE UPPER MIDWEST THROUGH
WEDNESDAY NIGHT. THE FLOW WILL THEN TURN TO THE SOUTHWEST AHEAD OF
AN APPROACHING POSITIVE TILT LONG WAVE TROUGH. A SURFACE LOW IS
EXPECTED IS EXPECTED TO FORM OVER THE DAKOTAS BY WEDNESDAY NIGHT
AHEAD OF THE LONG WAVE TROUGH WITH A WARM FRONT DEVELOPING TO THE
EAST ACROSS MINNESOTA INTO WISCONSIN. SHOWERS AND STORMS WILL
BECOME A POSSIBILITY ALONG THIS WARM FRONT AND THEN REMAIN IN THE
FORECAST FOR THE REMAINDER OF THE PERIOD AS THE SURFACE LOW MOVES
NORTHEAST INTO SOUTHEAST CANADA BUT THE COLD FRONT TRAILS
SOUTHWEST BACK OVER THE AREA AS IT BECOMES UNABLE TO MOVE EAST
THANKS TO HIGH PRESSURE SET UP OVER THE SOUTHEAST CONUS.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SUNDAY NIGHT)
ISSUED AT 1140 PM CDT SAT AUG 30 2014

CUMULUS HAS BEEN SLOW TO COMPLETELY ERODE AT BOTH TAF SITES WITH
AN OCCASIONAL CEILING AROUND 2500 FT AGL THE PAST FEW HOURS. HIGH
RESOLUTION MODEL GUIDANCE SUGGESTS THESE CLOUDS SHOULD STILL
DISSIPATE BY 31.08 TO 31.09Z...BUT ALSO SHOWS WINDS NEAR 2000 FT
AGL SUSTAINED AROUND 10 KTS THROUGH THE NIGHT. WITH 31.04Z DEW
POINT DEPRESSIONS STILL 3 TO 4 DEGREES AND LATEST HOURLY
VISIBILITY MODEL GUIDANCE NOT AS PESSIMISTIC...RAISED VISIBILITY
RESTRICTIONS INTO THE MVFR CATEGORY OVERNIGHT AT KLSE.

VFR CONDITIONS TO PERSIST AFTER 31.14Z...BUT INTRODUCED THUNDER
TEMPO GROUP AT KRST AFTER 01.02Z AS COLD FRONT APPROACHES FROM THE
WEST. BETTER CHANCES THAT THUNDERSTORMS WILL REMAIN WEST OF KLSE
THROUGH THE TAF PERIOD.


&&

.HYDROLOGY...
ISSUED AT 304 AM CDT SUN AUG 31 2014

CONCERNS ABOUT FLOODING POSSIBILITIES STARTING TO INCREASE WITH
THE SYSTEM COMING THROUGH TONIGHT AND MONDAY. PRECIPITABLE WATERS
ARE EXPECTED TO BE IN THE 1.75 TO 2 INCH RANGE WITH WARM CLOUD
DEPTHS AROUND 3.5KM. THIS SHOULD MAKE THE ACTIVITY EFFICIENT RAIN
PRODUCERS AND HAVE RAISED THE QPF AMOUNTS INTO THE 1 TO 1.5 INCH
RANGE FOR LOCATIONS GENERALLY WEST OF THE MISSISSIPPI RIVER. THIS
IS THE AREA THAT WOULD BE OF GREATEST CONCERN IF THE NOSE OF THE
LOW LEVEL JET BECOMES FOCUSED ON THAT REGION AS THE NAM SUGGEST
WILL HAPPEN BY 06Z MONDAY. HOWEVER...CONCERNED THAT THE HI-RES ARW
AND NMM SHOW A GOOD SHARE OF THE ACTIVITY STAYING SOUTH OF THE
AREA TONIGHT. THESE MODELS SUGGEST A MCS WILL FORM ALONG THE FRONT
OVER WESTERN AND CENTRAL IOWA IN THE DEVELOPING LOW LEVEL JET AND
THEN MOVE EAST OR SOUTHEAST IN THE BETTER CAPE ENVIRONMENT. THIS
IS ENOUGH OF A RED FLAG AND WITH 3 HOUR FLASH FLOOD GUIDANCE AT OR
ABOVE 2 INCHES...WILL HOLD OFF ON A FLASH FLOOD WATCH AND CONTINUE
TO INCLUDE THE HEAVY RAIN THREAT IN THE HWO AND WEB PRODUCTS.

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION...ROGERS
HYDROLOGY...04


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