Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 262003
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
303 PM CDT TUE AUG 26 2014

.SHORT TERM...(THIS EVENING THROUGH THURSDAY NIGHT)
ISSUED AT 303 PM CDT TUE AUG 26 2014

MAIN FORECAST CONCERN IS WITH THE RAIN CHANCES THROUGH THE PERIOD.

SURFACE ANALYSIS THIS AFTERNOON SHOWS HIGH PRESSURE WAS CENTERED
OVER EASTERN SOUTH DAKOTA. THE COLD FRONT THAT WENT THROUGH MONDAY
EXTENDED FROM NORTHERN INDIANA INTO CENTRAL MISSOURI.

TWO POTENTIAL AREAS FOR SOME RAIN TONIGHT. THE FIRST WILL COME
ACROSS THE NORTHERN SECTIONS OF THE FORECAST AREA WHERE A SHORT
WAVE TROUGH MOVING ACROSS NORTHERN NORTH DAKOTA AND SOUTHERN
CANADA WILL CONTINUE TO MOVE SOUTHEAST WITH THE TAIL END OF THE
WAVE CROSSING NORTHERN WISCONSIN. THE 26.12Z NAM SUGGESTS THIS
WAVE WILL PRODUCE WEAK TO MODERATE PV ADVECTION IN THE 500-300 MB
LAYER AS IT MOVES EAST. AT THE SAME TIME...THE LOW LEVEL RIDGE
WILL BE MOVING EAST AND IS EXPECTED TO BE OVER NORTHERN WISCONSIN
BY LATE TONIGHT. THE NAM BUFKIT SOUNDING FOR KRCX SHOWS SOME
DECENT LIFT ABOVE 700 MB FROM THE SHORT WAVE TROUGH WITH
RELATIVELY DRY AIR BELOW THIS FROM THE INFLUENCE OF THE RIDGE.
COULD BE A SHORT 1 TO 3 HOUR PERIOD WHERE SATURATION OF THIS DRIER
AIR COULD OCCUR TO PRODUCE SOME SPRINKLES OR A LIGHT SHOWER AND
WILL MAINTAIN A SMALL 20 PERCENT CHANCE ACROSS THE FAR NORTH FOR
THIS FEATURE.

THE SECOND AREA THAT MAY SEE SOME RAIN TONIGHT IS ACROSS THE FAR
SOUTH. AS THE LOW LEVEL RIDGE STARTS TO SLIDE EAST...THE FRONT
ACROSS MISSOURI WILL START TO RETURN NORTH WITH THE 26.12Z GFS AND
ECMWF SUGGESTING IT REACHES THE IOWA/MISSOURI BORDER BY LATE
TONIGHT. THE LOW LEVEL MOISTURE TRANSPORT INTO THE FRONT WILL BE
THE STRONGEST OVER NEBRASKA WHERE AN MCS IS EXPECTED TO FOTM. AS
THIS MOVES EAST...THE NORTHERN PART OF THE MCS COULD BRUSH THE
SOUTHERN SECTIONS OF THE FORECAST AREA AND WILL CONTINUE WITH A 20
TO 40 PERCENT CHANCE FOR THIS POSSIBILITY.

THE BEST CHANCE FOR RAIN WILL COME WEDNESDAY NIGHT INTO THURSDAY.
THE MODELS REMAIN CONSISTENT IN SHOWING A SHORT WAVE TROUGH COMING
OUT OF THE CENTRAL ROCKIES WEDNESDAY NIGHT AND REACHING THE LOWER
GREAT LAKES BY LATE THURSDAY. THIS IS EXPECTED TO BRING WEAK TO
MODERATE PV ADVECTION IN THE 500-300 MB LAYER RIGHT ACROSS THE
FORECAST AREA. IN THE LOW LEVELS...A SURFACE LOW SHOULD LIFT
NORTHEAST ACROSS THE REGION WITH THE SHORT WAVE TROUGH BRINGING
THE FRONT BACK INTO THE AREA. AS THIS HAPPENS...WEAK TO MODERATE
FRONTOGENESIS IN THE 1000-500 MB LAYER WILL SPREAD IN. THE WARM
AIR ADVECTION AHEAD OF THE SYSTEM WILL HELP TO PRODUCE 4 TO 6
UBAR/S OF UP GLIDE ON THE 305K ISENTROPIC SYSTEM. WITH THE
ABUNDANCE OF FORCING AHEAD OF THIS SYSTEM...EXPECTING MOST AREAS
WILL SEE RAIN START TO MOVE IN LATE WEDNESDAY NIGHT AND CONTINUE
THROUGH MUCH OF THURSDAY AND HAVE RAISED THE RAIN CHANCES UP TO 80
PERCENT ACROSS THE WEST FOR LATE WEDNESDAY NIGHT AND THEN WORK
THIS EAST ACROSS THE AREA DURING THE DAY THURSDAY.

.LONG TERM...(FRIDAY THROUGH TUESDAY)
ISSUED AT 303 PM CDT TUE AUG 26 2014

ONCE THE MID WEEK SYSTEM MOVES OFF TO THE EAST...THE AREA WILL NOT
BE DONE WITH THE RAIN CHANCES. THE MEAN LONG WAVE TROUGH WILL
STILL BE TO THE WEST OF THE REGION AND IS EXPECTED TO MOVE SLOWLY
EAST COMING ACROSS THE CENTRAL PART OF THE COUNTRY OVER THE
WEEKEND. THIS WILL SERVE TO MAINTAIN THE RAIN CHANCES THROUGH
SATURDAY NIGHT. AFTER A BRIEF BREAK SUNDAY...THE NEXT SYSTEM WILL
BE MOVING ACROSS SOUTHERN CANADA QUICKLY TURNING THE UPPER LEVEL
FLOW TO THE SOUTHWEST AND ALLOWING MORE SHORT WAVE TROUGHS TO MOVE
ACROSS THE UPPER MIDWEST AND BRINGING ANOTHER ROUND OF RAIN
CHANCES STARTING SUNDAY NIGHT AND MONDAY.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY AFTERNOON)
ISSUED AT 1218 PM CDT TUE AUG 26 2014

A QUIET TAF PERIOD AS A COOLER/DRIER AIRMASS BUILDS SOUTHEAST INTO
THE AREA. THE MAIN FORCING/LIFT FOR SHRA/TSRA WILL BE SOUTH OF THE
AREA THRU TONIGHT/WED MORNING. CLOUDS THIS AFTERNOON WILL MAINLY A
FEW DIURNAL CUMULUS AND SCT CIRRUS. SOME INCREASE OF MID CLOUDS
TONIGHT AS MOISTURE/DEBRIS CLOUDS...FROM CONVECTION EXPECTED TO
DEVELOP ACROSS THE SOUTHWEST HALF OF IA...SPREAD NORTHEAST ACROSS
SOUTHERN MN/NORTHEAST IA AND SOUTHWEST WI. THESE CLOUDS LATER
TONIGHT PLUS THE DRIER LOW LEVEL AIRMASS AND LIGHT NORTHEAST TO EAST
WINDS FOR BOUNDARY LAYER MIXING EXPECTED TO KEEP ANY LATE NIGHT/
EARLY MORNING BR/FG TO A MINIMUM.

&&

.HYDROLOGY...
ISSUED AT 303 PM CDT TUE AUG 26 2014

MODELS CONTINUE TO SUGGEST THE POTENTIAL FOR HEAVY RAIN WITH THE
SYSTEM THAT AFFECTS THE AREA WEDNESDAY NIGHT INTO THURSDAY.
PRECIPITABLE WATERS ARE EXPECTED TO BE IN THE 1.75 TO 2 INCH RANGE
WITH WARM CLOUD DEPTHS OF AT LEAST 4 KM...WITH THE GFS SUGGESTING
THESE COULD BE AS DEEP AS 4.5 KM. THIS WOULD MAKE THE SHOWERS AND
STORMS VERY EFFICIENT RAIN PRODUCERS. CURRENT INDICATIONS ARE THAT
THE HEAVIEST RAIN FROM THIS SYSTEM SHOULD FALL ALONG AND NORTH OF
INTERSTATE 90 THROUGH THURSDAY MORNING WHICH IS THE AREA THE FOR
THE MOST PART HAS NOT RECEIVED ANY DECENT RAINS LATELY. THE FOCUS
FOR THE RAIN THEN SHIFTS FARTHER SOUTH THURSDAY AFTERNOON. NOT
DISCOUNTING THE NEED TO LATER ISSUE AN ESF OR FLOOD/FLASH FLOOD
WATCHES BASED ON QPF FORECASTS. CURRENT FORECAST HAS WIDESPREAD 1
TO 2 INCH AMOUNTS WITH PARTS OF THE WESTERN AREA NEAR 2.5 INCHES.
THE RUNOFF FROM THIS SHOULD RESULT IN IN-BANK RISES AND WITH THIS
COMING OVER A SPREAD OUT TIME PERIOD...FLASH FLOODING SHOULD NOT
BE MUCH OF A PROBLEM.

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION.....RRS



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