Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 261044
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
545 AM CDT TUE AUG 26 2014

.UPDATED...
ISSUED AT 545 AM CDT TUE AUG 26 2014

850-700MB NOSE OF MOISTURE TRANSPORT HELPING FIRE A CLUSTER OF
SHOWERS/STORMS ACROSS PARTS OF NORTHEAST IA...ON THE NORTHERN
FRINGES OF ELEVATED INSTABILITY. SOME INDICATIONS IN THE RAP13 THAT
WEST-EAST RUNNING FRONTOGENETIC FORCING ALONG 900 MB LIES ACROSS
THIS REGION TOO. THIS WEAKENS SHORTLY AFTER 12Z. HAVE INCREASED RAIN
CHANCES ACROSS PARTS OF NORTHEAST IA INTO SOUTHWEST FOR THE EARLY
MORNING HOURS AS A RESULT.

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 403 AM CDT TUE AUG 26 2014

AT 3 AM...SCATTERED SHOWERS AND THUNDERSTORMS WERE MOVING EAST
ACROSS SOUTHWEST WISCONSIN AND PARTS OF NORTHEAST IOWA. THIS
PRECIPITATION WAS DUE TO A COMBINATION OF STRONG 900 TO 800 MB
FRONTOGENESIS ACROSS THIS AREA AND A 700 TO 500 MB SHORT WAVE
TROUGH MOVING THROUGH MINNESOTA AND WESTERN WISCONSIN. MEANWHILE
FURTHER SOUTHWEST A MESOSCALE CONVECTIVE SYSTEM /MCS/ WAS LOCATED
ACROSS SOUTHWEST IOWA AND EASTERN NEBRASKA. THIS WAS DEVELOPING ON
THE NOSE OF AN 850 MB JET.

FOR TODAY...THE 26.00Z MODELS ARE IN GOOD AGREEMENT THAT THE 900
TO 800 MB FRONTOGENESIS WILL SHIFT SOUTH OF THE AREA EARLY THIS
MORNING AS THE 700 TO 500 MB TROUGH MOVES EAST THROUGH WISCONSIN.
AS THIS OCCURS...THE SHOWER AND THUNDERSTORM CHANCES WILL END
ACROSS THE AREA. MEANWHILE THE MCS WILL QUICKLY DIE THIS MORNING
AS THE NOCTURNAL JET WEAKENS.

FOR TONIGHT...THE MODELS ARE A BIT SLOWER BRINGING THE SHORT WAVE
TROUGH /CURRENTLY LOCATED OVER COLORADO/ OUT OF THE CENTRAL
PLAINS. AS A RESULT...THE 925 TO 850 MB MOISTURE TRANSPORT DOES
NOT GET INTO THE AREA UNTIL AFTER MIDNIGHT. DUE TO THIS...SLOWED
DOWN THE INCREASE IN THE PRECIPITATION CHANCES MOVING INTO THE
AREA AND SHIFTED THEM A BIT FURTHER SOUTHWEST. ONE THING THAT IS
A CONCERN FOR TONIGHT IS THAT BOTH THE ARW...NMM...AND NESTED NAM
SUGGEST THAT THE SHORT WAVE TROUGH IS EVEN SLOWER AND THAT THE
STRONG 900 TO 800 MB FORCING IS EVEN FURTHER SOUTH. AS A
RESULT...THEY KEEP THE AREA DRY.

MEANWHILE FURTHER NORTH...A NORTHERN SHORT WAVE TROUGH MOVES
TROUGH CENTRAL MINNESOTA AND NORTHERN WISCONSIN. THIS SYSTEM HAS
LIMITED MOISTURE AND INSTABILITY...AND THE 700 TO 500 MB FORCING
LOOKS TO BE MUCH WEAKER SYSTEM TO THE SOUTH. THUS JUST INTRODUCED
A SLIGHT CHANCE OF SHOWERS TO NORTH CENTRAL WISCONSIN FOR THE
OVERNIGHT.

.LONG TERM...(WEDNESDAY THROUGH MONDAY)
ISSUED AT 403 AM CDT TUE AUG 26 2014

ON WEDNESDAY...THE 26.00Z DETERMINISTIC MODELS ARE IN GOOD
AGREEMENT THAT WEAK 850 MOISTURE TRANSPORT WILL CONTINUE ACROSS
SOUTHWEST WISCONSIN...NORTHEAST IOWA...AND SOUTHEAST MINNESOTA
DURING THE DAY. THIS TRANSPORT IS AHEAD OF A SHORT WAVE TROUGH
MOVING OUT OF THE NORTHERN PLAINS. AS A RESULT KEPT A 20 TO
30 PERCENT OF SHOWERS AND THUNDERSTORMS GOING THROUGHOUT THE DAY.

FROM WEDNESDAY NIGHT INTO THURSDAY NIGHT...THE MODELS CONTINUE TO
SHOW THAT THAT THE FRONT TO OUR SOUTH WILL MOVE NORTH AHEAD OF THE
APPROACHING SHORT WAVE TROUGH FROM THE NORTHERN PLAINS. IN
ADDITION...THE 925 AND 850 MB MOISTURE TRANSPORT WILL INCREASE
DRAMATICALLY AHEAD OF THIS SYSTEM. ALL OF THE MODELS ARE IN GOOD
AGREEMENT THAT CYCLOGENESIS WILL DEVELOP ACROSS THE AREA. THIS
WILL DRAMATICALLY INCREASE THE 0-3 KM AND 0-6 KM SHEAR ACROSS THE
AREA. WHILE THESE ARE INCREASING...THERE IS QUITE A BIT OF
UNCERTAINTY HOW MUCH INSTABILITY WILL BE LOCATED ACROSS THE AREA.
THE GFS HAS 0-3 KM SHEAR FAVORABLE FOR THE DEVELOPMENT OF LINES OF
CONVECTION. HOWEVER ITS ML CAPES ARE LESS THAN 1K J/KG. IF ANY
SEVERE WEATHER DID OCCUR...IT WOULD BE LIKELY ISOLATED. MEANWHILE
THE NAM HAS ML CAPES WEST OF MISSISSIPPI RIVER CLIMBING INTO THE
1500 TO 2500 J/KG RANGE WITH 0-6 KM SHEAR SUFFICIENT TO PRODUCE
SUPER CELLS. THIS WOULD RESULT IN MORE OF SEVERE WEATHER THREAT
ACROSS THE REGION. WHILE THE SEVERE WEATHER THREAT IS UNCERTAIN...
THE THREAT FOR HEAVY RAIN IS MORE CERTAIN DUE TO PRECIPITABLE
WATER VALUES AROUND 2 INCHES AND THE WARM CLOUD LAYER DEPTHS WILL
CLIMB TO 4.5 KM. WE MAY HAVE ISSUES OF FLOODING IN URBAN AREAS AND
IN AREAS WHERE THE STORMS REPETITIVELY MOVE ACROSS THE AREA.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z WEDNESDAY MORNING)
ISSUED AT 545 AM CDT TUE AUG 26 2014

VFR CONDITIONS TODAY INTO THIS EVENING...WITH A FEW HIGH CLOUDS HERE
AND THERE. CHALLENGES CREEP UP LATER TONIGHT ON WHETHER TO INCLUDE
SHRA/TS INTO THE FORECAST. WEST-EAST RUNNING FRONTAL BOUNDARY SITS
SOUTH ACROSS SOUTHERN IA/ILL...AND WILL BE THE MAIN FOCUS FOR PCPN
PRODUCTION. LOW LEVEL JET/MOISTURE TRANSPORT INTERACTION WITH THE
BOUNDARY WILL FIRE ANOTHER ROUND OF SHOWERS/STORMS OVER IA
TONIGHT...WITH THE MAJORITY OF THE FORCING STAYING THERE. GFS/EC/NAM
HINT THAT SOME LIGHT PCPN COULD WORK TOWARD THE I-90 CORRIDOR...THUS
IMPACTING KRST/KLSE. HOWEVER...MESO MODELS ARW/NMM/SPC WRF THINK IT
WILL STAY SOUTH. GOING TO TREND THE FORECAST TO DRY FOR
NOW...LETTING LATER MODEL RUNS CLARIFY ANY POTENTIAL THREAT.

&&

.HYDROLOGY...WEDNESDAY NIGHT INTO THURSDAY NIGHT
ISSUED AT 403 AM CDT TUE AUG 26 2014

THE 26.00Z MODELS CONTINUE TO SHOW THAT THERE WILL BE A VERY MOIST
AIR MASS IN PLACE ACROSS THE FORECAST AREA. PRECIPITABLE WATER
VALUES WILL BE AROUND 2 INCHES AND THE WARM CLOUD LAYER DEPTHS
WILL CLIMB TO 4.5 KM. THIS WILL MAKE THE SHOWERS AND THUNDERSTORMS
VERY EFFICIENT RAIN PRODUCERS. THIS COULD POSSIBLY RESULT IN FLASH
FLOODING IN URBAN AREAS AND WHERE THUNDERSTORMS TRAIN ACROSS THE
SAME AREAS. HPC/S 24 HOUR QPF FROM 12Z THURSDAY THROUGH 12Z FRIDAY
SUGGESTS THAT MUCH OF THE AREA COULD POTENTIALLY SEE 2 TO 3 INCHES
OF RAIN. THIS SEEMS VERY FEASIBLE. IF THIS TREND CONTINUES...MAY
HAVE TO CONSIDER A FLASH FLOOD WATCH.

&&

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

$$

UPDATED......RIECK
SHORT TERM...BOYNE
LONG TERM....BOYNE
AVIATION.....RIECK
HYDROLOGY....BOYNE


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