240
FXUS63 KARX 211923
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
223 PM CDT MON APR 21 2025
KEY MESSAGES
- SHOWER/T-STORM CHANCES RETURN TONIGHT (40-60%). RAINFALL
AMOUNTS MOSTLY 0.25" OR LESS (70-90% PROBABILITY).
- CONTINUAL PRECIPITATION AND STORM POTENTIAL THROUGH MOST OF
THE WORK WEEK.
DISCUSSION
ISSUED AT 201 PM CDT MON APR 21 2025
OVERVIEW:
SUNDAY'S RAINFALL INCLUDED A BROAD AREA OF 1.0 TO 2.0" ACROSS MUCH
OF THE FORECAST AREA. ADAMS AND RICHLAND COUNTIES HAD LESS THAN AN
INCH WITH AREAS OF 2 TO 3.25" ACROSS TREMPEALEAU CO AND SOME 2"+
AMOUNTS FOR PARTS OF WINNESHIEK, CLAYTON, CHICKASAW, ALLAMAKEE,
JACKSON, LA CROSSE, VERNON, CRAWFORD, CLARK, AND BUFFALO COUNTIES.
LATEST WATER VAPOR SATELLITE IMAGERY AND HEIGHTS SHOWED THE 500MB
TROUGH OVER NORTHERN WISCONSIN...A BRIEF AREA OF RIDGING OVER THE
PLAINS, WITH THE NEXT TROUGH STRETCHING FROM ALBERTA CANADA INTO THE
NORTHERN ROCKIES. THE LATEST SUBJECTIVE SURFACE ANALYSIS HAD AN A
DEEP AREA OF SURFACE LOW PRESSURE OVER THE GREAT LAKES WITH THE
OCCLUDED FRONT AND COLD FRONT THROUGH THE OHIO RIVER VALLEY. SURFACE
LOW PRESSURE WAS BEGINNING TO ORGANIZE OVER THE WESTERN HIGH PLAINS.
THE WARM FRONT WAS OVER SOUTHERN SOUTH DAKOTA INTO CENTRAL NEBRASKA.
LOCALLY, 18Z TEMPERATURES WERE IN THE 40S, HOWEVER 30S WERE STILL
NOTED FOR TAYLOR COUNTY.
TONIGHT THROUGH TUESDAY MORNING:
AS ONE STORM SYSTEM EXITS THE AREA, A LOW-AMPLITUDE TROUGH OVER THE
DAKOTAS WILL SWING EAST TONIGHT AND PIVOT NORTH ACROSS NORTHERN
WISCONSIN TUESDAY. MEANWHILE, ONE UPPER LEVEL JET OVER THE OHIO
RIVER VALLEY IS EXITING THE AREA WHILE ANOTHER JET SEGMENT IS
TRACKING FROM THE ROCKIES ACROSS THE PLAINS INTO THE LOCAL AREA. 700-
850MB WARM ADVECTION INCREASING TONIGHT INTO TUESDAY. THE MOISTURE
IS PROGGED TO BE 6 TO 8 DEGREES C AT 850MB WITH AROUND 0.80"
OF PRECIPITABLE WATER TUESDAY. MOISTURE TRANSPORT IS SIMILAR,
INCREASING WEST OF THE FORECAST AREA BY 06Z...WITH A NARROW AXIS
OF MOISTURE TRANSPORT MOVING IN 09-12Z AND EXITING BY 18Z. THE
THERMAL 850MB TROUGH OVER KANSAS EXTENDS NORTHEAST ACROSS
NORTHERN MISSOURI/WESTERN IOWA AT 12Z. BY 18Z, THE 850MB
DEWPOINT AXIS HAS SHIFTED EAST OF THE AREA. THERE ARE SHOWERS
OUT WEST ALREADY WITH A FEW STORMS IN MONTANA. THE HI-RES CAMS
SHOW THESE SHOWERS/STORMS PUSHING EAST TONIGHT ARRIVING SOMETIME
AROUND 05 OR 06Z, PUSHING EAST OVERNIGHT THEN SPLITTING WITH AN
AREA OF SHOWERS/STORMS DROPPING SOUTHEAST AND THE OTHER LIFTING
MORE NORTHEAST BY 12 OR 13Z WITH SOME RE-DEVELOPMENT THROUGH
THE MORNING HOURS FOR PARTS OF WI.
THE MU/MLCAPE AXIS IS JUST WEST OF THE FORECAST AREA BY 09Z THEN
PASSES THROUGH THE FORECAST AREA DURING THE MORNING. BY 18Z, THE
CAPE AXIS SETS UP ACROSS OUR SOUTHERN BORDER WITH DMX/DVN FORECAST
AREA. THE FORECAST SOUNDINGS FOR TONIGHT AND TUESDAY MORNING SHOW A
HIGH SHEAR ENVIRONMENT, BUT LACKING INSTABILITY. MLCAPE IS 300-
500J/KG, THUS WITH THE MOISTURE RETURN, SOME INSTABILITY AND DEEP
LAYER SHEAR OF 50-60KTS WILL WANT TO MONITOR STORMS. DUE TO THE
LONG HODOGRAPHS, HAIL WOULD BE THE PRIMARY CONCERN. PRECIPITATION
AMOUNTS SHOULD MOSTLY BE LESS THAN A QUARTER OF AN INCH (70 TO 90%)
PER THE 22.12Z HREF, HOWEVER WILL NEED TO MONITOR WHERE THE WARM
FRONT ENDS UP.
PRECIPITATION CHANCES TUESDAY NIGHT:
THE NARROW FILAMENT OF INCREASED LOW LEVEL THETA E ABUTS A
STATIONARY FRONT THROUGH TUESDAY NIGHT, ESTRANGED BY AN OPEN
PERTURBATION LIFTING NORTH OF THE FORECAST AREA. DIFFERENCES
BETWEEN LONG TERM GLOBAL ENSEMBLES IN BEHAVIOR OF THE PHASING
THROUGH SOUTHERN CANADA (EPS/GEFS) CAUSES VARIABILITY IN
LOCATION OF NARROW AXIS OF HIGHER PRECIPITATION OVERNIGHT. THE
MORE NORTHERN SOLUTION IN THE GEFS/GFS LEVERAGES HIGHER MOISTURE
AND ACCOMPANYING FRONTAL BOUNDARY SLIGHTLY FARTHER NORTH THAN
THE EPS. BESIDES THE INTER- MODEL VARIABILITY, INTRA- MODEL
VARIABILITY MUCH HIGHER IN THE GEFS THAN THE EPS. ACCOMPANYING
CONFIDENCE IN 0.1" OF PRECIPITATION OVER 24 HOURS OF 50-70%
ACROSS THE SOUTHERN HALF OF THE FORECAST AREA IS A DECREASE OF
10% FROM THE PREVIOUS RUN WHILE THE EPS HAS KEPT A STEADY 70-90%
GRAZING OUR SOUTHERN MOST COUNTIES. WHILE LOCATION REMAINS
UNCERTAIN, INITIAL IMPACTS OVERNIGHT INTO WEDNESDAY MORNING
EXPECTED TO BE LOW DUE TO DIURNAL TIMING AND UPSTREAM
CYCLOLYSIS. WHILE THE MAIN AREA OF INSTABILITY EXPECTED TO LAY
ACROSS OUR SOUTHERN HALF, COULD SEE A NARROW FILAMENT FARTHER
NORTH WITHIN THE WEAKENING, WARM SECTOR CAUSING SOME STORMS.
WEDNESDAY RAIN:
A SHORT LIVED DRY PERIOD EARLY WEDNESDAY MORNING AS THE SURFACE
FRONTAL BOUNDARY REGRESSES SLIGHTLY SOUTH MAKES WAY TO MORE
PRECIPITATION INITIALLY WEDNESDAY AFTERNOON INTO THE EVENING.
TUESDAY NIGHT'S ESTRANGED SURFACE LOW STAGNATES THROUGH THE
NORTHERN PLAINS, LEVERAGING INCREASED LOW LEVEL MOISTURE
TRANSPORT THROUGH THE UPPER MISSISSIPPI RIVER VALLEY. AS THE
UPPER LEVEL PATTERN REMAINS QUASI-REGRESSIVE, THE ACCOMPANYING
INVERTED LOW LEVEL TROUGH AMPLIFIES DRIVING A NEAR PERPENDICULAR
ORIENTATION BETWEEN TWO LOW LEVEL MOISTURE TRANSPORT AXES. THE
NARROW BAND OF HIGHER PRECIPITATION AMOUNTS EXPECTED TO ORIENTED
FROM WEST-SOUTHWEST TO EAST-NORTHEAST THROUGH THURSDAY. CURRENT
CLUSTER ANALYSIS PLACES THIS BAND OF 0.25" TO 1" OF QPF
ANYWHERE FROM CENTRAL MINNESOTA INTO SOUTHWEST WISCONSIN. AS
PREVIOUSLY MENTIONED, EXACT LOCATION OF QUASI-STATIONARY
BOUNDARY WILL GREATLY INFLUENCE ANY IMPACTS.
STORM CONFIDENCE WEDNESDAY:
DIFFERENCES IN TUESDAY'S FORECAST CONTINUE FORECAST DISCREPANCIES
FOR WEDNESDAY'S STORMS. SEPARATION TO THE BEST SYNOPTIC FORCING
WELL TO THE SOUTH LIMITS NORTHERLY EXTENT AND SPEED OF LIFTING
QUASI-STATIONARY/WARM SURFACE FRONT, KEEPING BEST INSTABILITY
AGAIN TO OUR SOUTH. THE AFOREMENTIONED NEAR PERPENDICULAR
ORIENTATION OF LOW LEVEL MOISTURE WILL INITIATE ELEVATED STORMS
ALONG AND NORTH OF THE SURFACE BOUNDARY, WHERE EVER IT DECIDES
TO LIE. FURTHER CHALLENGES SURROUND DRIER, COLDER AIR SAGGING
SOUTH AND MESOSCALE COLD POOL REINFORCEMENT. ENSEMBLE MEMBERS DO
BUILD SUFFICIENT INSTABILITY THROUGH SOUTHERN HALF OF THE
FORECAST AREA. QUESTION WILL BE AMOUNT OF DIURNAL HEATING TO
PROVIDE SUFFICIENT LIFTING.
PRECIPITATION THROUGH THURSDAY:
EVENTUALLY THE SURFACE LOW IS EXPECTED TO LIFT NORTHEAST ALONG
THE STATIONARY BOUNDARY, PERPETUATING HEAVY RAIN AND STORM
POTENTIAL THROUGH THURSDAY. LONG TERM GLOBAL ENSEMBLE CONFIDENCE
SUGGESTS BETWEEN 1" AND 2" OF TOTAL RAINFALL THROUGH THE END OF
THE WORK WEEK. FLOODING CONCERNS HARD TO NAIL DOWN AT THE
CURRENT FORECAST HOUR GIVEN THE CONVECTIVE NATURE HIGHLY
DEPENDENT ON EXACT LOCATION OF THE FLUCTUATING FRONTAL BOUNDARY.
AVIATION /18Z TAFS THROUGH 18Z TUESDAY/
ISSUED AT 1247 PM CDT MON APR 21 2025
THE LOW PRESSURE SYSTEM THAT BROUGHT THE WIDESPREAD RAIN TO THE
REGION IS EXITING THE AREA, HOWEVER THE NEXT STORM SYSTEM IS
ALREADY MOVING INTO THE WESTERN HIGH PLAINS. MVFR/VFR CONDITIONS
WILL GIVE WAY TO VFR CONDITIONS THEN DECREASING CLOUDS. MID
CLOUDS WILL MOVE IN WITH THE NEXT STORM SYSTEM WITH SHOWERS AND
A FEW STORMS POSSIBLE OVERNIGHT. FOR NOW INCLUDED SHOWERS AFTER
06Z AND CAN ADD THUNDER ONCE THERE IS HIGHER CONFIDENCE ON THE
TIMING AND AREAL COVERAGE OF THE STORMS. MVFR CONDITIONS DEVELOP
WITH TUESDAY MORNING WITH A FRONTAL BOUNDARY IN THE AREA. BRISK
NORTHWEST WINDS DIMINISH THIS EVENING BECOMING LIGHT SOUTHEAST.
ARX WATCHES/WARNINGS/ADVISORIES
WI...NONE.
MN...NONE.
IA...NONE.
DISCUSSION...JAR/ZAPOTOCNY
AVIATION...ZAPOTOCNY
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