Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 200750
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
250 AM CDT SAT SEP 20 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 250 AM CDT SAT SEP 20 2014

MAIN CONCERNS ARE ON EXITING MORNING CONVECTION AND THE RETURN FOR
MORE CONVECTION TONIGHT.

CURRENTLY AS OF 07Z...WATER VAPOR LOOP AND RAP 500MB HEIGHT ANALYSIS
SHOWED AN UPPER LOW JUST OFF THE CALIFORNIA COAST...RIDGING OVER THE
WESTERN THIRD OF THE U.S. INTO BRITISH COLUMBIA...AND TROUGHING OVER
CENTRAL CANADA INTO THE NORTHERN PLAINS/UPPER MIDWEST. A COUPLE OF
SHORTWAVES TRAVERSING THROUGH THIS TROUGH IN SOUTH DAKOTA AND MN...
COMBINED WITH A SURFACE TO 850MB COLD FRONT THAT STRETCHED FROM
DULUTH TO SIOUX CITY IA...HAVE AIDED IN MAINTAINING A BAND OF
SHOWERS AND THUNDERSTORMS. THIS BAND EXTENDS FROM WEST CENTRAL IA
THROUGH THE FORECAST AREA. MUCH OF THE ACTIVITY HAS BEEN ELEVATED IN
NATURE DUE TO CAPPING PRESENT OF BL PARCELS. PLENTY OF INSTABILITY
FOR THE CONVECTION WITH CAPE VALUES LIFTED FROM 1 KM BETWEEN 1000-
2000 J/KG. IN THE WAKE OF THE CONVECTION...THERE HAS BEEN A NARROW
AREA OF LOW STRATUS DEVELOPING...BEFORE CLOUDS CLEAR OUT OVER
CENTRAL MN. AIRMASS BEHIND THE COLD FRONT WAS NOT MUCH COLDER THAN
AHEAD...WITH 925MB TEMPS HOVERING BETWEEN 20-24C. HOWEVER...
PRECIPITABLE WATER DROPS SIGNIFICANTLY FROM 1.5-1.75 INCHES TO 1-
1.25 INCHES. UPSTREAM A MORE SIGNIFICANT COLD FRONT EXISTS OVER
NORTHERN NORTH DAKOTA AND NORTHWEST MN WHERE 925MB TEMPS DROP TO 12-
14C.

THE COLLECTION OF SHORTWAVES ACROSS SOUTH DAKOTA AND MN ARE PROGGED
TO CROSS THE FORECAST AREA THIS MORNING...PUSHING THE SURFACE-850MB
COLD FRONT THROUGH MUCH IF NOT ALL OF THE AREA BY 18Z. ANTICIPATING
MUCH OF THE CURRENT CONVECTION AHEAD OF THESE FEATURES TO MARCH
SOUTHEASTWARD OUT OF THE FORECAST AREA BY 13Z PER HIGH RESOLUTION
MODEL CONSENSUS. STILL CARRIED SOME LOW PRECIPITATION CHANCES OVER
THE SOUTHEAST HALF IN THE WAKE OF THE CURRENT ACTIVITY IN THE EVENT
ADDITIONAL SHOWERS/STORMS FORM AS THE SHORTWAVES MOVE IN.
OTHERWISE...SUBSIDENCE AND DRYING OF PRECIPITABLE WATER SHOULD ALLOW
SKIES TO CLEAR AFTER A BRIEF PERIOD OF MORNING STRATUS. THE SUNSHINE
COMBINED WITH 925MB TEMPS OF 18-20C FOR MUCH OF TODAY SHOULD ALLOW
FOR HIGHS TO TOP OUT IN THE 70S IN MOST LOCATIONS...WITH SOME LOW
80S IN VALLEYS.

SHORTLY AFTER 00Z THIS EVENING...ANOTHER LINE OF SHOWERS AND
EMBEDDED THUNDERSTORMS LOOKS LIKELY TO MOVE ACROSS THE REGION...
ESPECIALLY ALONG AND NORTH OF I-90. THIS LINE OF CONVECTION WILL BE
FORCED BY A POTENT SHORTWAVE OVER CENTRAL ALBERTA DROPPING THROUGH
THE UPPER MIDWEST TROUGHING. THE SHORTWAVE SHOULD ALSO HAVE A NARROW
CORRIDOR OF MOISTURE TO WORK WITH AS PRECIPITABLE WATER JUMPS UP TO
1-1.3 INCHES. INSTABILITY WISE...ITS FAIRLY MEAGER WITH VALUES OF
100-500 J/KG...MOSTLY IN THE EVENING DUE TO LOSS OF DAYTIME HEATING
COOLING THE BOUNDARY LAYER. ANTICIPATING MORE THUNDERSTORM ACTIVITY
SHOULD OCCUR TODAY UPSTREAM OVER NORTH DAKOTA INTO CENTRAL MN WHERE
THE JUXTAPOSITION OF HIGHER DIURNALLY BASED INSTABILITY INTERSECTS
THE SHORTWAVE. SHEAR PROFILES WOULD CERTAINLY SUPPORT BOTH A
DAMAGING WIND AND LARGE HAIL THREAT WITH CONVECTION...HOWEVER...THE
MEAGER INSTABILITY BY THE TIME THE CONVECTION MOVES INTO THE
FORECAST AREA SHOULD HELP MITIGATE SEVERE POTENTIAL. MAY STILL SEE
SOME STRONGER WIND GUSTS...THOUGH. HAVE RAISED PRECIPITATION CHANCES
UP FOR THE EVENING HOURS GIVEN VERY GOOD MODEL CONSENSUS FOR THIS
CONVECTION. KEPT SOME PRECIPITATION CHANCES GOING OVERNIGHT TOO
ALONG AND NORTH OF I-94 WHERE A FRONTOGENESIS BAND LOOKS TO SETUP ON
THE NORTH SIDE OF THE SHORTWAVE. LOOK FOR COOLER TEMPERATURES
TONIGHT AS COLDER AIR RUSHES IN BEHIND THE LINE OF CONVECTION. IN
FACT...925MB TEMPS COOL AS MUCH AS 10C ON A BRISK 30-40 KT 925MB
NORTHWEST WIND.

.LONG TERM...(SUNDAY THROUGH FRIDAY)
ISSUED AT 250 AM CDT SAT SEP 20 2014

MAIN CONCERNS ARE ON PRECIPITATION POTENTIAL FOR SUNDAY...FOLLOWED
BY TEMPERATURES AND VALLEY FOG POTENTIAL SUNDAY NIGHT AND BEYOND.

ANOTHER POTENT SHORTWAVE LOOKS TO FOLLOW QUICKLY ON THE HEELS OF THE
ONE MOVING THROUGH THE FORECAST AREA TONIGHT. THIS SHORTWAVE...
CURRENTLY IN NORTHERN SASKATCHEWAN...IS PROGGED TO DROP SOUTH
THROUGH THE FORECAST AREA SUNDAY MORNING. THIS SHORTWAVE WILL HELP
TO PULL THE FRONTOGENESIS BAND AND ASSOCIATED SURFACE COLD FRONT
SOUTHWARD THROUGH THE FORECAST AREA DURING THE MORNING HOURS.
APPEARS THE BULK OF THE RAIN/SHOWER ACTIVITY WITH THE BAND WILL BE
CONFINED TO WISCONSIN...BUT SOME MAY EVEN CREEP ACROSS THE MS RIVER
AS SUGGESTED BY THE 20.00Z ECMWF/CANADIAN. HAVE RAISED PRECIPITATION
CHANCES UPWARDS AND FURTHER INCREASES MAY BE NECESSARY. OVERALL
AMOUNTS LOOK TO BE FAIRLY LIGHT...THOUGH. THIS FRONTAL BOUNDARY
DROPPING SOUTH...925MB TEMPS ONLY TOPPING OUT FROM 10-12C AT 00Z
MONDAY...A BREEZY NORTH WIND AND A DECENT AMOUNT OF SUN NOT
OCCURRING UNTIL LATE IN THE DAY WILL YIELD MUCH COOLER CONDITIONS
COMPARED TO TODAY.

MODELS REMAIN IN GOOD AGREEMENT IN BUILDING HIGH PRESSURE INTO THE
FORECAST AREA FOR SUNDAY NIGHT THROUGH TUESDAY. THIS IS IN RESPONSE
TO THE UPPER RIDGE OVER THE WESTERN THIRD OF THE U.S. MARCHING EAST
INTO MN AND IA BY 12Z TUE. THE MAIN CENTER OF THE HIGH LOOKS TO
TRACK JUST SOUTH OF THE AREA...BUT THE GRADIENT RELAXES ENOUGH TO
ALLOW FOR DECOUPLING BOTH NIGHTS. SOME CLOUDS COULD DRIFT ACROSS
BOTH NIGHTS...BUT IT APPEARS ENOUGH PERIOD OF CLEAR SKIES SHOULD
EXIST FOR VALLEY FOG TO FORM...ESPECIALLY IN THE TRIBUTARIES. SUNDAY
NIGHT WILL BE THE COOLER OF THE TWO NIGHTS DUE TO AIRMASS
MODIFICATION TAKING PLACE ON MONDAY. THE TYPICAL COLD SPOTS IN
CENTRAL WI COULD DIP INTO THE MID/UPPER 30S SUNDAY NIGHT. 925MB
TEMPS OF 12-14C ON MONDAY WITH MORE SUN WILL ALLOW HIGHS TO TOP OUT
IN THE 60S TO LOW 70S...WARMEST IN VALLEYS. THESE 925MB TEMPS CLIMB
ANOTHER 1-2C ON TUESDAY...RESULTING IN A SIMILAR BUMP UP IN HIGHS.

IN GENERAL THE WEATHER REMAINS FAIRLY QUIET FROM TUESDAY NIGHT RIGHT
INTO NEXT WEEKEND...THANKS TO SIGNIFICANT UPPER RIDGING BUILDING UP
OVER THE EASTERN TWO THIRDS OF THE U.S.. IN FACT...MOST LONG RANGE
GUIDANCE IS SUGGESTING A 590 DAM 500MB HIGH TO FORM OVER MICHIGAN BY
00Z SATURDAY...WHICH WOULD BE ABOUT 2 STANDARD DEVIATIONS ABOVE
NORMAL. THIS IS ALL A RESULT OF A VERY DEEP TROUGH DEVELOPING ALONG
THE WEST COAST OF THE U.S....SOMETHING THAT HASNT BEEN SEEN FOR
QUITE SOME TIME. THE ONLY REAL ISSUE TO WORRY ABOUT IS THE CURRENT
UPPER LOW OFF THE CALIFORNIA COAST...WHICH MODELS TRY TO BRING
THROUGH THE FORECAST AREA IN A WEAKENING STATE CENTERED AROUND
WEDNESDAY. HAVE MAINTAINED SOME LOW PRECIPITATION CHANCES ASSOCIATED
WITH THIS FEATURE. DESPITE THE RISING 500MB HEIGHTS...IT TAKES
AWHILE FOR A SIMILAR RESPONSE TO 850MB TEMPS...PARTIALLY DUE TO THAT
UPPER LOW LIFTING THROUGH. THUS...TEMPERATURES HOLD NEAR SEASONABLE
NORMALS UNTIL THE END OF THE WEEK. FRIDAY...SATURDAY AND SUNDAY ALL
LOOK ABOVE NORMAL WITH HIGHS POSSIBLY APPROACHING 80.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SATURDAY NIGHT)
ISSUED AT 1134 PM CDT FRI SEP 19 2014

ACTIVITY HAS UPSCALED ACROSS NORTHWEST IOWA AND SOUTHERN
MINNESOTA SIMILAR TO WHAT THE MESO SCALE MODELS WERE SHOWING. AS
THE CURRENT ACTIVITY CONTINUES TO MOVE EAST...EXPECT THAT KRST
SHOULD BE IMPACTED BY SOME STORMS THROUGH ABOUT 07Z OR SO AND KLSE
THROUGH ABOUT 09Z WITH PRIMARILY MVFR CONDITIONS EXPECTED. SOME
UNCERTAINTY WHETHER ANY SHOWERS WILL LINGER/DEVELOP BEHIND THE
LINE OF STORMS. THE 20.02Z HRRR AND PREVIOUS RUNS HAVE BEEN
SHOWING GROWTH UP INTO WESTERN AND CENTRAL WISCONSIN WITH SOME
SHOWERS IMPACTING KLSE WHILE THE 20.00Z CR-NESTNAM KEEPS
EVERYTHING TO THE SOUTH. GIVEN THAT THE CURRENT ACTIVITY IS
FARTHER NORTH THAN THE CR-NESTNAM CURRENTLY INDICATES WILL HAVE
SOME LINGERING SHOWERS SIMILAR TO THE HRRR. VFR CONDITIONS SHOULD
THEN PREVAIL THROUGH SATURDAY BEHIND THE COLD FRONT WITH NORTHWEST
WINDS AND ENOUGH MIXING TO PRODUCE SOME GUSTS AT KRST. THE NEXT
SHORT WAVE TROUGH WILL RAPIDLY APPROACH FROM THE NORTHWEST
SATURDAY EVENING WITH MODERATE PV ADVECTION IN THE 500-300 MB
LAYER. THIS IS EXPECTED TO PRODUCE SOME SHOWERS AND HAVE INCLUDED
A VCSH FOR THE LAST HALF OF SATURDAY EVENING FOR BOTH SITES.


&&

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

$$

SHORT TERM...AJ
LONG TERM...AJ
AVIATION...04


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