Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 161104
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
604 AM CDT SAT AUG 16 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 253 AM CDT SAT AUG 16 2014

CURRENTLY AS OF 08Z...WATER VAPOR LOOP AND RAP 500MB HEIGHT ANALYSIS
SHOWED RIDGING OVER THE SOUTHWESTERN U.S....AN UPPER LOW IN NORTHERN
IDAHO...SHORTWAVES OVER THE DAKOTAS AND THE KANSAS CITY AREA...AND A
DEEP UPPER TROUGH OVER EASTERN CANADA INTO THE NORTHEAST U.S..
WITHIN THIS TROUGH...A FAIRLY POTENT SHORTWAVE WAS APPROACHING LAKE
SUPERIOR. THE FORECAST AREA LIES IN THE NORTHWEST FLOW ON THE
WESTERN EDGE OF THE TROUGH...BUT STILL NEAR THE INFLECTION POINT
FAVORING SUBSIDENCE AND HIGH PRESSURE AT THE SURFACE. THUS...ONLY
SOME SCATTERED CIRRUS CLOUDS HAVE BEEN TRACKING ACROSS THE FORECAST
AREA. 15.00Z MODELS DEFINITELY OVER-FORECAST THE PRECIPITABLE
WATER...INSTEAD OF BEING 1.5-1.75 INCHES IS ONLY 1-1.25 INCHES PER
GOES DERIVED IMAGERY. SURFACE DEWPOINTS ARE HIGHER...THOUGH...WHICH
WITH RADIATIONAL COOLING HAS LED TO SOME BR AND LOW STRATUS
DEVELOPMENT. THE NEAREST AREAS WHERE PRECIPITATION IS OCCURRING ARE
SOUTH OF I-80 IN IOWA ASSOCIATED WITH THE KANSAS CITY SHORTWAVE...
OVER THE CENTRAL DAKOTAS WITH THE SHORTWAVE THERE...AND THE MN
ARROWHEAD INTO UPPER MICHIGAN AHEAD OF THE APPROACHING SHORTWAVE.
THIS LAST PRECIPITATION AREA IS ALSO SUPPORTED BY A SURFACE COLD
FRONT TRACKING WITH IT.

500MB HEIGHTS ARE PROGGED TO RISE OVER THE NEXT 24 HOURS IN RESPONSE
TO THE UPPER LOW OVER NORTHERN ONTARIO PUSHING EAST INTO SOUTH
CENTRAL SASKATCHEWAN. TYPICALLY WHEN SEEING HEIGHT RISES ONE WOULD
EXPECT A WARM ADVECTION PATTERN. HOWEVER...IN THIS CASE...THE
SHORTWAVE APPROACHING LAKE SUPERIOR WILL DROP INTO THE EASTERN GREAT
LAKES THIS EVENING...SENDING ITS ASSOCIATED COLD FRONT THROUGH THE
FORECAST AREA.

MUCH OF THIS MORNING CONTINUES TO LOOK DRY AS ANY SHOWER ACTIVITY
ASSOCIATED WITH THE KANSAS CITY SHORTWAVE STAYS JUST TO THE
SOUTH...AS DEPICTED IN THE LATEST RAP/HRRR DATA. ANY FOG/STRATUS
SHOULD DISSIPATE WITHIN A FEW HOURS AFTER SUNRISE. HEADING INTO THE
AFTERNOON...DETERMINISTIC AND HIGH RESOLUTION MODELS REMAIN IN GOOD
AGREEMENT THAT THIS COLD FRONT WILL HELP INITIATE CONVECTION NORTH
OF I-94. THIS IS A RESULT TOO OF TEMPERATURES CLIMBING INTO THE 80S
FROM 850MB TEMPS AROUND 16C...AS WELL AS DEWPOINTS CLIMBING INTO THE
MID TO UPPER 60S...COMBINING TO PRODUCE MLCAPES OF 1000-2000 J/KG.
THIS CAPE IS OVERALL SKINNY AND 0-6 KM/0-3KM SHEAR ARE 20 KT OR LESS
WHICH WILL LIMIT SEVERE WEATHER POTENTIAL. HOWEVER...EQUILIBRIUM
LEVELS ARE NEAR 40000 FT AND WARM CLOUD DEPTHS APPROACH 4000
METERS...SO THE STORMS ARE LIKELY TO PRODUCE HEAVY RAIN. THE AMOUNT
OF CONVECTION SEEMS SCATTERED AT BEST SINCE THE BETTER FRONTAL
CONVERGENCE AND DYNAMIC FORCING ARE FARTHER EAST...MORE TOWARDS
LOWER MICHIGAN CLOSER TO THE SHORTWAVE TRACK.

CONVECTION NORTH OF I-94 IN THE AFTERNOON THEN LOOKS TO DROP SOUTH
IN THE EVENING WITH THE COLD FRONT. HOWEVER...COOLING TEMPERATURES
FROM LOSS OF DAYTIME HEATING AND THE FACT LITTLE CONVERGENCE EXISTS
ON THE FRONT WILL LIKELY RESULT IN A DIMINISHING TREND TO THE
CONVECTION. THERE MAY NOT BE MUCH LEFT AFTER 06Z AS THE 16.00Z GFS
SUGGESTS. IN THE WAKE OF THE COLD FRONT...IT APPEARS A LOW STRATUS
DECK MAY END UP DEVELOPING AND ADVECTING SOUTHWESTWARD. BASED ON
FORECAST SOUNDINGS...LOCATIONS SOUTHWEST OF I-94 APPEAR TO HAVE THE
GREATEST CHANCE OF SEEING THIS STRATUS DECK. COOLING 925MB TEMPS ON
THE EASTERLY FLOW SHOULD RESULT IN A COOLER NIGHT...ESPECIALLY IN
CENTRAL WI.

.LONG TERM...(SUNDAY THROUGH FRIDAY)
ISSUED AT 253 AM CDT SAT AUG 16 2014

MAIN ISSUE ON SUNDAY RELATES TO THE SHORTWAVE OVER THE DAKOTAS
CURRENTLY. THE 16.00Z GFS/NAM TRACK THIS SHORTWAVE ACROSS MN DURING
THE DAY...WHICH HELPS PRODUCE SOME SHOWERS AND EMBEDDED STORMS
MOSTLY JUST WEST OF THE FORECAST AREA. PART OF THE REASON THIS
PRECIPITATION DOESNT DEVELOP FARTHER EAST IS A DRY EASTERLY FLOW
PROVIDED BY HIGH PRESSURE BUILDING OVER UPPER MI IN THE WAKE OF
TONIGHTS COLD FRONT. ITS POSSIBLE SOME OF THE SHOWER ACTIVITY
CREEPS INTO SOUTHEAST MN...BUT CONFIDENCE IS NOT HIGH ENOUGH TO PUT
ANY CHANCES IN YET. OTHERWISE...SUNDAY LOOKS COOLER THAN TODAY DUE
TO THE EASTERLY FLOW...MORNING STRATUS AND 925MB TEMPS ABOUT 4C
COOLER.

SUNDAY NIGHT NOW POSES SOME FORECAST CHALLENGE. 16.00Z ECMWF IS
SLOWER THAN THE GFS/NAM TRACKING THE SHORTWAVE EASTWARD OUT OF
MN...AND IN FACT HAS THE WAVE MOVE RIGHT ACROSS THE FORECAST AREA.
ADDITIONALLY...IN THE ECMWF...A SURGE OF 850MB MOISTURE TRANSPORT
ACCOMPANIES THE WAVE...HELPING TO PRODUCE SHOWERS AND STORMS ACROSS
MUCH OF THE AREA AND ESPECIALLY AFTER 06Z. NOW...THE 16.00Z GFS ALSO
DEPICTS AN INCREASE OF 850MB MOISTURE TRANSPORT AND PRECIPITATION...
BUT ITS MORE OVER NORTHWEST SECTIONS OF THE ARE AND IN RESPONSE TO
UPPER TROUGHING COMING INTO THE DAKOTAS. IN ANY EVENT...GIVEN THAT
THESE 2 MODELS ARE PRODUCING PRECIPITATION...FELT A NEED TO INCLUDE
20 PERCENT CHANCES IN FOR AT LEAST THE WEST HALF OF THE FORECAST
AREA. THESE MAY NEED TO BE INCREASED IF MODEL TRENDS CONTINUE.

MONDAY THROUGH TUESDAY...AND IN PARTICULARLY MONDAY NIGHT...STILL
REMAIN THE BEST CHANCE FOR WIDESPREAD RAIN. ALL MODELS ARE IN
AGREEMENT THAT THE CURRENT UPPER LOW OVER NORTHERN IDAHO...AND
ASSOCIATED GENERAL UPPER TROUGHING...WILL MOVE THROUGH THE FORECAST
AREA DURING THIS TIMEFRAME. A SURGE OF 1.5 INCH PRECIPITABLE WATER
COMBINED WITH DPVA FORCING FROM THE TROUGH...AS WELL AS INSTABILITY
SHOULD RESULT IN SHOWERS AND STORMS. MONDAY NIGHT SEEMS TO BE THE
BEST JUXTAPOSITION OF THE AFOREMENTIONED PARAMETERS AND THUS HIGHEST
CHANCES AROUND 70 PERCENT ARE CENTERED THERE. 0-6KM/0-3KM SHEAR
PROGGED LESS THAN 20 KT SO BRIEF HEAVY RAIN WOULD LIKELY BE THE MAIN
CONCERN WITH THE STORMS.

THE REST OF NEXT WEEK FEATURES THE UPPER LOW AND TROUGHING LIFTING
OUT OF THE REGION ON WEDNESDAY...FOLLOWED BY RIDGING BUILDING UP
ACROSS THE PLAINS AS TROUGHING REFORMS OVER THE PACIFIC NORTHWEST.
THIS SETUP IS FAVORABLE FOR BOTH WARM AND MOIST AIR ADVECTION...AND
AS SUCH MODELS CONTINUE TO OSCILLATE ON WHERE AND WHEN STORMS COULD
FIRE ON THE NOSE OF THIS ADVECTION. THEREFORE...FORECAST DEPICTS A
CONSENSUS WITH 20-40 PERCENT CHANCES FROM WEDNESDAY THROUGH FRIDAY.
THERE ARE SOME INDICATIONS WEDNESDAY COULD PAN OUT DRY...THUS LOWEST
CHANCES ARE CENTERED THEN. SEVERE WEATHER POTENTIAL WISE...IF THE
ECMWF PANS OUT...THERE COULD BE A SHOT FOR SOME ON THURSDAY. PLENTY
OF UNCERTAINTY OUT THERE...THOUGH. TEMPERATURES LOOK TO GRADUALLY
INCREASE EACH DAY WITH MOST DAYS AT OR ABOVE NORMAL. IT SHOULD BE
NOTED THAT IF ANY DAY ENDS UP WITH LESS CLOUDS...TEMPERATURES COULD
END UP WARMER THAN FORECAST.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z SUNDAY MORNING)
ISSUED AT 604 AM CDT SAT AUG 16 2014

LOW STRATUS AND FOG WILL IMPACT THE TAF SITES THROUGH 15Z THIS
MORNING WITH IFR/LIFR CONDITIONS EXPECTED. PLAN ON CEILINGS OF
800 FT AT KLSE. CEILINGS WILL BE LOWER AT TIMES AT KRST...FALLING
TO AROUND 500 FT. ALSO...VISIBILITY COULD BE REDUCED TO AROUND 2
SM AT KRST. THE FOG AND STRATUS WILL DISSIPATE AROUND 15Z AS
MIXING INCREASES WITH DAYTIME HEATING. A COLD FRONT IS EXPECTED TO
DROP SOUTH OUT OF NORTHERN WISCONSIN THIS AFTERNOON AND WILL HEAD
FOR THE TAF SITES THIS EVENING. SHOWERS AND THUNDERSTORMS ARE
POSSIBLE ALONG THE FRONT BUT LOOK TO STAY NORTH OF THE TAF SITES.
WILL HAVE TO KEEP A CLOSE EYE ON KLSE WHERE AN ISOLATED SHOWERS OR
THUNDERSTORM COULD OCCUR EARLY IN THE EVENING BEFORE INSTABILITY
WANES. MVFR/IFR CONDITIONS ARE EXPECTED LATE TONIGHT INTO SUNDAY
MORNING AS A STRATUS DECK MOVES INTO THE TAF SITES. KRST MAY SEE
SOME DENSE FOG AS WELL...MAINLY AFTER 08Z.

&&

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

$$

SHORT TERM...AJ
LONG TERM...AJ
AVIATION...WETENKAMP


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