Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 170937
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
437 AM CDT SUN AUG 17 2014

.UPDATE...
ISSUED AT 437 AM CDT SUN AUG 17 2014

FOG IS BECOMING A BIGGER PROBLEM OVER NORTHEAST IOWA WITH MANY
SITES DROPPING TO OR BELOW 1/4 MILE VISIBILITY. MUCH OF THIS IS A
RESULT OF SKIES STILL CLEAR BECAUSE THE COOL FRONT HAS YET TO
MOVE INTO THE AREA. BEHIND THE FRONT ACROSS WISCONSIN...STRATUS
HAS HELPED KEEP THE FOG AT BAY. THERE HAS BEEN SOME SCATTERED
CLOUDS ACROSS SOUTHEAST MN AS WELL AHEAD OF THE FRONT KEEPING FOG
AT A MINIMUM THERE. SHOULD THESE CLOUDS CLEAR...THEN ITS POSSIBLE
THE THICKER FOG IN NORTHEAST IOWA COULD EXPAND INTO SOUTHEAST MN.

HAVE WENT AHEAD...ALONG WITH DMX AND DVN...AND ISSUED A DENSE FOG
ADVISORY FOR NORTHEAST IA. EXPIRATION TIME IS SET AT 9 AM...WHEN
THE WIDESPREAD NATURE OF THE DENSE FOG SHOULD BEGIN TO IMPROVE. IT
MAY NOT FULLY BE IMPROVED UNTIL 10 AM...THOUGH...PER 17.07Z HRRR
VISIBILITY PROGS.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 310 AM CDT SUN AUG 17 2014

CURRENTLY AS OF 08Z...WATER VAPOR LOOP AND RAP 500MB HEIGHT ANALYSIS
SHOWED AN UPPER LOW AND TROUGH TRAVERSING SOUTHERN SASKATCHEWAN AND
NORTHEAST MONTANA...A SHORTWAVE NEAR FARGO...AND RIDGING OVER
EASTERN MN. SUBSIDENCE UNDER THIS UPPER RIDGING HAS RESULTED IN
MAINLY DRY CONDITIONS OVERNIGHT IN THE FORECAST AREA. THERE WERE
SHOWERS LAST EVENING AROUND I-94 FROM CENTRAL MN INTO SOUTHEAST
WI...ASSOCIATED WITH LINGERING DAYTIME INSTABILITY AND CONVERGENCE
ALONG A BACKDOOR COLD FRONT. THOSE HAVE SINCE MOSTLY DISSIPATED DUE
TO LOSS OF DAYTIME HEATING. THAT COLD FRONT NOW STRETCHES FROM
SOUTHERN WI INTO SOUTHWEST MN AND TO A LOW IN SOUTHEAST NORTH
DAKOTA. AROUND THIS SURFACE LOW AND THE ASSOCIATED SHORTWAVE
ALOFT...MORE SHOWERS AND STORMS EXIST. GOES PRECIPITABLE WATER
IMAGERY DEPICTS PLENTIFUL MOISTURE POOLED ALONG THE FRONT...WITH
VALUES RANGING FROM 1.5-2 INCHES...HIGHEST IN THE CONVECTION NEAR
FARGO. THE MOISTURE HAS BEEN FELT AT THE SURFACE AS WELL WITH
DEWPOINTS IN THE MID TO UPPER 60S AROUND THE FRONT. THESE HIGH
DEWPOINTS HAVE LED TO SOME FOG AND STRATUS FORMATION THIS MORNING.
BEHIND THE FRONT OVER NORTH CENTRAL WISCONSIN THROUGH UPPER
MICHIGAN...DRIER AND COOLER AIR IS FILTERING IN AHEAD OF HIGH
PRESSURE OVER NORTHERN ONTARIO.

MUCH OF THE SHORT TERM PERIOD FOCUS IS ON THE SHORTWAVE NEAR FARGO
AS WELL AS THE COLD FRONT THAT HAS LAYED UP OVER THE SOUTHWESTERN
PORTION OF THE FORECAST AREA. REGARDING THE SHORTWAVE...MODELS
GENERALLY SUGGEST THE CORE NEAR FARGO ENDS UP TRACKING TO THE EAST
OVER THE NEXT 24 HOURS...REACHING CENTRAL UPPER MI BY 12Z MON.
HOWEVER...THE TAIL OF THE SHORTWAVE WILL END UP CROSSING THE
FORECAST AREA TONIGHT. DPVA FROM THE TAIL OF THE SHORTWAVE...DAYTIME
HEATING/INSTABILITY THAT DEVELOPS TO INTERACT WITH CONVERGENCE ALONG
THE COLD FRONT...AND INCREASING 850MB MOISTURE TRANSPORT TONIGHT
OVER THE FORECAST AREA ALL SUGGEST POTENTIAL FOR SHOWERS AND STORMS
OVER THE NEXT 24 HOURS. GIVEN THE TRACK OF THE SHORTWAVE TAIL AND
MOISTURE TRANSPORT SIGNAL...THE NORTHWEST HALF OF THE FORECAST AREA
STANDS TO HAVE THE BEST SHOT FOR PRECIPITATION. HIGH RESOLUTION
MODEL GUIDANCE SUGGEST MUCH OF THE ACTIVITY THIS AFTERNOON AND
EVENING SHOULD BE ISOLATED TO SCATTERED AT BEST. THE INCREASE IN
850MB MOISTURE TRANSPORT AFTER MIDNIGHT COULD ALLOW CONVECTION TO
INCREASE A BIT MORE IN COVERAGE...THUS HIGHEST CHANCES ARE 06-12Z
MONDAY.

REGARDING CONVECTIVE HAZARDS. 0-6KM AND 0-3KM SHEAR IS ALL BELOW 20
KT THROUGH TONIGHT...LIMITING SEVERE POTENTIAL. HOWEVER...
PRECIPITABLE WATER IS PROGGED IN THE 1.5-1.8 INCH RANGE WHICH WOULD
ALLOW THE STORMS TO PRODUCE BRIEF HEAVY RAIN.

TEMPERATURES TODAY SHOULD VARY QUITE A BIT ACROSS THE FORECAST AREA.
ACROSS THE SOUTHWEST END OF THE FORECAST AREA WHERE THE FRONT HAS
STALLED OUT...925MB TEMPS ARE SIMILAR TO YESTERDAY IN THE 21-22C
RANGE...SUPPORTING HIGHS INTO THE LOW 80S OR SO. MEANWHILE...IN
NORTH CENTRAL WI WHERE 925MB TEMPS ONLY TOP OUT AT 18C...HIGHS MAY
ONLY REACH THE MID 70S. SOUTHERLY FLOW ACROSS THE FORECAST AREA
TONIGHT SHOULD HELP KEEP BOTH TEMPERATURES AND DEWPOINTS UP...WITH
ONLY PRECIPITATION COOLING LOWS. THEREFORE...EXPECT ANOTHER MILD
AND SOMEWHAT HUMID NIGHT FOR MID AUGUST.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 310 AM CDT SUN AUG 17 2014

FOCUS FOR MONDAY THROUGH TUESDAY REMAINS ON THE UPPER LOW AND TROUGH
CURRENTLY OVER SOUTHERN SASKATCHEWAN AND EASTERN MONTANA. MODELS
CONTINUE TO BE IN GOOD AGREEMENT DROPPING THIS TROUGH INTO MN ON
MONDAY...INTO THE FORECAST AREA BY 12Z TUE...AND OVER EASTERN WI BY
00Z WED. THE TRACK OF THE TROUGH SUGGESTS THE BEST DYNAMICAL
FORCING FOR THE FORECAST AREA OCCURS FROM 18Z MONDAY TO 18Z
TUESDAY...MAXIMIZED DURING MONDAY EVENING. THERE SHOULD BE PLENTY OF
MOISTURE FOR THE TROUGH TO WORK ON TOO...WITH 1.5-1.75 INCHES OF
PRECIPITABLE WATER ALREADY IN PLACE AND IS PROGGED TO REMAIN STEADY
THROUGH THE TROUGH PASSAGE. GIVEN THE GOOD CONSISTENCY ON THIS
SIGNAL FOR THE TROUGH...FEEL FAIRLY CONFIDENT MUCH OF THE AREA WILL
SEE SHOWERS AND STORMS. DAYTIME HEATING INCREASING INSTABILITY FOR
MONDAY AND TUESDAY AFTERNOONS SHOULD ONLY HELP MATTERS. THEREFORE...
HAVE INCREASED PRECIPITATION CHANCES EVEN MORE. IT DOES APPEAR
LOCATIONS WEST OF THE MS RIVER SHOULD SEE PRECIPITATION DIMINISH FOR
A TIME FROM 03-12Z TUESDAY OVER THE WEST HALF OF THE FORECAST
AREA...THOUGH. 925MB TEMPS ON MONDAY IN THE 20-22C RANGE SHOULD
SUPPORT HIGHS IN THE UPPER 70S/LOW 80S...HELD DOWN BY
PRECIPITATION. IT WILL LIKELY FEEL A BIT HUMID TOO. 925MB TEMPS ONLY
TOP OUT FROM 18-20C ON TUESDAY...AND WITH THE POTENTIAL FOR
PRECIPITATION...HIGHS GET KNOCKED DOWN A FEW DEGREES.

ALL INDICATIONS ARE THAT THE UPPER LOW AND TROUGH WILL MOVE AWAY
FROM THE REGION TUESDAY NIGHT...ALLOWING ANY CONVECTION TO
DIMINISH/END TUESDAY EVENING WITH THE LOSS OF DAYTIME HEATING. WITH
THE AREA PROGGED TO LIE IN AN INFLECTION POINT BETWEEN THE DEPARTING
TROUGH AND UPPER RIDGING BUILDING UP ALONG I-29...SUBSIDENCE SHOULD
BE IN PLACE TO ALLOW FOR A DRY OVERNIGHT. CLEARING OF THE SKIES
COMBINED WITH RECENT RAINS AND DIMINISHING WINDS SHOULD PROMOTE
FOG DEVELOPMENT AND THUS ADDED AREAS OF FOG TO THE FORECAST. THE
DRY CONDITIONS COULD PERSIST THROUGH WEDNESDAY AS THAT UPPER RIDGE
SHIFTS EAST TO I-35. HOWEVER...THERE ARE SIGNS FOR SHOWER AND
THUNDERSTORM ACTIVITY TO DEVELOP ACROSS IOWA...NORTH OF A WARM
FRONTAL BOUNDARY PROGGED TO SET UP ALONG THE MO/IA BORDER. HAVE KEPT
SOME LOW CHANCES IN ACROSS THE FAR SOUTHERN PORTION OF THE FORECAST
AREA JUST IN CASE.

FOR WEDNESDAY NIGHT AND INTO NEXT WEEKEND...THERE IS GOOD MODEL
AGREEMENT AMONGST 17.00Z GUIDANCE AS WELL AS PREVIOUS MODELS FOR
TROUGHING TO DIG INTO THE PACIFIC NORTHWEST. IN RESPONSE...RIDGING
BUILDS DOWNSTREAM TOWARDS THE FORECAST AREA...RESULTING IN A
WARM/MOIST AIR ADVECTION PATTERN. THIS WILL LEAD TO MORE CHANCES OF
SHOWERS AND THUNDERSTORMS. ITS APPEARING LIKE LATE WEDNESDAY NIGHT
INTO THURSDAY MORNING COULD BE AN ENHANCED CHANCE BASED ON ECMWF/GFS
MODEL TRENDS. THIS IS ASSOCIATED WITH THE WARM FRONTAL BOUNDARY
ALONG THE MO/IA BORDER ON WEDNESDAY LIFTING INTO THE AREA. 1-6 KM
SHEAR IS 25 KT OR LESS...SO THINKING HEAVY RAIN SHOULD BE THE MAIN
CONCERN. WE MAY BE ABLE TO SQUEEZE IN A DRY DAY ON FRIDAY...AS A
SURFACE COOL FRONT PASSES...BUT HARD TO SAY AT THIS TIME. THEN
HEADING INTO THE WEEKEND...THERE ARE SOME UNCERTAINTIES WITH THE
PACIFIC NORTHWEST TROUGH. THE 17.00Z ECMWF NOW WANTS TO PUSH THE
TROUGH EASTWARD WHICH RESULTS IN A RELATIVELY STRONG STORM SYSTEM
LIFTING THROUGH THE REGION SATURDAY. THE RESULT COULD BE SOME SEVERE
WEATHER. THE GFS HAS A SIMILAR STORM SYSTEM BUT FARTHER OFF TO THE
WEST SINCE IT IS SLOWER TO MOVE THE UPPER TROUGH. EITHER SCENARIO
WOULD PULL WARMER AND MORE HUMID AIR INTO THE REGION. VERY HARD TO
SAY WHAT SUNDAY IS GOING TO LOOK LIKE WITH THE UNCERTAINTY ON UPPER
TROUGH PROGRESSION.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SUNDAY NIGHT)
ISSUED AT 1142 PM CDT SAT AUG 16 2014

LOOKS LIKE ANOTHER GOOD NIGHT FOR THE FORMATION OF FOG AND LOW
STRATUS ACROSS THE AREA. THE WEAK COLD FRONT CONTINUES TO MAKE
SLOW SOUTHWARD PROGRESS AND SHOULD EVENTUALLY PASS THROUGH BOTH
SITES. HOWEVER...THE WIND FIELD AHEAD OF THE FRONT IS SO LIGHT
WITH ONLY A MINOR INCREASE IN SPEEDS BEHIND THE FRONT...THAT IT
WILL NOT MAKE MUCH OF AN IMPACT. SOME CLOUDS CURRENTLY WITH THE
FRONT...BUT GENERALLY MOSTLY CLEAR AHEAD OF IT. EXPECT TO SEE SOME
FOG AND STRATUS DEVELOP BEFORE THE PASSAGE OF THE FRONT AND LOCAL
WEB CAM ALREADY SHOWING SIGNS OF THIS OVER THE MISSISSIPPI RIVER
CHANNEL. ONCE THE FRONT GOES THROUGH...THE 17.00Z NAM AND 17.03Z
RAP BOTH SHOW SOME INCREASE IN THE LOW LEVEL WINDS OUT OF THE EAST
TO THE POINT THAT SURFACE DEW POINTS MAY START TO DRY OUT SOME AT
KLSE SO THAT THE VISIBILITY MAY NOT GET TOO LOW. A VISIBILITY
BELOW A MILE STILL POSSIBLE AT KRST THOUGH. CONDITIONS EXPECTED TO
SLOWLY IMPROVE SUNDAY MORNING WITH A RETURN TO VFR CONDITIONS BY
EARLY AFTERNOON. THE NAM SUGGESTS THE POSSIBILITY OF SOME SHOWERS
AND STORMS IN THE AFTERNOON AS A SHORT WAVE TROUGH COMES OUT OF
THE DAKOTAS AND ACROSS MINNESOTA. HOWEVER...ITS FORECAST SOUNDINGS
SHOW VERY LITTLE VERTICAL MOTION AND OPTED TO STAY WITH A DRY
FORECAST. BETTER CHANCES FOR RAIN MAY COME AFTER 06Z MONDAY AS A
STRONGER SHORT WAVE TROUGH APPROACHES.


&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...DENSE FOG ADVISORY UNTIL 9 AM CDT THIS MORNING FOR IAZ008>011-
     018-019-029-030.

&&

$$

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



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