Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 302005
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
305 PM CDT SAT AUG 30 2014

.SHORT TERM...(TONIGHT THROUGH SUNDAY NIGHT)
ISSUED AT 305 PM CDT SAT AUG 30 2014

MAIN FORECAST CONCERNS THIS PERIOD...CLOUDS/FOG TONIGHT...HIGHS
SUNDAY...SHRA/TSRA CHANCES...SEVERE THREAT SUNDAY NIGHT.

18Z DATA ANALYSIS HAD LOW PRESSURE CENTERED OVER DOOR COUNTY WI
WITH A WEAK TROUGH WESTWARD INTO SOUTHERN MN. VIS IMAGERY/SFC OBS
SHOWED PLENTY OF LOWER STRATO-CU CLOUDS REMAINING ACROSS WI/MN/IA
IN THE SFC-850MB CYCLONIC FLOW. DIURNAL WARMING/MIXING AND SOME
STEEPER LAPSE RATES TO ABOUT 700MB ALLOWING FOR SCT/WDLY SCT -SHRA
ACROSS MUCH OF WI/EASTERN MN/NORTHEAST IA IN THE TRAILING TROUGH
AXIS/ CYCLONIC FLOW. THE STEEPER LOW LEVEL LAPSE RATES AND MINIMAL
INVERSION ABOVE THE CLOUD LAYER ALLOWING FOR SOME MIXING/BREAKUP
OF THE CLOUDS THRU THE EARLY AFTERNOON HOURS. TEMPS TODAY AROUND
10F COOLER THAN THOSE OF FRIDAY.

30.12Z MODELS INITIALIZED WELL. A SLOWING TREND ON THE TROUGH
EXITING EAST LATER THIS AFTERNOON/EARLY THIS EVENING. OTHERWISE
TIGHT CONSENSUS THRU SUNDAY AS THE NEXT TROUGH DIGS INTO THE
CENTRAL/NORTHERN ROCKIES THEN MOVES INTO THE CENTRAL/NORTHERN PLAINS
SUN. TREND THRU 00Z MON FAVORS A COMPROMISE OF THE EARLIER RUNS ON
STRENGTH/TIMING. 500MB CONSISTENCY REMAINS QUITE GOOD SUN NIGHT BUT
LOWER LEVEL...MESO-SCALE AND CONVECTIVE DETAIL DIFFERENCES SHOW UP
AS THE SFC-MID LEVEL TROUGH MOVES INTO THE UPPER MIDWEST. FCST
CONFIDENCE THIS PERIOD IS GOOD TONIGHT/SUN THEN WITH ALL THE DETAIL
DIFFERENCE AVERAGE FOR SUN NIGHT.

FOR THE SHORT TERM...QUIET...GENERALLY DRY WEATHER EXPECTED TONIGHT
AS A WEAK RIDGE OF HIGH PRESSURE QUICKLY MOVES EAST ACROSS THE
REGION. WEAK INSTABILITY...STEEP LOWER LEVEL LAPSE RATE AND MINIMAL
CIN WILL RESULT IN ISOLATED -SHRA ACROSS MUCH OF THE AREA INTO THE
EARLY EVENING HOURS. BIGGER QUESTION TONIGHT IS THE STRATUS AND
CLEARING OF THE CLOUDS OR NOT. PLENTY OF LOWER STRATUS UPSTREAM
ACROSS MN/IA EARLY THIS AFTERNOON...BUT ALL OF THIS IS EAST OF THE
SFC-850MB RIDGE AXIS. MODEL SOUNDINGS SHOWING LITTLE OR NO INVERSION
LATE THIS AFTERNOON/EARLY THIS EVENING TO TRAP THE MOISTURE WITH
DEEPER SUBSIDENCE ABOVE 850MB THRU MUCH OF THE NIGHT. WILL REMAIN
OPTIMISTIC FOR CLOUDS TO DECREASE/CLEAR TONIGHT. HOWEVER WITH THE
RECENT RAINS AND LIGHT WINDS THRU 700MB AS THE LOWER LEVEL RIDGE
COMES ACROSS...CLEARING SKIES SET THE STAGE FOR LATE NIGHT/EARLY
MORNING RADIATIONAL FOG. ADDED PATCHY FOG ACROSS THE ENTIRE AREA IN
THE 06Z-14Z TIMEFRAME FOR NOW. MAY YET NEED MORE AREAS TO WIDESPREAD
MENTION OF THE FOG BUT WILL CONTINUE TO WATCH CLOUD TRENDS. LOWER
LEVEL WARM ADVECTION SPREADS QUICKLY ACROSS THE AREA SUN WITH 925MB
TEMPS IN THE +22C TO +25C RANGE BY 00Z MON. SOUTH WINDS 10-20 MPH
SUN AFTERNOON SHOULD SUPPORT DECENT LOWER LEVEL MIXING...AND WITH
SOME SUNSHINE HIGHS WOULD BE SIMILAR TO THOSE OF FRI...IN THE 80S
APPROACHING 90 IN THE LOWER WI RIVER VALLEY.

STRONGER LOWER LEVEL THERMO-DYNAMIC FORCING AHEAD OF/ALONG THE SFC-
700MB TROUGH AXIS SPREADS INTO/ACROSS THE AREA LATE SUN AFTERNOON/
NIGHT. THIS UNDER STRONGER UPPER LEVEL PV ADVECTION AHEAD OF THE
SHORTWAVE TROUGH AND AGEOSTROPHIC FLOW/DIVERGENCE BETWEEN AT LEAST
PARTIALLY COUPLED 300MB JET MAXES. PW VALUES IN THE 1.5 TO 2 INCH
RANGE...ALONG WITH AN MUCAPE AXIS OF 500-1500 J/KG THRU SUN NIGHT.
LOWER LEVEL SHEAR AND FORCING PARAMETERS SUPPORT A LINE OF STRONG
TO SEVERE TSRA DEVELOPING WEST OF THE FCST AREA LATER SUN
AFTERNOON. NAM/GFS SOUNDINGS SHOWING A CAPPING INVERSION NEAR
850MB SUN AFTERNOON...WITH 50 TO 100 CIN OVER THE WEST SIDE OF THE
FCST AREA UNTIL AROUND 6 PM. GIVEN THIS...FAVORED MODELS AND
PREVIOUS CONSENSUS THAT ARE SLOWER TO BRING SHRA/TSRA INTO THE
AREA LATE SUN AFTERNOON/EVENING...MORE WITH THE FORCING AHEAD OF
THE LOWER LEVEL TROUGH/FRONT. DUE TO THE TIMING AND DECREASE OF
CAPE THRU THE EVENING AS THE LINE WOULD MOVE EAST INTO THE FCST
AREA...SEVERE RISK OVER MAINLY THE WEST 1/3 OF THE FCST AREA PER
SWODY2 LOOKS GOOD AT THIS TIME. SEE HYDROLOGY SECTION BELOW FOR
HEAVY RAIN CONCERNS WITH THE SUN NIGHT CONVECTION. USED A BLEND OF
THE GUIDANCE LOWS FOR TONIGHT/SUN NIGHT AND FAVORED WARMER OF
GUIDANCE HIGHS ON SUN.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 305 PM CDT SAT AUG 30 2014

FORECAST CONCERNS FOR MON THRU TUE NIGHT...SHRA/TSRA CHANCES MON AND
TUE...TEMPERATURES.

AGREEMENT BETWEEN 30.12Z MODELS REMAINS GOOD MONDAY AS THE MID LEVEL
TROUGH AXIS MOVES FROM THE MN/DAKOTAS BORDER TO EAST OF LK SUPERIOR.
IMPROVING CONSISTENCY FOR A SECONDARY SHORTWAVE/TROUGH TO MOVE
ACROSS THE AREA LATER MON NIGHT/TUE MORNING...A BIT FASTER THAN THE
EARLIER MODEL RUNS. CONSENSUS REMAINS GOOD TUE/TUE NIGHT FOR RISING
HGTS OVER THE NORTH-CENTRAL CONUS AS TROUGHING MOVES INTO THE PAC
NORTHWEST...WITH THE NEXT SHORTWAVE PASSING NORTH OF THE FCST AREA
TUE AFTERNOON/NIGHT. FCST CONFIDENCE IN THE MON THRU TUE NIGHT
PERIOD IS AVERAGE TO GOOD.

850-700MB TROUGH AXIS AND MID LEVEL TROUGH AXIS/SHORTWAVE PROGGED TO
BE WEST OF THE AREA AT 12Z MON...WITH MUCH OF THE PV ADVECTION AHEAD
OF THE SHORTWAVE AND SOME 850-500MB FN CONVERGENCE YET TO PASS.
RAISED SHRA/TSRA CHANCES TO 50-70 PERCENT /HIGHEST EAST/ FOR MUCH OF
THE FCST AREA AT LEAST EARLY MON MORNING. WITH THE SECONDARY
SHORTWAVE APPROACHING BY LATE IN THE DAY...CONTINUED A 20-40 PERCENT
SHRA/TSRA CHANCE MON AFTERNOON AND A SMALL CHANCE OVER THE FAR
SOUTHEAST END OF THE FCST AREA INTO MON EVENING. SECONDARY TROUGH
AXIS GENERALLY PASSES BY 12Z TUE. WITH A DRIER LOW LEVEL AIRMASS AND
HIGH PRESSURE TO BUILD INTO/ACROSS THE REGION FOR LATER MON NIGHT
THRU TUE NIGHT...DRIED OUT THE FCST FOR MOST OF THE FCST AREA FOR
THIS PERIOD. SOME WEAK LOWER LEVEL THERMO-DYNAMIC FORCING...AROUND
500 J/KG MUCAPE AND A MORE NORTHERN STREAM SHORTWAVE CLIPPING
NORTHERN WI SPREADS A SMALL SHRA/TSRA CHANCE ACROSS THE NORTH END OF
THE FCST AREA CENTERED ON TUE AFTERNOON. TEMPERATURES FOR MON THRU
TUE NIGHT LOOK TO TREND CLOSER TO NORMAL...AND THIS WELL HANDLED BY
THE MODEL/ENSEMBLE CONSENSUS. IF CLOUDS/PRECIP HANG ON LONGER
MON...THE CONSENSUS HIGHS MAY WELL BE A CATEGORY OR SO TOO WARM.

FOR WEDNESDAY THROUGH SATURDAY...DAYS 4 TO 7....

CONCERNS THIS PERIOD...SHRA/TSRA CHANCES WED NIGHT THRU SAT.

30.00Z AND 30.12Z MEDIUM RANGE MODEL RUNS IN DECENT AGREEMENT WED
WITH FLAT RIDGING/NEARLY ZONAL FLOW OVER THE CENTRAL/EASTERN CONUS
AND TROUGHING COMING ACROSS THE NORTHERN ROCKIES. TREND IS MORE
PROGRESSIVE WITH THE ENERGY COMING ACROSS THE NORTHERN ROCKIES.
BETWEEN MODEL AND RUN-TO-RUN AGREEMENT DECREASES FOR THU THRU SAT.
ECMWF/CAN-GEM TREND MORE PROGRESSIVE/STRONGER WITH A SHORTWAVE
TROUGH MOVING EAST ALONG THE US/CAN BORDER THU...THEN TOWARD MORE
WEST COAST RIDGING AND A NORTHWEST FLOW SHORTWAVE TROUGH DROPPING
TOWARD/INTO THE NORTH-CENTRAL CONUS FRI/SAT. FCST CONFIDENCE IS ON
THE GOOD SIDE WED...THEN BELOW AVERAGE FOR THU-SAT. WITH GFS LOOKING
LIKE AN OUTLIER...FAVORED THE ECMWF/GEM/ENSEMBLE MEAN CONSENSUS FOR
THE DAY 4-7 PERIOD.

FAVORING THE NON-GFS CONSENSUS...WEAK HIGH PRESSURE AND A DRIER
AIRMASS REMAIN OVER THE AREA WED AND LEFT THIS DAY DRY. WITH THE
MORE PROGRESSIVE ECMWF/GEM SOLUTIONS...A STRONGER ROUND OF LOWER
LEVEL WARM ADVECTION/ISENTROPIC LIFT AND MOISTURE TRANSPORT SPREADS
INTO THE UPPER MIDWEST WED NIGHT...CONTINUING INTO THU AS THE
SHORTWAVE TRACKS A SFC LOW ACROSS SOUTHERN CAN AND DRAGS A COLD
FRONT INTO THE REGION. 20-40 PERCENT SHRA/TSRA CHANCES THESE PERIODS
REASONABLE FOR NOW. THIS FRONT/TROUGH THEN PROGGED TO SAG SLOWLY
SOUTH ACROSS THE REGION THU NIGHT/FRI...WITH MORE SHRA/TSRA CHANCES
ACROSS THE AREA AS IT DOES. BIG QUESTION BY FRI NIGHT/SAT IS TIMING
AND HOW FAR SOUTH WILL THE FRONT GO...AND REMAIN THE FOCUS FOR
CONTINUED SHRA/TSRA CHANCES. GIVEN THE LOWER CONFIDENCE BY FRI/
SAT...USED THE MODEL/ENSEMBLE CONSENSUS PRECIP CHANCES. CONSENSUS
OF GUIDANCE HIGHS/LOWS FOR WED-SAT LOOK APPEAR REASONABLE AS WELL.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1240 PM CDT SAT AUG 30 2014

MAIN STORM CIRCULATION NOW WELL EAST OF THE AREA...BUT LOW CLOUDS
AND SHOWERS STILL WRAPPING AROUND THE CYCLONIC CIRCULATION. VISIBLE
SATELLITE IMAGERY SHOWS SOME BREAKS IN THE CLOUD AND INDEED MODEL
SOUNDINGS SHOW THE MOISTURE LAYER IS SOMEWHAT THIN. STILL...WHERE
CLEARING TAKES PLACE THE SKY QUICKLY SELF-DESTRUCTS WITH NEW CUMULUS
DEVELOPMENT. RADAR IMAGERY WAS SHOWING SOME LIGHT SHOWERS DEVELOPING
IN THESE AREAS AS WELL. SO ANTICIPATE AN AFTERNOON AND EARLY EVENING
OF MVFR TO VFR CEILINGS BETWEEN 25-35 KFT WITH OCCASIONAL LIGHT
SHOWERS. TONIGHT...MODELS SUGGEST LOW CLOUDS WILL DIMINISH AS
SUBSIDENCE SETS IN. FEEL IT WILL PROBABLY HAPPEN...BUT NOT AS
QUICKLY AS MODELS INDICATE. STILL...OPTED TO CLEAR OUT THE MVFR
CEILING FOR A TIME TONIGHT. THE PROBLEM WITH THAT...THERE IS STILL A
LOT OF BOUNDARY LAYER MOISTURE AROUND...AND AS WINDS GO
LIGHT...EXPECT FOG TO FORM. PREVIOUS FORECAST INCLUDED LIGHT FOG BUT
OPTED IN ENHANCE THAT FURTHER...ESPECIALLY AT KLSE. MIXING PICKS UP
SUNDAY MORNING AHEAD OF THE NEXT SYSTEM...SO THE FOG SHOULD QUICKLY
ERODE BY 14-15Z.

&&

.HYDROLOGY...
ISSUED AT 305 PM CDT SAT AUG 30 2014

SOME CONCERN FOR HEAVY RAIN CONTINUES WITH THE FRONTAL PASSAGE SUN
NIGHT INTO MON. PW VALUES PROGGED TO BE IN THE 1.5 UP TO 2 INCHES
RANGE WITH WARM CLOUD DEPTHS IN THE 3.5 TO 4 KM RANGE. THE SHOWERS
AND STORMS WILL HAVE THE POTENTIAL TO PRODUCE RAIN RATES OF 2
INCHES/HR OR MORE...BUT WITH THE CONVECTION EXPECTED TO BE
PROGRESSIVE...MOST RAIN AMOUNTS ARE LOOKING TO BE IN THE 1/2 TO 1
INCH RANGE FROM SUN EVENING THRU MON MORNING. IF ANY STORMS ARE
SLOWER MOVING OR REPEAT OVER A LOCATION...HEAVIER AMOUNTS COULD
OCCUR. IF A SLOWER MOVING STORM WOULD MOVE ACROSS AN URBAN AREA OR
CITY...STREET FLOODING COULD DEVELOP RATHER QUICKLY.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....MW
HYDROLOGY....RRS


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