Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 300405
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1105 PM CDT WED APR 29 2015

.SHORT TERM...(TONIGHT THRU THURSDAY NIGHT)
ISSUED AT 305 PM CDT WED APR 29 2015

MAIN FCST CONCERNS THIS PERIOD...ANY -SHRA/SPRINKLE CHANCE
TONIGHT...TEMPERATURES AND CONTINUED DRYNESS THRU THE PERIOD.

DATA ANALYSIS AT 18Z HAD HIGH PRESSURE RIDGING FROM HUDSONS BAY
ACROSS MN TO TX. A WEAK WAVE OF LOW PRESSURE WAS LOCATED NEAR
NORTHWEST IL...IN RESPONSE TO A SHORTWAVE DROPPING SOUTH ACROSS WI
TODAY. PER WV IMAGERY BULK OF THE SHORTWAVE ENERGY WAS PASSING
ACROSS THE CENTRAL GREAT LAKES. LIFT/CONVERGENCE FROM THE SFC WAVE
AND SHORTWAVE PRODUCING HIGH-BASED CUMULUS OVER THE SOUTHEAST 1/3 OF
THE REGION...WITH GENERALLY CLEAR SKIES ACROSS NORTHWEST WI/IA AND
MUCH OF MN. EARLY AFTERNOON TEMPS NEAR TO A BIT ABOVE NORMAL.

29.12Z MODELS INITIALIZED WELL AND OFFER A TIGHTER CONSENSUS OF
SOLUTIONS AS A MID LEVEL LOW DROPS SOUTH ACROSS THE GREAT LAKES
TONIGHT THEN RATHER ROBUST SHORTWAVE RIDGING BUILDS INTO THE REGION
FROM THE WEST THU/THU NIGHT. TREND FAVORS A COMPROMISE ON THE
TIMING/TRACK OF THE 500MB LOW TONIGHT THEN STRONGER OF THE EARLIER
RUNS WITH THE SHORTWAVE RIDGING THU/THU NIGHT. SHORT-TERM FCST
CONFIDENCE IS GOOD THIS CYCLE...AS DRY/TRANQUIL WEATHER ENDS THE
MONTH OF APRIL.

FOR THE SHORT TERM...INDICATIONS IN A FEW OF THE MODELS FOR SOME
WEAK 500-300MB PV ADVECTION...700-500MB FN CONVERGENCE AND 850-
700MB MOISTURE TO DROP INTO THE FCST AREA LATER TONIGHT INTO THU
MORNING. HOWEVER...BIGGER PICTURE SHOWS STRENGTHENING MID LEVEL
SUBSIDENCE/DRYING ACROSS MN/IA/WI THRU TONIGHT/THU AHEAD OF/UNDER
THE SHORTWAVE RIDGING BUILDING IN FROM THE WEST. SFC-850MB HIGH
PRESSURE IS BUILDING IN WITH WEAK COLD ADVECTION IN THE LOWER
LEVELS. MODELS SOUNDINGS NEARLY ISOTHERMAL IN THE SFC-600MB LAYER
AT 12Z THU. WITH ANY WEAK FORCING/LIFT NOT HAVING STEEPER MID
LEVEL LAPSE RATES TO WORK WITH...WILL LEAVE TONIGHT/THU MORNING
DRY. SFC RIDGE AXIS AND CONTINUED DRY LOW LEVEL AIRMASS DRIFTS
ACROSS THE REGION THU/THU NIGHT. RH/S AGAIN IN THE 25 TO 35
PERCENT RANGE THU AFTERNOON UNDER THE INFLUENCE OF THE DRY HIGH
PRESSURE. WINDS TO REMAIN LIGHT TONIGHT THRU THU NIGHT WITH THE
HIGH NEARBY/WEAK GRADIENTS OVER THE REGION...KEEPING FIRE WEATHER
CONCERNS IN CHECK. THE DRY LOW LEVEL AIRMASS...LIGHT WINDS AND
GENERALLY CLEAR SKIES WILL CONTINUE TO ALLOW FOR DIURNAL TEMP
CHANGES OF 25 TO AS MUCH AS 40F. LOWS WILL BE NEAR NORMAL TONIGHT/
THU NIGHT...WITH EVEN SOME LOWS NEAR 32F IN THE NORMALLY COLDER
LOCATIONS ALONG/NORTHEAST OF I- 94...WHILE HIGHS THU LOOKING TO BE
ABOVE NORMAL. USED A BLEND OF THE GUIDANCE LOWS TONIGHT/THU
NIGHT...TRENDING TOWARD COLDER OF GUIDANCE LOWS ALONG/NORTHEAST OF
I-94. TRENDED TOWARD WARMER OF GUIDANCE HIGHS THU WITH PLENTY OF
SUNSHINE EXPECTED AND THE DRY AIRMASS.

.LONG TERM...(FRIDAY THROUGH WEDNESDAY)
ISSUED AT 305 PM CDT WED APR 29 2015

MAIN FCST CONCERNS IN THE FRI THRU SAT NIGHT PERIOD...SHRA CHANCES
FRI WITH APPROACH OF A WEAKENING MID LEVEL TROUGH...SHRA/TSRA
CHANCES LATE SAT/SAT NIGHT...TEMPERATURES THRU THE PERIOD.

29.12Z MODELS OFFER IMPROVING AGREEMENT FRI...FOR SHORTWAVE TO PUSH
THE SHORTWAVE RIDGING EAST OF THE REGION...BUT WEAKEN IN THE
PROCESS. TREND FROM PREVIOUS RUNS IS WEAKER WITH THIS SHORTWAVE AS
IS MOVES IN/PASSES FRI. REASONABLE AGREEMENT FOR BROAD/WEAK
SHORTWAVE RIDGING OVER THE REGION SAT...BUT IMPROVING SIGNAL FOR A
WEAK SHORTWAVE THRU QUASI-ZONAL FLOW ACROSS THE NORTHERN CONUS TO BE
APPROACHING THE REGION LATE SAT AND MOVING ACROSS THE AREA SAT
NIGHT. TREND IS STRONGER WITH THE LATE SAT/SAT NIGHT SHORTWAVE
FEATURE. GIVEN WEAK/WEAKENING FEATURES AND POTENTIAL IMPACTS...OR
NOT...ON THE SENSIBLE WEATHER...FCST CONFIDENCE IN THE FRI THRU SAT
NIGHT PERIOD IS AVERAGE THIS CYCLE.

LITTLE SFC REFLECTION OF THE APPROACHING MID LEVEL TROUGH ON
FRI...BUT SOUTHWEST FLOW AHEAD OF THE 850-700MB TROUGH DOES PULL
INCREASING MOISTURE NORTHWARD INTO THE REGION AHEAD OF IT.
MOISTURE AXIS WITH PW VALUES AROUND 3/4 INCH PROGGED OVER THE FCST
AREA FRI AFTERNOON. EVEN WITH THE WEAKENING SHORTWAVE...MODELS
PRODUCE A MODEST ROUND OF 500-300MB PV ADVECTION ACROSS THE FCST
AREA FRI AFTERNOON. MODEL SOUNDINGS SHOWING BULK OF MOISTURE ABOVE
ABOUT 750MB FRI AFTERNOON...WITH MODEST UPWARD MOTION BUT NO CAPE
IN THE 700-400MB LAYER. ANY -SHRA FRI WOULD BE FALLING FROM A MID
CLOUD DECK. 20-40 PERCENT -SHRA CHANCE CENTERED ON FRI AFTERNOON...
WITH ANY PRECIP QUITE LIGHT...GOOD FOR NOW. PASSAGE OF THIS
SHORTWAVE AND WEAK SHORTWAVE RIDGING ALOFT WITH SOUNDINGS SHOWING
CAPPING AROUND 700MB LOOKING TO KEEP FRI NIGHT/SAT MORNING DRY.
HOWEVER...MOISTURE BROUGHT NORTH WITH THIS WAVE REMAINS OVER THE
AREA AND IS ADDED TO BY MOISTURE BEHIND PULLED NORTHWARD BY THE
NEXT SHORTWAVE HEADED INTO THE REGION. PW VALUES IN THE 3/4 TO 1
INCH RANGE BY SAT EVENING. CAPE INCREASES WITH THIS ROUND OF
MOISTURE MOVING INTO THE AREA...WITH MUCAPES OF 500-1000 J/KG
OVER AT LEAST THE WEST HALF OF THE FCST AREA BY 00Z SUN. MODEL
SOUNDINGS SHOWING LITTLE IF ANY CAPPING/CIN AT 00Z SUN. ADDED A
SMALL SAT AFTERNOON SHRA/TSRA CHANCE TO THE SOUTHWEST END OF THE
FCST AREA. CAPE OF UP TO 500 J/KG REMAINS OVER THE FCST AREA SAT
NIGHT. WITH APPROACH OF A SFC TROUGH/FRONT AND SOME AT LEAST WEAK
THERMO-DYNAMIC FORCING AHEAD OF IT...15 TO 40 PERCENT SHRA/TSRA
CHANCE ACROSS MUCH OF THE FCST AREA REASONABLE FOR SAT NIGHT. WITH
THE SOUTHWEST FLOW/WARM ADVECTION MUCH OF THIS PERIOD...850MB
TEMPS IN THE +6C TO +9C RANGE AT 00Z SAT THEN +9C TO +12C RANGE...
SUPPORTING HIGHS WELL ABOVE NORMAL BY SAT. USED A BLEND OF THE
GUIDANCE HIGHS/LOWS FOR FRI THRU SAT NIGHT.

FOR SUNDAY THRU WEDNESDAY...DAYS 4 TO 7...

MAIN FCST CONCERNS THIS PERIOD...SHRA/TSRA CHANCES THRU THE PERIOD...
TEMPERATURES.

MEDIUM RANGE MODEL RUNS OF 29.00Z/29.12Z IN GOOD AGREEMENT SUN/MON
AS SOUTHWEST FLOW ALOFT SLOWLY DEVELOPS OVER THE CENTRAL CONUS...
TRANSITIONING TO A MORE UNSETTLED...AND WETTER PATTERN FOR THE UPPER
MIDWEST NEXT WEEK. UNDULATIONS IN THE SOUTHWEST FLOW ALOFT OCCUR MON
NIGHT THRU WED AS TROUGHING DEEPENS OVER THE WESTERN CONUS AND
PIECES OF ENERGY EJECT OUT OF THE BROADER LONGWAVE TROUGH. RATHER
GOOD CONSISTENCY AMONG THE MODELS FOR SHORTWAVE RIDGING TO BUILD
OVER THE REGION TUE NIGHT...AHEAD OF A STRONGER PIECE OF ENERGY
LIFTING OUT OF THE WESTERN TROUGH AND TOWARD THE UPPER MIDWEST WED.
SOME TIMING/STRENGTH DIFFERENCES OF COURSE BY TUE/WED BUT GIVEN THE
BETTER MODEL CONSISTENCY...DAY 4-7 FCST CONFIDENCE IS AVERAGE TO
GOOD THIS CYCLE.

FALLING HGTS OVER THE WESTERN CONUS AND SHORTWAVE ENERGY TRACKING
EAST ALONG THE US/CAN BORDER BRINGS THE SFC TROUGH/FRONT SLOWLY
INTO/ACROSS THE FCST AREA SUN/SUN NIGHT. SOUTH/SOUTHWEST FLOW AHEAD
OF THIS TROUGH CONTINUES TO SPREAD INCREASING MOISTURE/LIFT INTO THE
UPPER MIDWEST SUN/SUN NIGHT. PW VALUES LOOKING TO BE IN THE 1 TO
1.25 INCH RANGE BY LATE SUN...AHEAD OF/WITH THE TROUGH/FRONT. MU
CAPE VALUES OF 500-1500 J/KG PROGGED INTO THE AREA BY LATE SUN AHEAD
OF THE FRONT/TROUGH. MODEL CONSENSUS OF SHRA/TSRA BECOMING LIKELY BY
SUN NIGHT APPEARS QUITE REASONABLE. LESSER SHRA/TSRA CHANCES MON
THRU TUE SEEM REASONABLE AS HIGHER CAPE AIRMASS SHOULD BE PUSHED
SOUTH OF THE FCST AREA...HOWEVER MODELS DIFFER ON WHERE THE TROUGH/
FRONT ENDS UP ON MON-TUE. ENOUGH TIMING/DETAIL DIFFERENCES MON THRU
TUE THAT 20-60 PERCENT SHRA/TSRA CHANCES...HIGHEST SOUTH...PER THE
MODEL CONSENSUS LOOK OKAY. STRONGER SHORTWAVE TROUGH/ENERGY PROGGED
TO LIFT NORTHEAST THRU THE PLAINS WED...SPREADING ANOTHER ROUND OF
INCREASING MOISTURE/LIFT/INSTABILITY TOWARD/INTO THE FCST AREA.
CONSENSUS SHRA/TSRA CHANCES IN THE 20-50 PERCENT RANGE FOR TUE NIGHT
WED GOOD FOR NOW BUT IF PRESENT TRENDS CONTINUE THESE WILL HAVE TO
BE RAISED. WITH THE SOUTHWEST FLOW...TEMPS FOR SUN THRU WED CONTINUE
TO TREND ABOVE NORMAL...BUT ARE LIKELY TO BE IMPACTED IF CLOUDS/
SHRA/TSRA DOMINATE...WITH WARMER LOWS AND COOLER HIGHS THAN IN THE
FCST GRIDS. MODEL/ENSEMBLE CONSENSUS OF HIGHS/LOWS HAS THEM WELL
TRENDED FOR NOW.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)
ISSUED AT 1105 PM CDT WED APR 29 2015

VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD. SCATTERED
ALTOCUMULUS WILL SLIDE ACROSS THE TAF SITES OVERNIGHT WITH CLOUD
BASES OF 8 TO 10 KFT. HIGH PRESSURE WILL PROVIDE DRY AND QUIET
WEATHER AT THE TAF SITES DURING THE DAY ON THURSDAY PERSISTING
INTO THURSDAY EVENING.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...WETENKAMP



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