Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 291937
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
237 PM CDT SAT AUG 29 2015

.SHORT TERM...(TONIGHT THRU SUNDAY NIGHT)
ISSUED AT 237 PM CDT SAT AUG 29 2015

MAIN FCST CONCERNS THIS PERIOD...LATE NIGHT/EARLY MORNING
RADIATIONAL FOG POTENTIAL...TEMPERATURES.

18Z DATA ANALYSIS HAD LOW PRESSURE OVER SOUTHEAST IA/NORTHWEST IL
AND OVER CENTRAL SASKAT. WEAK HIGH PRESSURE EXTENDED FROM EASTERN
NEB TO NORTHERN WI/LOWER MI BETWEEN THEM. WV IMAGERY SHOWED
SHORTWAVE ENERGY AND A SECONDARY TROUGH AXIS DROPPING ACROSS IA AND
SOUTHEAST WI...WITH SUBSIDENCE/RISING HGTS SPREADING ACROSS MN/WI
BEHIND IT. SUBSIDENCE WAS STRENGTHENING AN INVERSION NEAR 925MB...
HELPING TO TRAP LOWER LEVEL MOISTURE/CLOUDS ACROSS MUCH OF THE
SOUTHEAST HALF OF THE UPPER MIDWEST. CLOUDS SLOWLY ERODING ACROSS
SOUTHERN MN/CENTRAL WI WITH DIURNAL WARMING/MIXING AND SOME WEAK SFC-
925MB ANTICYCLONIC FLOW. STUBBORN CLOUDS HOLDING TEMPS DOWN WITH
WARMEST TEMPS ACROSS THE REGION ONCE AGAIN OVER THE NORTHWEST HALF
OF MN WITH SOME SUNSHINE.

NO BIG PROBLEMS NOTED WITH 29.12Z MODEL INITIALIZATIONS. SOLUTIONS
TRENDING TOWARD A TIGHTER CONSENSUS AS HGTS RISE/RIDGING ALOFT
BUILDS INTO THE UPPER MIDWEST TONIGHT/SUN...THEN AS HGTS FALL A BIT
AHEAD OF A STRONGER SHORTWAVE TROUGH MOVING INTO WESTERN ND BY 12Z
MON. TREND IS TOWARD A COMPROMISE OF THE EARLY RUNS WITH THE RIDGING
OVER THE REGION AND TOWARD STRONGER OF EARLIER RUNS ON THE SHORTWAVE
INTO WESTERN ND LATE SUN NIGHT. WITH THE TIGHTER CONSENSUS...SHORT
TERM FCST CONFIDENCE IS GENERALLY GOOD THIS CYCLE.

FOR THE SHORT TERM...LOWER CLOUDS/MOISTURE TRAPPED IN WEAK FLOW
UNDER A MDT/STRONG INVERSION NEAR 925MB PROVING PROBLEMATIC.
STRENGTHENING THE INVERSION ABOVE 925MB MAKING THE LOW CLOUD
LAYER MORE RESISTANT TO MIXING/EROSION. DIURNAL WARMING/MIXING
WAS WORKING ON IT THIS AFTERNOON...THOUGH MUCH SLOWER THAN EARLIER
EXPECTED. CLOUD COVER HAS BEEN INCREASED OVER MUCH OF THE FCST
AREA THRU THE EVENING...ESPECIALLY OVER THE SOUTHEAST HALF.
REMAINED OPTIMISTIC WITH A DECREASE OF CLOUDS THRU THE LATE
EVENING...EVEN AS WINDS BECOMING QUITE LIGHT /LESS THAN 5 TO
10KTS/ THRU 700MB BY/AFTER MIDNIGHT. CLOUDS AND HOW LONG THEY STAY
TONIGHT WILL HAVE IMPACTS ON THE RADIATIONAL FOG DEVELOPMENT.
STRONG SIGNAL AMONG ALL MODELS FOR WIDESPREAD FOG LATE TONIGHT
INTO SUN MORNING. EARLIER CLEARING WOULD ALLOW FOR MORE
WIDESPREAD FOG...WITH GREATER POTENTIAL/COVERAGE OF DENSE FOG
BETWEEN 09-14Z SUN. SLOWER CLEARING/LINGERING LOW CLOUDS TONIGHT
WOULD KEEP THE FOG MORE PATCHY WITH THE FOG /SOME DENSE/ MAINLY IN
THE RIVER VALLEYS. WILL LEAVE THE FOG GRIDS TONIGHT AS IS WITH
AREAS/WIDESPREAD IN THE VALLEYS/MORE FOG PRONE AREAS AND PATCHY
ELSEWHERE. HOWEVER DID EXPAND AT LEAST SOME PATCHY FOG MENTION IN
TIME FROM 05Z-15Z. DUE TO THE STRENGTH OF THE INVERSION AND
SATURATION BELOW IT...WILL HAVE TO WATCH FOR SOME OF THE FOG
PERSISTING PAST 15Z. GIVEN UNCERTAINTIES WITH LOW CLOUDS LATE
THIS AFTERNOON/EVENING...WILL NOT ISSUE A DENSE FOG ADVISORY AT
THIS TIME BUT ONE MAY YET BE NEEDED CENTERED AROUND 12Z SUN.

SFC-850MB RIDGE AXIS MOVES A BIT SOUTH OF THE AREA FOR SUN
AFTERNOON/NIGHT...WITH SFC-850MB WINDS MORE SOUTHWEST IN THE
5-15KT RANGE FOR MORE LOWER LEVEL MIXING. PLENTY OF DRY AIR ABOVE
THE INVERSION WITH DIURNAL WARMING/MIXING LOOKING GIVE US A SUNNY
SUNDAY AFTERNOON. MODEL SOUNDINGS INDICATING 15-20KTS ABOVE THE
NOCTURNAL INVERSION SUN NIGHT...WITH ANY RADIATIONAL VALLEY FOG
LOOKING MUCH LESS WIDESPREAD. GIVEN THE MOIST BOUNDARY LAYER FROM
RECENT RAINS...DID ADD A PATCHY FOG MENTION TO THE VALLEYS AND
MORE FOG PRONE AREAS BETWEEN 09-14Z MON. 925MB TEMPS IN THE 21C-
24C RANGE SUN AFTERNOON. WITH SOME SUNSHINE AND THE MIXING...HIGHS
SUNDAY TRENDING NEAR TO A BIT ABOVE NORMAL. USED A BLEND OF THE
GUIDANCE LOWS/HIGHS FOR TONIGHT THRU SUN NIGHT.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 237 PM CDT SAT AUG 29 2015

FOR MONDAY THRU TUESDAY NIGHT...

MAIN FCST CONCERNS THIS PERIOD...ABOVE NORMAL TEMPERATURES...SMALL
SHRA/TSRA CHANCE TUE NIGHT.

29.12Z MODELS IN GOOD AGREEMENT AT THE ND SHORTWAVE LIFTS INTO
NORTHWEST ONT MON...PUSHING THE MID LEVEL RIDGE AXIS INTO MI/EASTERN
ONT. DECENT AGREEMENT THEN FOR BROAD SOUTHWEST FLOW ALOFT OVER THE
NORTH CENTRAL CONUS MON NIGHT INTO TUE NIGHT. IMPROVING CONSENSUS
FOR A SHORTWAVE TO RIPPLE NORTHEAST OUT OF THE SOUTHERN PORTION OF
THE WESTERN CONUS TROUGHING AND TOWARD/INTO THE UPPER MIDWEST TUE
NIGHT. TREND WITH THE APPROACHING SHORTWAVE TUE NIGHT IS TOWARD
STRONGER OF THE EARLIER RUNS.

PERSISTENT WESTERN TROUGHING KEEPS LEE TROUGHING OVER THE PLAINS
THRU THIS PERIOD...WITH THE REGION IN THE BROAD SOUTHWEST LOW LEVEL
FLOW AHEAD OF IT. LOWER LEVELS CONTINUE TO WARM THIS PERIOD AND WITH
A CONNECTION TO THE GULF MEX MOISTURE CONTINUES TO INCREASE AS WELL.
THIS PERIOD TO FEEL RATHER SUMMERY AS HIGHS REACH INTO THE MID 80S
TO AROUND 90 AND DEW POINTS RISE INTO THE MID/UPPER 60S. THIS
AIRMASS WILL BECOME UNSTABLE...ESPECIALLY DURING THE AFTERNOONS WITH
AS MUCH AS 1500-2500 J/KG MUCAPE. LITTLE IN THE WAY OF TRIGGERS FOR
CONVECTION SEEN OVER/CLOSE TO THE AREA MON THRU TUE AS THE ND
SHORTWAVE AND ITS ATTENDANT LOWER LEVEL TROUGH SHEAR NORTHEAST INTO
CAN. POTENTIAL FOR AS MUCH AS 100-150 J/KG CIN OVER THE AREA BOTH
MON/TUE AFTERNOONS AS WELL. WILL LEAVE MON-TUE DRY. APPROACHING
SHORTWAVE TUE NIGHT SPREADS SOME INCREASE OF LOWER LEVEL MOISTURE
TRANSPORT INTO THE AREA AND SOME REDUCTION OF THE CAPPING WITH DUE
TO WEAK 700-500MB COOLING. SMALL SHRA/TSRA CHANCE ACROSS MAINLY THE
NORTHWEST HALF OF THE FCST AREA TUE NIGHT OKAY FOR NOW. 925MB TEMPS
MON/TUE PROGGED IN THE 24C-27C RANGE...SUPPORTING HIGHS IN THE MID
80S TO NEAR 90...10F-15F ABOVE NORMAL. TRENDED TOWARD WARMER OF
GUIDANCE HIGHS MON/TUE WHILE FAVORING A BLEND OF THE GUIDANCE LOWS
MON/TUE NIGHTS.

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

MAIN FCST CONCERNS THIS PERIOD...SMALL SHRA/TSRA ALMOST ANY
PERIOD...ABOVE NORMAL TEMPERATURES.

29.00Z/2.12Z MEDIUM RANGE MODELS SHOW A GOOD CONSENSUS FOR RATHER
STRONG EASTERN NOAM RIDGING/WESTERN NOAM TROUGHING WED. IMPROVING
AGREEMENT FOR THE TUE NIGHT SHORTWAVE ENERGY TO MOVE ACROSS THE
UPPER MIDWEST WED. DECENT AGREEMENT FOR THE NOAM FLOW TO AMPLIFY
THU/FRI AS THE WEST COAST TROUGH DEEPENS AND RIDGING STRENGTHENS
OVER THE MID/UPPER MS VALLEY. TREND THU/FRI FAVORS STRONGER OF
EARLIER RUNS WITH THE WESTERN NOAM TROUGHING AND THE LESS ROBUST
WITH THE MID CONUS RIDGING. DECENT AGREEMENT FOR THE PATTERN TO
THEN SLOWLY PROGRESS SAT WITH THE WESTERN TROUGH MOVING INLAND AND
THE UPPER RIDGE AXIS SHIFTING EAST OF THE REGION. DAY 4-7
CONFIDENCE REMAINS GOOD FOR ABOVE NORMAL TEMPERATURES BUT AVERAGE
FOR THE SHRA/TSRA CHANCES.

UNDER THE RIDGING ALOFT...A WARM/MOIST LOWER LEVEL AIRMASS /PW
VALUES AND 850MB TEMPS OF 1 TO 2 STD DEVIATIONS ABOVE NORMAL/ IS
OVER THE REGION DURING THE DAY 4-7 PERIOD. SOME 1K-2K J/KG MUCAPE
PROGGED OVER THE AREA IN THIS AIRMASS DURING THE AFTERNOONS.
QUESTIONS MOST DAYS WILL BE TIMING OF THE SHORTWAVES THRU THE FLOW
AND HOW MUCH CAPPING/CIN OVER THE AREA. WITH THE SIGNAL FOR A
TRACKABLE SHORTWAVE TO MOVE ACROSS THE AREA WED...SMALL SHRA/TSRA
CHANCES THIS DAY REASONABLE FOR NOW. AREA REMAINS NEAR THE WEST/
NORTH PERIMETER OF THE MID LEVEL RIDGE THU THRU SAT...WITH MORE WEAK
AND POORLY TIMED SHORTWAVES LOOKING TO RIPPLE UP AND OVER THE WEST
SIDE OF THE RIDGING. CAPPING FURTHER UNDER THE RIDGE AXIS MAY KEEP
THE AREA PROTECTED FROM SHRA/TSRA CHANCES THU/FRI. WITH THE RIDGE
AXIS SHIFTING EAST A BIT FOR SAT...CAPPING MAY BE LESS WITH
POTENTIAL FOR A WEAK SHORTWAVE OR 2 TO SPREAD A SHRA/TSRA CHANCE
INTO THE AREA. GIVEN LESSER CONFIDENCE IN THE SMALLER SHORTWAVES AND
MESO-SCALE DETAILS IN THE WED-SAT PERIOD...WITH STAY WITH THE
MODEL/ENSEMBLE CONSENSUS SHRA/TSRA CHANCES FOR DAYS 4-7. LOWER LEVEL
TEMPERATURE PROFILES CONTINUE TO SUPPORT TEMPS AROUND 10F TO 15F
ABOVE NORMAL NEXT WEEK. MODEL/ENSEMBLE CONSENSUS HIGHS/LOWS FOR WED-
SAT GENERALLY LOOK GOOD BUT DID RAISE HIGHS IN THE MS/WI RIVER
VALLEYS A COUPLE DEGREES ABOVE THE CONSENSUS.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1201 PM CDT SAT AUG 29 2015

JUST HOW LONG THE MVFR CEILINGS WILL HOLD ON IS THE FIRST CONCERN
WITH THE FORECAST. SOME THINNING OF THE CLOUDS EVIDENT ON VISIBLE
SATELLITE...BUT THE 29.12Z NAM FORECAST SOUNDINGS SUGGEST THE CLOUDS
WILL HOLD ON WELL INTO THE EVENING. WITH WEAK SUBSIDENCE OVER THE
AREA...WILL BE PESSIMISTIC AND KEEP THE MVFR CEILINGS INTO THE
EVENING. SKIES SHOULD THEN CLEAR/SCATTER OUT FOR A SHORT WHILE THIS
EVENING BEFORE IT LOOKS LIKE FOG WILL FORM. PLENTY OF LOW LEVEL
MOISTURE FROM THE RECENT RAINS AND A DEEP LAYER OF LIGHT WINDS AND
WILL TAKE BOTH TAF SITES DOWN TO IFR WITH LOW VISIBILITIES AND
CEILINGS. THIS COULD TAKE MOST OF SUNDAY MORNING TO TOTALLY
DISSIPATE...ESPECIALLY IN THE VALLEY FOR KLSE AND WILL HOLD THE IFR
CONDITIONS FOR MUCH OF THE MORNING.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....04


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