Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 272304
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
600 PM CDT THU AUG 27 2015

.SHORT TERM...(TONIGHT THRU FRIDAY NIGHT)
ISSUED AT 253 PM CDT THU AUG 27 2015

MAIN FCST CONCERNS THIS PERIOD...SHRA/TSRA CHANCES AND RAIN AMOUNTS
THRU THE PERIOD.

DATA ANALYSIS AT 18Z HAD LOW PRESSURE OVER SOUTH-CENTRAL NEB...IN
RESPONSE TO A SHORTWAVE TROUGH AND PIECES OF ENERGY MOVING EAST FROM
WY INTO SD/NEB. STRONGER OF THE 925-700MB MOISTURE TRANSPORT/THETA-E
CONVERGENCE AHEAD OF THIS LOW/TROUGH WAS FOCUSED INTO EASTERN
SD/SOUTHWEST MN EARLY THIS AFTERNOON. SHRA/TSRA ON THE INCREASE IN
FAR SOUTHWEST MN AS OF 18Z...WHILE PRECIP ELSEWHERE ACROSS THE
REGION WAS WANING. MORE CLOUDS ACROSS MUCH OF THE UPPER MIDWEST
TODAY AHEAD OF THE PLAINS LOW AND SHORTWAVE TROUGH COMING EAST...BUT
INCREASING SOUTHERLY SFC-850MB FLOW SPREADING WARMER AND MORE MOIST
AIR INTO THE REGION.

NO MAJOR PROBLEMS NOTED WITH 27.12Z MODEL INITIALIZATIONS. GFS WAS
AROUND 5F TOO LOW WITH SFC DEW POINTS IN THE MOISTURE PLUME AHEAD OF
THE PLAINS LOW. PER WV IMAGERY...NAM/GFS/CAN-GEM APPEARED TO STRONG
WITH A SHORTWAVE INITIALIZED OVER NORTHEAST CO. EVEN WITH SOME
SMALLER DIFFERENCES AT THE START...MODELS GENERALLY CONVERGING ON A
TIGHTER CONSENSUS AS THE WY/CO SHORTWAVE ENERGY MERGES AND DEVELOPS
A SMALL MID LEVEL LOW OVER SOUTHWEST MN BY 12Z FRI. TREND AT 12Z FRI
IS A BIT WEAKER WITH THE MID LEVEL LOW. TREND FRI FAVORS FASTER OF
THE EARLIER RUNS WITH THIS LOW THEN OPENING UP AND MOVING EAST
ACROSS THE REGION FRI/FRI NIGHT. GFS WITH THE BETTER RUN-TO-RUN
CONSISTENCY LATER TONIGHT INTO FRI NIGHT. SHORT-TERM FCST CONFIDENCE
IS GENERALLY GOOD THIS CYCLE.

FOR THE SHORT TERM...BIGGER QUESTION LATER TONIGHT INTO FRI NIGHT IS
HOW MUCH IS IT GOING TO RAIN AND WHERE WILL THE HEAVIEST RAIN FALL.
LOWER LEVEL THERMODYNAMIC FORCING/LIFT CONTINUES TO COME TOGETHER
FOR LATER TONIGHT INTO FRI NIGHT. THIS WITH STRONGER LIFT/DIVERGENCE
ALOFT AHEAD OF THE SHORTWAVE/TROUGH AND BETWEEN PARTIALLY COUPLED
250-300MB JET MAXES. THIS DEEP LAYERED FORCING/LIFT ALONG...WITH PW
VALUES IN THE 1.5 TO 1.75 INCH RANGE AND MDT/STRONG 925-700MB
MOISTURE TRANSPORT/THETA-E CONVERGENCE OF THIS HIGHER PW AIRMASS
INTO THE AREA...SPREADS ACROSS THE FCST AREA CENTERED ON FRI. MODEL
CONSENSUS REMAINS GOOD FOR THIS SYSTEM TO PRODUCE A BAND OF 1 TO 3
INCHES OF RAIN ACROSS THE FCST AREA. THE TREND TOWARD A WEAKER
OVERALL 500MB LOW/TROUGH AND THE STRONGER CO/SOUTHERN SHORTWAVE HAS
SHIFTED THE AXIS OF HEAVIER RAIN SOUTHWARD...NOW MORE OVER SOUTH 2/3
OF THE FCST AREA...ALONG/SOUTH OF I-90. THE SLIGHTLY WEAKER SYSTEM
ALSO DELAYS ARRIVAL OF THE RAIN INTO THE AREA A BIT TONIGHT.
LOWERED/SLOWED RAIN CHANCES SPREADING ACROSS THE AREA THIS EVENING
THEN RAISED THEM TOWARD 100 PERCENT LATE TONIGHT/FRI AS THE
STRONGEST/DEEPEST LIFT MOVES ACROSS THE AREA. WITH A FASTER OPENING
UP OF THE TROUGH AND EXIT EAST...TRENDED RAIN CHANCES DOWN MORE
QUICKLY FRI NIGHT. MODEL CONSENSUS CONTINUES TO SHOW LITTLE IF ANY
MUCAPE OVER THE AREA TONIGHT/FRI. ANY TSRA TONIGHT/FRI LOOK TO HAVE
TO BE DRIVEN BY THE STRONGER/ DEEP FORCING/LIFT. APPEARS MUCH OF THE
PRECIP WILL COME ACROSS THE AREA AS A BROAD SHIELD/BAND OF RAIN/SHRA
WITH EMBEDDED/ISOLATED TSRA. LIMITED TSRA MENTION TONIGHT/ FRI/FRI
NIGHT TO ISOLATED. USED A BLEND OF THE GUIDANCE LOWS/HIGHS FOR
TONIGHT THRU FRI NIGHT. A COOL DAY FRI WITH RAIN/THICK CLOUDS OVER
THE AREA MUCH OF THE DAY.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 253 PM CDT THU AUG 27 2015

FOR SATURDAY THRU SUNDAY NIGHT...

MAIN FCST CONCERNS THIS PERIOD...LINGERING SHRA CHANCES SAT
MORNING...TEMPERATURES THRU THE PERIOD.

27.12Z MODEL RUNS CONSISTENT IN THE MID LEVEL TROUGH AXIS MOVING
EAST OF THE AREA SAT MORNING. MODELS REMAIN IN DECENT AGREEMENT
WITH THE TROUGH AXIS TO REMAIN OVER EASTERN WI SAT AFTERNOON AS A
SECONDARY/NORTHERN STREAM SHORTWAVE WOULD DROP DOWN THE WEST SIDE
OF THE TROUGH. MODEL THEN IN GOOD AGREEMENT FOR RISING HGTS AND
STRONGER MID LEVEL RIDGING TO BUILD QUICKLY INTO THE UPPER MIDWEST
SAT NIGHT INTO SUN NIGHT. TREND FAVORS A COMPROMISE OF THE EARLY
RUNS ON THE STRENGTH/PLACEMENT OF THE RIDGING ALOFT BY SUN/SUN
NIGHT. FCST CONFIDENCE FOR SAT THRU SUN NIGHT IS GOOD THIS CYCLE.

FASTER OPENING/MOVEMENT EAST OF THE 500MB TROUGH HAS THE DEEPER
FORCING/LIFT WELL EAST OF THE AREA BY LATE SAT MORNING. LIMITED
MOST OF THE LINGERING SHRA CHANCES WITH THE EXITING SYSTEM TO THE
12-15Z PERIOD SAT MORNING. DID LINGER A SMALL -SHRA CHANCE OVER
THE FAR EAST/SOUTHEAST SIDE OF THE FCST AREA SAT AFTERNOON...WHICH
BLENDS WELL WITH NEIGHBORING GRIDS. THIS DUE TO THE STRONGER
SECONDARY/NORTHERN STREAM SHORTWAVE DROPPING SOUTH ACROSS WI SAT
AFTERNOON...SOME LINGERING LOWER LEVEL MOISTURE AND RATHER DEEP
CYCLONIC FLOW. THIS EXITS QUICKLY SAT EVENING...WITH HIGH PRESSURE
AND THE RIDGING ALOFT THEN BUILDING ACROSS THE REGION THRU SUN
NIGHT. MAY YET NEED SOME LATE NIGHT/EARLY MORNING VALLEY FOG
MENTION CENTERED ON 12Z SUN AND MON. WILL LEAVE THIS OUT FOR NOW
AND LET THOSE POTENTIAL DETAILS COME INTO BETTER FOCUS ONCE THE
SYSTEM FOR FRI IS BY THE REGION. WEAK LOW LEVEL COLD ADVECTION
BEHIND THE LOW FOR SAT...WITH WEAK/NEUTRAL TEMPERATURE ADVECTIONS
FOR SAT NIGHT THRU SUN NIGHT. 925MB TEMPS IN THE 16C-20C RANGE SAT
AFTERNOON...SUPPORTING HIGHS MAINLY IN THE 70S. WITH THE RISING
HGTS/WARMING ALOFT...LOWER LEVEL AIRMASS LOOKS TO WARM IN PLACE
SUN...WITH 925MB TEMPS SUN AFTERNOON SUPPORTING HIGHS IN THE UPPER
70S/LOWER 80S. USED A BLEND OF THE GUIDANCE HIGHS/LOWS FOR SAT/SAT
NIGHT AND SUN. TRENDED TOWARD WARMER OF GUIDANCE HIGHS SUNDAY WITH
PLENTY OF SUNSHINE EXPECTED AND THE RIDGE BUILDING ALOFT.

FOR MONDAY THROUGH THURSDAY...DAYS 4 OT 7...

MAIN FCST CONCERNS THIS PERIOD...TEMPERATURES AND HOW WARM WILL IT
GET BY THE MIDDLE OF NEXT WEEK.

MEDIUM RANGE MODEL RUNS OF 27.00Z/27.12Z IN RATHER GOOD AGREEMENT
FOR MID LEVEL RIDGING TO BUILD OVER THE REGION MON. GOOD AGREEMENT
FOR HE UPPER MIDWEST/GREAT LAKES TO THEN PERSIST THRU THE WEEK AS A
REX-TYPE BLOCKY FLOW PATTERN DEVELOPS OVER NOAM. WITH DECENT AND
IMPROVING MODEL CONSENSUS...DAY 4-7 FCST CONFIDENCE IS AVERAGE TO
GOOD THIS CYCLE.

THE RIDGING ALOFT/REX-BLOCK PATTERN WOULD LOOK TO SET THE FCST AREA
UP FOR A WARM/DRY PERIOD NEXT WEEK. HOWEVER...THE AREA IS CLOSE TO
THE PERIMETER OF THE RIDGING FOR TUE-THU. PERSISTENT TROUGHING ON
THE WEST COAST AND SOUTHWEST FLOW ALOFT ACROSS THE ROCKIES AND
PLAINS WILL SEND SHORTWAVE RIPPLES THRU THE FLOW AND INTO THE UPPER
MIDWEST. CANNOT RULE OUT A STRONGER ONE OF THESE WAVES SPREADING A
CHANCE OF SHRA/TSRA INTO MAINLY THE NORTH HALF OF THE FCST AREA FROM
TUE ONWARD. ONE OF THESE WAVES COULD DEVELOP A CONVECTIVE COMPLEX
OVER ND/NORTHWEST MN ANY OF THE NIGHTS...WITH CORFIDI VECTORS
INDICATING PROPAGATION TO THE SOUTHEAST/SOUTH ACROSS MN/WI DURING
THE OVERNIGHT/EARLY MORNING HOURS. WILL STAY WITH THE CONSENSUS AND
LEAVE MON-THU MAINLY DRY FOR NOW. PERSISTENT SOUTH-SOUTHWEST LOW
LEVEL FLOW/WARM ADVECTION RAISES 925MB TEMPS INTO THE 24C-28C RANGE
FOR THE MON-TUE PERIOD...1 TO 2 STANDARD DEVIATIONS ABOVE NORMAL.
MON-THU CONTINUES TO TREND TOWARD A VERY WARM PERIOD WITH TEMPS OF
10F TO 15F ABOVE NORMAL...ESPECIALLY THE HIGHS. RAISED HIGHS MON-THU
A BIT OVER THE CONSENSUS BLEND...MAINLY IN THE RIVER VALLEYS.
CONSENSUS BLEND OF THE DAY 4-7 GUIDANCE LOWS LOOKS GOOD FOR NOW.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z FRIDAY EVENING)
ISSUED AT 600 PM CDT THU AUG 27 2015

UPPER LEVEL SHORTWAVE TROUGH SLATED TO SWING ACROSS THE REGION ON
FRIDAY...BRINGING WHAT SHOULD BE A PROLONGED PERIOD OF RAIN AND
ACCOMPANYING MVFR/IFR CIGS AND VSBYS. CURRENTLY A WEAKLY FORCED
FRONTOGENETIC BAND OF LIGHT SHOWERS FROM MID LEVEL VFR CLOUDS
STRETCHING INTO SOUTHEAST MN. MINIMAL IF AN IMPACT. THE BULK OF THE
FORCING AND THUS PCPN LOOKS TO MOVE IN AFTER 12Z...MORE FAVORED
AROUND 18Z. MODELS HAVE BEEN TRENDING A BIT SLOWER ON TIMING OF THE
MAIN PCPN AREAS /AND SOUTH/...BUT LATEST HOPWRF SUGGESTING FASTER.
GOING TO STICK WITH CURRENT TIMING IN THE FORECAST...BUT WORK LOWER
CIGS/VSBYS IN FOR FRI AFTERNOON. NO INSTABILITY TO SPEAK OF...BUT
STRONG FORCING COULD RESULT IN SOME ISOLATED THUNDER IN WITH THE
RAIN. NOT ENOUGH OF A CHANCE TO INCLUDE IN FORECAST THOUGH.


&&

.HYDROLOGY...TONIGHT THROUGH FRIDAY NIGHT
ISSUED AT 346 PM CDT THU AUG 27 2015

MODEL/ENSEMBLE CONSENSUS CONTINUES TO PRODUCE 1 UP TO 3 INCHES OF
RAIN ACROSS MUCH OF THE FCST AREA FROM LATER TONIGHT THRU FRI
NIGHT. THIS FALLING ACROSS A LANDSCAPE WHERE MOST OF THE CROPS/
VEGETATION ARE IN NEED OF A GOOD DRINK OF WATER. ONE HOUR FLASH
FLOOD GUIDANCE VALUES ARE IN THE 1.8 TO 2.7 INCH RANGE WHILE 6HR
FF GUIDANCE VALUES ARE IN THE 2.8 TO 4.5 INCH RANGE. MOST OF THE
PRECIP EXPECTED TO FALL AS RAIN AND SHOWERS WITH ISOLATED TSRA.
MOST RAIN RATES LOOKING TO BE IN THE 1/10 TO 1/3 INCH/HR
RANGE...PERHAPS UP TO 3/4 INCH/HR WITH ANY TSRA. GIVEN THE
GENERALLY DRY ANTECEDENT CONDITIONS AND MAINLY MODERATE RAIN
RATES...MOST OF THE RAINFALL LATE TONIGHT THRU FRI NIGHT SHOULD TO
SOAK INTO THE SOIL WITH MINIMAL RUNOFF EXPECTED.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....RIECK
HYDROLOGY....RRS


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