248
FXUS63 KARX 240838
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
338 AM CDT THU JUL 24 2008
SHORT TERM
TODAY THROUGH SATURDAY NIGHT
FORECAST CONCERNS THIS PACKAGE...PRECIPITATION CHANCES TODAY THROUGH
FRIDAY EVENING...TEMPERATURES.
DATA ANALYSIS AT 06Z HAD HIGH PRESSURE RIDGING FROM JAMES BAY SOUTH
INTO WI AND IL. LOW PRESSURE WAS LIFTING NORTH OF LK WINNIPEG WITH
A TRAILING TROUGH/COLD SOUTH THRU THE DAKOTAS TO EASTERN CO. FOG-
PRODUCT/IR IMAGERY/SFC OBS SHOWED WIDESPREAD MID/HIGH CLOUDS ACROSS
MN/IA/WI AS MOISTURE PUSHED INTO/THRU THE UPPER RIDGE AXIS. WEAK TO
MDT 850-700MB MOISTURE TRANSPORT/THETA-E CONVERGENCE RESPONSIBLE FOR
BAND OF SCT -SHRA FROM NORTHWEST OF KMSP TO SOUTHEAST IA. STRONGER/
DEEPER CONVECTION WAS IN THE KFSD/KOMA AREAS ON NOSE OF LOW LEVEL
JET/STRONGER 850MB MOISTURE TRANSPORT AND INSTABILITY. ALL THE
SHRA/TSRA MOVING SOUTHEAST EARLY THIS MORNING...FOLLOWING CORFIDI
VECTORS QUITE NICELY.
NO LARGE ERRORS NOTED WITH 24.00Z NAM/GFS INITIALIZATIONS...WITH
THESE 2 MODELS PRESENTING RATHER SIMILAR SOLUTIONS FOR TODAY THRU
SAT NIGHT. DPROG/DT OF 500MB HGTS AT 24.00Z SHOWED NAM/GFS RUNS OF
22.00Z AND 23.00Z VERIFIED QUITE WELL ACROSS NOAM...WITH EDGE AND
BETTER RUN-TO-RUN CONSISTENCY TO GFS OVER EASTERN NOAM AND TO
NAM WITH THE TROUGHS OVER OR/WA AND SASKAT/ND. THRU 36HRS TREND IS
TOWARD MORE PROGRESSIVE WITH THE EASTERN CONUS TROUGH AND A BIT
SLOWER/STRONGER WITH THE TROUGHS NEAR LK WINNIPEG AND OFF THE WEST
COAST AT 12Z FRI. FOR 36-84HRS TREND IS FURTHER WEST/SLOWER WITH
THE UPPER LOW NEAR JAMES BAY AND WITH THE NEXT TROUGH NEARING THE
BC COAST. THIS LEAVES THE AREA MORE UNDER THE INFLUENCE OF THE
EASTERN CAN UPPER LOW/NORTHWEST FLOW ALOFT FOR THE WEEKEND. CHECK OF
OBS VS. MODEL DATA AT 06Z SHOWED NAM/GFS WITH REASONABLE DEPICTIONS
OF THE SFC MASS FIELDS ACROSS CENTRAL NOAM. PER WV IMAGERY MODELS
REASONABLE WITH THE SHORTWAVE DETAILS ACROSS THE CONUS...WITH NO
CLEAR FAVORITE ON THE DETAILS. GFS GENERALLY BETTER WITH THE 00Z-
06Z PRECIP ACROSS THE REGION. NO CLEAR DETERMINISTIC MODEL OF CHOICE
THIS CYCLE. WITH MODEL SOLUTIONS QUITE SIMILAR...INCLUDING UKMET/
ECMWF AND LCL/RUC WRF MODELS...FAVORING ONE OVER THE OTHER AGAIN
LOOKS TO HAVE LITTLE IMPACT ON THE SENSIBLE WEATHER FOR TODAY INTO
SAT NIGHT. AGAIN FAVORED A MODEL/ENSEMBLE COMPROMISE THIS CYCLE.
SHORT TERM FORECAST CONFIDENCE ON THE GOOD SIDE.
FOR THE SHORT TERM...GENERALLY WEAK 850-700MB MOISTURE TRANSPORT/
THETA-E CONVERGENCE PRODUCING THE SCT -SHRA FROM EAST CENTRAL MN TO
SOUTHEAST IA WEAKENS OR DROPS SOUTHEAST OF THE FORECAST AREA BY
15Z THIS MORNING. STRONGER THERMODYNAMIC FORCING FOR CONVECTION
OVER SOUTHEAST SD/WESTERN IA WEAKENS/DROPS SOUTHEAST ACROSS IA THIS
MORNING AS WELL. UPPER RIDGE AXIS IS OVER THE AREA TODAY WITH
RELATIVELY WARM MID LEVEL TEMPS AND AN INVERSION REMAINING NEAR
800MB. THIS AND CONSIDERABLE DEBRIS MOISTURE/CLOUDINESS LIMIT CAPE
DEVELOPMENT OVER THE AREA TODAY. ALL THIS LEAVES THE AREA WITH
LITTLE IN THE WAY OF FORCING/LIFT/INSTABILITY FOR CONVECTION
TODAY/TONIGHT AND LOWERED MOST RAIN CHANCES THESE PERIODS TO SLIGHT
CHANCE. RESURGENCE OF LOW LEVEL JET/MOISTURE TRANSPORT TONIGHT RE-
FIRES CONVECTION ACROSS CENTRAL IA...BUT GOOD MODEL CONSENSUS ON THE
DETAILS THAT THIS FORCING WILL REMAIN/PASS SOUTHWEST OF THE FORECAST
AREA TONIGHT. DID LEAVE A BIT HIGHER RAIN CHANCE ACROSS NORTHEAST IA
TONIGHT WITH THAT AREA CLOSER TO THE CONVECTIVE FORCING/INSTABILITY.
MOISTURE/INSTABILITY INCREASE OVER THE AREA FRI...AHEAD OF THE
TROUGH/FRONT. MIXED LAYER CAPES INCREASE TO THE 1K-2K J/KG RANGE FOR
FRI AFTERNOON. 0-6KM SHEAR INCREASES AS WELL WITH SWODY2 LOOKING
REASONABLE. WITH THE TREND TOWARD A SLOWER/STRONGER TROUGH/FRONTAL
PASSAGE ON FRI...AND THE MDT/STRONG CAPE/SHEAR...RAISED SHRA/TSRA
CHANCES FOR FRI. DUE TO THE SLOWER FRONTAL PASSAGE AND SOME
LINGERING INSTABILITY...SHRA/TSRA CHANCES FRI EVENING LOOK WELL
TRENDED AND RAISED THESE A BIT AS WELL. HIGH PRESSURE AND A DRIER
AIRMASS IN THE WAKE OF THE FRONT SLIDE INTO THE AREA FOR SAT/SAT
NIGHT. THE DRY FORECAST FOR THESE PERIODS CONTINUES TO LOOK GOOD.
GENERALLY FAVORED A BLEND OF GUIDANCE HIGHS TODAY. SHOULD BE
PLENTY OF DEBRIS CLOUDINESS TODAY...BUT ANY HOLES IN THE CLOUDS THIS
AFTERNOON WOULD ALLOW TEMPS TO JUMP QUICKLY. GUIDANCE LOWS TONIGHT/
FRI NIGHT SIMILAR AND LOOK GOOD. TRENDED TOWARD WARMER OF GUIDANCE
HIGHS FRI...AHEAD OF THE TROUGH/FRONT. GUIDANCE HIGHS SIMILAR AND
REASONABLE FOR SAT. WITH THE DRIER HIGH BUILDING IN...TRENDED LOWS
DOWN A BIT FOR SAT NIGHT TOWARD THE 24.00Z GFS MEX-MOS GUIDANCE.
LONG TERM
SUNDAY THROUGH WEDNESDAY
24.00Z MEDIUM RANGE MODELS IN GENERALLY GOOD AGREEMENT WITH EACH
OTHER FOR THE SUN-WED PERIOD. RUN-TO-RUN CONSISTENCY REMAINS GOOD ON
SUN. BY MON/TUE THIS STARTS TO DECREASE AS MODELS TREND TOWARD A
STRONGER SHORTWAVE PUSHING THRU THE WESTERN NOAM RIDGE AXIS MON...
THEN DROPPING ACROSS THE REGION TUE. THIS TREND TOWARD 23.00Z
ECMWF RUN...WHICH LOOKED LIKE AN OUTLIER AT THE TIME. SOME TIMING
DIFFERENCES WITH THIS WAVE DROPPING ACROSS THE REGION...BUT BOTTOM
LINE LOOKS TO BE THE AREA REMAINS UNDER NORTHWEST FLOW ALOFT THRU
THE MIDDLE OF NEXT WEEK. STILL SOME THREAT FOR CONVECTION ON SUN...
HOWEVER MODEL CONSENSUS INCREASING FOR THE FORCING FOR THIS TO
REMAIN SOUTHWEST OF THE FORECAST AREA. DID ADD A SMALL SHRA/TSRA
CHANCE TO TUE WITH THE INCREASING MODEL SIGNAL/CONSENSUS FOR A
TROUGH/FRONTAL PASSAGE. WITH THE CONTINUED NORTHWEST FLOW ALOFT...
TEMPS LOOK TO REMAIN NEAR TO A BIT BELOW NORMAL. THIS WELL TRENDING
IN THE DAY 4-7 GRID-SET AT THIS TIME.
AVIATION
TODAY AND TONIGHT
CONTINUE TO EXPECT VFR CONDITIONS THROUGH AT LEAST MUCH OF THE
AFTERNOON...BEFORE TRENDS TO POTENTIAL MVFR CEILINGS AND PERHAPS
VSBYS COME INTO THE FORECAST. BANDS OF MAINLY -SHRA TO CONTINUE TO
WORK EAST ACROSS THE TAF SITES THIS MORNING...BUT RAINFALL SPOTTY
AND LIGHT AND CEILINGS/VSBYS EXPECTED TO REMAIN LARGELY VFR. MAINLY
LGT SLY FLOW FOR THE TAF SITES TODAY...WITH THIS BRINGING A HIGHER
MOISTURE AIRMASS INTO THE TAF SITES LATER THIS AFTERNOON/EVENING.
INDICATIONS IN THE BOTH DETERMINISTIC AND PROBABILISTIC DATASETS FOR
AN MVFR STRATOCUMULUS FIELD TO DEVELOP WITH THIS LOW LEVEL THETA-E
ADVECTION...AND POTENTIAL FOR HAZE/BR TO BE WIDESPREAD IN THE 4-6SM
RANGE OVERNIGHT. THERE/S A MORE SUBTLE INDICATION IN THE ABOVE
DATASETS FOR IFR FOG TO DEVELOP AFTER 06Z GIVEN LIGHT FLOW...AND
WEAK CONVERGENCE AHEAD OF A COLD FRONT. WOULD BE DEPENDENT ON LOWER
MVFR CEILINGS BREAKING UP...WHICH MAY NOT HAPPEN. FOR NOW...WILL
CONTINUE TRENDS OF LOWERING VIS INTO MVFR CATEGORIES TONIGHT AND
CONTINUE MVFR CEILINGS ALREADY FORECAST.
ARX WATCHES/WARNINGS/ADVISORIES
WI...NONE.
MN...NONE.
IA...NONE.
SHORT/LONG TERM...RRS
AVIATION..........BINAU
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