000
FXUS63 KARX 061955
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
255 PM CDT SAT SEP 6 2008
.SHORT TERM...THROUGH TUESDAY NIGHT
FORECAST CONCERNS WERE PRIMARILY FOCUSED ON RAIN CHANCES
TONIGHT INTO SUNDAY...THEN AGAIN MONDAY AND MONDAY NIGHT.
AFTERNOON SURFACE ANALYSIS HAD HIGH PRESSURE ACROSS THE MID-MISSOURI
VALLEY. WATER VAPOR IMAGERY INDICATED FORECAST AREA WAS IN BETWEEN
UPPER LEVEL SYSTEMS. LAST NIGHTS SHORT WAVE SEEN IN WATER VAPOR
DEPARTING ACROSS SOUTHEAST CANADA. MEANWHILE...NEXT SHORT WAVES
LOCATED ALONG NEBRASKA/SOUTH DAKOTA BORDER...AS WELL AS DIVING
INTO THE NORTHERN PLAINS. RADAR AND SURFACE DATA INDICATED SHOWERS
AND ISOLATED THUNDERSTORMS ASSOCIATED WITH BOTH FEATURES.
DETERMINISTIC NCEP AND EUROPEAN MODELS IN THE 06.12Z DATA SUITE
CONTINUED TO INDICATE A COUPLE OF WAVES SHOULD BRING RAIN TO THE
FORECAST AREA IN THE NEAR TERM. CONFIDENCE WAS FAIRLY HIGH IN OVERALL
DETAILS OF FIRST SYSTEM AFFECTING THE FORECAST AREA SUNDAY...WITH
GOOD AGREEMENT IN REGARD TO QPF AND AREAL EXTENT OF RAIN CHANCES.
DIFFERENCES NOTED WITH TIMING AND STRENGTH OF SECOND SYSTEM MONDAY
AND MONDAY NIGHT LOWERS CONFIDENCE A BIT IN EXTENT OF RAIN CHANCES
AND QPF AMOUNTS. HOWEVER...BASED ON HPC GUIDANCE AND 06.15Z SREF...
GFS AND NAM/WRF HIGHEST AMOUNTS APPEAR TOO FAR NORTH. THUS...QPF
IN THE FORECAST AREA WAS ADJUSTED LOWER THAN NCEP MODELS...WHICH
AGREED WITH HPC GUIDANCE AND EUROPEAN MODELS.
FOR TONIGHT/SUNDAY...SHORTWAVE ENERGY AND WEAK SFC-700MB WARM
ADVECTION/THETA-E CONVERGENCE ALONG LOW LEVEL TROUGH AXIS...
RESPONSIBLE FOR THE AREAS -RA/SCT SHRA/TSRA OVER EASTERN SD/NEB
TODAY...SLIDES EAST/SOUTHEAST TONIGHT. THE WEAK FORCING/LIFT WITH
THESE FEATURES CONTINUES TO BATTLE DRIER LOW/MID LEVEL AIR AS THEY
PASS JUST SOUTHWEST OF THE AREA THIS EVENING. DESPITE LOSS OF
HEATING/WEAK INSTABILITY DID LEAVE A SMALL -SHRA CHANCE ACROSS THE
SOUTHWEST HALF OF THE FORECAST AREA THIS EVENING.
NEXT STRONGER UPSTREAM SHORTWAVE PUSHES INTO/ACROSS THE AREA LATE
TONIGHT/SUN. LOWER LEVEL THERMO-DYNAMIC FORCING/LIFT REMAIN WEAK AND
MOISTURE ON THE LIMITED SIDE. BULK OF FORCING/LIFT LATER TONIGHT/SUN
THE DYNAMICS/PV ADVECTION WITH THE SHORTWAVE. UNLIKE FRI EVENING...
300MB JET DIVERGENCE APPEARS TO BE LACKING FOR LATE TONIGHT/SUN.
DEEPER MOISTURE AND BETTER BUT WEAK LIFT SLIDES ACROSS THE NORTHERN
HALF OF THE FORECAST AREA SUN AFTERNOON...AND CARRIED HIGHEST RAIN
CHANCES THERE. CONTINUED MENTION OF ISOLATED TSRA SUN AFTERNOON...AS
BOTH NAM/GFS INDICATE MOST UNSTABLE CAPE OF 500 J/KG.
MONDAY...FORCING WAS IMPRESSIVE WITH UPPER LEVEL AND SURFACE WAVE.
THIS WAS MANIFESTED BY MID LEVEL FRONTOGENESIS...ALONG WITH UPPER
LEVEL DIVERGENCE ASSOCIATED WITH RIGHT REAR QUADRANT OF 300MB JET.
CONFIDENCE WAS HIGH ENOUGH TO INCREASE RAIN CHANCES INTO THE LIKELY
CATEGORY...MOSTLY IN NORTHEAST IOWA AND SOUTHWEST WISCONSIN...BASED
ON PROXIMITY TO STRONG FORCING. WE BELIEVE A FURTHER INCREASE IN
RAIN PROBABILITIES WILL BE WARRANTED IN THESE LOCATIONS.
DUE TO EXPECTED CLOUD COVER AND/OR RAIN...TEMPERATURES SHOULD NOT
HAVE VERY MUCH OF A DIURNAL RANGE ON MONDAY. THEREFORE...CONFIDENCE
WAS HIGH ENOUGH TO LOWER MAXIMUM TEMPERATURES CONSIDERABLY. THIS WAS
CORROBORATED BY GFS AND NAM MOS GUIDANCE...WHICH SUGGEST EVEN
FURTHER LOWERING MAXIMUM TEMPERATURES ON MONDAY MAY BE WARRANTED.
MONDAY NIGHT...WE BELIEVE A PROGRESSIVE SOLUTION OF THE UPPER
LEVEL AND SURFACE SHOULD BE THE MOST PROBABLE OUTCOME. THUS...
FORCING SHOULD EXIT STEADILY EASTWARD. AS A RESULT...CONFINED RAIN
CHANCES TO 09.00Z-09.06Z TIME PERIOD MAINLY IN SOUTH AND EAST
SECTIONS OF THE FORECAST AREA.
.LONG TERM...WEDNESDAY THROUGH SATURDAY
PRIMARY FORECAST CONCERNS WERE ASSOCIATED WITH RAIN CHANCES DURING
THE MID TO LATE WEEK TIME PERIOD. THERE HAS BEEN A CONSISTENT SIGNAL
OF AN UPPER LEVEL AND SURFACE WAVE TO AFFECT THE UPPER MISSISSIPPI
VALLEY IN THE DETERMINISTIC AND PROBABILISTIC DATA SUITE. 12Z GFS AND
ECMWF HAVE CONTINUED TO FOLLOW SUIT...WHICH LENDS FAIRLY HIGH
FORECAST CONFIDENCE IN A RAIN EVENT ACROSS THE FORECAST AREA. IN
FACT...OUR CONFIDENCE WAS HIGH ENOUGH TO INCREASE RAIN CHANCES
WEDNESDAY NIGHT INTO THURSDAY.
06.12Z DETERMINISTIC GFS AND ECMWF WERE AT ODDS WITH REGARD TO TIMING
OF UPPER LEVEL AND SURFACE FEATURES BY DAY 7...SATURDAY. AT THIS
TIME...CHOSE TO HAVE A DRY PERIOD FRIDAY THROUGH SATURDAY...WITH
WESTERLY FLOW EXPECTED TO RETURN AT THE SURFACE AND ALOFT. CURRENT
DATA BASE INDICATED THIS ALREADY...SO NO CHANGES WERE MADE IN THIS
PORTION OF THE EXTENDED OUTLOOK.
&&
.AVIATION...TONIGHT AND SUNDAY
BULK OF SCT-BKN 3K-4K FOOT DIURNAL CUMULUS DECK UNDER COOL 850MB
TEMPS SHOULD DISSIPATE WITH LOSS OF HEATING LATE THIS AFTERNOON/
EARLY THIS EVENING. MID/HIGH CLOUDS ALREADY INCREASE ACROSS THE
TAF SITES LATER THIS EVENING/OVERNIGHT AS NEXT SHORTWAVE ENERGY
APPROACHES THE AREA. THICKENING CLOUDS AND SOME LIGHT SOUTH/
SOUTHWEST WIND FLOW EXPECTED TO KEEP LATE NIGHT/EARLY MORNING FOG TO
A MINIMUM BUT MAY STILL SEE SOME LOCAL 4SM-6SM VSBYS IN BR ACROSS
SOUTHEAST MN/WESTERN WI.
SHORTWAVES PUSH ACROSS THE AREA SUNDAY...BUT AGAIN MEAGER MOISTURE
TO WORK WITH AND DRIER AIR IN THE SFC-850MB LAYER. AREAS -RA/SCT
-SHRA EXPECTED AT/AROUND THE TAF SITES MAINLY FROM MID MORNING THRU
SUN AFTERNOON. MUCH LIKE FRI EVENING WITH LIMITED LOW LEVEL MOISTURE
AND FORCING/LIFT...BULK OF CIGS/ VSBYS EXPECTED TO REMAIN VFR IN ANY
RAIN ON SUN. A MORE POTENT SYSTEM LOOMS FOR MON/MON EVENING WITH
POTENTIAL FOR WIDESPREAD MVFR/LCL IFR CIGS IN -RA/EMBEDDED -TSRA.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&
$$
SHORT/LONG TERM...RRS/THOMPSON
AVIATION..........RRS
