000
FXUS63 KILX 072343
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
643 PM CDT SUN SEP 7 2008
.DISCUSSION...
ISSUED 258 PM CDT SUN SEP 7 2008
18Z SFC MAP SHOWED WEAK HIGH PRESSURE STRETCHING FROM NRN MO EAST
THROUGH THE OHIO VALLEY. COLD FRONT EXTENDED FROM CENTRAL MN BACK
INTO CENTRAL NEB. A FEW STORMS WERE DEVELOPING AHEAD OF THE FRONT
IN NRN IA...WHILE ISENTROPIC LIFT BEHIND THE FRONT IN NEB WAS
CAUSING AN EXPANDING BAND OF SHOWERS. ALOFT...A BROAD LONGWAVE
TROUGH DOMINATED THE CENTRAL CONUS...WITH A SHORTWAVE OF INTEREST
DROPPING THROUGH THE NRN ROCKIES WHICH WILL BE OUR WEATHER-MAKER
FOR TOMORROW. MAIN FORECAST CONCERN IS SYSTEM MOVING THROUGH
MON/MON NIGHT WITH ASSOCIATED POPS AND SEVERE WX POTENTIAL...THEN
MUCH BELOW NORMAL TEMPS FOR TUE AND WED. PATTERN COULD POTENTIALLY
TURN ACTIVE AGAIN FOR LATE-WEEK/WEEKEND BUT THIS IS DEPENDANT ON
TROPICAL ACTIVITY.
SHORT TERM...TONIGHT THROUGH MONDAY NIGHT
QUIET CONDITIONS OVERNIGHT AS STORM ORGANIZES OVER THE PLAINS.
UPPER JET STREAK OF 90+KTS AND POTENT MID-LEVEL SHORTWAVE WILL
PHASE WITH BAROCLINIC ZONE STRETCHING ACROSS THE MIDWEST TO
DEVELOP A SFC LOW FROM AN EXISTING INVERTED TROUGH. SYSTEM WILL BE
INTENSIFYING THROUGH MON. LARGE PRECIP SHIELD NW OF TROUGH/LOW
EXPECTED TO SPREAD UP FROM MO AND IA...INTO NW IL MONDAY MORNING.
DECENT ISENTROPIC LIFT AND FRONTOGENETIC FORCING WILL AFFECT MAINLY
THE FAR NW WITH STEADY RAINS BY MIDDAY...WITH THIS AREA SLOWLY
ADVANCING EASTWARD THROUGH THE AFTERNOON. FARTHER SOUTH THE ATMOSPHERE
WILL BECOME MODERATELY UNSTABLE BY MID-AFTERNOON. FCST SOUNDINGS
SUGGEST CAPES APPROACHING 2K J/KG...ALONG WITH 60 KTS OF 0-6KM
SHEAR SOUTH OF THE AXIS OF STRONGEST DIFFERENTIAL HEATING...MAINLY
SOUTH OF I-74 AND WEST OF I-55. WITH SYNOPTIC SCALE LIFT
INCREASING...THE POTENTIAL EXISTS FOR SOME LOW-END SEVERE
THUNDERSTORMS TO DEVELOP ACROSS NRN MO INTO WEST-CENTRAL IL BY MID
AFTERNOON AND TRACK EAST THROUGH THE EARLY EVENING. FCST AMOUNT OF
WIND SHEAR AND INSTABILITY WOULD PROMOTE ORGANIZED MULTI-CELL
CLUSTERS CAPABLE OF PRODUCING NEAR SEVERE DOWNBURSTS...ALONG WITH
SOME SMALL HAIL GIVEN LOW WET-BULB ZERO HEIGHTS. THIS WILL BE
HIGHLY DEPENDENT ON AFTERNOON HEATING AND THE DEVELOPMENT OF A SFC
LOW TO INCREASE LIFT AND SHEAR. SOUTH OF I-70...DRY CONDITIONS ARE
EXPECTED UNTIL LATE AFTERNOON OR EVENING WHEN LIFT INCREASES FROM
THE NW. WITH ALL THESE FACTORS IN PLAY...EXPECT A LARGE TEMP
GRADIENT WITH HIGHS RANGING FROM THE LOW/MID 60S FAR NW UNDER
STRATIFORM RAIN...TO MID/UPPER 80S SE UNDER PARTLY SUNNY SKIES. AS
INSTABILITY DECREASES THROUGH THE EVENING...GENERAL SHOWERS AND
THUNDERSTORMS ASSOCIATED WITH THE FRONT WILL TRACK SOUTHEAST...AND
CLEAR THE FAR SE COUNTIES LATE MON NIGHT. AREAS THAT CLEAR OUT IN
THE NW COULD DROP INTO THE MID/UPPER 40S BY TUE AM.
LONG TERM...TUESDAY THROUGH NEXT SUNDAY
THE TUE THROUGH THU MORNING PERIOD WILL BE DOMINATED BY A LARGE SFC HIGH
PRESSURE SYSTEM CROSSING THE AREA FROM WEST TO EAST. THIS WILL
BRING A STRETCH OF NICE EARLY FALL WEATHER...WITH MOSTLY CLEAR
SKIES AND SEASONABLY COOL TEMPS. LOWS WED AM COULD BE THE COOLEST
OF THE SEASON SO FAR WITH WIDESPREAD MID/UPPER 40S. AS THE HIGH
EDGES INTO THE EASTERN GREAT LAKES BY THU...SOUTHERLY FLOW ON THE
BACK-SIDE OF THIS SYSTEM WILL BEGIN TO TRANSPORT WARMER AND MORE
HUMID AIR INTO THE AREA. AT THE SAME TIME...A DEEP TROUGH IS
EXPECTED TO FORM OVER THE ROCKIES...AS SHORTWAVE ENERGY FORMS A
SFC LOW ALONG BAROCLINIC ZONE IN THE PLAINS THU. PRECIP ASSOCIATED
WITH THIS SYSTEM WILL BRING A CHANCE OF SHOWERS AND ISOLATED
STORMS MAINLY THU NIGHT. BY FRIDAY...MAJOR DISAGREEMENT SHOWS UP
BETWEEN THE ECMWF AND GFS. GFS SOLUTION FAVORS A DRIER AND WARMER
PERIOD INTO THE WEEKEND AS WEAK UPPER RIDGING BUILDS ACROSS THE
EASTERN THIRD OF THE COUNTRY...WHILE TROUGH RE-LOADS OVER THE
ROCKIES. ECMWF ON THE OTHER HAND WOULD FAVOR A WETTER AND COOLER
PERIOD AS THE COLD FRONT TURNS STATIONARY ACROSS THE AREA INTO THE
WEEKEND...AND IMPULSES RIDING THROUGH THE SW FLOW BRING REPEATED
RAINS FRI AND SAT. THIS DIFFERENCE MAY BE CAUSED BY THE HANDLING
OF IKE...AS GFS TRACKS SYSTEM SIMILAR TO GUSTAV...BUILDING RIDGING
AND SUBSIDENCE AHEAD OF THE FEATURE UNTIL REMNANTS POTENTIALLY
REACH US LATE IN THE WEEKEND. LATEST 12Z ECMWF RUN ACTUALLY TAKES
IKE INTO NRN MEXICO THIS WEEKEND ALLOWING THE MID-LATITUDE SYSTEM
TO SET UP OVER OUR REGION. WITH LITTLE RUN-TO-RUN OR MODEL-TO-MODEL
AGREEMENT HAVE MADE VERY FEW CHANGES BEYOND FRI...WITH SLGT CHC
POPS GOING FRI-SUN.
SCHAFFER
&&
.AVIATION...
ISSUED 643 PM CDT SUN SEP 7 2008...FOR THE 00Z TAFS
DIURNAL CU FIELD WILL CONTINUE TO DISSIPATE THIS EVENING...BUT A
MID CLOUD DECK IS MOVING IN FROM THE WEST. A FEW SHOWERS ARE
ALREADY HITTING THE GROUND IN SE IOWA...BUT THEY SHOULD REMAIN
VERY ISOLATED UNTIL TOMORROW MORNING. THE CURRENT BAND OF AC WILL
AFFECT THE NORTHERN CWA...SO WILL LEAVE THE VCSH IN PIA AFTER
MIDNIGHT AND KEEP A VCSH FOR THE MORNING IN BMI AND SPI. THE
BULK OF THE PRECIP SHOULD HOLD OFF UNTIL AFTERNOON FOR MOST SITES
BESIDES PIA. INSTABILITY AND SHEAR PARAMS POINT TWRD SOME
THUNDERSTORMS FOR THE AFTERNOON...ESPECIALLY IN THE SE 2/3RDS OF
THE CWA...ALONG AND SOUTH OF THE BAROCLINIC ZONE. WILL PUT SOME
VCTS AT ALL SITES IN THE AFTERNOON. VIS AND CIGS SHOULD REMAIN
MVFR FOR THE MOST PART.
SHIMON
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
