Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 230257
AFDILX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
857 PM CST SUN NOV 22 2009

.DISCUSSION...
ISSUED 857 PM CST SUN NOV 22 2009

TEMPS ARE FALLING LITTLE FURTHER AND SOME SITES ALREADY CLOSE TO
FORECASTED MINS...MAINLY IN THE NORTHEAST. HIGH CLOUDS CONTINUE
OVER THE WEST AND EASTERN PARTS OF THE CWA SO THIS SHOULD CAUSE
TEMPS TO BECOME STEADY OVERNIGHT. SO WILL BE MAKING SOME
ADJUSTMENTS TO TEMP GRIDS TONIGHT. NOT SURE YET IF IT WILL AFFECT
TEXT PRODUCT...BUT IF DOES...SHOULD ONLY AFFECT EASTERN AND
NORTHEAST GROUPS.

AUTEN
&&

.AVIATION...
ISSUED 527 PM CST SUN NOV 22 2009

VFR CONDITIONS WILL CONTINUE TONIGHT BUT THEN LIGHT FOG WILL
DEVELOP AT ALL SITES AROUND 06Z LIKE IT DID LAST NIGHT. LIGHT FOG
WILL BURN OFF IN THE MORNING FOLLOWED BY A MID DECK ADVECTING INTO
THE AREA. PIA...BMI AND SPI COULD SEE SOME LOWER SC CLOUDS AT
3.5KFT WHILE DEC AND CMI WILL JUST SEE A MID DECK TOMORROW. LIGHT
EAST WINDS WILL BECOME SOUTHEAST TOMORROW AND CONTINUE INTO THE AFTERNOON.

AUTEN
&&

.PREV DISCUSSION...
ISSUED 309 PM CST SUN NOV 22 2009

A PLEASANT FALL DAY IS UNFOLDING ACROSS CENTRAL AND SOUTHEAST
ILLINOIS...COURTESY OF A LARGE AREA OF HIGH PRESSURE CENTERED OVER
NEW ENGLAND. ONE MORE PRETTY NICE DAY IS ON TAP UNTIL SLOW MOVING
STORM SYSTEM ARRIVES FOR TUESDAY AND LINGERS IN THE AREA THROUGH
THANKSGIVING. THIS SLOW MOVING STORM SYSTEM...AND THE ASSOCIATED
PRECIPITATION THREAT AND IMPACT ON TEMPERATURES...IS OUR PRIMARY
FORECAST CONCERN TODAY.

12Z MODELS IN REASONABLE AGREEMENT THROUGH THE SHORT TERM.
HOWEVER...THEY ARE TRENDING SLOWER WITH OUR APPROACHING STORM
SYSTEM. HAVE TRENDED THE FORECAST TOWARD THE MIDDLE OF THE MODEL
ENVELOPE...WHICH IS QUICKER THAN THE NAM....BUT SLOWER THAN THE
GFS.

SHORT TERM...TONIGHT THROUGH WEDNESDAY...OUR STORM SYSTEM IS
ALREADY BEGINNING TO DEVELOP OVER THE CENTRAL PLAINS...AS
EVIDENCED BY THE LOW PRESSURE CENTER STARTING TO DEVELOP OVER
KANSAS. HOWEVER...IT WILL NOT REALLY DEEPEN OR START MOVING OUR
DIRECTION UNTIL IT INTERACTS WITH THE VIGOROUS UPPER WAVE
CURRENTLY DIVING THROUGH THE PACIFIC NORTHWEST. IN THE MEANTIME...
QUIET AND MILD LATE NOVEMBER CONDITIONS WILL PERSIST INTO MONDAY
NIGHT AS RETURN FLOW INCREASES ON THE BACK SIDE OF THE DEPARTING
AREA OF HIGH PRESSURE. THE DIURNAL TEMPERATURE RANGE MAY BE A BIT
SMALLER THROUGH MONDAY NIGHT THAN THE PAST FEW DAYS AS THE SYSTEM
BEGINS TO APPROACH AND CLOUD COVER INCREASES.

THE APPROACHING STORM AND TRAILING FRONTAL BOUNDARY IS STILL
EXPECTED TO MOVE THROUGH THE REGION ON TUESDAY...BUT THE FRONTAL
SPEED HAS SLIPPED BY ABOUT 6 HOURS. SO...AN 18Z TUE-06Z WED
PASSAGE LOOKS MORE LIKELY NOW THAN THE PREVIOUSLY EXPECTED 12Z
TUE-00Z WED PASSAGE. THE MAIN SURFACE LOW WILL LIFT INTO THE GREAT
LAKES TUESDAY NIGHT...WITH THE TRAILING COLD FRONT EXPECTED TO
CLEAR THE FORECAST AREA BY AROUND 06Z WEDNESDAY. THIS SYSTEM
SHOULD BE PRETTY DYNAMIC...THOUGH ALREADY OCCLUDED BY THE TIME IT
GETS HERE...WITH DECENT JET FORCING PROGGED OVER THE FORECAST AREA
ON TUESDAY COUPLED WITH THE SURFACE FROPA. HAVE KEPT LIKELY RAIN
CHANCES IN THE WESTERN HALF OF THE FORECAST AREA FOR TUESDAY...WITH
THE EASTERN HALF PROBABLY NEEDING A BUMP ONCE THE TIMING IS MORE
FIRM. HOWEVER...IT IS HARD TO JUSTIFY LIKELY POPS IN THE EAST AT
THIS TIME AS THE FROPA STILL APPEARS POSSIBLE IN MORE THAN ONE
FORECAST PERIOD.

ONCE THE FRONT CLEARS THE FORECAST AREA TO THE EAST...THERE SHOULD
BE A LULL IN THE PRECIPITATION THREAT. WHILE WRAP AROUND
PRECIPITATION IS POSSIBLE WEDNESDAY...THE BULK OF IT SHOULD HOLD OFF
UNTIL THE EXTENDED RANGE OF THE FORECAST PERIOD. THE SLOWER
PROGRESSION OF THE SYSTEM AND ASSOCIATED ARRIVAL OF COLDER AIR
SHOULD KEEP ANY PRECIPITATION FALLING AS RAIN THROUGH WEDNESDAY.

LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY...MODELS/ENSEMBLES
INITIALLY IN FAIRLY GOOD AGREEMENT WITH RESPECT TO FEATURES
EXPECTED TO IMPACT FORECAST AREA. HOWEVER...SPREAD INCREASES
SIGNIFICANTLY FROM FRIDAY TO THE END OF THE PERIOD.
UNFORTUNATELY...ONE ITEM THAT HAS DEVELOPED SINCE YESTERDAY IS
THAT IT LOOKS INCREASINGLY LIKELY THAT PRECIPITATION...MOSTLY
SNOW...IS POSSIBLE ON THANKSGIVING. THE SURFACE LOW THAT BROUGHT
THE RAINS ON TUESDAY WILL STILL BE OVER THE GREAT LAKES AT THE
START OF THE PERIOD. A STRONG UPPER WAVE WILL DIVE DOWN THE
BACKSIDE OF THIS VERTICALLY STACKED SYSTEM...LIKELY PRODUCING A
ROUND OF PRECIPITATION LATE WEDNESDAY NIGHT INTO THANKSGIVING DAY.
SOUNDING PROFILES/THICKNESS SCHEMES SUGGEST THAT ALL THE WARM AIR
WILL BE NEAR SURFACE BASED WHEN THIS PRECIPITATION WOULD FALL. SO
THE PRECIPITATION WILL BE FALLING AS SNOW...AND IT APPEARS THAT
THE ABOVE FREEZING NEAR SURFACE LAYER WILL BE TOO SHALLOW TO MELT
THE SNOW BEFORE IT REACHES THE GROUND. HOWEVER...AT THIS POINT...
LITTLE OR NO ACCUMULATION IS EXPECTED WITH THE SNOW. UPPER/SURFACE
RIDGING START TO BUILD IN FRIDAY INTO SATURDAY ENDING THE
PRECIPITATION AND ALLOWING TEMPERATURES TO MODERATE A BIT. THE
ARRIVAL OF THE NEXT WEATHER SYSTEM IS MUCH LESS CLEAR AT THIS
TIME. THE GFS SOLUTIONS TEND TO DEVELOP A MORE PHASED DEEP TROF
OVER EASTERN NORTH AMERICA...WHILE THE ECMWF SPLITS THE FLOW AND
CUTS OFF A LOW OVER THE SOUTHWEST UNITED STATES. THIS RESULTS IN
THE GFS BEING QUICKER AND STRONGER WITH THE NEXT SYSTEM BY LATE IN
THE WEEKEND. WILL CONTINUE TO SIDE WITH THE MORE CONSISTENT ECMWF
AND LEAVE THE END OF THE WEEKEND DRY AT THIS TIME.

BAK

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$







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