Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 280751

Area Forecast Discussion
251 AM CDT Sun Sep 28 2014

.SHORT TERM...(Today)

A shallow layer of moisture below a subsidence inversion will allow
some patchy ground fog to linger near river valleys early this
morning (IL and Wabash rivers). Any fog should dissipate by 830

The 580dm 500mb low centered over NW Iowa will become an open wave
and drift southeast across IL later today. That will bring scattered
cirrus clouds with it, but sunshine should generally prevail across
IL today. That will help high temps climb above all guidance numbers
again today, as the dry low levels warm quickly. Temps should top
out in the low to mid 80s this afternoon. Winds will remain light
under a weak surface pressure gradient, with east to southeast winds
of 5 to 8 mph again today.


.LONG TERM...(Tonight through Saturday)

Quiet weather expected for the start of the work week. Shortwave
currently entering western Saskatchewan will be digging as it
approaches Lake Superior, forming a closed low over Lake Huron by
early Tuesday. This will drive a cold front southward, but general
model consensus only brings it into northern Illinois before
beginning to lift back north. 850 mb temperatures in the 11-13C
range will keep our highs above normal, with some modest cooling on
Tuesday as the front approaches. Have favored the warmer MAV MOS
guidance during this period.

Large upper low currently spinning over western Nevada still
expected to swing northeast and weaken early this week, with a small
closed low lifting through the Dakotas on Tuesday. This will help
draw the previously mentioned front back north, before a stronger
cold front moves in from the northwest. Still looks like Wednesday
will largely be dry, before showers increase Wednesday night ahead
of the front. Shear profiles increase nicely as the front gets
closer on Thursday, with 0-6km bulk shear progged to reach 40-50
knots across much of the CWA Thursday afternoon per the ECMWF model,
while the GFS is on the lower end of that range. Will need to
continue watching the threat for some stronger storms Thursday
afternoon and evening. Highest PoP`s will be during the evening
hours, with a quick decrease in rain from west to east after
midnight following the passage of the front itself.

Strong surge of cooler air to plunge into the Midwest on Friday in
the wake of the front. Some model discrepancies exist at this range,
as the GFS was more prominent on driving a surface low northeast
along the front and amplifying as a significant upper low drops into
Wisconsin. The ECMWF is continues with a smaller but strong open
wave that races through the region on Friday. Both keep the coolest
air to our north, but the GFS would be more of a threat with
lingering wraparound showers. For PoP`s, have continued to mention
them in the 30-40% range, but currently think this would mainly be
early in the morning. Temperature-wise, the GFS brings the 850 mb
zero isotherm into central Illinois by late Friday and dipping below
zero by early Saturday, while the ECMWF is already spreading warmer
air back in by that point. With this discrepancy, the GFS would
indicate highs only in the 50s on Saturday, while the ECMWF favors
mid to upper 60s. Have favored more of a blend with Saturday as the
coolest day, with highs mostly 60-65.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)

Main forecast concern will be in the 09z-12z time frame with the
potential for some brief MVFR vsbys in fog. Will continue to carry
a brief period of MVFR vsbys (~5SM) early Sunday morning with the
higher probability for some patchy fog acrs the west where late
evening temp-dew point spreads were the lowest compared with the
observations sites east. Whatever fog we do see early in the
morning should dissipate by 13z with VFR conditions for the
remainder of the forecast period. Surface winds will remain light
easterly tonight and Sunday with speeds of 3 to 7 kts.




LONG TERM...Geelhart
AVIATION...Smith is the U.S. government's official web portal to all federal, state and local government web resources and services.