Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 271158

Area Forecast Discussion
558 AM CST Thu Nov 27 2014

.SHORT TERM...(Today)

08z/2am surface analysis shows clipper system tracking southeastward
through the Ohio River Valley, while high pressure builds into the
Northern Plains.  Accumulating snow associated with the clipper has
ended, leaving behind mostly cloudy and cold conditions.  Main
short-term forecast challenge will be timing the clearing trend
later today.  Current IR satellite imagery shows clouds blanketing
the region, with the back edge evident across southeast Minnesota
into north-central Iowa.  Based solely on satellite timing tools,
clearing would arrive in the Illinois River Valley between 15z
and 17z.  However, with continued deep-layer northwesterly flow
and very cold air aloft, think clouds may tend to re-develop along
the back edge of the main cloud area, thus slowing the overall
eastward progression of the clearing.  Negative Cu-rule values
support this theory, as does the latest HRRR.  As a result, have
decided to go with an overcast morning across the board, followed
by a slow northwest to southeast clearing this afternoon.  Some
locations along/east of I-57 will likely remain cloudy through the
entire day.  Due to the cloud cover and gusty northwesterly winds,
high temperatures will remain in the 20s across most of the area,
with a few lower 30s south of I-70.


.LONG TERM...(Tonight through Wednesday)

After chilly low temperatures in the teens tonight, a major
warming trend will get underway by the end of the week.  High
pressure will shift east of the region on Friday, allowing gusty
southerly winds to develop.  Despite partial sunshine and strong
WAA, warming trend will be mitigated by existing snow cover along
a Robinson line.  Have therefore
lowered highs across this area by 3-4 degrees, with readings
reaching the lower 40s.  Much warmer weather is expected
everywhere on Saturday as strong southerly flow continues.  Models
are showing a fetch of increasing low-level moisture from the
western Gulf of Mexico northward into central Illinois, with
surface dewpoints increasing from the lower 30s early in the day
to well into the 40s to near 50 degrees late.  Forecast soundings
are showing saturation below 850mb, which suggests increasing
low-level cloudiness.  Have therefore gone with a mostly cloudy
forecast with high temperatures reaching the lower to middle 50s.

Another warm day is expected on Sunday, although an approaching
cold front may trigger scattered showers across the central and
southern zones as the day progresses.  Front will drop into the Ohio
River Valley by Sunday night, with a colder airmass settling
southward early next week.  Model discrepancies still exist Sunday
night, with the ECMWF trying to develop some light precip
along/north of the boundary into the colder airmass.  Meanwhile, the
GFS keeps the light precip confined to areas along/south of the
front.  With no clear upper wave to support the post-frontal precip,
have opted to side with the GFS here, resulting in dry weather
across all but the far SE CWA where a few rain showers may linger.

After that, major model differences arise by the middle of next week
as all are in poor agreement concerning the evolution of a closed
upper low over the central Pacific.  00z Nov 27 GFS shows this
feature shearing eastward across the central CONUS, eventually
triggering overrunning precip across Illinois by
Wednesday/Thursday.  The ECMWF takes the low and gradually absorbs
it into a stronger low over the Gulf of Alaska, with a downstream
trough providing cool/dry weather to Illinois on
Wednesday/Thursday.  Meanwhile, the GEM keeps the low off the coast
of California the entire time and also features a cool/dry forecast
for Illinois.  With so much uncertainty, decided to make very few
changes to the going forecast beyond Monday.  For now will maintain
dry weather through Wednesday with a gradual warming trend.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)

VFR conditions across the central Illinois terminals will quickly
degrade to MVFR this morning as a band of SC spreads in from the
northwest. Then, skies will scatter out this afternoon as a ridge
of high pressure begins to build into the area. However, mid/high
level VFR cigs will spread into the region tonight ahead of the
next weather disturbance. A period of 10-15 kt northwest winds
will exist to start the day, with a few gusts to 20 kts possible
into midday. Then, winds will trend light/variable as the ridge of
high pressure builds in for tonight.




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