Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 250450

Area Forecast Discussion
1150 PM CDT Fri Oct 24 2014


Dense fog advisory issued for areas west of the Illinois River. As
a shortwave trough moves off to the southeast late tonight,
clearing skies should allow good radiational cooling to resume in
a slightly moister air mass, bringing a good scenario for fog
formation. Observations of very low visibilities in fog over
eastern Iowa indicate west central Illinois should develop dense
fog overnight as the clearing sets in. Later updates may be
needed to adjust for fog coverage.


.SHORT TERM...(Tonight)

19z/2pm satellite imagery shows low clouds that persisted for much
of the day across west-central Illinois have finally dissipated,
with just one small patch remaining from south of Lincoln to near
Taylorville.  Based on satellite trends, this patch will dissipate
over the next hour or so as well, leading to mostly clear skies late
this afternoon into the early evening.  Main short-term forecast
concern will be the potential re-development of low clouds/fog
tonight as a weak trough of low pressure moves through the area.
While the overall depth of the atmosphere will be very dry, a
shallow surface-based moist layer will remain in place.  With no
deep-layer moisture to work with, precip is not expected to develop
along the trough despite the NAM hinting at some overnight. As winds
become light/variable and cloud cover associated with the passing
trough begins to clear from northwest to southeast after midnight,
fog will begin to develop.  HRRR is showing widespread fog once
again forming across central/eastern Iowa, then spreading into the
Illinois River Valley toward dawn.  As a result, will carry areas of
fog along/west of the Illinois River, with patchy fog as far east as
Champaign/Danville.  Due to cloud cover and light southerly winds,
low temperatures will be quite a bit warmer than in recent nights,
with readings remaining in the lower to middle 50s.


.LONG TERM...(Saturday through Friday)

Saturday will start off with some morning fog, mainly N of I-70. The
fog should burn off around mid-morning, as a cold front pushes east
across IL. Some cooler air associated with the front will be mainly
confined to northern IL, with our 850mb temps still remaining the
12-14C range in the post-frontal airmass. Dry-adiabatic warming of
that layer to the surface, coupled with some sunshine should help
push high temps into the low 70s N of I-74 with upper 70s S of
I-70. NW winds will still develop behind the front, so a brief
chill may be added to the air at times.

Mid level warming will develop Saturday night into Sunday, as 850mb
temps climb toward +16C. That warm layer will linger into Monday.
Southerly winds will develop Sunday afternoon once the surface ridge
axis moves east of IL, so highs should once again climb into the 70s
across the board. On Monday, a few 80 degree readings will possible
on Monday across southwest and southeast portions of the forecast
area, which would be nearly 20 degrees above normal for late
October. Record highs for Monday are in the mid 80s and currently
appear safe.

A long wave trough is forecast to move across the northern
Plains on Monday night, with cold front progressing across IL
Tuesday morning and early afternoon. The ECMWF indicates a more
dynamic system developing in the western Great Lakes on Tuesday,
while the GFS remains a bit more progressive and less strong with
the frontal passage in our area of IL. We did maintain widespread
chance PoPs Monday night, as moisture pooling and lift still appear
to be  sufficient a band of rain to affect central IL. We continued
the likely PoPs in SE Illinois for Tuesday, but chances should
diminish Tue afternoon as the main forcing for precip departs into
Indiana, and dry air quickly filters into the area aloft.

A significant cool down is agreed upon by the GFS and ECMWF for
Tuesday and Tuesday night, as 850mb temps plummet to 0 to -3C by
midday Wednesday. That will bring temps back below normal for highs
Wed, with upper 50s north of I-74, with low 60s south near
Jacksonville to Lawrenceville.

Another cold front is indicated for Thursday, with better agreement
on timing of the front between the GFS and ECMWF. Confidence is
increasing that a band of light rain showers may accompany that
front. So low chances for rain was added to the Thurs forecast,
mainly from the Illinois river to I-55.

The GFS is a about 4-5 degrees colder than the ECMWF behind the
front, with 850mb readings in the -2 to -4C range Friday afternoon.
We will continue with the cooling trend for late week. Some freezing
temps could develop for Friday night N of I-74 if the GFS scenario
pans out.


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

Mainly VFR conditions continue across central/SE Illinois late
this evening as a shortwave trough and weak cold front moves
across the region, bringing cigs just above 3 kft AGL. This
feature will move off to the east overnight, then clearing of the
cloud cover will allow fog to develop overnight. Given a similar
air mass to last night and winds expected to become light/variable
in the vicinity of the trough, vsbys between 1 and 3 miles
included for most central Illinois TAF locations, with LIFR
category included for KPIA. Gradual clearing with winds NW 8-10
kts expected after 14Z.


DENSE FOG ADVISORY from 3 AM to 9 AM CDT Saturday FOR ILZ027>029-



LONG TERM...Shimon
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