Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 300813

Area Forecast Discussion
313 AM CDT Tue Sep 30 2014

.SHORT TERM...(Today)

Short-term forecast concerns include potential for dense fog across
the southern KILX CWA, as well as, approaching deck of low clouds
across the north.  Clear skies/light winds have once again led to
the development of fog early this morning, particularly from the Saint
Louis area eastward across southern Illinois.  HRRR shows fog
becoming dense across this area over the next few hours before
dissipating after 8am.  So far the fog has not become widespread or
dense, but based on forecast soundings and the HRRR, have included
patchy dense fog south of I-70.  Have also issued a Special Weather
Statement to highlight the low visbys.

07z/2am surface analysis shows cold front settling southward into
central Illinois, with a widespread area of low clouds noted to the
north of the boundary across the Great Lakes into north-central
Illinois.  Latest IR satellite imagery shows leading edge of clouds
along the I-74 corridor.  Main questions today will be how far south
these clouds will push and whether or not they will clear out this
afternoon.  Satellite timing tools suggest the clouds will reach a
Rushville to Mattoon line by daybreak, but additional southward
progress appears unlikely as frontal boundary stalls.  Will
therefore go with a cloudy forecast across the northern two-thirds
of the CWA this morning, with mostly sunny skies across the far
south.  Despite continued northeasterly flow behind the front,
forecast soundings indicate a gradual dissipation of low-level
moisture as the day progresses.  Have gone with decreasing clouds
through the afternoon accordingly.  Due to the cloud cover and shift
in wind direction, temps will be quite a bit cooler today than in
recent days, with afternoon highs ranging from the upper 60s
along/north of I-74 to around 80 far south.


.LONG TERM...(Tonight through Monday)

Winds will veer to the southeast on Wednesday in response to an
approaching upper wave and its associated cold front.  As a result,
temperatures will rise back into the upper 70s and lower 80s.  As a
deep upper trough slowly approaches from the west, surface low
pressure is expected to develop over the Southern Plains on
Wednesday then lift northward into the Great Lakes by Thursday
night.  Warm-advection showers/thunder will develop ahead of this
system Wednesday night: however, all model guidance keeps any precip
focused mainly across the western half of the CWA.  Given this
slowing trend, have removed PoPs for all locations along/east of a
Paris to Effingham line.  Further west, will go with likely PoPs
across the Illinois River Valley after midnight.  As surface low
deepens and tracks just west of the area on Thursday, strong S/SW
flow will provide one more very warm day with highs in the upper
70s/lower 80s.  Showers and thunderstorms will develop in advance of
a slowly approaching cold front, mainly focused across the west.
With strong upper dynamics in place and increasing amounts of
moisture flowing northward from the Gulf of Mexico, a few strong to
severe thunderstorms will be possible Thursday afternoon across
west-central Illinois.  Once low lifts into the Great Lakes, front
will get a solid push eastward by Thursday evening.  Will go with
categorical PoPs across the board as the boundary interacts with a
moist and moderately unstable environment.

Cold front will push into Indiana by early Friday morning, with
perhaps a few lingering showers across the Wabash River Valley.
Main weather story late in the week will be the sharply cooler
conditions.  850mb temps are progged to drop into the 0 to -2C range
by Saturday morning, supporting daytime highs only in the lower
to middle 60s and overnight lows in the lower 40s this weekend.
Airmass will slowly moderate by early next week, with temperatures
climbing back to around 70 by Monday.


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

Cold front has just passed Pontiac and continues to make steady
progress south-southwest into our forecast area. Based on the
present movement, it appears it will push across PIA and BMI
between 05-06z with our southern TAF sites seeing FROPA in the
08z-10z time frame. Winds initially after frontal passage will
gust up to 20 kts from the northeast (020-050) but will begin
to diminish an hour or two after passage. Based on the latest
satellite trends, the MVFR and local IFR cigs will follow within
an hour of the wind shift with cigs in the 500-1000 foot range,
especially at KBMI with even the possibility for a little drizzle
and vsby restriction in fog for an hour or two as the lower clouds
settle in. The real challenge will be how quickly the lower cigs
are able to lift late tomorrow morning or early afternoon with
the low level moisture trapped under a developing subsidence
inversion. Latest guidance off the NAM-WRF and HRRR continues to
suggest we will see some breaks develop by late morning or early
afternoon so will continue with the previous trends in the TAFs
in scattering out the lower cigs by afternoon with some broken
VFR cigs into the late afternoon hours.

Ahead of the front, surface winds will be light and variable and
then switch into the northeast after frontal passage with speeds
in the 10 to 15 kts with a few gusts around 20 kts for a brief
time. Winds on Tuesday will be from the northeast at 8 to 15 kts.




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