Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 301540

Area Forecast Discussion
1040 AM CDT Tue Sep 30 2014


Updated the forecast earlier this morning to have patchy fog into
midday along and south of a Quincy to Jacksonville to Taylorville
to Robinson line and locally dense this morning from Jacksonville
sw. Also adjusted cloud cover and highs today, with low clouds
getting into southeast IL as they slowly drift south toward
highway 50 from Flora to Olney to Lawrenceville. Cloud bases of
500-1500 ft and even less than 500 feet in southern counties where
fog is present. Pilot reports show cloud deck about 1-2k ft thick
and starting to break up in ne counties late this morning where
drier air has worked in on ne winds as dewpoints slipped into
upper 40s and lower 50s. Low clouds should continue to break up from
the ne and sw with skies becoming partly to mostly sunny during
the afternoon. Temps currently 55-60F except low to mid 60s se of
I-70 and Mount Carmel airport up to 72F and still enjoying sunny
skies. The cold front has pushed just south of CWA with ne winds
6-12 mph and low clouds blanketing much of CWA. Cold front to
weaken as it pushes into southern IL this afternoon/evening and
low clouds to decrease during the afternoon. Highs mostly in the
low to mid 70s over central IL, but to range from 65-70F from I-74
northeast, to the lower 80s along and south of highway 50.


.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 12Z TAFS through 12Z Wednesday morning)

The cold front has pushed south of all TAF sites, with northeast
winds around 10-14kt in its wake. A short period of LIFR
fog and ceilings have developed along the leading edge of the
clouds at SPI and DEC with 200FT ceilings and 1/2sm FG. Behind the
front, IFR conditions with ceilings of 400-800 ft and p6sm vis have
developed within 2 hours of FROPA.

Based on the latest observation trends, ceilings should improve
to the MVFR by late morning. The main concern is how soon the
clouds will erode to VFR, with all 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. 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. Southern TAFS of DEC and SPI should see VFR the soonest,
and possibly as early as 11 am or Noon.

The gusty northeast winds immediately behind the front will
settle out in the 8 to 14 kt range for much of the day. Mixing
heights will be limited by the low ceiling heights, so gusts
should not get to 20kt at any point.




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