Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 171113
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
513 AM CST Fri Nov 17 2017

.SHORT TERM...(Today and tonight)
ISSUED AT 235 AM CST Fri Nov 17 2017

Southeast wind flow helping to keep temperatures relatively steady
early this morning, mainly in the lower 30s. The weakest of this
flow is near the Indiana border, closer to the periphery of high
pressure centered over western Ohio. Large expanse of stratus has
been advecting west-northwest recently, and several of the high-
res models show this moving into southeast Illinois toward
sunrise. Have added some mention of fog for this morning from
about Paris southward, as these low clouds move in.

Little activity on radar mosaics as of 2 am, though warm-air
advection precipitation should get going west of the Mississippi
River after sunrise. Have made some adjustments to the timing of
the arrival of these showers, which are progged to be moving into
west central Illinois mid to late morning. Substantial dry wedge
below 800 mb is seen on forecast soundings over eastern Illinois
until midday, so will keep the forecast dry there until early
afternoon.

Cold front will begin moving into the forecast area after
midnight. Strong warm air advection ahead of it will result in
slowly rising temperatures much of the night, reaching near 60
degrees in many areas south of I-72. While rain becomes widespread
during the evening, thunder threat will be spreading from west to
east. Areas east of I-57 more likely to see any thunder after
midnight, though an isolated storm may occur late evening as
elevated low CAPE`s start to move in.

.LONG TERM...(Saturday through Thursday)
ISSUED AT 235 AM CST Fri Nov 17 2017

A sharp drop in temperatures is on tap for Saturday as the front
passes. Temperatures will be falling from the start of the day
northwest of the Illinois River, but should be able to reach the
lower 60s across the southeast CWA. In that area, the front should
be arriving toward midday, and the GFS shows surface based CAPE`s
of 500-1000 J/kg in the morning. If anyone in the CWA had any
severe weather from this system, it would probably be in this
area; the SPC Day2 outlook has a marginal risk south of I-70.
However, widespread clouds and weak lapse rates would minimize any
significant threat.

Precipitation will linger in the deformation zone behind the front
a good portion of the afternoon. Across the northwest CWA, some
concern for a rain/snow mix or changeover to snow late afternoon.
However, forecast soundings show rapid drying in the lower and
middle atmosphere, so the precipitation may cut off before a
changeover occurs. Will continue with the mixed precipitation
mention across the northern CWA for late afternoon. By sunset,
temperatures should be in the 30s west of I-55, and low-mid 40s
east.

The remainder of the forecast looks quiet. Periodic slugs of
colder air will spread across the Great Lakes. However, the latest
European model brings a substantial upper trough into the
Mississippi Valley by next weekend. Dry weather will prevail for
the pre-Thanksgiving travel rush, with a large expanse of high
pressure over the central U.S.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 513 AM CST Fri Nov 17 2017

Watching an area of IFR ceilings below 1,000 feet advecting
northeast toward KCMI. AWIPS timing tool would suggest an arrival
around 15Z, though HRRR guidance suggests some erosion on the
northwest flank around that time. Will mention only a scattered
layer at 1200 feet for now.

Otherwise, generally VFR conditions should prevail into early
afternoon, before ceilings below 2,000 begin to take hold.
Scattered showers will become more widespread this evening, at
which time IFR conditions are likely to develop. Gusty southerly
winds will continue into the evening, before beginning to diminish
late in the period as a cold front moves in from the west.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Geelhart
LONG TERM...Geelhart
AVIATION...Geelhart


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