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FXUS63 KILX 240240

National Weather Service Lincoln IL
840 PM CST MON NOV 23 2015


Weak high pressure centered over Iowa southeast thru central
Illinois will drift to the east of the area by Tuesday morning.
Satellite data indicates quite a bit of high level clouds
streaming in from Iowa early this evening with quite a bit of
high and mid level clouds seen further northwest to the Dakotas.
Quite a bit of moisture added to the boundary layer today from
the melting that occurred with the mild temperatures. Main
question overnight is what affect the band of cirrus has, if any,
on the potential for fog development as winds will be quite
light under the surface ridge axis.

What would otherwise be an excellent setup for fog is still in
question this hour as forecast soundings off the RAP were not
nearly as aggressive with the fog threat as the HRRR which
highlights an area just east of a Bloomington to Decatur line with
some low visibilities by morning. Still appears there may be enough
breaks in the cirrus to allow good radiational cooling overnight
and at least patchy fog by early morning, especially along and
north of the I-74 corridor. The current forecast reflects this
quite well so other than some minor tweaks to the hourly temps,
the rest of the forecast is in good shape. No evening ZFP update
will be needed at this time.


.SHORT TERM...(Tonight)

Main short-term forecast concern continues to be potential for fog
development tonight as high pressure slides overhead and winds go
light/variable. Several factors point to at least patchy fog,
including nearly calm winds after sunset and increasing boundary
layer moisture due to snow melt. However, the possible fly-in-the-
ointment will be increasing high clouds currently seen on visible
satellite imagery streaming from the Dakotas into Iowa.
All models suggest these clouds will spread into Illinois late
this afternoon then persist through much of the night. Whether
they will be thick enough to limit radiational cooling and prevent
widespread fog is still in question. Several models such as the
NAM and GFS indicate little or no restriction to visby tonight,
while the HRRR is showing fog developing from mid-evening through
the overnight hours. At this point, will mention patchy fog over the
snow cover, generally along/north of the I-74 corridor. Overnight
lows will be in the middle to upper 20s.

.LONG TERM...(Tuesday through Monday)

Upper level ridging will build into central IL Tuesday providing for
dry conditions with some mid and high cloud cover. The warming trend
will continue slowly with highs reaching the low to mid 40s I-74
northward where snow cover the low to mid 50s south of
I-70. Lows Tuesday night should drop to only the mid to upper 30s as
increased winds and cloud cover prevent further cooling at the

By Wednesday...this ridge will slip east of IL allowing increased
southerly flow and a plume of Gulf moisture to nudge eastward into
Illinois. The result will be a continued warming trend with highs
reaching around 50 north of I-74 to the upper 50s from I-70
southward. Chances for precipitation will be arriving in western
portions of the state by evening. 12Z models currently keep this
precipitation light until Thursday/Thanksgiving evening...then
spread heavier amounts around 0.50 inches per 6 hours across the
state from Thursday afternoon through Friday morning. Total rainfall
amounts look to be around 1 to 1.25 inches southeast of Champaign-
Decatur, and 1.5 to 1.75 inches to the northwest. Could see a mix of
light snow with the light rain over central IL Friday night before
ending. Highs Thursday should be in the mid 50s to around 60,
cooling to the low to mid 40s over the IL river valley to the mid to
upper 50s in southeast IL.

High pressure will build into the Midwest region on Saturday for a
good chance for dry conditions, although GFS is quicker returning
moisture back into central IL and perhaps some precipitation will
return Saturday night into Sunday if this turns out. Chance for
precipitation with this next system looks to linger through Monday.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)

The main challenge this forecast period will be with the potential
for MVFR, or lower vsbys in fog later tonight into Tue morning.
With the very mild temperatures across the area this afternoon, a
good deal of snow was melted across the I-74 corridor and north
which may lead to some patchy fog overnight as winds become
quite light as a ridge of high pressure drifts over the area. The
one negative for widespread fog would be some high level clouds
tracking southeast out of Iowa, which should be in our area
tonight. Will continue to keep the vsbys in the MVFR range across
PIA, BMI and CMI and not have any restrictions in vsbys at SPI and
DEC. What fog we do see overnight should burn off quickly by 15z
Tuesday with VFR conditions prevailing for the rest of the day.
Light and variable winds tonight with become light east to
southeast on Tuesday at 5 to 10 kts.


.ILX Watches/Warnings/Advisories...


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