Area Forecast Discussion
Issued by NWS Central Illinois

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366
FXUS63 KILX 060549
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1149 PM CST Mon Dec 5 2016

.UPDATE...
Issued at 903 PM CST Mon Dec 5 2016

Quite the forecast challenge this evening with respect to
temperatures, sky cover and fog.

The 00z ILX sounding showed the strong low level inversion in
place with plenty of moisture trapped below it. A weak east to
southeast wind flow at and below the inversion has helped to push
the low clouds from much of central IL toward the WNW to the
Illinois River Valley this evening. This resulted in clearing the
past couple of hours which has caused temperatures to plunge into
the upper 20s/lower 30s in a few spots. Have adjusted the
temperature grids and low temperatures to account for this.

The other challenge is the sky cover. Low clouds around the IL
River Valley and the MS River valley will mainly stay in those
areas the next few hours, but then start expanding back across
much of central Illinois, which will cap temperatures and then may
cause them to rise a bit. In addition, lower clouds are advecting
northward toward southeast IL. Regional radar showing light rain
starting to developing in a warm advection zone in parts of SE MO
and toward Belleville. The forecast still looks on track for the
light rain to expand into parts of central and southeast IL after
midnight, primarily east of I-55 with the best chances south of
I-70.

With respect to fog, already seeing it from BMI-PNT, with lighter
fog scattered in west central and central IL. At this point it
looks like any dense fog that develops will be isolated, so will
stick with areas of fog across all of central IL and more patchy
fog with the spotty light rain in eastern IL.

&&

.SHORT TERM...(This evening through Tuesday)
ISSUED AT 313 PM CST Mon Dec 5 2016

Through the day today, southeast and east parts of central IL have
had some clearing. central and northern sections remain overcast.
Expect cloud development to come back in with next system coming
up from the south into the OH valley. This will bring chance of
pcpn into the southeast half of the region overnight. Toward
morning, temperatures will be near freezing in the north central
areas briefly, and so will be slight chance of freezing rain.
Amounts though will be very light.

Still expect chance of patchy dense fog developing overnight with
ample moisture over region.

.LONG TERM...(Tuesday night through Monday)
ISSUED AT 313 PM CST Mon Dec 5 2016

High pressure building into Illinois from the Plains Tuesday night
will provide continued clearing of low clouds from west to east
during the evening, along with some colder air. However, high cirrus
clouds will begin advancing northward into Illinois late Tuesday
night as a wave of energy and moisture advance from the Plains
toward Illinois. The 12z models have continued the downward trend in
forcing for precipitation and moisture content Wednesday afternoon
and night. The only model that shows any measurable precipitation in
our area is the Canadian, and that is only south of I-72 Wed
afternoon and evening. The GFS, NAM, and ECMWF all keep
precipitation south and north of our area. If any precipitation
develops, there appears to be enough cold air in place to keep it as
primarily snow. The chances in the grids were dropped to just slight
chances /20%/ with the latest update, so little to no measurable
snow is expected with that wave Wed afternoon/Wed evening.

Much colder air arriving with the Canadian surface high Wed night
and Thursday will provide several days of below normal temperatures,
along with dry conditions to close out the week. Low temps will drop
into the teens for three nights from Wed night through Friday night,
with wind chills dipping into the single digits each night. Wind
chills late Wed night may even drop below zero from Peoria to
Galesburg and north.

A system is taking aim on the area next weekend. The progressive GFS
and Canadian bring the system through earlier than the ECMWF,
starting precip on Saturday, and are more aggressive with precip
development. The EC takes most of the energy north of our
forecast area on Sunday and Sunday night, with the bulk of its
precip development delayed until a cold front stalls across
Illinois Sunday night into Monday.

With the timing differences, PoPs in the blended extended cover from
Saturday afternoon through Monday, but the actual event will likely
be contained in one 12-18 hour period. Enough warm air is projected
to arrive ahead of that system for some of the precip to fall as
rain, especially south of I-74. High temps are expected to climb
into the mid 30s to low 40s for Sunday and Monday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1150 PM CST Mon Dec 5 2016

Plenty of forecast challenges with the 06z TAFS tonight. The
latest IR/Fog satellite loop continues to show the edge of the
LIFR ceilings and visibility near the PIA and BMI TAF sites. In
the past hour PIA has varied from 4SM to 1SM and BMI from 1/4SM to
2SM. Believe the average visibility will range from 1-2 miles
through 08-09z with broken LIFR ceilings being predominant. The
CONSShort blend seems to have finally caught on to the ceiling and
visibility trends, so there is fairly high confidence that LIFR
conditions will dominate at PIA and BMI through daybreak.

Primarily looking at the development of MVFR conditions across the
rest of central Illinois the next few hours with low level warm
advection and ceilings around 1 Kft upstream. Visibility is only
expected to bottom out around 2-3SM around 10-11z and then rebound
quickly by mid-morning.

After a shift in the wind to a northwest direction after daybreak we
should see the strong low level inversion get eaten away slowly
during the morning hours. This will allow for more mixing, which
should result in a dissipation of the fog and low ceilings by
late morning/early afternoon.

&&

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

$$

UPDATE...Miller
SHORT TERM...Goetsch
LONG TERM...Shimon
AVIATION...Miller



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