Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 130535
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1135 PM CST Sun Nov 12 2017

.UPDATE...
Issued at 906 PM CST Sun Nov 12 2017

The main issues tonight center on the timing and coverage of
clearing, and the subsequent formation of dense fog. Fog as
actually already started to form under the stratus layer, mainly
north of a line from Champaign to Lincoln to Peoria. Utilizing
timing tools on the satellite loops shows the clearing line may
reach Peoria by 330 am, I-55 corridor by 7-730am, then down to
I-70 by early afternoon. However, HRRR updates are indicating a
blanket of stratus clouds could remain across all of our forecast
area into Monday afternoon. Not ready to shift that far toward
slowing the clouds, so have trended slower based on satellite
trends. The last couple of runs of the HRRR has also reduced the
coverage and eastward extent of dense fog. Have expanded Patchy
Fog to cover the current foggy areas, and limited the Areas of Fog
to NW of the Illinois river, where HRRR still focuses thicker fog.
Can`t rule out the possible need for a dense fog advisory NW of
the IL river, but confidence is not high enough to issue that with
the evening update.

Clearing could help low temps reach or forecast lows in the NW
counties, but the delayed clearing along and south of I-72 tonight
will create warmer lows there. Have increased low temps across the
south by several degrees.

The remainder of the forecast looks on track. Updated forecast
info is already available.

&&

.SHORT TERM...(This evening through Monday)
ISSUED AT 316 PM CST Sun Nov 12 2017

Short-wave trough evident on 20z/2pm water vapor imagery over
northeast Illinois will track into Ohio by this evening. Synoptic
subsidence in the wake of the wave will lead to clearing skies
tonight:however, it appears this will be a slow process. Latest
visible satellite imagery shows the back edge of the low cloud
shield just now advancing east past Des Moines, Iowa. Based on
satellite timing tools, clearing will not arrive in west-central
Illinois until around midnight...and may not reach the Indiana
border until closer to daybreak. Once skies clear, the potential
for fog exists overnight thanks to very light winds and a moist
boundary layer. HRRR has been consistently showing widespread fog
along/west of the Mississippi River tonight...as has been trending
further east into central Illinois with subsequent runs. Have
therefore decided to include patchy fog in the forecast after
midnight along/west of a Champaign to Shelbyville line. With low
temperatures bottoming out in the upper 20s and lower 30s, there
could potentially be a few slick patches on sidewalks and
bridges/overpasses for the Monday morning commute. Once any early
morning fog dissipates, mostly sunny skies will prevail with high
temperatures climbing into the middle 40s.

.LONG TERM...(Monday night through Sunday)
ISSUED AT 316 PM CST Sun Nov 12 2017

Another mild and dry day will be on tap for Tuesday, with highs
reaching the upper 40s and lower 50s. The next approaching storm
system will come into the picture Tuesday night into Wednesday as
a short-wave trough and its associated cold front quickly swing
through the region. All models show FROPA around midday on
Wednesday, so have continued to focus highest PoPs late Tuesday
night into Wednesday morning. With modest MUCAPE values of
100-200J/kg in advance of the front, have included a slight chance
for thunderstorms across the western two-thirds of the KILX CWA
late Tuesday night. Once the front passes, rain chances will
quickly diminish from west to east by Wednesday afternoon/evening.
Dry conditions will prevail on Thursday before the mid-week
frontal boundary returns northward as a warm front Thursday night.
Model consensus keeps the area largely dry until late Thursday
night when showers and perhaps a few thunderstorms develop along
the advancing boundary.

Big questions still remain for the end of the period...with the
12z Nov 12 GFS continuing its recent slowing trend and the once
consistent ECMWF now slowing its solution by nearly 12 hours. The
latest GFS features an area of low pressure tracking out of the
Plains into southeast Wisconsin by 00z Sat...while the ECMWF takes
a similar track with the low arriving in southeast Wisconsin by
12z Sat. Both solutions place central Illinois within the warm
sector of the system Friday and/or Friday night...with the
potential for thunderstorms. Given considerable model spread, have
limited slight chance thunder to mainly during the day
Friday...lingering across east-central Illinois into Friday
evening. This obviously may need to be adjusted if the slowing
trend continues.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1135 PM CST Sun Nov 12 2017

The upper low that triggered rain earlier today has shifted east
into Indiana, with widespread subsidence in the wake of the low. A
subsidence inversion has trapped moisture near the surface across
Illinois. However, satellite loops show a clearing line at the NW
corner of Knox County at 0530z, progressing to the SE at a slow
but steady pace. It is projected to reach PIA between 08z-09z,
BMI/SPI around 11z, and eventually to DEC/CMI between 13z-14z.
The HRRR updates continue to keep IFR clouds entrenched across all
of central IL through early afternoon, with the only exception
some brief clearing for PIA late tonight. The HRRR actually has a
decent handle on the short term clearing trends, but halts the
clearing at PIA, then fills clouds back in, expanding stratus back
to the NW into Iowa again.

Depending on clearing timing and coverage, dense fog could
develop from west to east in the wake of the clearing. Have
kept the only inclusion of LIFR fog at PIA for now, and the latest
HRRR supports that scenario, with limited eastward expansion of
dense fog. Will monitor cloud and visibility trends closely for
dense fog potential.

Winds will be a non-factor to aviation over the next 24 hours due
to light north or variable winds.

&&

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

$$

UPDATE...Shimon
SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Shimon



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