Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 211749
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1149 AM CST Sun Jan 21 2018

.UPDATE...
Issued at 1120 AM CST Sun Jan 21 2018

Lingering dense fog mostly associated with the warm frontal
feature that is lifting into northern IL this morning. Some
additional fog developing to the SE further complicating advisory
headlines. Have updated for the clearing in Central IL...expanding
to the south as a result. Plenty of RH with the warm air advecting
into the region this morning. Not expecting much clearing for the
clouds. As for the fog, areas to the north eventually lift early
in the afternoon...and improve slightly in the southeast. However,
the HRRR is pretty consistently showing redevelopment along and
near I-70...and also along and north of I-74. Minor updates
throughout the morning...but in the larger scale, fog remains the
main concern. &&

.SHORT TERM...(Today and tonight)
ISSUED AT 331 AM CST Sun Jan 21 2018

Fog is developing across parts of central Illinois early this
morning...primarily the N/NE KILX CWA. Visibilities across this
area have already dropped to one quarter of a mile, with all model
guidance suggesting the fog will persist/spread through much of
the morning. As a result, have issued a Dense Fog Advisory
along/northeast of a Lincoln to Mattoon line through noon. The
advisory area will likely need to be expanded westward into the
Illinois River Valley to include the Peoria metro, but held off
on issuing that far west due to current temp/dewpoint spreads of
3-4 degrees. Think fog/drizzle will eventually spread across the
entire area, then persist for the balance of the day into tonight.
Despite the fog and low overcast, southerly winds will boost high
temperatures into the upper 40s and lower 50s.

A potent short-wave trough currently over the Four Corners region
will shift eastward today, resulting in surface cyclogenesis
in the lee of the Rockies across eastern Colorado. The low will
then track E/NE, reaching northeast Iowa by 00z Tue. As the system
approaches, a corridor of deep-layer moisture and elevated
instability will push into the area overnight into Monday morning.
00z Jan 21 NAM is suggesting a slightly more unstable airmass
than previously forecast, with MUCAPE values reaching 400-500J/kg
late tonight. As a result, have carried categorical PoPs for
showers and a few thunderstorms.

.LONG TERM...(Monday through Saturday)
ISSUED AT 331 AM CST Sun Jan 21 2018

The initial wave of showers/thunder will be along/east of the I-55
corridor by 12z Mon...followed by a lull in precip coverage as the
mid-level dry slot works into the region. The big question will be
if any additional convection develops on the edge of the dry slot
Monday afternoon, as NAM shows MUCAPEs in excess of 500J/kg along
the advancing cold front. This may be a bit overdone, but given
this instability combined with moderate shear, have decided to
mention thunder through Monday afternoon. High temperatures will
soar well into the 50s before the cold front arrives late in the
day.

Colder air will filter back into the region Monday night once the
front passes to the east. Meanwhile, showers will become more
prevalent as wrap-around moisture from the system spills into
central Illinois from the northwest. Have featured likely PoPs
along/north of a Taylorville to Danville line as the low tracks
into the Great Lakes. Thermal profiles will initially be warm
enough to support rain early Monday evening, with the precip
mixing with and changing to snow overnight. Any snow accumulation
will be minor, as QPF will be light and the ground will be wet.

After a few lingering snow-showers early Tuesday morning,
cooler/drier weather will be on tap for Tuesday and Wednesday.
Highs on Tuesday will be in the middle to upper 30s...moderating
to the upper 30s and lower 40s by Wednesday. Meanwhile, another
vigorous upper wave will come onshore along the West Coast on
Wednesday...inducing downstream ridging across the Midwest by the
end of the week. As a result, temperatures will warm back into
the 50s by Friday. 00z Jan 21 models are in excellent agreement
with the initial speed of the system, with the GFS/ECMWF/GEM all
showing a cold front pushing through central Illinois on Saturday.
The main difference is that the ECMWF then develops a wave along
the departing front across the Ohio River Valley on Sunday while
the other models do not. At this point, will feature high chance
to likely PoPs for rain Friday night into Saturday as the front
passes. Will continue to monitor future model runs to see if the
ECMWF holds on to its wave solution.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1120 AM CST Sun Jan 21 2018

Not much of a change in the overall forecast, but continuing the
fog/VLIFR in a few TAFs at least through 20Z. Models inconsistent
in handling improvement in the vis but CMI BMI the most likely to
see a continuation of the poor conditions. HRRR tries to improve
the vis slightly late this afternoon, only to reestablish LIFR
conditions. As is, TAFs may be far too optimistic, but the
clearing to the SW for SPI and DEC is more than currently picked
up by models as well. Trying to balance between the two, with
southern flow pushing warmer temps into the region. A lot will
ride on this afternoons crossover temps with regards to the
overnight vis. In addition, developing showers with isolated
thunder possible after 06z in the TAFs.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 2 PM CST this afternoon for ILZ038-
043>046-053>057-061>063-066-067-071-072.

&&

$$

UPDATE...HJS
SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...HJS



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