Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 231742

Area Forecast Discussion
National Weather Service Lincoln IL
1242 PM CDT Mon Oct 23 2017

Issued at 1018 AM CDT Mon Oct 23 2017

The deformation zone associated with a closed upper level low
lifting northeastward from western TN into eastern IN today
remains over eastern Illinois producing widespread light rain and
drizzle from near the I-55 corridor eastward. This area of rain
will slowly slip northeastward with the low, reaching around I-57
and points east by late afternoon. However, the next upper level
trough and cold front will be approaching from the northwest will
arrive by evening northwest of the Illinois River, bringing a
return of showers tonight, leaving only a short break in wet
conditions for areas west of I-57 today. Current forecast is on
track with this scenario, so no significant updates are needed
this morning.


.SHORT TERM...(Today and tonight)
ISSUED AT 320 AM CDT Mon Oct 23 2017

07z/2am surface analysis shows 1010mb low over southeast
Illinois...with widespread showers lifting northward across the
eastern half of the KILX CWA. The low will track northeastward
today and will reach southern Michigan by 00z/7pm. Most models
show deformation zone precip associated with the low persisting
across east-central and southeast Illinois for much of the day,
with areas along/west of the I-55 corridor gradually drying out
by midday. High temperatures today will range from around 60
degrees along the Indiana the middle 60s across the
southwest CWA around Jacksonville and Springfield.

A short-wave trough currently evident on water vapor imagery over
the Northern Plains will dive southeastward later today,
providing the necessary kick to eject a closed upper low over the
Ozarks northeastward. As the trough approaches and amplifies,
scattered showers will develop along/northwest of the Illinois
River late this afternoon...then will spread across the remainder
of the area and become more widespread tonight. Have carried
likely PoPs along/north of a Paris line
accordingly. Low temperatures tonight will drop into the lower to
middle 40s.

.LONG TERM...(Tuesday through Sunday)
ISSUED AT 320 AM CDT Mon Oct 23 2017

Tuesday will be a cool/showery day as the upper wave digs into the
region. Likely to categorical PoPs are warranted, with high
temperatures remaining in the 40s. As the wave begins to depart,
the showers will end and skies will clear from west to east
Tuesday night. While low temperatures are expected to drop into
the lower to middle 30s, think W/NW winds of 8-10mph will prevent
frost formation.

A series of short waves will drop southeastward out of Canada,
re-enforcing the mean upper trough in place over the Great
Lakes/Midwest through the remainder of the extended. Heights will
temporarily rise in advance of the next significant wave expected
to arrive at the end of the week, allowing high temperatures to
climb well into the 60s on Thursday. As the wave approaches, a
cold front will sweep through Illinois Thursday night into Friday
morning, but will be devoid of precip due to an initially dry
airmass. As stronger upper support arrives, a few showers will
develop along/behind the departing front Friday afternoon and
evening...mainly east of the I-57 corridor.

Main concern later in the period will be forecasting a potential
frost/freeze over the weekend. 00z Oct 23 models have come into
much better agreement concerning the timing of the Friday
wave...but some minor discrepancies still exist for another
short-wave expected Saturday night/Sunday. The GFS is considerably
stronger with this feature, resulting in lower heights and colder
temperatures. Meanwhile the ECMWF is slightly weaker and further
north and therefore not quite as cold. ECMWF numeric guidance
suggests lows around 30 degrees both Friday and Saturday
night...while the MEX shows even colder readings down into the
middle 20s. At this point, will play it conservatively and
forecast lows in the lower to middle 30s Friday night...then the
upper 20s to around 30 degrees Saturday night. These numbers will
be fine-tuned with later forecasts...and frost/freeze headlines
may eventually be needed as well.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1242 PM CDT Mon Oct 23 2017

Scattered IFR/MVFR cigs persist from KDEC-KBMI eastward with VFR
to the west. IFR/MVFR will slowly shift east and should clear out
of KCMI around 21Z. Next weather system approaching rapidly from
the NW and should cause scattered showers to develop around 00Z
and persist through the remainder of the forecast period.
Cigs/vsby should slowly trend downward to MVFR by 08-12Z. Winds
WNW 6-12 kts increasing to 10-15 kts with gusts around 25 kts
after 12Z.




LONG TERM...Barnes
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