Area Forecast Discussion
Issued by NWS Central Illinois

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599
FXUS63 KILX 220419
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1119 PM CDT Sat Oct 21 2017

.UPDATE...
Issued at 842 PM CDT Sat Oct 21 2017

The latest surface analysis indicates a cold front just east of
Des Moines this evening and will continue to track slowly towards
our area overnight. A band of showers and thunderstorms was
located along the frontal boundary this evening and will also
track east towards west central Illinois around around or just
after the Midnight hour based on the latest Rapid Refresh model
depiction, which has trended a bit slower with the onset of the
showers from earlier runs. Then once the showers arrive, it
appears based on the storm movement which is mainly north-
northeast, that west central Illinois will have the highest POPs
through the early morning hours as the front will be very slow to
push into our area Sunday morning.

The short term models suggest areas east of I-55 may not see much
in the way of rainfall until 6:00 to 8:00 A.M., with areas closer
to I-57 not seeing much, if any rainfall, from the frontal band
until later Sunday morning. Not ready to jump on that just yet but
the higher POPs we have depicted across the west looks reasonable
at this time with a steady decrease in rain chances early Sunday
the further east you go across our area. Better instability
remains along and west of the Mississippi River this evening with
models suggesting a limited amount of elevated Cape and decent mid
level lapse rates over west central IL as the band of showers and
storms move in late tonight and early Sunday morning. A few of
the stronger storms, if they make it into far west central
Illinois late tonight, may produce some gusty winds and small
hail, but the convective allowing models have been very consistent
the past 24 hours in showing a weakening trend to the storms late
tonight as they push into west central IL.

Have already sent out an update addressing some isolated showers
that developed over southeast IL late this afternoon and early
this evening. Further adjustments made to the timing of the precip
across the east have been made for very late tonight. The updated
ZFP should be out by 9:10 PM.

&&

.SHORT TERM...(This evening through Sunday)
ISSUED AT 310 PM CDT Sat Oct 21 2017

Isolated showers lingered over eastern IL at mid afternoon due to
a weak short wave that will exit ne of CWA next few hours. Clouds
breaking up especially west of I-57 along with breezy south winds
gusting 20-35 mph was helping warm temperatures back into the mid
to upper 70s. Dewpoints were in the upper 50s and lower 60s, with
some mid 50s near the Wabash river valley.

A strong cold front over far western MN to near the NE/IA border
into eastern KS will push east into nw/west central IL around
12Z/Sun and push into nw IN and se IL by 00Z/7 pm Sunday. Latest
models have continued to trend a bit slower with arrival of
showers and thunderstorms overnight and likely not reaching the
IL/IN border after 12Z/7 am Sunday. SPC day1 outlook for late
evening into overnight has marginal risk of severe storms west of
a Galesburg to Rushville line while slight risk remains west of
IL over parts of IA and central and western MO. Mild lows tonight
in the lower 60s. Have likely to categorical pops on Sunday with
thunderstorms possible in the morning and in eastern IL into
Sunday afternoon. Highs Sunday range from lower 60s by Galesburg
to the low to mid 70s in eastern IL.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 310 PM CDT Sat Oct 21 2017

12Z forecast models develop a cutoff 567 dm 500 mb low near the
AR/MO border by overnight Sunday night and ejects into western KY
by 18Z Monday and then opens up as it lifts ne into the upper Ohio
river valley by 06Z/Tue. This will likely keep chances of rain
showers around east of the IL river Sunday night and Monday.
Lingering chances of showers in eastern IL Monday night. Highs
Monday of 64-68F. Then strong upper level trof digs down into
central IL later Monday night and Tue ushering in even colder air.
Chances of rain showers spread se across central IL during the day
Tue with lingering chances in ne CWA Tue evening. Highs Tue in the
low to mid 50s with areas from I-74 ne struggling to reach 50F.
Lows Tue night in the mid to upper 30s and could be some patchy
frost late Tue night if nw winds weaken enough. Cool highs Wed in
the low to mid 50s, except upper 50s from Jacksonville sw. Lows
Wed night in the upper 30s, with some lower 40s western/sw CWA.
Patchy frost could be a risk late Wed night near the Wabash river.

Strong upper level trof pulls way from IL from Wed into Thu while
another cold front pushes se toward nw IL by early Thu evening.
Temperatures to moderate into the low to mid 60s on Thu ahead of
this front. Cold front to push se thru central IL during Thu night
and near the Ohio river by early Fri afternoon. Have 20-30%
chances of rain showers se of I-55 overnight Thu night into Fri
morning. Then 20-30% chance of rain showers over most of CWA from
Fri afternoon thru Sat. Highs Fri range from mid 50s from IL river
nw to the lower 60s in southeast IL. Highs only in the low to mid
50s on Sat while lows 35-40F Fri and Sat nights and another risk
of patchy frost.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1110 PM CDT Sat Oct 21 2017

Little change from previous discussion. The main forecast
challenge will be timing of the main precipitation shield
currently out to our west into our TAF area. Initially, it
appears the PIA and SPI TAF sites will be affected by the showers
and storms, with a gradual shift further east Sunday morning as
the front edges across the area. VFR cigs will be replaced by
MVFR cigs at PIA and SPI in the 10z-12z time frame, at BMI and
DEC in the 12z-15z period and by 18z at CMI.

Timing of the cold front from west to east still appears to be in
the late morning at SPI and PIA and by late afternoon, in the
19z-22z time frame at CMI as the front slows down its eastward
progress later in the day. Southerly winds ahead of the front will
continue to gust up to 25 kts at times at most sites except DEC
and CMI where we will continue to hold on to LLWS at 1500 feet for
winds out of the south at 45 kts (based off the ILX VAD wind
profile) for several more hours until the precip arrives. Winds
will shift into the northwest at 10 to 15kts after FROPA with
model soundings showing drier air advecting in behind the front
resulting in a return to VFR conditions from west to east tomorrow
afternoon and evening with CMI the last to see their cigs
improve.

&&

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

$$

UPDATE...Smith
SHORT TERM...07
LONG TERM...07
AVIATION...Smith



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