Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 210430

Area Forecast Discussion
National Weather Service Lincoln IL
1130 PM CDT THU OCT 20 2016

Issued at 904 PM CDT Thu Oct 20 2016

Showers progressing toward Indiana, with the band of showers
extending from Effingham to Paris at 9 pm. Timing tools show all
precip should depart our forecast area by midnight. The next
focus is on low temps tonight and potential for frost. Clouds are
slowly continuing to clear from west to east in western Illinois,
with winds diminishing below 10mph after clearing develops.
Surface dewpoints upstream in Iowa are in the 32 to 35F range, so
we should see our frost advisory areas have enough time later
tonight to dip into the mid 30s. Updated the cloud clearing trends
to slow them down slightly from previous forecast. That may allow
for slightly higher low temperatures east of I-57. Made minor
adjustments there, otherwise low temps look on track. High
pressure should provide mostly clear and cooler conditions
tomorrow, with highs only in the mid to upper 50s, with persistent
NW winds.


.SHORT TERM...(This evening through Friday)
ISSUED AT 303 PM CDT Thu Oct 20 2016

A vigorous short-wave trough evident on 19z/2pm water vapor imagery
over southern Iowa/northern Missouri will continue to track eastward
across the area this afternoon and evening.  Enough synoptic lift is
being generated by this feature that weak radar returns are showing
up, suggesting the possibility of a few light showers or sprinkles.
Both the NAM and GFS show measurable precip developing across the
E/SE KILX CWA this evening as the wave passes through.  Think this
may be a bit too aggressive, but will mention isolated showers
along/east of the I-57 based on current radar trends.  Clouds will
hold firm into the early evening hours before gradually clearing
from west to east overnight.  HRRR has been handling cloud trends
quite well, so have followed its solution closely.  Skies will clear
to the I-55 corridor by midnight, then to the Indiana border by dawn
Friday.  With skies clearing and a cool/dry airmass flowing into the
region, overnight lows will dip into the middle to upper 30s across
the western CWA.  Think winds will decrease enough to allow the
formation of frost across the Illinois River Valley toward dawn.
Have therefore issued a Frost Advisory for all counties along and
west of the Illinois River.  Further east, clouds will hold on long
enough to keep lows in the lower 40s.  High pressure will slide
overhead on Friday, resulting in sunny skies and cool high
temperatures in the middle to upper 50s.

.LONG TERM...(Friday night through Thursday)
ISSUED AT 303 PM CDT Thu Oct 20 2016

High pressure ridge will sit right over the CWA Friday night, and
with clear skies and light winds, frost will be a concern. Based on
the forecasted ridge axis, believe the best area for frost will be
in the eastern half of the CWA. Will probably need an advisory for
Fri night, but will wait til tomorrow to issue. High pressure will
dominate the area with dry weather through most of the weekend. A
frontal system will slide across northern half of the CWA on Sunday,
but there will be a lack of moisture, so no precip is expected when
this moves through. Behind this weak front, cooler high pressure
will move into the area for the first couple of days of the week.
Dry weather will continue as well. The next real chance of precip
will be beginning Tue night after midnight and then continuing into
Wed night. Earlier model runs differed on timing and location of the
precip; however, now models in better agreement. This system is
expected to move through quickly, so Thursday of next week should be

Besides the cool temps Fri night, seasonable temps will continue for
Sat and Sat night. Temps will then warm to above normal for Sunday,
before becoming seasonable again after the second high pressure area
moves in for next week.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1129 PM CDT Thu Oct 20 2016

High pressure advancing into Illinois will provide VFR conditions
over the next 24 hours. Showers have departed well southeast of
CMI to DEC, and clouds will continue to clear from west to east
the rest of the night. CMI should be the last to clear out by 08z.
No TAF sites have MVFR clouds in place, and satellite loops and
upstream OBS indicate that should remain the case as clouds clear

NW wind gusts of up to 23kt re-developed at DEC and CMI in the
last hour, but they should diminish by 07z. A relatively tight
surface pressure gradient appears sufficient to keep sustained NW
winds around 10-12kt for the rest of the night for all sites
except PIA. During the day tomorrow, NW winds will hover in the
08-12kt range under mostly clear skies. Some lake effect clouds
could drift over CMI from mid-morning to early afternoon at 2.5K
ft levels, but any ceilings should remain east of CMI. Clear skies
and light NW winds are expected by sunset tomorrow and through
the evening.


.ILX Watches/Warnings/Advisories...
Frost Advisory from 4 AM to 8 AM CDT Friday for ILZ027>031-036-



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