Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 280455
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1155 PM CDT TUE SEP 27 2016

.UPDATE...
Issued at 900 PM CDT Tue Sep 27 2016

Minor update to the forecast tonight with clouds spreading
southward a bit quicker and continued slight chances of showers
spreading southward across central IL overnight. Lows in the upper
40s and lower 50s overnight. 551 dm 500 mb low over central upper
MI will track southward to southern Lake MI and near Chicago by
sunrise Wed. A surface cold front moving southeast toward the
IL/IA border will push southeast across CWA overnight and bring
isolated light rain showers along with increasing clouds.

&&

.SHORT TERM...(This evening through Wednesday)
ISSUED AT 249 PM CDT Tue Sep 27 2016

Bone-dry air in the lower levels of the atmosphere have allowed for
ample mixing down from around 850 mb, pushing temperatures into the
mid-upper 70s across the forecast area this afternoon. Widespread
dew points in the mid-upper 30s with some near freezing dews from
about Macomb-Peoria northward. Wind gusts have increased to around
30 mph in the last 1-2 hours as the pressure gradient tightens and
some of the stronger winds near the 850 mb level get mixed down as
well.

Surface obs and satellite imagery show low pressure spinning over
northeast Lake Superior this afternoon, with an associated cold
front dropping south through Minnesota and Wisconsin. This front
will quickly track across the forecast area after midnight, and
should be through most of the CWA by sunrise. Forecast soundings
continue to show a fairly substantial dry layer at the lowest
levels, but decent 850-700 mb lapse rates immediately along the
boundary to eke out a few showers. Have added some isolated showers
for at least a couple hours after midnight over most of the CWA and
went as high as about 30% east of I-55 as the front will be
intensifying as it gets further southeast.

Upper low is progged to reach northern Indiana by about mid-morning
Wednesday, with another trough quickly swinging through the forecast
area late morning into early afternoon. Any showers from the
original front should taper off early morning, but by midday and
afternoon, additional showers are possible across the eastern CWA as
lapse rates will be steeper in that area due to the core of the 500
mb temperatures bullseye moves southeast across Indiana. Should feel
a lot like fall, with plenty of clouds, brisk west wind and highs
only in the lower-mid 60s.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 249 PM CDT Tue Sep 27 2016

The extended period of cooler than normal temperatures will be in
full swing by Wednesday night, as the occluded low pressure system
finishes its southward progress into central Kentucky. There is good
agreement in the 12z models that the low will linger there on
Thursday before beginning to drift NW toward IL on Friday. The low
appears to linger near the Illinois/Indiana border Friday night
before lifting toward lower Michigan on Saturday. Beyond that, the
consensus in the models is for the low to drift NE through the Great
Lakes on Sunday into Monday before dissipating as the next long wave
trough progresses from the West Coast toward the Rockies.

The main change in the 00z models appears to be a more westward
progression to the low in general, and especially on Friday as the
low begins its departure phase. That has prompted an increase in the
coverage of Chance PoPs farther west toward the IL river on Thursday
through Friday night. Forecast soundings continue to show a
relatively shallow layer of cloud moisture for Wednesday night
through Thursday night, so any precipitation during that time frame
should remain very light. Friday afternoon and evening the soundings
become saturated through a deeper layer as the low reaches its
closest to our forecast area. So rainfall amounts could climb toward
a tenth of an inch in some areas, especially closer to the Indiana
border. Precip chances decrease on Saturday, with Chance PoPs mainly
in our northern counties east of Peoria. Dry conditions should
return for Saturday night and continue through the rest of the
extended forecast.

High temperatures will be below normal from Thursday through Sunday,
with readings in the upper 60s to low 70s Thursday and Friday, and
lower 70s across the board Saturday and Sunday. Southerly flow
developing on Monday and Tuesday ahead of the next low pressure
system will push high temperatures quickly back above normal.
Blended guidance is showing mid to upper 70s for Monday, with
upper 70s to lower 80s on Tuesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1155 PM CDT Tue Sep 27 2016

Large 552 dm 500 mb low over southeast upper MI will track sse
into northcentral Indiana by 18z/1 pm Wed and into central Indiana
Wed evening as it weakens to 562 dm. Associated cold front over nw
IL will slide southeast across central IL during overnight
bringing isolated light rain showers to mainly the I-74 corridor.
Carried VCSH for 3-4 hours to account for this at PIA, BMI and
CMI. Broken to overcast mid/high clouds have spread southward of
I-70 at midnight and will see lower clouds of 2.5-5k ft spread
southward across central IL behind cold front later tonight into
Wed morning. The MVFR ceilings will be more likely along I-74.
Carried VCSH at CMI Wed afternoon closer to cutoff low moving over
Indiana. Broken to overcast clouds expected on Wed and ceilings
should lift from west to east during the afternoon/evening and
predominately be VFR. Light sw winds ahead of cold front will turn
NW to NNW during overnight and increase to 10-14 kts with gusts
17-21 kts on Wed, then be around 10 kts after sunset Wed out of
the NNW.

&&

.ILX Watches/Warnings/Advisories...
NONE.

&&

$$

UPDATE...07
SHORT TERM...Geelhart
LONG TERM...Shimon
AVIATION...07



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