Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 180412

Area Forecast Discussion
National Weather Service Lincoln IL
1112 PM CDT Tue Oct 17 2017

Issued at 855 PM CDT Tue Oct 17 2017

Surface high pressure continues to slowly move off to our east
this evening. As a result, southerly winds continue over most
of the area early this evening, however, over extreme east and
southeast Illinois, closer to the surface ridge axis, winds
have dropped off enough to allow temperatures to fall at a much
quicker pace than out west. Still think as the high edges further
off to our east, winds will turn more southerly and pick up enough
to keep early morning lows in the 40 to 45 degree range. A few of
the more favored "cold" locations may see the mercury dip into the
upper 30s, but overall, it appears the going forecast lows look

Another nice day shaping up for Wednesday as southerly winds
increase some by afternoon which should help push temperatures
into the lower 70s under a sunny sky. Based on the current
evening trends, very few changes were made to the grids for
the overnight hours, as a result, no evening update will be
needed at this time.


.SHORT TERM...(This evening through Wednesday)
ISSUED AT 300 PM CDT Tue Oct 17 2017

Brisk southwest winds this afternoon will diminish this evening as
decoupling occurs with sunset. An steady pressure gradient will
keep overnight winds from becoming calm, as winds become southerly
at 5 to 10 mph with time.

High pressure anchored to the southeast of Illinois will keep
skies mostly clear through tomorrow, with the exception of a few
cirrus clouds at times. A weak upper level disturbance in the 500
mb flow will keep slightly lower thickness over Illinois tomorrow.
However, we still could see high temperatures climb a degree or
two warmer than today. Southerly winds at 10-20 mph will add some
cooling effect to the day, especially in the shade.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 300 PM CDT Tue Oct 17 2017

The longwave ridge in the Plains will begin to translate eastward
Wed night and Thursday, bringing rising heights and steady warm
air advection. Coupled with high surface pressure, dry and
gradually warming conditions are forecast through Friday night.
The next chance of rainfall is still projected over the weekend.
However, there are some differences in the extended models about
the timing of the cold front into central Illinois. The GFS and
Canadian GEM bring the front across IL Saturday night, while the
ECMWF delays the FROPA about 6 hours. The onset of persistent
rains will likely be affected by a later FROPA, but all 3 models
show pre-frontal showers developing on Saturday across our
forecast area. So we did not alter the timing from consensus.

Due to decent agreement with Sat night being the primary precip
period, we left some categorical PoPs /80+%/ in our western
counties after midnight Sat night. Otherwise, Likely /60-70%/ PoPs
were advanced across our forecast area Sat night and Sunday
morning. Thunder potential is not high, with MLCAPE values of
250-300 J/kg, but we kept a mention of thunder due to the
combination of marginal shear parameters with the instability.

High temps on Friday and Saturday should be the warmest of the
next week. Any areas that see significant sunshine on Saturday
will climb a few degrees warmer than Friday, into the upper 70s
to near 80.

Showers will make an exit on Sunday as the cold front departs to
the east. Cooler conditions will develop on Sun and Mon, as highs
drop about 10 deg from Saturday. A reinforcing push of cold air
will arrive on Tuesday, limiting highs to the upper 50s to low
60s. Spotty showers or sprinkles could develop Monday and Tuesday
under the cold pool, especially with the upper trough axis and
waves of energy extending into Illinois during that time.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1105 PM CDT Tue Oct 17 2017

VFR conditions are expected through this forecast period. High
pressure off to our east late tonight will continue to dominate
the weather over the forecast area through 00z Thursday. A weak
frontal boundary will approach the southern Great Lakes late in
the period, but other than some scattered high level clouds, no
significant weather is expected. Southeasterly winds tonight at
10 kts or less will be south at 10 to 15 kts on Wednesday with
a few gusts around 20 kts at times Wed. afternoon.




LONG TERM...Shimon
AVIATION...Smith is the U.S. government's official web portal to all federal, state and local government web resources and services.