Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 300414

Area Forecast Discussion
1114 PM CDT Mon Sep 29 2014


The cold front has slipped south of Joliet over the past hour and
continues to make steady progress towards our area this evening.
Not much in the way of weather other than a wind shift and an
increase in clouds. Based on the present movement, it appears the
boundary will shift south thru Peoria and Bloomington between
Midnight and 2 am. Radar mosaic showing a decreasing area of
showers north of the boundary, so not expecting any measureable
precip with the front, other than the possibility for a brief
period of drizzle as the low clouds settle south into the forecast
area after midnight. Other than some adjustments to the evening
temperature trends, no other changes were needed to the grids that
would require an evening update.


.SHORT TERM...(Tonight)

Weak high pressure will continue fair weather into this evening as
few to scattered cumulus clouds with 5-7k ft bases away from the IN
dissipate around sunset. Patchy shallow ground fog will develop
again near rivers and streams in southeast IL during overnight.
A cold front over southeast WI and central IA will push south into
central IL overnight, reaching I-70 by sunrise Tue, and weaken over
southeast IL on Tuesday. Stratus clouds behind/north of the front
will move south into central IL during overnight and especially
affect areas from I-74 north while southeast IL likely stays clear
most of tonight. Light winds into this evening will become northeast
at 5 to 10 mph during overnight over central IL. Lows tonight will
mostly be in the mid 50s with lower 50s north of Peoria.


.LONG TERM...(Tuesday through Monday)

A dry front, located just north of the state is forecast to slowly
drop into the area tonight through tomorrow. NAM-WRF model has been
consistent with forecasting an increase in low clouds across the
northern half of the state tonight and tomorrow. It looks like it
could get to I-74 by morning and could progress further south
tomorrow. How far south it gets will depend on how much of the
southern edge of the cloud deck gets eroded away by the strong sun
during the day tomorrow. So, current thinking is it sill get past
I-74 and could get to I-72 in the eastern part of the CWA. During
the day tomorrow, the sunshine should dissipate a lot of the clouds,
like what is occurring in parts of SD/MN/and IA today. Then mostly
clear and dry conditions are expected tomorrow night through Wed.

A weather system in the western US will the lift northeast into the
northern plains and will have a trailing cold front that will extend
down into the southern plains. Showers and thunderstorms are
expected to develop along and ahead of this front Wed well west of
the CWA. However, these storms will advect eastward into the state
beginning Wed night. A second low pressure area will develop along
the front and will move northeast into the Midwest and bring the
cold front through the area Thur and Thur night. Additional
thunderstorms will develop along the front Thursday and move across
the area during the day and through Thur night. Conditions do not
look right for any of the storms to produce severe weather, but this
could change in the next couple of days. However, lightning and
brief heavy rainfall will be likely with any storms that move
through the area late Wed night and Thursday. Once the front and pcpn
pass, there could be some lingering showers behind the front, so
will be keeping some chance pops in the east for Friday.

Beyond Friday, conditions will improve as a drier and cooler air
mass pushes into the area for the weekend. There is one small, weak
wave that could bring some light showers to the northeastern parts
of the area, along and north of I-74, but its too far away to have
much confidence, so will just have slight chance pops for Sunday,
which will not be mentioned in the worded forecast.

Temps will remain above normal tomorrow, when the sunshine is able
to break through the clouds, and Wed as well. Then clouds and rain
for Thur will keep temps milder and around normal. Once the front
moves through at the end of the week, temps will become much cooler
and actually be below normal for the beginning of Oct.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)

Cold front has just passed Pontiac and continues to make steady
progress south-southwest into our forecast area. Based on the
present movement, it appears it will push across PIA and BMI
between 05-06z with our southern TAF sites seeing FROPA in the
08z-10z time frame. Winds initially after frontal passage will
gust up to 20 kts from the northeast (020-050) but will begin
to diminish an hour or two after passage. Based on the latest
satellite trends, the MVFR and local IFR cigs will follow within
an hour of the wind shift with cigs in the 500-1000 foot range,
especially at KBMI with even the possibility for a little drizzle
and vsby restriction in fog for an hour or two as the lower clouds
settle in. The real challenge will be how quickly the lower cigs
are able to lift late tomorrow morning or early afternoon with
the low level moisture trapped under a developing subsidence
inversion. Latest guidance off the NAM-WRF and HRRR continues to
suggest we will see some breaks develop by late morning or early
afternoon so will continue with the previous trends in the TAFs
in scattering out the lower cigs by afternoon with some broken
VFR cigs into the late afternoon hours.

Ahead of the front, surface winds will be light and variable and
then switch into the northeast after frontal passage with speeds
in the 10 to 15 kts with a few gusts around 20 kts for a brief
time. Winds on Tuesday will be from the northeast at 8 to 15 kts.




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