Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 181721

Area Forecast Discussion
1221 PM CDT Sat Oct 18 2014


Vigorous short-wave evident on 1330z water vapor imagery is
dropping southward out of Wisconsin this morning. This feature is
bringing widespread cloud cover to the Great Lakes and Upper
Midwest, with the leading edge of the clouds currently sinking
into north-central Illinois. Based on satellite timing tools and
forecast soundings, clouds will overspread much of the KILX CWA
over the next 1 to 2 hours, with areas along/south of I-70
remaining mostly sunny until early afternoon. Further north
beneath the short-wave, scattered showers are noted on radar
imagery across southern Wisconsin into northeast Illinois.
Trajectory of wave and associated precip is generally to the S/SE
into northern/central Indiana: however, think the far NE CWA
around Champaign and Danville may see a few sprinkles from midday
into mid-afternoon before the wave passes to the south. Once the
wave drops into the Ohio River Valley, subsidence on its back side
will lead to clearing skies from northwest to southeast across the
region this afternoon/evening. Despite widespread cloud cover
currently upstream, models insist that skies will become clear
across the board by early evening. Will continue to monitor
satellite over the next several hours to see if clearing trend
needs to be tempered, but for now will follow NAM/HRRR idea of
rapid clearing in the wake of the wave later today. Have adjusted
sky grids to better reflect current trends and have nudged high
temps into the lower 60s along/south of I-70 where sunshine will
prevail for a few more hours before clouds arrive. Have also
expanded the Frost Advisory to include the entire CWA tonight, as
clear skies/light winds will allow for lows in the middle 30s and
areas of frost everywhere.


.SHORT TERM...(Today)

Persistent/deep cyclonic northwest flow exists across central and
southeast Illinois early this morning, and it is expected to last
through the day. The low cloud deck in this flow regime has some
sizable breaks in it at the moment, with parts of the forecast area
and upstream into the upper Midwest seeing mostly clear skies for a
time. Expect one more surge of widespread overcast skies later today
as a compact/potent upper wave (currently near the U.S./Canada
border of the upper Midwest/western Great Lakes region) and
associated surface trof swing through the region. However,
upper/surface ridging building in late today should help to scour
out the cloud cover for later tonight. The persistent low level cold
advection, expected breezy conditions, and considerable cloud cover
today is expected to result in chilly highs mainly in the 50s.


.LONG TERM...(Tonight through Friday)

A couple items for consideration during this forecast range:

1) Frost potential tonight -- High pressure expected to settle into
the Ohio Valley late tonight, with winds dropping off to nearly
calm. Earlier cloud cover expected to be out of the area before
midnight. Most areas will be dropping into the mid 30s for lows.
About the east 1/2 of the CWA would be vulnerable for some patchy
frost, with the far southwest less likely as southwest winds already
start up by late night. After collaborating with neighboring
offices, will issue a frost advisory over most of the CWA except for
the far southwest (i.e. Rushville to Jacksonville).

2) Shower potential Sunday night/Monday -- Shortwave expected to dig
across the northern Plains on Sunday as upper ridging amplifies over
the Rockies before shifting eastward. Latest ECMWF starting to come
more toward the GFS solution with an increase in showers after
midnight Sunday night, lingering across the southeast CWA into
Monday. Have not made much change aside from splitting the PoP grids
into 6-hour periods to focus the best chances during these two time

Otherwise, fairly quiet weather expected through the period. As the
week progresses, a blocking pattern will be setting up, as the
Monday wave digs even further and develops a large closed low over
the mid-Atlantic states by mid week. This feature only slowly drifts
east late week as a large surface cyclone develops. While an upper
wave tries to drop down into the Midwest toward the end of the week,
there will not be any moisture advection, as a surface low forms
along the trailing front over the western Caribbean and forces a
northeast flow across the Gulf. Thus, will keep the forecast dry
from Monday night onward.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)

Low VFR/high MVFR cloud cover is currently in place across central
Illinois, thanks to an upper-level wave dropping southward through
the region. Further upstream, a large area of clearing has
developed in the wake of the wave across northern/central
Wisconsin. Think this clearing will accelerate southward during
the afternoon and reach KPIA by around 23z. Latest HRRR shows
similar timing, with skies not completely clearing out across the
eastern terminals until perhaps 03-04z. Based on forward speed of
the clearing in the subsident region behind the wave, think this
may be a bit too slow. Have therefore introduced clear skies at
KCMI by 02z. High pressure will build into the region tonight,
providing clear skies and light/variable winds. As the high begins
to shift off to the east, a light SW return flow will develop by
Sunday morning.


FROST ADVISORY from 3 AM to 8 AM CDT Sunday FOR ILZ027>031-



LONG TERM...Geelhart
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