Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 232331

Area Forecast Discussion
National Weather Service Lincoln IL
631 PM CDT Wed Jul 23 2014

ISSUED 253 PM CDT Wed Jul 23 2014

SHORT TERM...Tonight through Saturday

Cold front currently along the Ohio River will continue to sag
southward tonight, as high pressure builds into the region from
the Northern Plains. N/NE flow around the high will bring a much
cooler/drier airmass southward, resulting in overnight lows
dipping into the middle to upper 50s. Dewpoints will lower
significantly as well, so am not expecting any issues with fog
overnight. Sunny and cool weather will persist on Thursday, with
afternoon highs remaining in the 70s.

A more active weather pattern will begin to develop Thursday night
through Saturday, as a series of upper short-waves ride up and
over a prevailing ridge centered over the Plains. The first wave
is expected to trigger a thunderstorm complex across southern
Minnesota/northern Iowa Thursday night. Model solutions vary
concerning the exact track and timing of the complex, so forecast
confidence is slightly below normal with this feature. NAM
appears to be the furthest south and fastest model, while the
GFS/ECMWF/GEM are all further north and slower. Given presence of
dry airmass beneath surface ridge axis stretching from the eastern
Great Lakes to the Ohio River Valley, think NAM is too aggressive
in driving precip into Illinois late Thursday night. Prefer the
slower/more northern model consensus. As a result, will only carry
a slight chance PoP along/northwest of the Illinois River after
midnight Thursday night. Will spread low chance PoPs across the
northern half of the KILX CWA on Friday as upper wave passes to
the north and weak warm front lifts northward through the area. A
second upper wave may enhance convection across parts of the area
Friday night: however, models are in poor agreement here as well.
Will therefore maintain just chance PoPs at this time.

As surface low tracks into the Great Lakes, a trailing cold front
will drop southward into central Illinois on Saturday.
Temperatures ahead of the front will climb into the upper 80s to
around 90 degrees, producing CAPE values of 2000 to 3000J/kg.
Despite good instability and 0-6km shear increasing to between 40
and 50kt by afternoon, airmass will remain capped. As a result,
will continue with just chance PoPs through the day. Best chance
for widespread showers/storms arrives Saturday night, as
approaching upper wave weakens cap and associated surface low
pressure tracks along the slow-moving frontal boundary. With all
the ingredients for convection coming into better focus, have
bumped PoPs to likely Saturday night.

LONG TERM...Sunday through Wednesday

Scattered showers/storms will linger into Sunday, before cold
front clears the area later in the day. After that, extended
forecast will be dominated by large upper low centered over the
Great Lakes. Northerly flow will bring another unseasonably cool
July airmass into the region, with highs only reaching the lower
to middle 70s on Monday. Cool/dry weather will prevail through
Wednesday before another wave potentially brings precip chances
back into the picture by the end of next week.



ISSUED 631 PM CDT Wed Jul 23 2014

VFR conditions will prevail at all TAF sites next 24hrs. Some CU
is still around at PIA and BMI, but this will dissipate quickly in
next 1-2 hrs. Then clear skies overnight at all sites is expected.
Expecting some CU to develop again tomorrow, but not as much as
today; though still will have scattered in the TAFs. Winds will be
north to northeast tonight with speeds decreasing and becoming
lighter, especially at CMI where gusts may still be there for an
hour or two. DEC observation is missing at 23z, but data should be
getting into the system via FSS. Since ATC personnel will be
leaving later tonight, observation will not be available at 02Z.
So will have AMD NOT SKED from 03Z til 11Z in DEC TAF.




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