Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 211131
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
631 AM CDT THU JUL 21 2016

.SHORT TERM...(Today and tonight)
ISSUED AT 237 AM CDT Thu Jul 21 2016

Initial concern in the short term is around fog for early this
morning. Winds are light and variable, with skies remaining mostly
clear. Locally dense fog has formed in our eastern areas near
Danville and Paris already, with another area of less dense fog
farther south from Flora to Robinson and Lawrenceville. Dewpoints
depressions range from 0 to 3 degrees across the board. See enough
support to continue mention of fog through just after sunrise, but
confidence not high on how much dense fog will occur.

Next concern surrounds thunderstorm chances over the next 24 hours.
The latest Day 1 QPF forecast from WPC expanded rainfall amounts
from northern IL into central IL, with 1.20" of rain as far south as
Peoria for the time frame between 7 am today and 7 am Friday. Radar
analysis shows an active MCS in MN progressing to the E-SE, and most
high resolution models progress that toward our forecast area later
today. Some dissipate the line more than others, but it appears
there is enough support for some storm activity to make it all the
way to central IL either late this morning or early afternoon.
Therefore, we added some precip chances to today`s forecast. Most
coverage today should be isolated in nature.

However, another complex of storms is projected to develop north of
our area tonight and progress southward into our CWA. Current model
trends show the higher potential being after 08z/3am tonight, and
have expanded and increase PoPs after midnight into the chance
category from Peoria and northward.

The other concern today is how much any convection and cloud cover
will hold down high temperatures. Guidance numbers were going about3-
4 degrees less hot than our previous forecast. Have not cut our
forecast quite that drastically, but did lower todays highs a degree
or two across the north and east where storm potential appears
higher. Expect that even filtered sunshine under this strong of an
upper ridge and warm low-mid layers should be enough to push highs
into the low 90s.

Lows tonight should be held up in the mid 70s by the very moist
boundary layer, with patchy fog possible again.

.LONG TERM...(Friday through Wednesday)
ISSUED AT 237 AM CDT Thu Jul 21 2016

Primary concern remains the intense heat. Will extend the Excessive
Heat Warning into Sunday for areas from about Rushville-Danville
southward, as the front continues to slow down a bit and is more
likely to pass through Sunday evening. A little more in the way of
clouds and scattered storms, which will keep the heat index more in
the 105-110 range vs. the 109-112 on Friday and Saturday, but 850 mb
temperatures are still around 24C at that range, and lesser clouds
than expected could easily bump the heat index up a few notches.

That being said, Friday could be a bit tricky with the heat,
primarily over the northern CWA. Most of the model guidance
continues to favor an MCS track over the central and northern Great
Lakes, with some westward development/retrograding along a frontal
boundary or leftover convective boundaries. The Canadian and ECMWF
are more in line with that philosophy, along with the NAM to a
lesser extent. The GFS is much further southeast and aggressive,
given its CAPE`s in the vicinity of 6000 J/kg on the Peoria forecast
sounding, but 700 mb temps near 12C suggest a significant cap. Have
added some 30% PoP`s along and north of I-74 during the day, with
mainly isolated storms at best elsewhere. Should the clouds be more
widespread, highs in the mid 90s would be tough to attain, but the
tropical dew points would still be present unless a cold pool from
the northern storms tracks into the area.

As the weekend begins, the upper flow becomes more west-east and
subtle ripples may serve as a focus for some convection, addressed
with the previous scattered PoP`s around 30% Saturday. The incoming
front on Sunday may prove to be more of a catalyst with the rain
chances, though.

Much quieter as the new work week begins, and temperatures return
closer to normal as high pressure drifts through the region.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 631 AM CDT Thu Jul 21 2016

MVFR visibility will linger for the first hour or two of this TAF
period, due to muggy dewpoints in the mid 70s and light winds.
Any areas of dense fog appear to be confined to areas closer to
the Indiana border, and should not affect any terminal site.

A warm frontal boundary lifting north through Illinois today could
become a focus for a few thunderstorms north of I-74 this
afternoon and evening. However, there is a chance the models are
over-doing convection since mid-level warming should help to
suppress storm development. Therefore no mention of thunder was
included with the 12z issuance.

A late night MCS may approach the northern TAF sites after 09z-
10z, and a VCSH was included due to better agreement on that
scenario in the models.

Some MVFR fog may redevelop late tonight, but it should only be
for a couple hours toward sunrise once again.

Surface winds will start out light southerly this morning, then
become south-southwest and increase to 8-13kt from late morning
through afternoon.

&&

.ILX Watches/Warnings/Advisories...
Excessive Heat Warning from noon today to 7 PM CDT Sunday for
ILZ040>057-061>063-066>068-071>073.

Excessive Heat Warning from noon today to 7 PM CDT Saturday for
ILZ027>031-036>038.

&&

$$

SHORT TERM...Shimon
LONG TERM...Geelhart
AVIATION...Shimon


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