Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 210422

Area Forecast Discussion
National Weather Service Lincoln IL
1122 PM CDT WED JUL 20 2016

Issued at 830 PM CDT Wed Jul 20 2016

A weak frontal boundary stretching from an area of low pressure
over central South Dakota east-southeast through southwest
Illinois is expected to lift north of the area by Thursday
morning. Dew points along and to the south of this boundary were
in the 75 to 80 degree range late this afternoon with surface
temperatures in the mid 90s, producing heat index values of
106-112 degrees out to our west. This is going to be the main
weather story over the next several days as an excessive heat
warning is in effect from Noon tomorrow through Saturday evening.
An expansive upper level ridge will amplify a bit to the north
into the Midwest before a couple of shortwave trofs start to knock
it down some later this week and into the weekend. The 00z ILX
sounding indicated the 700 mb temp warmed about 4 degrees, up to
+11 degrees C which effectively capped the atmosphere from any
widespread convection this afternoon despite the strong instability
present across the region.

A remnant MCV from the storms that developed over northwest IL
late this morning into this afternoon before the capping inversion
arrived, was located just to our north and a few of the hi-res
convective allowing models were developing some widely scattered
storms near this feature. The short term hi-res models have been
doing poorly with this very moist atmosphere and it seems as if it
doesn`t take much to generate precip with these models. The main
area of lift tonight will be well to our north associated with
the initial Northern Plains shortwave which will drag a weak
frontal boundary and convection south into the upper Great Lakes
Thursday morning. With the strong cap in place and better lift to
our north, will continue to keep the overnight portion of the
forecast dry. We should have the update out by 900 pm.


.SHORT TERM...(This evening through Thursday)
ISSUED AT 259 PM CDT Wed Jul 20 2016

Convection with torrential rain in the southwest part of the
forecast area has finally weakened and moved out of the region early
this afternoon. Another batch of showers/isolated t-storms north of
I-80 is expected to move around the NE side of the upper level ridge
and into parts of central IL early this evening, before weakening.

Many of the short range models are in agreement that as the upper
ridge continues to build over our area tonight, the potential for
convection will be quite minimal. With continued warm advection
aloft into Thursday, the atmosphere should be strongly capped, which
will keep any showers and t-storms from forming.

Moderate levels of atmospheric mixing is expected to be offset by
increasing low level moisture, resulting in mid 70s dew points in
all of central and southeast IL by Thursday afternoon. With high
temperatures expected to be in the mid 90s, heat index values will
climb to 105 to 112 by mid-afternoon. As a result, have upgraded the
Excessive Heat Watch to an Excessive Heat Warning starting Thursday.

The last time central and southeast IL had heat index values this
high was June 29-July 1, 2012.

.LONG TERM...(Thursday night through Wednesday)
ISSUED AT 259 PM CDT Wed Jul 20 2016

Main forecast focus remains with the high heat and humidity
through this weekend, as strong 598 dm 500 mb subtropical high
over OK stays near IL where 500 mb heights 594 dm or higher. Will
upgrade the excessove heat watch to an excessive heat warning from
Thursday afternoon through Saturday afternoon across central and
southeast IL. And this heat headline may likely need to be
extended through Sunday afternoon southeast of the IL river. Local
warning criteria calls for 110F+ peak heat index for two days with
nighttime heat index at least 75F. Meanwhile the alternate
criteria of 105F+ heat index for at least four days supports an
excessive heat warning which appears likely form Thu thru Sunday
over most of CWA. Highs will be in the low to mid 90s through this
weekend, though a bit cooler nw of IL river on Sunday where around
90F from IL river nw. Tropical dewpoints in the mid to upper 70s
supports these high heat indices averaging 105-112F during the
afternoon hours into this weekend. Lows through this weekend will
be near dewpoints in the mid to upper 70s.

Mid-level capping may break enough north of I-74 by Friday afternoon
and further south into central IL during Friday night into Saturday
to have a few thunderstorms to form. This due to weak short wave
moving into the upper MS river valley and western great lakes by
Saturday morning. SPC day3 outlook has marginal risk (5%) of severe
storms north of I-72 on Friday afternoon into Friday night mainly
for winds but small hail, heavy rains and frequent lightning also
risk. Locally heavy rains supported by precipitable water values at
or above 2 inches. A cold front will push into northern IL on Sunday
and then slowly into central IL Sunday night, and southeast IL and
approaching Ohio river on Monday. This will bring more chances of
showers and thunderstorms early next week, though shifting into
southeast IL south of I-72 on Monday. Temps cool closer to normal
for late July in the mid to upper 80s on Monday and Tuesday with
dewpoints slipping into the upper 60s northern CWA Monday and across
CWA on Tue. Dry conditions generally expected Monday night through
Tue night with next chance of showers and thunderstorms arriving
next Wed.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1115 PM CDT Wed Jul 20 2016

The main concern will be with the potential for some MVFR vsbys
in fog in the 09z-13z time frame. Plenty of low level moisture
around due to the recent rains coupled with a very light wind
regime and a mostly clear sky will lead to some patchy fog during
the early morning hours. Once the fog lifts around 13z, some
models were suggesting the threat for wdly scattered TSRA along a
northward moving warm front around 12-13z. However, the high res
convective allowing models have been struggling quite a bit in
this saturated air mass with a different model solution shown with
each model run. So confidence on that occurring is rather low at
this time so will not include in this set of TAFs. Forecast
soundings suggest some few to scattered cumulus to develop during
the day on Thursday, but like today once the temperatures warmed
aloft, the cumulus were few and far between and we think that will
be the case on Thursday as 850-700 mb temps will be just as warm
if not warmer than todays.

Surface winds will once again be a non-factor this forecast period
with light southerly winds tonight, with southerly winds increasing
a bit to between 8 to 13 kts Thursday.


.ILX Watches/Warnings/Advisories...
Excessive Heat Warning from noon Thursday to 7 PM CDT Saturday
for ILZ027>031-036>038-040>057-061>063-066>068-071>073.



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