Area Forecast Discussion
Issued by NWS Central Illinois

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425
FXUS63 KILX 201959
AFDILX

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

.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 18Z TAFS through 18Z Thursday afternoon)
Issued at 1250 PM CDT Wed Jul 20 2016

Synoptic stationary front extended from central IA into east
central MO. Elevated showers and t-storms to the northeast of the
front are finally starting to show a dissipating trend in west
central IL. Will continue to see spotty shra/tsra near SPI with
ceilings just barely into the VFR category.

There continues to be low confidence in the shorter range models,
which are showing various areas of convection popping up in parts
of central and eastern IL this afternoon. Can`t completely
discount it due to increasing levels of instability, as well as
various low level boundaries that could act as a focusing
mechanism for convection. Thus, kept the trend of VCTS in the
forecast through much of the afternoon - and into early evening
from PIA-BMI-CMI.

Overnight, could see areas of light fog develop with high levels
of low level moisture, saturated ground in some areas, and light
wind. Thus, MVFR visibility was put in a few of the TAF sites
where rainfall has been more prevalent the past day.

&&

.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.

&&

$$

SHORT TERM...Miller
LONG TERM...07
AVIATION...Miller



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