Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 222014
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
314 PM CDT Sat Jul 22 2017

.SHORT TERM...(This evening through Sunday)
ISSUED AT 314 PM CDT Sat Jul 22 2017

19z/2pm surface analysis shows a weak frontal boundary extending
from southern Lake Michigan westward to the Iowa/Missouri border.
The airmass ahead of the front is still recovering from early
morning convection: however, latest LAPS data indicates CAPE
values have increased to 2500-3500J/kg...with a max of over
4000J/kg noted south of I-70. Despite the increasing instability,
storms have yet to re-develop. One reason is because much of the
KILX CWA is currently under a region of subsidence in the wake of
the morning storm complex, as evidenced by an anticyclonic curl on
water vapor imagery across southeast Illinois. As this feature
tracks further east and the cold front sinks southward, widely
scattered thunderstorms will develop along/north of I-74 within
the next 2-3 hours. Storms will become more widespread across
east-central/southeast Illinois by mid to late evening, with a few
of the storms potentially becoming strong to severe east of I-57.
Have carried low chance PoPs everywhere during the evening, with
high chance across the east accordingly. The showers/storms will
drop further southward and will mainly be confined to the SE
counties after midnight.

The cold front will settle into the Ohio River Valley on Sunday:
however, drier air will only trickle southward into the region...
with dewpoints still expected to be in the lower 70s. Meanwhile, a
vigorous short-wave trough noted on latest water vapor over
southern Manitoba will track southeastward into Michigan.
Increasing lift along/ahead of the trailing vort max will interact
with the ample boundary layer moisture to trigger widely scattered
thunderstorms across eastern Iowa into north-central Illinois
Sunday afternoon. With CAPE values ranging from 2500-3500J/kg and
0-6km bulk shear increasing to around 40kt, a few of the cells
could potentially produce marginally severe wind gusts and hail.
Have included low chance PoPs along/north of a Jacksonville to
Danville line during the afternoon...then further south to just
north of I-70 in the evening. Do not think areal coverage will be
that substantial...and think any storms that fire will dissipate
after sunset as daytime instability wanes.

&&

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 314 PM CDT Sat Jul 22 2017

Once the upper wave passes to the east, high pressure will build
into the Midwest early next week...bringing cooler and less humid
conditions to central Illinois. High temperatures will drop into
the middle to upper 80s for Monday and Tuesday before rebounding
back into the 90s by Wednesday. Models are showing a significant
trough digging across the Great Lakes by late in the period,
which will allow another airmass change to arrive by the end of
the week. 12z Jul 22 GFS has sped up the initial short-wave and
its associated cold front, with scattered showers/thunder pushing
into west-central Illinois as early as Wednesday afternoon.
Meanwhile, the ECMWF remains about 6-12 hours slower with precip
arrival. At this point, it appears the best rain chances will
materialize Wednesday night through Thursday night, with all
solutions taking the front south of the region by Friday. As a
result, will carry high chance PoPs during that time, with
cooler/drier weather returning for Friday and Saturday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1253 PM CDT Sat Jul 22 2017

Cloud cover from early morning convection has largely dissipated,
with some lingering mid/high clouds noted on latest visible
satellite imagery. A cold front will sink southward into the
region later this afternoon, triggering widely scattered
thunderstorms. Based on latest high-res models, have introduced
VCTS at KPIA/KBMI after 21z...then further south at KSPI/KDEC
after 23z. Any late afternoon/evening storms will dissipate after
dark...followed by dry conditions overnight into Sunday morning.
Winds will initially be S/SW at around 10kt, then will veer to the
N/NW tonight in the wake of the cold front. Despite FROPA, plenty
of boundary layer moisture will linger, so think patchy fog will
be possible late tonight. Have lowered visbys to around 5 miles
after 09z accordingly.


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM CDT this evening for
ILZ027>029-036-037-040>042-047>054-061-062-066>068-071>073.

Heat Advisory until 8 PM CDT this evening for ILZ030-031-038-
043>046-055>057-063.

&&

$$

SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Barnes


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