Area Forecast Discussion
Issued by NWS Central Illinois

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301
FXUS63 KILX 251608
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1108 AM CDT Tue Jul 25 2017

.UPDATE...
Issued at 1106 AM CDT Tue Jul 25 2017

High pressure and the associated dry air mass remain in control of
the weather across central an southeast Illinois this morning.
This scenario will persist into tonight, providing pleasant and
tranquil summer weather. Going forecast was in good shape with
only minor hourly tweaks necessary.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 308 AM CDT Tue Jul 25 2017

Quiet weather expected in the short term, as high pressure drifts
further east across the Great Lakes. As it moves away, more of a
southeast/south flow will be setting up by tonight. Dew points
will start creeping upward, the start of a warming trend as the
upper ridge over the southern Plains starts to edge eastward.
Temperatures should be fairly close to normal.

.LONG TERM...(Wednesday through Monday)
ISSUED AT 308 AM CDT Tue Jul 25 2017

Temperatures still look to reach the upper 80s/lower 90s on
Wednesday ahead of the cold front to our northwest. Heat index
expected to rise a bit above 100 over the southwest edges of the
CWA. However, main concern during this period is with heavy rain
and severe weather potential Wednesday-Thursday. After that
period, conditions should be similar to what we have experienced
the last 24 hours or so, as a broad scale trough sets up over the
eastern half of the CONUS this weekend.

1) Heavy rain potential:

Latest NAM and GFS continue to produce tremendously high
precipitable water values of 2.5-2.75 inches, near record
territory for this area. The NAM in particular hard hits an area
roughly along I-80 in eastern Iowa and I-74 in Illinois, rolling
an MCS through our area as soon as Wednesday evening and exiting
the eastern CWA late Thursday morning. Raw model QPF output
suggests perhaps a bit too much fine scale elements this far out,
with storm total QPF of 4-6 inches. The latest ECMWF is more of a
northern outlier at this point and keeps the heaviest rain over
the northern third of the state. Overall track of the GFS is close
to the NAM but a bit further north and lighter with its QPF axis
(though still over 3 inches). Given such high PWAT values,
tropical rainfall amounts certainly can`t be ruled out at this
point, but the run-to-run consistency has not consistently
pinpointed a particular corridor for the heaviest rain. While a
flash flood watch may be needed at some point across the northern
CWA, the uncertainty leans against issuing it at this point, and
we will issue a Hydrologic Outlook instead to highlight the
concerns.

2) Severe weather potential:

Slight risk of severe weather remains in place for generally
Wednesday night, with the latest Day2 outlook highlighting the
slight risk as far south as I-70. The potential for some
convective feedback in the NAM is resulting in very significant
0-6km bulk shear values over a large area of the northern CWA (50
knots) during the evening, while the GFS focuses it more
immediately ahead of the surface low. However, the NAM soundings
show more of a skinny CAPE profile and loaded with moisture. Freezing
levels around 15,000 feet Wednesday night would suggest less of a
hail threat and more of a damaging wind issue.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 535 AM CDT Tue Jul 25 2017

Largely clear at sunrise across central Illinois, with scattered
clouds south of KSPI. Cu-Rule amongst the models suggest this area
around KSPI/KDEC will be most likely to develop broken cloud cover
by late morning as diurnal development kicks in. Forecast
soundings indicate this may start as a period of MVFR conditions
with ceilings around 2500 feet before clouds lift around midday.
Areas further northeast from KPIA-KCMI won`t have quite as much of
a moisture advection going, and will keep skies scattered in this
area. The diurnal clouds will fade around sunset, but high clouds
will slowly spread in from the north late in the period.


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Bak
SHORT TERM...Geelhart
LONG TERM...Geelhart
AVIATION...Geelhart



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