Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 170429
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1129 PM CDT Sat Sep 16 2017

.UPDATE...
Issued at 850 PM CDT Sat Sep 16 2017

Initial band of convection that developed over northern Missouri
into central and east central Iowa late this afternoon into early
this evening has diminished in intensity as it encounters a more
stable air mass. Additional convection developed over the past few
hours along the weak cool front which was analyzed just west of
Des Moines. Short term models suggest any additional showers will
have a tough time pushing east of the Mississippi River later
tonight as the air mass in our area is rather dry in the low and
mid levels of the atmosphere as seen on the 00z ILX sounding. The
current forecast has this well handled for the evening and overnight
hours with only a slow eastward progression to the rain chances
with overall, our best chance for scattered showers and storms
coming along on Sunday, as the front settles southeast across the
area. No significant changes were made to the overnight grids, as
a result, no ZFP update will be needed at this time.

&&

.SHORT TERM...(This evening through Sunday)
ISSUED AT 300 PM CDT Sat Sep 16 2017

A gradual increase in deep moisture will accompany an approaching
cool front - which extended from northeast MN through western IA
to the OK panhandle at 2 pm. There is quite a bit of dry air in
the mid levels of the atmosphere, however, that will need to be
overcome first before any of the precipitation makes it to the
ground. Forecast soundings and most of the short range models
indicate this won`t happen in west central IL until well after
midnight, and east of the IL River until daybreak or shortly
afterwards.

The front is expected to reach the IL River valley early Sunday
afternoon, and east of I-55 by dusk. This will result in
increasing t-storm chances in eastern IL during the afternoon, and
in parts of southeast IL toward evening. The clouds and periods of
rain will keep temperatures on Sunday a good 10 degrees cooler
than today`s highs, but still a bit warmer than normal for mid-
September.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 300 PM CDT Sat Sep 16 2017

Our warmer than normal pattern is expected to continue into next
weekend, with unsettled weather into mid-week.

The 12z model runs are all pushing the cool front a bit farther
south late Sunday - toward the I-70 corridor - than their
previous runs. This looks reasonable based on the expected upper
level ridge axis just north of the Ohio River valley. The impact
on our weather would be for showers/t-storms to remain in eastern
IL through Sunday evening and primarily in southeast IL Sun night.

The models are also fairly consistent in showing a shortwave
trough ejecting from the upper trough axis near the Rockies toward
the Midwest Monday. This should allow the front to lift back north
as a warm front, and give the forecast area the best chances for
thunderstorms with moderate/heavy rain Monday. Moderate
instability and increasing deep layer shear Monday afternoon-
evening could result in some strong storms north of a Springfield-
Danville line, although the best ingredients appear to be off to
the west (as seen in the SPC Day 3 outlook).

With Hurricane Jose making its way up the eastern seaboard, our
weather pattern into mid-week will be rather stagnant with plenty
of warm and humid air, and summer-like, diurnally driven showers
and t-storms Tuesday and Wednesday.

Mid and upper level ridging is expected to strengthen from the
eastern Great Lakes to the lower Mississippi Valley by Thursday,
in response to a digging upper level trough in the intermountain
region. This will keep any synoptic low level boundaries well to
our west. The resulting weather will be more of the same with
unseasonably warm and humid temperatures and perhaps and isolated
t-storm into early next weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1125 PM CDT Sat Sep 16 2017

A cold front near Des Moines late this evening is expected to
push across the Illinois River Valley between 14 and 17z Sunday
morning, and by 22z at CMI. Scattered showers and thunderstorms
will accompany the front but it appears coverage looks limited
enough to continue with VCTS/VCSH. There continues to be a low
probability for some brief MVFR cigs not only with any TSRA that
affects a TAF site but also along and just behind the weak cold
front tomorrow. Soundings have backed off that idea in the last
several runs so other than including a scattered lower group, will
not include any reductions to MVFR at this time. The front and
shower chances will continue to slowly push east and southeast
across the remainder of the TAF sites during the afternoon and
evening. Winds ahead of the front tonight will be light southerly
with speeds of less than 10 kts, with winds veering more into a
southwest to west direction as the front approaches, and into a
light northwest direction after FROPA with speeds during the day
generally 10 kts or less.

&&

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

$$

UPDATE...Smith
SHORT TERM...Miller
LONG TERM...Miller
AVIATION...Smith



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