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Area forecast discussion
National Weather Service Lincoln Illinois
1145 PM CDT Fri Sep 20 2019

issued at 236 PM CDT Fri Sep 20 2019

Heavy rainfall is forecast over central Illinois this weekend due
to a slow moving cold frontal system. The heaviest rain of 2 to 3
inches will be focused across the Illinois River valley...with
amounts decreasing to around 1 inch further southeast along and
south of I-70.


issued at 901 PM CDT Fri Sep 20 2019

Central Illinois will remain under a weakly forced humid and
unstable air mass overnight. MUCAPES of 1500-2000 j/kg has been
enough to allow a few lingering convergent areas to produce
showers. The predictability of the location of those showers this
evening has been Akin to the whack-a-mole game. Isolated showers
have been bubbling up sporadically in the north, southeast and
eastern portions of our forecast area so far. Lighting this
evening has been minimal if at all. Cams are pointing toward more
of the same the rest of the night.

A more well defined forcing mechanism is projected to approach
western Illinois late tonight, increasing rain and storm chances
west of spi to pia by sunrise. The corridor of highest moisture
content tomorrow looks to be on a SW to NE axis across the northwest
third of IL, where our highest pops and Flash Flood Watch reside.
As evidenced with this evenings showers, there will likely be
isolated to scattered showers east of I-55 from time to time as
well, during the heat of the day on Saturday.

Periodic clouds tonight and steady S-southeast flow will keep low temps
very close to 70 degrees, as dewpoints remain in the upper 60s to
low 70s as well. Patchy fog can not be ruled out, but increasing
cloud cover in western counties should preclude fog from getting
out of hand toward becoming dense fog.

Update this evening was to the pops/weather grids primarily, to
capture the current and expected short term trends in shower/storm
coverage. The remainder of the forecast appeared on track.


Short term...(this evening through saturday)
issued at 246 PM CDT Fri Sep 20 2019

Models continue similar trend as past of bringing major upper trof
into region on weekend, with ample moist air flowing in ahead of
the upper feature. Cold front associated with system will approach
area early Sunday morning and cross area during day Sunday. Still
some minor differences on frontal timing, but the result will
still be most significant rainfall amounts over the northwest
cwa(2-3 inches), with lesser amounts to the east to I-55. As
front moves across region late Sunday, exiting system will produce
near 1 inch amounts. 850mb moisture transport today focusing good
plume of moisture advection into the Kansas-NE area and expect this
same good advection through day tomorrow further to the east, so
available moisture should not be a problem with strong upper
dynamics. For these reasons, have simply made some minor
adjustments to match coordinated probability of precipitation and quantitative precipitation forecast with surrounding
offices, using blend of wpc and nbm and coordinating weather forecast office amounts.
Will also keep the current Flash Flood Watch going for the western
half of County Warning Area.

Long term...(monday through friday)
issued at 156 PM CDT Fri Sep 20 2019

Cold front that brings the heavy rain to central Illinois over the
weekend is progged to drive south of the Ohio River valley early
next week while high pressure builds across portions of the
Midwest in its wake. Dry weather and more seasonable temperatures
will be in place behind the front early next week. Expect
afternoon highs in the mid to upper 70s while overnight lows will
dip into the mid 50s Monday night into Tuesday morning.

The latter half of the upcoming week looks to become more
progressive and active as a series of waves push across the
region. At this time, there remain rather significant differences
between the models, particularly with how far north the baroclinic
zone lifts; however, there is a general theme that the local area
will see a wave midweek and another late in the week. While there
is a decent likelihood of a couple rounds of showers and storms
through the latter half of the week as these waves transit the
region, timing will still need to be refined. Have fairly low
confidence in the temperatures, though, given differences in the
storm track and location of the baroclinic zone on any given day.


Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1145 PM CDT Fri Sep 20 2019

VFR conditions are expected at all taf sites in the first 6 hrs
of the 06z taf period, then MVFR clouds will begin to advance
into western Illinois Saturday morning, along with increasing
chances of showers and storms. Pia is the only taf with MVFR
clouds forecast in the morning for now. Pia and spi will see the
highest rain chances by later morning and through the afternoon.
The primary forcing for precip will reside west of the Illinois River,
but scattered storms will develop in the humid and unstable air
mass east of there during the day, mainly west of I-57.

We delayed rain for Dec and cmi until after 00z Sat evening.

Steady rains look likely overnight Sat night, especially west of
I-55, with coverage expanding eastward.

Winds look to increase from south Sat morning, with afternoon
gusts climbing to 20-24kt at times. Wind gusts will diminish with
sunset, but sustained winds look to linger in the 10-12kt range.


Ilx watches/warnings/advisories...
Flash Flood Watch from Saturday evening through late Sunday
night for ilz027>031-036-037-040-041-047>050.




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