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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1233 am CDT Sat Sep 21 2019

Update...
830 PM CDT

During this evening, we have continued to assess timing of rain
and potential thunderstorms during the day Saturday. Bottom line
up front is that the chances for rain are highest during the day
across north central Illinois in the morning, and this includes
embedded thunderstorms. The chance for scattered showers and a
few storms continues through the afternoon and early evening
before a notable increase in the rain footprint later Saturday
evening into overnight. While certainly some efficiency to rain
rates could be realized with any showers during the day, the
primary heavy rain threat remains after dark on Saturday.

Satellite imagery, including the water vapor band and the air mass
rgb, indicate a moist air mass expanding northward from the Ozarks
region. This was sampled at near 1.8 precipitable water on 00z
lzk and sgf soundings. An upper jet measured at 100 kt at abq is
propogating northeastward and will continue to result in a
poleward flux of this moisture tonight. There should be a gradual
increase in showers and some thunderstorms across northern
Missouri, southern Iowa, and western Illinois into the overnight
in response to the juxtapose of upper forcing for ascent and
moisture transport with the low-level jet. In fact, already have a
couple convective cells southeast of the Quad Cities.

The primary chances for north central Illinois look to be after 4
a.M. And persisting through the morning as a mid-level wave
associated with the speed Max moves northeast across the region.
The 00z dvn sounding already indicated 1000 j/kg with modifying
for saturating at 850 mb, or basically a proxy for the low-level
jet layer proxy. So think some thunder (e.G. Scattered) is a
fairly good bet including into the a.M. Over north central
Illinois.

While the deep moist plume will spread across the entire area by
midday, isentropic ascent weakens some further east, so showers
will likely remain scattered or even just isolated into northeast
Illinois and Northwest Indiana late morning into the afternoon.
This is why the primary coverage time during the day Saturday in
the County Warning Area looks like the a.M. And especially north central Illinois.
That said, suspect there will be some spotty afternoon showers and
possibly a couple storms.

Any other modifications to the forecast into tomorrow were minor.

Mtf

&&

Short term...
220 PM CDT

Through Saturday...

For the remainder of the afternoon, main concerns will be the
potential for isold shra/tsra across the region and the potential
for any development along a lake breeze boundary pushing inland.

The environment remains warm/moist/modestly unstable, but the lack
of any significant sfc focusing mechanism will keep and shra/tsra
development relatively isolated. The exception could be the
potential for shra/tsra development along the lake breeze
boundary, which could provide some sfc forcing. However, the
environment across nern Illinois is a little more stable and has been
under more extensive cloud cover through much of the day, so any
convective development along the lake breeze should be low chance
as well. Latest radar imagery shows some isolated shra invof the
Kankakee River. Some of the shra could have the potential to
produce brief heavy rainfall. The isolated activity will also be
somewhat diurnal in nature, so pcpn potential should diminish at
sunset. Highs this afternoon should top out in the low to middle
89s for most area, except far nern Illinois where sely flow off of the
lake will limit temps to the middle to upper 70s. Conditions
should be quiet overnight with lows in the middle to upper 60s.

For Saturday, deep layer moisture will be on the increase as ssely
flow sets up between an approaching cold front/trough from the
west and high pressure parked over the mid-Atlantic coast. Latest
guidance is consistent in building a plume of pwats of 2.0 inches
across the local area in a sustained fetch of moisture from the
western Gulf of Mexico. Pcpn chances should increase through the
day with strengthening warm advection in advance of the frontal
trough with strong, gusty sswly winds, with gusts up to 30 mph
developing by late morning. With the strongest focus expected to
be with the sfc cold front, which should still be well west of the
cwa, convective activity should be more scattered, but ramping up
in coverage from west to east through the afternoon. While
isolated stronger storms cannot be ruled out, and the potential
for severe weather still appears low at this time. The main
threat will once again be heavy downpours.

&&

Long term...
308 PM CDT

Sunday through Friday...

The primary focus continues to reside around the increasing risk
for heavy rainfall and possible flash flooding across the area,
particularly Saturday night through Sunday evening.

A large upper trough over the western Continental U.S. Will shift eastward
across The Rockies and over the plains states by Saturday night.
As this occurs, strong and deep south-southwesterly flow will be
induced across much of the Mississippi River valley and western
Great Lakes region. While the primary larger scale dynamics with
this approaching system will remain to our northwest Saturday
night, we are still likely to see an increase in the coverage of
showers and storms over northern Illinois. This initial period of
showers and thunderstorms looks like it may focus over far
northern Illinois Saturday night into Sunday morning, especially along
and north of I-80. An additional wave of showers and
thunderstorms are then expected Sunday afternoon into the evening
in association with the approach of a surface cold front and the
main upper trough.

Given the fact that the steering layer flow is expected to remain
nearly parallel to the orientation of the frontal zone, training
showers and storms will be favored over the area ahead of the
front into Sunday evening. This is especially concerning given
the presence of a prefrontal tropical airmass, which will feature
close to 2" precipitable water values and deep warm cloud depths.
Very efficient warm rainfall processes will thus be favored with
these showers and storms, which should support rainfall rates in
excess of 1.5" per hour Saturday night and Sunday. Total rainfall
amounts of 2-3 inches looks likely across most of northern Illinois and
into far northwestern in. However, I would not be surprised to
see some areas end up with 4+ inches.

While confidence is increasing the the area will experience some
heavy rainfall, there are still some small uncertainties on the
exact placement of the heaviest rainfall. With this in mind, we
have opted to hold off on issuing a Flash Flood Watch at this
time. However, we have hit the messaging a bit harder, with a
situation report issued. We will also continue to highlight the
potential in an esf (hydro outlook).

Surface high pressure is expected to build across the area Monday
following Sunday evenings frontal passage. This should result in a
couple of quiet and pleasant weather days early next week before
the possibility for showers and storms returns to the area mid to
late next week.

Kjb

&&

Aviation...
for the 06z tafs...

Aviation weather concerns are minimal through about daybreak this
morning, but numerous weather impacts due to a combination of low
cigs and showers/thunderstorms are possible later this morning,
again later this afternoon, and then late tonight and into Sunday.

VFR conditions are expected to prevail tonight as better low and
mid-level moisture begins to get pumped northward ahead of a
vigorous storm system translating eastward into the central and
northern Great Plains. High and then mid-level cloud cover will
thicken up early this morning with increasing chances for
precipitation in an initial band of warm advection. Forecast
soundings through late morning reveal very little in the way of
usable instability with profiles quickly trending towards moist
adiabatic. As a result, opted to drop the thunder wording and go
with vcsh and tempo rain showers wording for this initial wave of ascent.
Can't rule out a couple strikes, but think the chances for ts are
just too low to carry a mention in the tafs at this time.

This initial wave of activity may decrease during the early-
afternoon hours, but another wave of lift may encroach on the
region during the mid-late afternoon and early-evening hours.
There may be a slightly higher potential for electrification with
this activity, but still am not confident enough to move -tsra out
of a prob30 group. This secondary wave is low confidence overall
in timing and spatial orientation/extent.

Winds may also become gusty today, favoring a 210-230 direction as
momentum Transfer of 35-40 kts of flow aloft occurs. Wouldn't be
surprised to see some occasional gusts north of 30 kts into the
Chicago-area terminals as a result.

As this second wave exits, we may experience a brief lull in
activity into the late-evening before the main round of forcing
arrives. Have inserted a mention of prevailing showers and
vicinity thunder after 22/10z to account for this.

Carlaw

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...10 am Saturday to 1 PM Sunday.

&&

$$

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