Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202008

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
308 PM CDT Fri Sep 20 2019

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 IL 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 IL 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.


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 CONUS 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 IL. This initial period of
showers and thunderstorms looks like it may focus over far
northern IL 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 IL 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.



For the 18Z TAFs...

Continued subtle forcing and the lack of any significant surface
features make for a somewhat uncertain forecast for the remainder
of the afternoon, in particular for any shra/tsra chances. Latest
radar imagery shows some sct shra developing well south of the
terminals where there is a little higher instability and less high
level cloud cover. While there is a slight chance for a pop-up
shower along a lake breeze pushing inland this afternoon, chances
of any pcpn directly impacting the terminals is too low to include
in the TAFs. Winds should be light and generally swly through the
afternoon except turning sely following the passage of the
aforementioned lake breeze. For the overnight hours, expect
dry/vfr conditions with light winds trending to sly-ssely.

For tomorrow, concerns will begin to focus on the next storm
system approaching the region. Through the morning hours, winds
should strengthen quickly as the pressure gradient strengthens
between high pressure parked over the Mid-Atlantic coast and the
approaching frontal trough. In an environment of increasing deep
layer moisture and warm advection, sct shra and perhaps an isold
tsra may begin begin to approach RFD by late morning and the
Chicago area terminals by mid-day. However, the greater chance of
shra/tsra will likely be on the increase through the late
afternoon hours.





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