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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 210130 AAA

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

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 

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 CWA 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. 



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 00Z TAFs...

632 PM...Forecast concerns include...

Chance of thunderstorms mid morning thru early afternoon Saturday.
Chance of thunderstorms Saturday night.
Strong/gusty southwest winds Saturday.
Mvfr cigs Saturday.

A lake breeze stalled east of ord/mdw and then began moving back
to the lake this afternoon. This boundary will dissipate with
sunset but winds are still expected to shift to light southeast
this evening. Winds will then turn back to south/southwest
overnight with speeds slowly increasing and likely to 10kts by
sunrise. Winds will remain south/southwesterly Saturday with
speeds/gusts increasing. Maintained gusts in the mid 20kt range
but forecast soundings do show the potential for gusts to 30kt.

The next best chance for showers and a few thunderstorms appears
to be centered on mid morning for rfd and then late morning into
early afternoon for the Chicago terminals. While thunder coverage
may remain isolated...opted to carry thunder mention in the prob
groups. As trends emerge the timing will likely be narrowed. After
this activity...the rest of the day looks generally quiet. There
is still a chance of an isolated thunderstorm Saturday afternoon
but confidence is too low to maintain previous prob mention.

Chances for showers and scattered thunderstorms will begin to
increase again after sunset Saturday evening. There is still some
uncertainty for a start time so opted for just prob mention at
ord/mdw but eventually prevailing showers/rain with some mention
of thunder can be expected late Saturday night into Sunday
morning...with periods of heavy rain possible.

Mvfr cigs will be possible Saturday...especially across northwest
IL and at rfd. How far east these mvfr cigs reach is uncertain
but are possible into dpa/ord by late morning. cms