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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 211951

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
251 PM CDT Sat Sep 21 2019

248 PM CDT

Through Sunday night...

Significant concerns for areas of heavy rain and flash flooding
continue to increase over much of northern IL for tonight through
Sunday evening. This as a tropical origin airmass, featuring near
record precipitable water values around 2", interacts with an
approaching strong synoptic scale short wave trough and surface 
cold front. Overall, little there is little change in the forecast
thinking, with no changes planned to the watch area. It still
appears that the main axis of heavy rain will orient right across
northern IL, generally along and north-northwest of the I-55
corridor. Thus both the Rockford and the Chicago metro areas will
likely be impacted by this heavy rainfall through Sunday evening. 

Warm moist advection continues to increase across the region this
afternoon in response to the continued enhancement of the lower 
level wind field across the central US. This is occurring in
response to the development of a rather impressive 110+ kt south-
southwesterly upper level jet streak on the eastern periphery of
a very dynamic upper trough over the Rockies. Overall, the main
result of this lower-level response in the wind field has been to
support the development of convective showers and a few storms  
over much of Missouri, eastern IA and into IL this afternoon.
These trends are likely to continue into this evening over the

In spite of the fact that many of these showers are void of
lightning, they are very efficient rainfall producers due to the 
deep warm cloud depths in excess of 12,000 ft. For this reason, 
these showers could easily drop a quick half to 1 inch over some 
isolated areas late this afternoon and evening. While any hydro
concerns should remain minor over the next few hours, these 
showers are only increasing the vulnerability to flash flooding 
tonight and Sunday. These showers and some embedded thunderstorms
should increase in areal coverage over northern IL overnight and
into Sunday morning, and with the steering layer flow remaining
parallel to the frontal orientation, expect these showers to 
track northeastward over the same areas of northern IL tonight and
on Sunday. 

Forecast guidance does continue to slow the timing of the frontal
passage on Sunday, with it now likely not moving across eastern 
IL until mid to late Sunday evening. For this reason, the threat
of heavy rainfall will continue Sunday evening. We may even end 
up having some stronger showers and storms right ahead of the
front Sunday evening. While instability will be marginal, strong 
dynamic forcing for ascent, associated with the stout mid-level 
trough, is likely to compensate. For this reason, we could not 
only have a continued heavy rain threat Sunday evening, but also 
the possibility for some strong wind producing storms or even an 
isolated tornado. 

The showers and storms should end from west to east during the
evening on Sunday as the cold front shifts eastward across the
area. Total rainfall amounts continue to look very impressive, 
with an areal average of 2 to 4 inches likely across much of 
northern IL in the flash flood watch area. There will also likely 
be areas that see more than 4 inches, and these areas could be 
near the Chicago metro area. This certainly does add concern for 
the possibility of some significant flooding across portions of 
the area, especially if these high rainfall amounts fall over the
urban areas of the Chicago metro area.



251 PM CDT

Monday through Saturday...

A more quiescent stretch of weather is expect to start off the 
work week with surface high pressure in charge for a time. Monday 
and Tuesday look like pretty nice days with highs in the low to 
mid 70s and dewpoints hanging out in the mid to upper 50s. This 
surface high will drift just south of the region Monday night and 
into Tuesday morning and will result in slackening winds 
overnight. Given the copious rainfall expected this weekend and 
Sunday night, would normally be concerned for a fog potential late
Monday night, but the combination of possibly plentiful upper-
level cloud cover and about 15 kts of flow immediately off the 
deck should help greatly curtail what might otherwise be a 
widespread dense fog potential. 

This surface high will head east of the region by Wednesday with mid-
level shortwave ridging giving way to the next rather robust 
shortwave. By Wednesday morning, guidance indicates that mid-level 
lapse rates will be on the increase ahead of this next system, 
supporting a pretty quick increase in instability immediately to our 
north and west. The presence of a pretty potent jet streak nosing 
across the Upper Midwest and into the Great Lakes region may help 
drive a zone of enhance low-level warm advection somewhere near the 
area. We'll have to keep an eye on a thunder potential as a result. 
While the mid-level flow will be quite fast, instability overall 
looks to be lacking which should help keep the threat for strong to 
severe convection to a relative minimum, at least at this juncture. 

Looks like we'll get another brief reprieve from any precipitation 
chances towards Thursday before the yet another system encroaches 
towards Friday and into next weekend.



For the 18Z TAFs...

A rather messy period of weather setting up for the period with
main concerns being timing of periods or shra/tsra, some of which
may be heavy at times through the period, and wind trends with
gusty sswly winds for the remainder of the day and, possibly,
continuing into the overnight hours.

Currently, several waves of shra/tsra are moving across the region
in an area of broad warm advection/isentropic lift in advance of
the main synoptic cold front/sfc trough. At issuance time, the
Chicago area terminals are split by areas of pcpn moving to either
the west or east of the terminal sites. This will likely be a
short minimal in the pcpn as additional development is expected
later this afternoon, overnight and for much of tomorrow. However,
while confidence is high in periods of shra/tsra, timing and
coverage remain low confidence until the synoptic cold front
approaches and then pushes across the region at which point,
convective activity will be more organized and less scattered.

Given the tropical nature of the environment, any showers or
thunderstorms will be capable of producing very heavy rainfall
rates/amounts in a short time, sharply reducing cigs/vis. Expect 
that vsbys will occasional drop to 1 sm or lower in +RA/+TSRA 
through Sunday with periods of ifr cigs possible as well. 

Surface winds should be gusty and, generally sswly through the
period, with gusts to 25 kt likely through this afternoon and
occasional gusts to around 30 kts will be possible. A wind shift
to nwly will occur following the passage of the sfc cold front
which is not expected until tomorrow evening.


IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     PM Saturday to 1 AM Monday.

LM...Small Craft Advisory...nearshore waters 
     until 1 PM Sunday.