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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 201820

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

314 AM CDT

Through Saturday...

Main forecast concerns/challenges are with thunderstorm potential
along/south of I-80, and then with increasing chances for more 
widespread showers/storms on Saturday. Severe threat through the 
period appears low, with heavy downpours the main threat.

Latest radar imagery depicting scattered showers/storms 
continuing to fester across parts of north central and east 
central IL early this morning. These storms are developing along 
lingering weak boundary/surface trough stretching across parts of 
northern IL. Earlier development was also supported with a weak 
mid level vort, but with its departure, additional support is 
stemming from weak low/mid level flow over this boundary/trough. 
Convergence and flow both expected to remain relatively weak, and 
should assist with keeping coverage limited with additional 
development staying confined to areas along/south of I-80 early 
this morning. Additionally, intensity and storm organization will 
also remain limited with lacking flow and bulk shear, and with 
weak lapse rates in place. However, with PWAT axis overhead and 
with weak steering flow, heavy rainfall with localized flooding 
will be a concern this morning.

Should see any remaining convection diminish through mid morning,
as boundary/trough becomes more diffuse. An isolated storm can't 
be ruled though, with this potential once again staying confined 
to areas along/south of I-80 in IL. Skies will start off partly 
sunny to cloudy for most areas, as cloud cover from the early 
morning precip remains overhead. Will see this cloud cover scatter
through midday/early afternoon, with more partly cloudy skies 
anticipated this afternoon. As these skies clear, guidance showing
that instability will be on the rise with little to no cap this 
afternoon. Chances for isolated storms this afternoon appear low 
at this time, with lacking focus. However, have continued isolated
wording in the forecast given the moist and unstable conditions 
likely in place this afternoon. Despite early morning cloud cover,
still expect temps to rebound with highs well into the 80s. Any 
thunderstorm development later tonight expected to remain to the 
west of the area, with a dry forecast still appearing likely.

Will see moisture really ramp up on Saturday, with high PWATs 
approaching two inches expected to move overhead throughout the 
day. Meanwhile, more energetic flow and surface trough/boundary 
approaching from the west will provide increasing chances for 
storms. Potential exist for storms across north central IL 
Saturday morning, but more widespread storms look to arrive later 
in the afternoon. Highest chances Saturday afternoon will remain 
across north central IL. Can't rule out an isolated stronger storm
or two, but severe weather appears low at this time. The main 
threat will once again be heavy downpours and flooding. Will need 
to monitor this period for possible flooding/flash flooding, 
especially as additional storms are appearing likely into the 
remainder of the weekend.



318 AM CDT

Saturday night through Thursday...

The primary focus of the long term forecast remains on the
potential for several rounds of rainfall--some heavy--and an 
attendant threat for flash flooding this weekend, followed by 
renewed river rises into next week. 

The weather system which will ultimately be responsible for 
delivering our next appreciable precipitation chances is currently  
spiraling across the Great Basin (and barely moving). However, by 
the start of the long term portion of the forecast on Saturday 
evening, the parent trough axis should be translating eastward 
into the Nation's heartland and will begin to shed several 
disturbances into the fast southwesterly flow which will act to 
ramp our precipitation chances up considerably through the end of 
the weekend. 

While the best large scale forcing for ascent looks to remain 
displaced well to our north and west through much of Saturday 
evening and overnight, the approach of a robust 100+ kt jet streak 
into Iowa and Minnesota will initiate a notable low-level mass 
response through the evening hours as the southwesterly 925-850 mb 
flow ramps up considerably. This burgeoning low-level jet should 
result in an expanding area of warm advection into parts of 
Missouri and western Illinois. This initial wave of warm 
advection/isentropic upglide may support an area of convection 
into the northwestern half of our CWA through Saturday night. 
While low-level flow will remain robust through the night, there 
are some signs that an expansive area of stronger or even severe 
convection may develop down across Kansas and western Missouri 
which may have some impact on poleward moisture transport into 
Illinois which could potentially help put a lid on rainfall rates 
during this period. 

That said, PWATs will likely have surged into near-record or even 
record territory for this time of year with guidance supporting 
values locally exceeding 2 inches. Tall and skinny MLCAPE
profiles, very deep warm cloud depths pushing 14 kft, and a rich 
tropical-like airmass will certainly support a potential for 
extremely efficient warm rain processes in showers and storms. 
Additionally, some degree of low- and mid-level shear could 
support a level of mid-level rotation which would act to further 
increase rainfall rates. At least through Sunday morning, the 
heaviest rainfall potential looks to remain relegated essentially 
north and west of I-55. Things may temporarily wane through the 
mid-late morning hours on Sunday as initiating wave of warm 
advection heads off to the north and east. 

The bulk of the synoptic-scale support from this system looks to 
charge into the region through Sunday afternoon and evening as
60-80 m/12 hour height falls begin to overspread the area. As 
this occurs, we could see a blossoming of showers and 
thunderstorms along the slowly-moving cold front. The concern for 
Sunday is that the cloud- bearing flow is forecast to orient 
essentially parallel to the incoming front/low-level theta-e axis 
which will support a training potential as renewed updrafts drift 
over the same areas. Still pretty hard to pick out exactly where 
this main axis will set up, but the potential exists for areas in 
our CWA to pick up multi-inch rainfall totals before all is said 
and done by Monday morning. This event does have some hallmarks of 
a Maddox Synoptic Flash Flood set up, so this certainly bears 
watching, especially considering the plentiful rainfall portions 
of our area have received over the last 2 weeks. Aforementioned 
uncertainties regarding the placement of the primary training 
axis, however, precludes the issuance of a Flash Flood Watch at 
this still early juncture, but one is certainly on the table in 
future forecast issuances. 

High pressure will take control of our sensible weather Monday 
afternoon and into Wednesday before broad cyclonic flow begins to 
brush up against our region towards the middle and end of the
week.  Attendant increases in warm advection will support at 
least low-grade precipitation chances into the end of the 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.