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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 200538

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1238 AM CDT Tue Aug 20 2019

315 PM CDT

Through Tuesday...

There are a few spotty showers and possibly a couple storms 
through 8 p.m. or so, but the primary attention through Tuesday 
is on the potential for organized, severe storms Tuesday morning, 
especially along/south of I-80.

The now nearly stationary surface front is draped from just north
of Chicago through far northern LaSalle County. Isolated cells
have developed along this in our CWA, with a more scattered
presentation southeast of the Quad Cities. This activity has 
almost entirely been void of lightning, with clouds tops in the 
forecast area on only one cell dropped below 0C thus far and only 
slightly at that. With some dry air entrainment on updrafts and 
limited surface convergence and confluence, feel that sustained 
and deeper updrafts for electrification will be limited. With the 
combined effects of the weak surface front, a weak mid-level 
perturbation, and dew points around 70 yielding mixed layer CAPE 
values slightly over 1,000 J/kg this isolated activity is expected
to continue through 8 p.m. or so with an east-southeast 

Tonight looks primarily quiet for the local area, thought the high
dew point air and the calming wind field will provide some concern
for fog, possibly dense. The sky should become clear for much of 
the late evening into early overnight before a combination of low-
level jet induced clouds and cirrus are probable. Fog may develop
in that pre-cloud time, and if clouds are not as plentiful at 
daybreak as model guidance is indicating, some dense fog could 
develop and linger beyond daybreak. Confidence in this is overall 
low given the cloud potential so have not messaged this too hard 
and evening shift will have the chance to assess more.

Attention through the night will be on convective development and
evolution to the west. A strong upper level speed maximum and
associated short wave disturbance across the South Dakota/Nebraska
border area this afternoon will migrate eastward, intersecting a 
low-level jet nose into Iowa overnight. Rapidly steepening mid-
level lapse rates will help to favor robust convective 
development evolving into an MCS. There is some uncertainty at 
what latitude this will form in Iowa, and that will play into how
much of our area is threatened Tuesday morning. But MCS 
maintenance parameters, such as organized lift, deep layer shear,
ample MUCAPE, rich low-level and deep layer moisture, and cold 
pool development are more than juxtaposed for sustenance into 
Illinois Tuesday morning. Convection allowing model (CAM) guidance
is in high agreement on this form of evolution, with a fairly 
good signal in forward propagation in line with environmental 
steering fields.

The challenge is just how this evolves, with some guidance
indicating more southeasterly and others more easterly and
impacting more of the CWA. The parent short wave is favored to 
develop more eastward in line with the upper level winds and
potential vorticity fields, and looks to not clear the CWA until 
the afternoon hours. However given the forward propogating nature 
anticipated, a quicker movement is presently expected, with the 
mid-late morning hours most favored for some CWA impacts. The 
southeast-leaning instability gradient and low-level jet
orientation with respect to the mid-level flow supports MCS 
prorogation vectors of an east-southeast to southeast movement. 
This would favor mainly the southern and possibly just the 
southwestern CWA for the convective bowing segment apex and the 
greater potential for damaging wind gusts. North of any bowing 
segment apex with the convectively enhanced shortwave there is 
likely to be scattered convection though, possibly embedded 
within stratiform activity. This includes into or at least near 
Chicago. Given the instability and shear spectrum space, these 
could be strong and cannot rule out severe too. 

Overall confidence is somewhat low on just how much of the CWA is
impacted by any bowing segment, and beyond 10 am or so if the 
convection will show a weakening trend, which can occur if 
outrunning the primary forcing. Also further north the convective
presentation is expected to remain more scattered, and the degree
of strong to severe potential is somewhat uncertain, but isolated 
hail and wind would be potential threats.

One item to note with the convective enhanced short wave track to
the north of the MCS, is that it could favor a wake low developing
and tracking over some part of the CWA, resulting in a brief 
period of gusty sustained winds behind showers and storms during 
the late morning/early afternoon. That feature is plenty difficult
to predict even a couple hours in advance let alone 18-24, so 
confidence in this is low but some signals are there.

Going into the mid-late afternoon Tuesday, the subsidence in the
wake of any morning activity should keep the lid on the mixed
layer pretty well even though there could be some return flow
aloft resuming. While temperatures will likely fall for a brief
period later morning they should bounce back to the mid 80s or so.



330 PM CDT

Tuesday Night through Monday...

Similar to how the organized convection this past Sunday morning
largely precluded renewed convective development through Sunday
evening and night, think Tuesday's likely organized daytime 
convection will make any renewed development with approaching cold
front less likely through Tuesday evening and night. Can't rule 
out isolated to perhaps widely scattered convection, particularly 
over or east of our northwest Indiana counties. It will otherwise 
be another warm and muggy overnight. The cold front will move 
across the area on Wednesday, but this will be more of a wind 
shift, with stronger cool and dry advection not coming until 
Wednesday night into Thursday. Thus Wednesday will be the last 
warm and humid day in this stretch, with highs in the lower-mid 
80s away from Lake Michigan (upper 70s lakeside due to winds 
turning onshore). Chances for showers and thunderstorms appear 
better south of the CWA where there will be stronger convergence, 
but low (slight to low chance) PoPs south of I-80 appear 
reasonable given moderate instability in presence of lower 70s dew

As a stronger trough axis from mid-upper low over eastern Canada 
and secondary cold front pivots south across the region Thursday,
still can't rule out isolated showers and storms well south of
I-80 where there will some lingering moisture and instability. For
the rest of the area, it will be a pleasant and cooler day with
highs in the 70s and dew points dropping back to the 50s.  At 
this time, Friday and through the weekend is looking rather 
pleasant and dry with near to slightly below normal temperatures 
for late August and low humidity as dewpoints remain in the 50s. 
Some low shower and storm chances will spread back east across the
region towards the very end of this forecast (primarily Monday) 
as sprawling high pressure exits off to our east. 



For the 06Z TAFs...

Active pattern expected to bring showers/thunderstorms 
near/across the terminals later this morning into midday, and 
potentially again later this afternoon. Monitoring area of 
showers/thunderstorms currently developing across 
northwest/northern IA at this time, as these storms will likely 
impact the area this morning. Expect further development over the 
next several hours into more of an organized complex of storms, 
which will then track east southeast. At this time, it still 
appears that that the bulk of this development will stay just to 
the south of the terminals. Have continued current timing of -shra
with vcts with this in mind, however, it is still possible that 
slightly more thunder could be observed. Monitoring the potential 
to include at least a tempo for tsra, and am monitoring trends to 
see if a slightly sooner arrival time will be needed. Mostly dry 
conditions then expected later this afternoon through much of the 
end of the period. Will need to monitor later trends, as some 
isolated thunderstorm development could occur this afternoon. Low
confidence on this possibility at this time. Outside of precip 
and some fog development this morning, mainly VFR conditions 
expected. Winds will likely be highly variable, with the expected 
precip. A wind shift to more of a southerly direction is 
anticipated this morning ahead of the storms, and think this 
direction will be the more preferred direction through the period.