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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 171616

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1116 AM CDT Sat Aug 17 2019

1115 AM CDT

Minor updates this morning as a wing of showers and thunderstorms 
progress northeastward across northeast IL and northwest IN. This
potential continues into the early afternoon. While no severe 
weather is imminent, can't entirely rule out a near-severe gust 
and small hail through the remainder of the morning.

Behind this initial wave, satellite/radar trends along with many 
hi-res CAMs suggest somewhat of a lull in activity early-mid 
afternoon. However, by mid-late afternoon, focus turns to another 
incoming wave (currently across southeast IA/northeast MO), once 
again increasing the chance for scattered showers and 
thunderstorms across much of the forecast area. The most likely 
time frame for this development looks to be between 20z - 00z 
before activity exits east of the CWA leaving another break in the
action for much of the evening hours. Once again, a non-zero
threat that any additional convection later this afternoon may 
include gusty winds and hail.

Heading into tonight, latest trends continue to suggest convective
development across eastern IA/northwest IL before rolling eastward
toward the forecast area after midnight. Will continue to monitor
that threat through the afternoon with additional updates to 



323 AM CDT

Through Today...

Main forecast challenges/concerns are with likely isolated 
thunderstorms early this morning for areas along and south of 
I-80, and then additional more widespread showers and storms 
expected later this morning through midday for areas south of I-88
in IL and northwest IN.

In the near term, have observed a few severe storms with large 
hail and damaging winds develop just south of I-80. These storms 
are focused right along surface/lower level boundary situated 
across the area, where higher moisture/dewpoint air is pooling. 
Large scale support has been in the form of a fairly stout mid 
level vort swinging right through the middle of the CWA. Expect 
the current warned storms to persist as the move east into 
northern IN over the next hour, as this large scale forcing slides
east. The high shear environment with steepening mid level lapse 
rates will continue to support the threat of large hail and 
damaging winds to around 70 mph, especially as these storms are 
forming along the MLCAPE axis with limited to no CIN and with even
some higher DCAPE noted over the last hour. As these storms shift
east of the area over the next hour, may see a lull in the 
overall precip early this morning across the area. Can't rule some
isolated showers or storms, but should see the intensity of any 
storms be weaker with limited forcing/focus.

Turn attention towards upstream precip and focus, which will 
likely provide additional thunderstorm chances for a good portion 
of the CWA through midday. Upstream mid levels remain highly 
active and energetic per latest satellite trends, and with the 
abundance of convection noted across the central Plains. Several 
well defined waves/mid level speed maxes noted on satellite will 
track east through early to mid morning. Previously mentioned 
boundary which is in place from IN/IL back into northern MO will 
once again be a focus for blossoming thunderstorms across the 
region, with large scale forcing once again increasing and with 
low/mid level WAA focusing more towards our area. This blossoming 
thunderstorm development will be along and ahead of the current 
storms over MO, with latest trends and short term guidance 
providing higher confidence of these storms/development to work 
their way into the southern CWA in the 8- 10 am time frame. As 
these storms move east this morning, they will likely encounter 
increasing ascent to the north, and it's appearing highly likely 
that northward development will occur and impact areas further to 
the north in the CWA. These storms will likely reach the I-80 to 
I-88 corridor by mid morning, with most locations along/south of 
I-88 observing showers and thunderstorms. Confidence lowers with 
how far north these storms will reach, however, it's possible that
locations just north of I-88 in IL also briefly observe this 
arching line of storms by mid to late morning. Confidence with 
intensity of these anticipated storms later this morning is 
somewhat lower, but given trends right now, think the threat of 
strong to severe storms is definitely there. The environment will 
likely not change, with high shear and steep lapse rates once 
again supporting organized development. Large hail and damaging 
winds of at least 60 mph will be possible with any storm through 
mid/late morning.

These storms should move east of the area by early to mid 
afternoon with most locations drying out. However, additional 
energy is expected to once again move overhead this afternoon, 
with at least isolated to widely scattered storms possible across 
far northeast IL and northwest IN. Confidence on this development 
and intensity is even lower, but with with plenty of moisture and 
instability still remaining overhead, do think a strong to severe 
storm or two will again be possible this afternoon into early 



347 AM CDT

Tonight through Friday...

Pattern will remain active at the start of the period, with 
additional widespread thunderstorms appearing likely late tonight 
into early Sunday morning. Seemingly no real change to the overall
pattern/environment tonight, with highly active/energetic mid 
levels and a strong LLJ supporting strong WAA across the region 
still expected to be in place. Will also see increasing 
instability/moisture during this time, which will all support 
thunderstorm development tonight. There may be some isolated to 
widely scattered thunderstorms across the area this evening, but 
don't expect more widespread storms to arrive until after 
midnight. Expect a complex of storms to develop across IA this 
evening, and then track east into northern IL and northwest IN 
later in the night into early Sunday morning. Most guidance in 
good agreement with this overall solution, though some do vary 
with coverage and intensity of the approaching storms. Do feel 
fairly confident that most locations across the CWA will observe 
showers/thunderstorms, but with some lower confidence on 
intensity. However, with the expected environment and recent 
trends, feel that a more probable solution will be for a severe 
threat for much of the CWA and will continue to message this. Hail
will be a threat, but with damaging winds more of a likely 
threat. The high PWAT air and higher coverage of storms does 
provide some concerns for heavy downpours with at least isolated 

Should see these storms exit Sunday morning, with some dry 
conditions expected during the day. With lingering surface 
trough/boundary across the region, additional thunderstorms will 
be possible Sunday night. The pattern expected at the start of 
next week will support warm/humid conditions with highs in the 
upper 80s to around 90. Expect periodic thunderstorm chances 
during this time, with MCS development appearing probable across 
the region. Should see a front swing through by midweek and bring 
a change to the pattern, with cooler/drier conditions expected at 
least through the end of the work week.



For the 12Z TAFs...

Numerous aviation weather concerns over the next 24-30 hours (and
beyond), with, in general, fairly low confidence especially with
respect to timing of ts at the terminals. 

The main concerns revolve around:

-Convective potential mid-late morning and timing/coverage of
-Additional shower/thunderstorm potential this afternoon.
-Robust convective development very late tonight and into tomorrow

Latest satellite imagery continues to show a series of impulses 
upstream embedded within a ribbon of fast mid and upper-level
flow. One wave is enhancing thunderstorm activity moving through
central IL and gradually building to the northeast. While this
particular area of thunderstorms is not likely to impact the
terminals, another, strong isentropic lift aloft is generating and
east-west oriented line of convection is filling in from SQI-GYY.
Individual cells along the line are moving to the east at arnd 30
kt, while the entire line is slowly translating to the north. 
Bast on the latest trends, MDW/GYY will be the first of the 
terminals to be impacted by this line and certainly cannot rule 
out the potential for the line to translate farther north, 
reaching ORD/DPA/RFD. Main adjustment to the TAFs is to go with 
prevailing shra and tempo tsra for the late morning/early 
afternoon hours. Given some uncertainty on how solid that the line
will fill. There is the possibility that prevailing tsra may be 
needed, at least for a short time.

This activity should then move east of the area by early 
afternoon. Confidence continues to drop thereafter with potential
subsidence behind this wave, tempering afternoon convective 
chances. Alternatively, yet another series of waves currently over
the Central Plains could provide the impetus for additional 

Finally, robust convection is expected to develop across our
region very late tonight and into tomorrow, some of which could 
be strong to severe. Given the extended range, have gone with just
a prob30 mention for TSRA for some of the morning hours, which
could be extended later into the afternoon.