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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 170239

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
939 PM CDT Thu Aug 16 2018

902 PM CDT

Continue to monitor complex of storms pushing into areas
along/west of I-39 at this time. These storms have been slow to
reach the area, with a general weakening trend noted over the last
one to two hours. Although this weakening trend has been noted,
development continues at this time, and think this will likely
continue late this evening through the midnight time frame as 
they push further into north central IL. At this time, these
storms will be capable of producing gusty winds up to around 45 
MPH and heavy downpours.

These storms are developing in/around surface trough and front 
situated over western IL, north through IL and WI, with stronger 
mid/upper level flow in place ahead of an approaching upper level
low. As this upper level system continues to pivot southeast, a 
more favored location for continued support will slowly sag south 
of I-80 later tonight. So as the current complex near the I-39 
corridor continues further to the east over the next 2-3 hours, it
should see more of a diminishing trend. Do think some scattered 
showers/thunderstorms will reach the Chicago metro, but confidence
is low to the extent of coverage. Intensity during this time
should wane.

Once again, a more favored location for continued development
later tonight into early Friday morning will be south of I-80.
Already seeing this development occur at this time across west
central IL, with this development expected to spread east into 
the southern CWA tonight. Upper level flow and ascent will be 
stronger in this location, with instability axis situated across 
the area. This may support a continued threat for a stronger 
storm, with a wind threat the likely hazard. Any storm will also 
continue to produce heavy downpours, with some threat of localized
flooding remaining.



154 PM CDT

Through Tonight...

Slow moving showers and thunderstorms continue to develop this
afternoon, with greater coverage closer to the stronger upper 
trough across the plains. While there is no lighting in our area
yet, weaker shortwave forcing ahead of this low combined with 
1000-1500 J/Kg of MLCAPE and moist low- levels suggests scattered 
showers and some storms will continue and maybe expand a bit. 
Greater coverage is still anticipated over Wisconsin and in Iowa.
Shear is low, and thus severe weather is

As the main upper low over Iowa and a lead wave ahead of this 
move over the area this evening into overnight, festering or 
redeveloping showers and some storms are probable, though low 
confidence on what coverage would be. The mid-level jet associated
with the primary upper low will rotate eastward across central 
Illinois later this evening into overnight, and this is where 
there is better potential for some organized storms and wet 
microburst potential, though cannot rule it out into parts of the 
western and southern CWA.

Guidance is not hitting fog and low clouds too hard, but with the
weak wind field there may be pockets of it around tonight also
given the moist conditions and weak wind field. 



259 PM CDT

Friday night through Thursday...

On Friday, the slow-moving upper low will track across northern IL 
in the morning and then lift northeast across lower Michigan during 
the afternoon hours.  As the low lifts newd tomorrow afternoon, it 
will weaken to an open trough with an axis extending into the lower 
Ohio Valley by Friday evening.  There will be a continued chance for 
sct showers and a few isolated thunderstorms across the area, with 
the greatest chance for any thunderstorms likely to have a diurnal 
trend and should be more likely in the afternoon to early evening 
hours.  General conditions for Friday will not be a whole lot 
different than today, though temperatures should be a few degrees 
lower as the upper low moves overhead.  As the upper low and 
associated sfc low lift into lower Michigan, surface winds will 
trend to nely/nly, helping to keep lakefront locations a bit lower 
than inland area.  Highs Friday should be generally be arnd 80 F, 
with upper 70s along the lake front.  By Friday evening, any 
lingering showers should come to an end and cloud cover should be on 
the decrease as upper ridging begins to build across the region. 
Through the weekend, warmer, dry conditions are expected as the 
upper ridge continues to build across the upper Midwest and Western 
Great Lakes regions.  Temperatures should rebound back into the 
middle 80s for much of the area.  However, broad sfc high pressure 
pressure and a very weak pressure gradient will be in place, so lake 
breeze development is likely both Saturday and Sunday, limiting 
highs along the lake front to the upper 70s.

The next significant chance for pcpn will come on Monday as a nrn 
stream shortwave dropping out of the nrn plains phases with a srn 
stream shortwave lifting out of the south high plains.  the phasing 
shortwaves will lead to a deepening sfc low tracking across the 
Middle Mississippi Valley on Monday.  Timing of the pcpn spreading 
from west to east across the region will be dependent on the timing 
of the shortwave phasing and sfc low development, but the general 
idea from the longer range guidance is pointing to increasing 
coverage of showers and thunderstorms across the region through the 
day on Monday and into Monday evening.  Following the passage of 
this system, a return to drier, but cooler conditions is expected 
for the middle of next week.  From Tuesday through the Thursday, 
highs are expected to only be in the 70s.


For the 00Z TAFs...

649 pm...Main forecast concern is the chance of thunderstorms 
through Friday morning.

An upper level low over IA will slowly move across northern IL on
Friday morning. Current thunderstorm activity near the IL/WI state
line and west of the terminals is struggling to move into
northeast IL with less favorable conditions. Eventually...however
there is expected to be a slow increase in thunderstorm activity 
by mid/late evening but confidence regarding location is low. Its 
possible that thunderstorms affect northwest IL and rfd and then 
pivot southeast...mainly south of the Chicago terminals. From this
distance have opted for vicinity mention for the Chicago 
terminals from mid evening through early Friday morning. Changes 
to this timing are possible as trends emerge.

As the upper low crosses the southern end of Lake Michigan midday
Friday...there is some potential for additional showers or
thunderstorms after sunrise across northeast IL. Confidence is low
for this time period and trends will need to be monitored
overnight for possible thunder mention for Friday morning.

East winds will continue behind the lake breeze at ord/mdw early
this evening and then light and variable winds are expected at all
locations tonight with north/northeast winds developing Friday
morning. Some light fog is possible overnight/into Friday morning
along with ifr/low mvfr cigs. The most favored locations currently
appear to be mainly along/south of a rfd/dpa/igq line. cms


259 PM CDT

A weak trough moving south of Lake Michigan has resulted in light 
onshore flow over southern Lake Michigan and more brisk northeast to 
north flow across the northern portions of the lake. North winds 
will overspread the entire lake overnight tonight as low pressure 
lifts through Lower Michigan. North flow is expected to persist 
through the day on Saturday. High pressure will build across the 
region Saturday night into Sunday with southerly flow returning 
Sunday night into Monday. A cold front is expected to move down the 
lake Monday night and is expected to usher in strong northerly winds 
by Tuesday. Winds could be near gale force through the day and are 
expected to diminish overnight.