Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192007

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
307 PM CDT Wed Jul 19 2017

307 PM CDT

Through Thursday...

The short term forecast will be updated shortly.


306 PM CDT

Thursday night through Wednesday...

Biggest concern in the extended is convection that may lead to heavy
rain Friday and Friday night. Chances of storms continue through the
weekend before a dry period sets up for the first half of next week.

Expecting daytime convection to continue into Thursday evening/night
ahead of the cold front.  The cold front will stall over the far
southern portion of the forecast area.  Additional convection is
expected Friday. There will be two areas of focus: the stationary
front south of I-80 and an upper level disturbance shifting
northeast from Iowa into southern WI.  The GFS suggests PWAT values
will be 2-2.25 inches, which is well above climatology and near the
upper end of daily maxima for mid to late July.  Mid level flow is
around 40 MPH so while storms should be moving at a decent pace,
heavy downpours are likely.

The GFS appears to be capturing the current situation the best so
weighed the forecast in its direction. While thunderstorms are
possible across the forecast area Friday afternoon, think the best
chance will be south of a Rockford to Valparaiso line. However,
convection will spread to include all of the forecast area in the
evening, and an organized convective complex will likely shift
southeast through the region Friday night. This complex will lead to
additional heavy rain that parts of the area do not need.  Will
continue to highlight our thoughts about heavy rain in an ESF and
the hydrology section below.

Flow off the lake will likely keep temps along the lake in the 80s
while the rest of the region will be in the mid 80s to around 90.
Dewpoints in the 70s will lead to heat indices from 90-100 north
of I-80 to 100-106 south of I-80.

I have medium confidence in Saturday`s forecast as the convective
complex should still be overhead Saturday morning. Convection should
work over the atmosphere, and I am unsure how quickly we will be
able to recover Saturday afternoon. If we can recover, thinking the
best chance for additional storms will be along and east of I-55.
The hot, sticky conditions continue Saturday with highs in the mid
80s to the low 90s and heat indices in the mid 90s to up to 106

Cooler and drier air arrives Sunday with highs in the 80s. High
pressure slowly shifts east and reaches Lake Michigan Tuesday
afternoon. I have dry conditions in the forecast through at least
Tuesday night if not into Wednesday. Models are slowing the next
system mid week so Wednesday may be drier than currently forecast.



405 AM CDT

The Hydrologic Outlook (ESF) for along and north of I-88 has been
continued for concerns of multiple rounds of storms through early
this weekend. Observational trends are supporting the forecast in
terms of a very moist atmosphere over the area from now through
Saturday. The confidence on receiving a burst of heavy rain with
a quick-moving storm complex in far northern Illinois overnight
has increased, uncertainty still resides in exact track of this
system. Where the center of it passes certainly will have
potential for 1"-2" in around 1-2 hours. Regularly with such
quick movement, even if intense, these tend to produce widespread
averages more of 0.50"-1.00". If there were to be some slower
movement or backbulding, concerns would raise for flooding.

Wet antecedent conditions particularly over the Des Plaines and
Fox River basins provide concerns through the rest of the week as
storm chances inch back up Thursday night and into Friday.
Location of this rainfall is even more uncertain, however there
may be higher concern of flooding with that given it is a regime
that could favor training storms.



For the 18Z TAFs...

Main forecast concern is thunderstorm potential/timing tonight
into Thursday morning. Another concern will be timing of a lake
breeze/wind shift to northeast this afternoon and wind speed
following the lake breeze passage.

For this afternoon, the main concern will focus on the timing of a
wind shift and wind speeds following the passage of a weak cold
front/lake breeze boundary. Latest radar imagery shows the
boundary steadily pushing inland. Winds have been lgt/vrbl this
morning, but should shift to nely following the passage of the
boundary. There is not much push behind the boundary, so expect
that winds will remain under 10 kt following the passage of the
boundary, with prevailing 7-8 kt speeds likely, though an isold
gust up to 10-12 kt is possible for an hour or so following the
lake breeze passage.

The next concern will be thunderstorm trends tonight and tomorrow.
A complex of thunderstorms has developed over ern SD/swrn MN
this afternoon. This cluster of storms is expected to track
generally esely along a corridor of enhanced instability. The
various models, including the high res convection allowing models
and the larger scale models have a general trend of tracking the
thunderstorm complex into nrn IL late tonight, but are trending a
bit slower than previous runs. While there is a relatively decent
consensus on the general trends of the convective complex, there
are significant differences on the exact track as well as the ts
coverage of the system by the time it is close enough to impact
the terminals. So, the latest update has mainly to push back the
prob30 ts groups, but have maintained the prob30 mention due to
the continued uncertainty in the timing and coverage. some
guidance is also indicating the potential for secondary
development following the passage of the initial round of ts, so
have included a vcsh mention for the morning hours tomorrow,
though some isold ts is not out of the question. Thunderstorms
late tonight into early tomorrow morning will only be the first
round of thunderstorms to impact the area as a pattern continues
to evolve there multiple rounds of thunderstorms may impact the
region. Latest guidance suggests a second round of organized
convection moving into the area tomorrow afternoon as the weak
cold front/lake breeze boundary that pushed through the area this
afternoon lifts back north as a warm front with weak frontal
convergence lining up with the persistent corridor of instability.
So, have added a prob30 mention to the 24-30hr forecast period of
the ORD TAF.


306 PM CDT

Looking at a relatively quiet period over the lake through next
week. Weak high pressure currently over the lake will continue east
tonight. Winds become southerly behind the high early Thursday
morning and then continue to veer through the day. The baggy
pressure gradient will likely result in periods of variable wind
directions across the lake.

Another weak high moves over the lake Friday morning. Winds become
southeast by the evening, and speeds may increase to 15-25 kt over
the southern end of the lake. Guidance indicates the gradient may
increase over the southern end of the lake Friday evening, but do
not have a lot of confidence in the pattern.

Low pressure passes over the lake this weekend with westerly flow
becoming northerly across the lake by Monday morning. North winds may
reach 15-25 kt for a time Sunday afternoon into Monday. Northerly
flow stays over the lake until the high moves over the lake Tuesday
evening. Persistent northerly flow may result in waves hazardous to
small craft Sunday night into Monday.






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