Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191135

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

335 AM CDT

Through Thursday Morning...

Forecast concerns in the short term center on potential storm
complex moving into the area tonight, and with it an attendant
wind and heavy rain threat. Have expanded the likely thunderstorm
mention in the forecast for overnight in northern Illinois north
of I-80/88 and boosted rainfall amounts some. Still modest to high
uncertainty for 24 hours out in the more precise path of this
storm complex from the Upper Mississippi Valley toward our region,
as well as the rate of any weakening overnight/early Thursday
morning. Also, while this would produce a period of very heavy
rainfall, confidence in this propagating quick is higher, so that
will limit duration of such rain.

The humid air has invaded the area early this morning with dew
points generally in the upper 60s/lower 70s. A surface trough/weak
cool front is draped across southern Wisconsin and will move
slowly southward this morning, and become stationary this
afternoon as it re-orients as a warm front. With very rich
moisture in the low-levels (850mb-surface), spotty storms are
possible this morning and afternoon. The focus this afternoon will
be primarily along this boundary. Convergence looks to be quite
weak during peak heating, however given the moisture and mixed
layer CAPEs (1500 J/kg), it may not be too difficult to trigger
a couple storms.

Temperatures today will depend on that boundary location, with
northeast winds expected northeast of this. The boundary is
presently forecast to be near a Rockford-to-Valparaiso line, with
mid 80s northeast (upper 70s lakeside), and around 90 south. With
that forecast, heat index readings in the south/southwest will
dabble around 100.

With regard to storms overnight, initiation of storms is expected
in the eastern Dakotas/southern Minnesota region this afternoon
ahead of a potent short wave on the northern periphery of the
strong, summer ridge. While the upper jet of 115+ kt moves nearly
due east and the mean layer winds are closer to due east, expect
that the forecast organized storm complex will ride the
instability gradient, taking it more southeast. This means into
or clipping northern Illinois overnight. In addition, Corfidi
vectors which take into account replenishing inflow, take the
complex more southeast on all guidance. So have followed closer to
the 19.00 GFS and NAM solutions, with highest PoPs in northeast
Illinois (east of I-39 and north of I-80). While the HRRR
solutions are bringing the storm complex into northern Illinois by
late evening, feel they are too quick, and favor more so after

The maintenance parameters for this mesoscale convective system
(deep layer shear, shear balance, instability, low-level jet) all
favor the MCS to hold together fairly well to at least near the
northern CWA edge. A damaging wind threat does exist with this as
far southeast as our area especially if the system accelerates.
The plethora of CAMs generally indicate a well-defined both at
least to our northwest, and some into our area. For rainfall, see
the hydrology discussion for more.



346 AM CDT

Thursday Afternoon Through Tuesday...

Certainly for Thursday afternoon and night, and likely even into
Friday, will depend on how the overnight MCS pans out, as its
associated outflow/modulated frontal boundary will be key and have
a domino effect lasting 24-48 hours. Not only will this impact
storm chances and focus areas but also temperatures, especially

Assuming the MCS has some in-roads into the CWA late tonight, this
lowers confidence quite a bit in upper 80s to lower 90s on
Thursday. It will not take much recovery time though if we do
have morning clouds/cooler air, as the thermal ridge of 850mb
temperatures above 20C will be directly over the area. If the
southern CWA is not affected at all, there could be mid 90s and
near heat advisory.

As the boundary begins to move back northward, most likely on
Thursday night, the baroclinic zone will serve as a focus for
isolated/scattered storms. These would have a heavy rain/training
concern, though very difficult to say where in the region that
would be. For Friday, a stronger short wave and associated speed
max in the westerlies is forecast and that would likely yield more
storm coverage and robustness into Friday night. Presently this is
in southern Wisconsin/far northern Illinois, and if that were to
pan out verbatim there would be heightened flooding concerns, but
confidence remains lower. The boundary is forecast to shift into
the area during the start of the weekend and clear the CWA by the
middle to end of the weekend. A lot of uncertainty in this period,
but needless to say, thunderstorm chances maintained.

For late week heat, the northern extent of the highest
temperatures will depend heavily on convective influence. The
high dew points will be found areawide, and may even be a little
low especially Friday into Saturday, as moisture pooling ahead of
the boundary could occur. Also with crops now fully mature,
evapotranspiration could boost surface moisture a bit further than
a guidance blend indicates. The southern/southwestern forecast
area may need a heat advisory for Friday and possibly Saturday
based on the current forecast (heat index readings 103-107). As
for Rockford to Chicago, less confidence exists in uninhibited
heating for temperatures to reach 90 and heat index readings to
exceed 100.



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 12Z TAFs...

635 am...Main forecast concern is thunderstorm potential/timing
late this evening into Thursday morning. Additional concerns
include cloud heights this morning and a lake breeze/wind shift
to northeast late this morning/early this afternoon.

A complex of thunderstorms is expected to develop near the SD/MN
border mid/late this afternoon. Fast upper level winds will likely
move this complex quickly to the east/southeast through mid
evening. Short term/Hi res guidance show more of a turn to the
southeast then the regional/global models and are also faster.
Have followed this trend with the expectation that this complex
will turn southeast around the edge of the upper ridge just
southwest of the area. These thunderstorms should be in a gradual
weakening phase but may still have a strong outflow boundary.
However...confidence in this solution is only medium and its
possible this activity dives further south...west of the terminals
or moves more easterly...north of the terminals. Thus...opted to
use just prob mention with this forecast and later forecasts can
adjust accordingly as trends emerge. There is also uncertainty
regarding any additional showers or thunderstorms that may
develop/fester behind this expected first line.

Westerly winds to start will slowly turn more northwesterly this
morning and are still expected to shift northeast but confidence
regarding timing and wind speeds has decreased. What was looking
like a front is now shaping up to be a lake breeze which will
likely be later than previously expected. Maintained 17z timing in
this forecast for ord/mdw but tweaks/refinement can be expected
with later forecasts. Wind speeds may briefly reach 10kts with the
lake breeze but prevailing speeds will likely remain under 10kts.
Wind speeds will turn more easterly by late afternoon and then
southeasterly this evening.

There is a ribbon of ifr/mvfr cigs across southern WI that will
likely drift south into far northern IL but now that heating has
begun with daylight...confidence is low regarding mvfr cigs or
lower at the terminals. Trends will need to be monitored and its
possible some mvfr cigs may develop before lifting above 3kft
later this morning. cms



224 am...A weak cold front will move south across Lake Michigan
this morning shifting winds northerly. But this shift will be
brief as winds turn back southerly over northern and central
portions of the lake this evening. Exact wind directions become
problematic late this evening and overnight with the expectation
that thunderstorms will be moving across southern portions of the
lake and will likely cause erratic winds with outflows. Thus...
confidence is low for wind directions...particularly on the
southern part of the lake. A weak area of high pressure is
expected to move across the northern and central Great Lakes
region Thursday night and Friday as a broad area of low pressure
develops over the northern and central plains. This low will move
across the Great Lakes region on Sunday as high pressure builds
across the upper midwest Sunday night into Monday.

Areas of fog have developed over northern portions of Lake
Michigan early this morning and these appear like they will
continue and possibly move south as the front moves south this
morning. Confidence is low...especially regarding how dense this
fog may become so trends will need to be monitored. cms





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