Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190724

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

911 PM CDT

It appears a modest MCV had driven convection from eastern IA
into western IL. With only modest shear, diurnally diminishing
surface based and ML instability, and limited forcing, the storms
that had taken aim on areas west of I-39 in northwest IL has
quickly lost steam. Outflow from the decaying storms could drive
some isolated/scattered showers and storms into areas north of
I-80 but now more likely to remain outside of the Chicago metro
this evening. With higher instability farther south, the
convection has remained more robust in west central Illinois.
Would still expect an overall weakening trend, but will need to
monitor radar trends for additional updates.

Overnight into Wednesday morning, the focus turns to storms from
northeast IA to across northern and central Lake Michigan
associated with a weak cold front boundary. Am having doubts on
much of this convection making it into the CWA, and anticipate
what does make it near or into far northern Illinois to be weaker
than it is now. This will be due to unfavorable diurnal timing
(diminishing instability) and lack of a strong low level jet per
00z RAOBS to help storms tap into and sustain with available
elevated instability. With this in mind, kept PoPs mainly in lower
chance range for areas mainly north of I-80 after midnight into
early-mid Wednesday morning. Do not expect any severe risk with
storms that do make it into northern IL. A warm and muggy night
is in store otherwise in store. The cold frontal boundary could
take on some lake enhancement Wednesday morning, with a north-
northeast wind shift possibly taking place during the morning.
This will keep shore areas comfortably cooler on Wednesday than
the heat and humidity inland, especially southwest CWA.

The rest of Wednesday, with a lack of focus/forcing, it appears
the threat for any additional storms is small (slight chance),
with a much higher threat possible overnight into Thursday with a
potential robust MCS.



218 PM CDT

Through Wednesday...

For the remainder of the afternoon and through this evening,
increasingly warmer, more humid conditions are setting up as high
pressure over the region begins to slide to the east and prevailing
southerly to southeasterly flow sets up in the lower levels.  The
main forecast concern this afternoon will be the inland progression
of a lake breeze boundary.  Winds have turned onshore along Lake
Michigan, with cooler air spreading inland.  latest high res
guidance suggests that the lake breeze may reach inland as far as
O`Hare and Midway and has already pushed through Waukegan.
Temperatures this afternoon should top out in the upper 80s, with a
few locations possibly pegging the 90 F mark.  With the lake breeze
pushing inland, the lake front will only see temps in the middle to
upper 70s.

The next concern will be timing of chances of showers and
thunderstorms late tonight.  This activity should be along and in
advance of a cold front, which extends from western Lake Superior
through the upper Missouri Valley.  The longer range guidance is
relatively consistent in slowly pushing the front southward
overnight, reaching the Rockford area by around midnight and then
continuing south and east, pushing south of the CWA durg the early
morning hours.  There should be a fairly widespread area of showers
and thunderstorms in advance of the front, in a zone of strong
warm/moist advection, but the front will be pushing through nrn IL
and into nrn Indiana durg the overnight to early morning hours,
which would be a diurnally unfavorable time for frontal convection.
However, while confidence in timing and coverage of thunderstorms is
relatively low, currently feel that there will likely be at least
scattered showers and thunderstorms overnight.  The front should
stall out across the CWA, and then lift back north again as low
pressure deepens over the northern plains.  With upper ridging
building across the upper midwest tomorrow, temperatures will likely
be at least a couple degrees higher, with a more widespread area see
90+ degrees.  Weak onshore flow persisting through the day should
keep the lakefront cooler, with highs likely only reaching the upper
70s along the IL lakefront and low 80s along the nrn IN lakefront.
With sfc dewpoints expected to climb into the lower 70s, heat index
reading may reach or exceed 100 F, especially for locations south of
I-80, where temps and dewpoints are likely to be highest.


311 PM CDT

Wednesday night through Tuesday...

Main forecast concerns/challenges are with periodic thunderstorm
chances beginning from the start of the period into the weekend,
with the possibility of severe weather, heavy rainfall and
flooding. Hot and humid conditions will also be possible by the
end of the work week into the start of the weekend.

Confidence continues to be on the lower side with the exact
details, however, overall setup is going to favor periodic
thunderstorm development across the region. Ridging across the
central CONUS will provide a setup for periodic waves of energy to
stream across the region into the weekend. Also, instability and
moisture axis shift back overhead, with strong instability and
high PWATS of around 2 inches a definitely possibility. Timing and
placement of this possible forcing will be key with gaining a
better understanding of where the thunderstorm complexes will
track. Guidance continuing to focus in on another mid level wave
moving throughout the region late Wednesday. MCS development would
be a likely scenario from this forcing, with any MCS then
progressively moving through the region. Guidance still varying on
exactly where this will track with some guidance keeping the
complex of storms to the north, while other guidance pushing it
into northern Illinois by early Thursday morning. Once again
instability/moisture axis is forecast to be situated right into
northern Illinois. This setup would support a more southerly
track, with the possibility for any storms to move through the
instability axis into the CWA. So, am thinking these thunderstorms
will have decent chance of getting into northern Illinois and
northwest Indiana, but still have lower confidence with exact
placement. This is the first period to keep note of as severe
weather and heavy rainfall will be possible. This will be
especially troublesome in terms of the heavy rainfall and possible
flooding, if this were to track through areas north of I-88 where
very wet conditions are in place and flooding/elevated rivers are
already occurring. A warm day expected Thursday but convection
trends will control the extent of this warmth. Air mass will be
supportive of highs around 90 and with dewpoints likely reaching
well into the 70s, heat indices above 100 still appear possible
particularly across the southern CWA.

MCS development appears possible once again Thursday night into
Friday, with the CWA remaining in the general track of any
approaching storms. Still lower confidence with exact trends, but
still another period to monitor as setup would favor additional
thunderstorms with heavy rainfall, and possible severe weather.
Despite this lower confidence, several synoptic features in place
that would support wetter conditions over dry conditions. No real
big change in the pattern into the start of the weekend, with
additional chances of storms. It`s not until late in the weekend
into early next week that a change in the pattern is possible.



315 PM CDT

Continued Hydrologic Outlook (ESF) for concerns of multiple
rounds of storms in an atmosphere that favors heavy rainfall, and
wet antecedent conditions particularly over the Des Plaines and
Fox River basins. Precipitable waters will be consistently above
1.60" for much of Wednesday night-Saturday night and so the
moisture will be present with fairly high confidence. The
uncertainty resides in placement and timing of storms, which given
the forecast weather pattern, is a particularly high uncertainty.
Nonetheless, computer models have indicated that the general
southern Wisconsin/northern Illinois region should see storms late
Wednesday night into early Thursday, and then possibly additional
multiple rounds Thursday night into Friday. These could yield
quite a gradient in rainfall amounts with much lower south (south
of I-88) to higher north.

The bottom line is that there are a lot of storm chances coming
up in an environment that can produce copious rainfall, with a
few periods of likely storms within that probable as the time
period draws nearer.



For the 06Z TAFs...

1236 am...Two main forecast concerns this period which are
thunderstorm chances/timing and a wind shift/boundary to the
northeast mid/late this morning. Additional concerns are light
fog and mvfr cigs.

Convection over southern WI is slowly weakening and short term
guidance is in good agreement showing this trend so not expecting
any activity from southern WI to move south into northern IL early
this morning. Additional showers with perhaps some thunder is
festering over west central IL. Low confidence regarding what this
activity will do but its likely to drift east/southeast and remain
south of the terminals and may begin to dissipate in the next few
hours. Nevertheless...this will need to be monitored for the
potential of some showers later this morning. Isolated showers and
thunderstorms may develop early/mid this afternoon along a frontal
boundary which will be moving southwest and away from the Chicago
area terminals. If anything developed it would most likely remain
southwest of ord/mdw but confidence of anything developing is low.
Focus then shifts to the upper midwest this afternoon with new
thunderstorm development that will turn southeast and move toward
northern IL after 06z early Thursday morning...thus only possibly
affecting the 30 hr ord taf. While thunderstorms near the Chicago
terminals by 12z Thursday morning are possible...have stayed dry
with this forecast to watch trends early this morning. Some
thunder mention in all the tafs around/just after sunrise Thursday
morning is possible with later forecasts...though confidence
regarding timing/coverage remains low.

A wind shift/weak cold front will move south later this morning
and likely be enhanced by the lake. Southwest winds to start will
turn more west/northwest after sunrise and may continue turning
more northerly but a shift to the northeast is expected by
mid/late morning. High confidence for this shift...but only medium
confidence for timing...thus some tweaks to timing can be expected
with later forecasts. Winds will remain east/northeast into this
evening then are expected to shift light southerly later tonight
into Thursday morning...though outflow from any thunderstorms
Thursday morning could shift winds northeasterly.

A dissipating outflow boundary from convection earlier this
evening across eastern IA is moving across the area currently. A
brief increase in southwesterly winds to 10-15kts is occurring
along with a narrow ribbon of scattered 3kft clouds. These winds
and clouds should be brief. Mid/high clouds are expected through
the period but a period of mvfr cigs is possible later this
morning as the boundary/wind shift moves across the terminals.
Models are also hinting as some light fog by sunrise this morning
while this is possible confidence is low so will need to monitor
trends early 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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