Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220648

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
148 AM CDT Tue Aug 22 2017

831 PM CDT

Little change to going forecast this evening, though severe
threat continues to appear minimal in the wake in the wake of
extensive morning MCS cold pool and cloud cover across eastern IA
and northern IL/IN. SPC has removed the northern 2/3 and northwest
IN from the marginal risk for the remainder of the overnight

Evening surface analysis depicts weak surface low pressure over
the Upper Mississippi Valley, along a cold front which stretches
from the northern Great Lakes to the Central Plains. Strong low-
level baroclinic zone was noted across Missouri and downstate IL
however, in association with strong outflow boundary from MCS
which decayed across the region earlier in the day. Despite warm
and humid warm sector air mass ahead of the cold front, RAP
mesoanalysis and 00Z soundings indicate relatively stable/capped
near-surface conditions, with weak low-level lapse rates north of
the outflow boundary. Remnants of strong linear MCS over IA
continues to weaken as it propagates east-southeast toward the

While the threat of severe weather diminishes, especially across
northern parts of the cwa, shower and thunderstorm coverage should
continue to increase across the area through late evening as
forcing strengthens in association with a digging/amplifying upper
trough across the Upper Midwest. The resulting increase in
southwesterly low level jet, large scale height falls and the
increasingly favorable upper jet position is expected to allow
showers/storms to spread across the region overnight in advance of
the cold front. While some potential for isolated strong wind
gusts will persist across the southern tier or so of counties,
locally heavy rainfall looks to be the primary concern with
overnight storms. PW`s around 2 inches and southwesterly low level
jet parallel to the cold frontal zone may produce some
training/back-building in some spots which could produce isolated
rainfall amounts in excess of 1-1.50 inches. Thunderstorm
coverage should diminish from the north/northwest toward morning,
with the focus quickly shifting to the east/south of the forecast
area after sunrise.



313 PM CDT

Through Tuesday...

Main concern is showers and thunderstorms this evening and
overnight and associated risk for primarily heavy downpours and
localized flooding. Current subsident regime behind earlier
short-wave has yielded dry conditions for the time being this
afternoon. Fairly quiet conditions should continue through about
sunset. Thereafter, in the evening into the overnight, trend
should be for increasing coverage of showers and storms tied to
additional short-wave energy arriving from the Plains. Main wave
is tied to well defined MCS over NW IA, with another arriving from
KS/MO area. A signal on high-res and WRF guidance for a subtle
disturbance and perhaps better lower-level convergence could
provide an earlier evening focus for convection breaking out in
mainly NW Indiana (possibly extending into parts of NE IL).

Later in the evening and into the overnight as activity expands in
from the west, strengthening low level jet could help sustain more
robust showers and storms, with PWAT of 2-2.25" supportive of very
heavy downpours and even localized flooding in stronger storms
depending on residence time/training. Overall storm motions do
appear to be quick enough to preclude a more significant flooding
risk. Severe risk appears to be unfavorably timed diurnally, with
an isolated gusty/damaging wind risk (wet microburst/precip
loading) and deep layer shear possibly supportive of a hail risk
(marginally severe on the high end).

A cold front will sweep across the area on Tuesday, with lingering
isolated-scattered storms possible in the morning (12z-15z) and
focus shifting to mainly southeast of I-55 in the early-mid
afternoon for any additional showers/isolated storms immediately
tied to frontal passage. Clouds will clear from the northwest
behind the front in dry air advection and CAA, with breezy west
to northwest winds. Highs will reach the upper 70s to lower 80s.



241 PM CDT

Tuesday night through Monday...

Transition to drier and much cooler weather occurs mid-week and
lingers through the end of the week before a gradual warm-up
brings temps back to near normal for late August.

Surface cold front clears the forecast area to the southeast by
Tuesday evening, with breezy northwest winds bringing a cooler and
less humid air mass into the region. This trend persists through
the end of the week, as a long-wave upper trough deepens across
the eastern CONUS, and keeps us in deep northwest flow. Surface
high pressure spreads across the region during the period, though
models have been fairly consistent in depicting a mid-level short
wave which digs across the Upper Midwest and Great Lakes region
late Wednesday night into early Thursday, with a reinforcing push
of cooler air and a surface cold front passage. Guidance continues
to indicate the potential for some spotty showers early Thursday
as 850 mb temps drop to about +6C over the warm late-August waters
of Lake Michigan. Otherwise, dry conditions are expected to
persist into the weekend as the surface high strengthens across
the western Lakes.

925 mb temps support daytime highs in the low-mid 70`s, with
overnight lows dropping into the low-mid 50`s. A few upper 40`s
are possible in our typical cool spots away from the immediate
Chicago metro area and warmer lake front areas.

The eastern CONUS upper trough eventually is progged to drift off
to the east this weekend, with the surface high pressure ridge
also moving off and allowing winds to become more southeasterly.
Medium range solutions of the ECMWF and GFS both indicate a short
wave trough propagating from the Northern Plains southeast into
the Midwest Saturday night into Sunday, though there are
differences with the evolution beyond that time. The EC develops a
deep closed upper low over the area which lingers into the
following Monday, while the GFS is much less amplified. In general
however, a gradual moderation in low-level temps and a modest
increase in low-level moisture is likely.



For the 06Z TAFs...

Main forecast concern will be ts/pcpn trends, and associated
cig/vis impacts, for the remainder of the overnight hours. The
remainder of the day should be quiet.

Latest radar trends and short range guidance suggests now that the
greatest ts potential is along the leading edge of an old cold
pool/outflow from earlier thunderstorm complexes. The line of TS
is now south and east of all terminals, from north-central Indiana
through central Illinois. Latest obs invof the convective line
indicate nwly outflow gusts, suggesting that the line will
continue to sag south and east, and will be less of a threat to
the terminals. So, have removed all mention of ts from the TAFs.

An area of more stratform rain and showers associated with
remnants of convection over srn Iowa and east-central IL is still
spreading east under fast westerly flow aloft and the terminals
could still see some rain or showers for the remainder of the
overnight hours, but the potential for any additional ts is
diminishing as the main forcing conntinues to push to the
southeast. Cigs/vis could still be a concern with any additional
rain in an already very moist environment. Cigs/vis reduction
should generally be at mvfr levels.

There may be some lingering showers at daybreak, but conditions
should quickly dry out with cigs/vis impproving as high pressure
building into the northern plains spreads east across the middle
and upper Mississippi Valley. Winds will shift from sely early
this morning to nwly by mid day with ocnl gusts to 18-20 kt. The
nwly flow will usher in a cooler, drier air mass through the
afternoon hours and expect no significant operation impacts for
the remainder of the period as clear skies and light nwly winds
set up by early Tuesday evening and continue Tuesday night.


241 PM CDT

Main marine forecast concerns continue to be on a period of
northerly winds down the length of the lake Tuesday night into
Wednesday. While not overly strong, the long fetch will likely
result in small craft advisory conditions for the shores of
southern Lake Michigan.

In the near term, a cold front has become nearly stationary near
the north shore of the Lake in the Upper Peninsula. Low pressure
is expected to develop along this front over Wisconsin tonight,
moving northeast over far northern Lake Michigan early Tuesday
morning and trailing a cold front which will push down the lake
during the day. The low is expected to deepen later in the day as
it pulls away into Quebec, setting the stage for breezy northwest
winds across the lake. These north-northwest winds will continue
into Wednesday before weakening as high pressure spreads into the
region. The high eventually late Thursday, allowing winds to
diminish and waves to subside. The strongest winds, 20-25 kts, are
expected late Tuesday and Tuesday night, with small craft
advisory conditions expected along the southern shores of the lake
from Tuesday afternoon through Wednesday.



IN...Beach Hazards Statement...INZ002...1 PM Tuesday to 4 PM

LM...Small Craft Advisory...LMZ744-LMZ745...1 PM Tuesday to 4 PM




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