FXUS63 KLOT 212014

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
314 PM CDT Mon Aug 21 2017

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 18Z TAFs...

Wind trends are the initial focus followed by thunder potential,
primarily tonight. Earlier thunderstorm complex to the west has
decayed but wind fields remain disturbed as what's left of it
passes the terminals. Expect a period of west to southwest winds
at the Chicago area terminals early this afternoon before turning
south and even south-southeast. Rockford has already been easterly
and is trending southeast and will end up south-southeast or south
in the next hour or two. South to southwest winds will then
prevail this evening turning southwest late as a cold front
approaches. May need to bump us speeds slightly this evening and
tonight. Winds will then shift northwest late Tuesday morning
behind the front with speeds increasing somewhat with gusts to
near 20 kt.

Scattered showers and possibly an isolated storm will move across
the terminals through mid this afternoon. Some new development is
possible but the chance is low. Additional isolated activity is
possible into early this evening but the chance is also low.
Attention turns to late this evening and especially overnight as
the front approaches. Thunderstorms will become more likely with
time from about late evening onward with the best chance after
06z and probably closer to 08z as storms approach from the west.
Gusty winds may occur with the storms along with reduced
visibility which could briefly fall below the 2SM advertised in
the current TAFs. Showers may linger through daybreak but then
exit with MVFR clouds expected. Improvement to VFR is expected
through late morning.



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.






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