Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152119

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
319 PM CST Wed Nov 15 2017

134 PM CST

Through Thursday...

At 100 pm this afternoon, a cold front was in place from near UGN
south-southwest to near PNT. Light rain and drizzle will continue
out ahead of the front but will quickly taper behind the front with
a few peeks of sunshine developing across the western CWA this
afternoon. The front should clear the eastern counties of the CWA by
around 4pm this afternoon with dry conditions in place through the
rest of the day. RAP/NAM low level moisture profiles indicate
that low stratus may thicken again overnight and prefer the
cloudier model solutions based on current cloud cover over MN and
northern IA which has been gradually spreading southeast through
the day. Bumped up overnight lows a touch due to the expected
increased cloud cover. Gusty west to northwest winds will pick up
behind the front this afternoon and evening, peaking with gusts to
around 30 kt through late this evening. Cold advection through
the overnight hours will help to maintain steep low level lapse
rates as well as some gustiness through the overnight hours.

High pressure will build across the mid and upper Mississippi Valley
midday Thursday then slide east across northern Illinois and
northwest Indiana through the afternoon. Winds will veer to the
north and diminish as this occurs, and eventually switch to an east
to southeasterly direction behind the ridge axis during the
afternoon. Expect afternoon highs to top out around 40F.



318 PM CST

Thursday night through Wednesday...

Ensembles and their deterministic components are in reasonable
agreement regarding the handling of progressive mid-latitude flow
through late in the weekend.  Friday night into Saturday would
appear to have the most active weather in this timeframe, though
still in the form of rain, perhaps with a bit of thunder as well. A
50 kt low level jet ahead of an approaching frontal zone will bring
seasonably mild temperatures and considerable moisture...greater
than one-inch PWATs...into the region beginning late Friday. In
fact, temperatures Friday night are looking warmer than during the
daytime on Friday and Saturday. Forecast soundings are mostly moist
adiabatic Friday night and not indicating a lot of instability, but
with the pronounced moisture convergence and warm nose around 850mb
it would not be surprising to generate some convection, especially
in southern portions of the forecast area.  The frontal zone should
be clear of the area, or nearly so, early Saturday, but rain chances
may linger into the afternoon under the passage of the upper
shortwave energy trailing the front.

Apart from the early weekend activity the extended period looks
relatively quiet under the influence of weak upper ridging.  Looking
out toward the middle of next week and the busy Thanksgiving travel
days, there is nothing at this point suggesting significant issues
for the Midwest, at least in terms of a large midlatitude cyclone
making its way through the area.



For the 18Z TAFs...

A cold front is advancing across northern Illinois late this
morning and early this afternoon with light rain, drizzle, and
IFR/LIFR conditions ahead of the front. Conditions will begin to
improve once the front passes through the area and winds swing
around to the northwest. Precipitation should end with fropa
allowing vsby to improve and expect a steady improvement of
ceilings back to MVFR. Northwest winds behind the front will
become gusty peaking in the mid to high 20 kt range mid afternoon
through this evening.

There may be a window where we improve to VFR and possibly clear
out this evening and overnight but a broad stratus deck is
currently spreading southeast across MN into northern IA and
expect this to continue into Illinois and Indiana overnight.
Ceilings will remain or fall back to MVFR and cannot rule out IFR
by Thursday morning. Winds are expected to become light out of the
north Thursday afternoon as a ridge builds across the Mississippi
Valley, then expect winds to become light easterly mid to late in
the afternoon as the ridge axis passes overhead.



318 PM CST

A fairly active pattern will produce multiple periods of gales
through the weekend.  The passage of a cold front today is being
followed by a period of northwest gales through mid-morning
tomorrow.  Winds then relax and veer to southeasterly and southerly
during the Thursday evening and overnight hours. The approach of
another cold front will tighten the gradient and support another
period of southerly gales from Friday morning into Friday evening.
Following this front the winds relax and veer to northwesterly
overnight Friday into early Saturday, eventually increasing again to
likely gales beginning Saturday afternoon and persisting into early



LM...Gale Warning...LMZ777-LMZ779 until 6 AM Thursday.

     Small Craft Advisory...IN nearshore waters until 11 PM Thursday.

     Small Craft Advisory...IL nearshore waters until 4 PM Thursday.




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