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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
626 am CDT Wed Oct 23 2019

Short term...
320 am CDT

Through Thursday...

Winds have begun to abate after several days of rather breezy
conditions, though we still have a bit of a pressure gradient
draped across the region supporting a modest southwest wind. A
peek out the window and at the satellite shows skies have, at
temporarily, cleared behind the extensive stratocumulus field that
gave yesterday that real fall feel and look.

Looking upstream, GOES water vapor imagery indicates a quick-
moving short wave trough digging southeast from northern plains.
This disturbance will shift eastward into the Midwest this
afternoon, with an associated area of weak surface low pressure
propagating across northern Illinois late in the day/evening. After a
sunny start to the early morning, clouds are expected to quickly
increase as warm air advection (waa) and associated ascent
develops ahead of the approaching short wave. While best chances
of precipitation look to focus from northern Iowa into southern WI,
various guidance does develop light showers/sprinkles into
northern Illinois from mid- morning Onward due to the strong warm air advection. .
guidance does highlight the mid- morning through mid-afternoon
period as having the greatest coverage, though persistent warm
advection and approach of the main mid- level trough/vorticity
axis supports some lingering potential into the late afternoon and
evening hours as well, especially across far northern Illinois near the
WI border. With some decent low level lapse rates and dry speckled
layers, a few of the showers could have locally gusty winds.

The disturbance associated frontal forcing will weaken and shift
off to the northeast overnight, and the surface cold front will
then push into central Illinois. However the upper jet will
remain pointed toward the lower/eastern Great Lakes through the
day Thursday. Initially the moisture advection will remain
shunted to the south, but a bit of this moisture may sneak into
areas along and southeast of I-57 Thursday night for another
narrow area of light rain.



Long term...
320 am CDT

Friday through Tuesday...

Height rises will ensue on Friday, and surface high pressure will
nose in and effectively shift the cold front, which is the main
precip focus area, well south of the area. The day will start with
cool readings in the 30s, maybe some sub freezing areas northwest
of Chicago. Skies will clear allowing for some warming, but it
will be fairly cool with highs on either side of 50. It appears we
could get readings back down into the lower 30s Friday night, but
we will get back into warm air advection and increased cloud cover ahead of fast
moving southern stream disturbance that may graze the area with
some light rain this weekend. Model handling of this disturbance
and its northward extent is not consistent. Saturday later
afternoon into early Sunday would be a favored time based on a
model mean.

There is still fairly good agreement in large ridge building a
ridge off to our west in the Gulf of Alaska, and this would allow
another system off the Pacific coast to dig into the
central/western US. This pattern typically supports at least some
periods of active weather across the central Continental U.S.. for this
reason we continue to monitor the possibilities for more
additional precip into next week. And depending how quickly/how
much the upper trough to our north digs and how it takes up
residence to our west still needs to be handled and this will
determine precip timing and precip type.



for the 12z tafs...

West-southwest winds with sporadic gusts to 20 knots are expected this morning
under increasing mid-level clouds. A low pressure system will track
east-northeast across northern Illinois late this afternoon and evening, with
the low passing near or over rfd around 00z. Isolated to scattered
rain showers are expected ahead of the low this afternoon for all sites,
though confidence on overall coverage is low. This is especially
true early in the afternoon as the initial -shra will encounter dry
air below the mid-level clouds. Winds will back to the S for the
Chicago Metro sites and south-southeast at rfd for the afternoon hours. There is
the potential that winds could become south-southeast at Ord/mdw/dpa for a few
hours during the late afternoon if the low tracks farther south than
currently forecast. However, some of the -shra could contain south-southwest
wind gusts to 25 knots as low-level winds above the surface increase
mid to late afternoon. The low-level winds will continue to increase
this evening and may result in low level wind shear for the Chicago Metro sites for
several hours before a cold front crosses the area late in the

MVFR ceilings will likely spread across the area behind the cold
front and possibly persist into Thursday morning.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...Winthrop Harbor to Gary in until 10 am

Small Craft Advisory...Gary to Michigan City in until 1 PM Wednesday.



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