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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
214 PM CDT sun may 28 2017

Short term...
151 PM CDT

Through Monday...

A cold front has pushed across northern Illinois and Northwest
Indiana earlier today with convection early this afternoon in the
vicinity of the front, primarily over northern Indiana, but
clipping Benton County. Behind the front, northwest winds have
overspread most of the County Warning Area with modest instability in place, but
weakly capped or uncapped per rap soundings. There will be two
areas to keep an eye on through the afternoon and evening for
potential showers and thunderstorms. The first will be associated
with the lake breeze in and around the Chicago Metro area this
afternoon. A strong thunderstorm has already developed on the
boundary and additional isolated showers and storms will be
possible through the afternoon. Farther west, a secondary surface
trough is evident in obs while a cu field has developed within the
vicinity of the trough that stretches from central WI into far northwest
Illinois an eastern Iowa. There is potential for isolated to widely
scattered convection as this area moves across the County Warning Area mid to late
this afternoon and into the early evening. With only 500-700 j/kg
MLCAPE and 20-30kt deep layer shear in place, severe threat is
not a big concern, but cannot rule out the potential for a few
storms to become strong. Cloud to ground lightning and gusty winds
would be a concern for any outdoor events.

Threat for precipitation should wane diurnally with dry conditions
expected overnight into Monday morning. Upper low is expected to
pivot across the western Great Lakes tomorrow while a sheared vort
lobe drops into northern Illinois. Forecast soundings tomorrow
are not too terribly different than today indicating steep low
level lapse rates with modest instability and little or no
convective inhibition. Given the favorable diurnal timing of the
shortwave, anticipate widely scattered showers and thunderstorms
will once again be a possibility. Temperatures will also be
fairly similar tomorrow with highs in the mid 70s. Deep mixing and
breezier conditions will be in place though, so there is a lower
likelihood of a lake breeze.



Long term...
213 PM CDT

Monday night through Sunday...

The upper level trough spreads over the region Monday night. Cooler
air will lead to lows in the low 50s. Soundings are pretty dry so
kept a dry forecast going after any lingering showers exit to the
east Monday evening. Another vorticity streamer rotates around the
upper level low and it may kick off showers and storms Tuesday
afternoon. Tuesday will also be cooler with highs around 70.

High pressure builds over the plains Tuesday night and Wednesday,
and the upper level low slowly shifts east. Wednesday will be
dry and cooler day with highs around 70.

My confidence is lower when it comes to thursday's forecast. The
GFS features a shortwave trough and convection spreading across the
region Thursday afternoon. The 12z emcwf, on the other hand, has a
much subtler wave, but does have a little quantitative precipitation forecast Thursday afternoon.
Decided to keep a chance of showers and storms in for Thursday, but
think the GFS is suffering from convective feedback. Warmer air
moves in aloft, but not sure how much cloud cover we will have, so
kept temps in the 70s.

Late this week looks active with many periods of showers and storms.
A low is forecast to form and move through the Midwest Saturday
night with another push of cooler air moving in behind it Sunday.



for the 18z tafs...

A cold front has pushed across the terminals today with northwest
winds picking up behind the front early this afternoon. Showers
and thunderstorms are expected to redevelop along the front this
afternoon from roughly a vpz to rzl to tip line and areas east. A
few isolated showers and thunderstorms may redevelop farther
northwest over Wisconsin and northern Illinois, passing near the
terminals, but coverage is expected to remain on the lower side
and confidence of precip on station is too low for more than a
vcsh mention at this time.

Other concern today will be the movement of a lake breeze that has
developed and pushed inland over Northwest Indiana...currently
through gyy and on the doorstep of mdw with veered winds
east/north of the boundary. Some of the high-res models do
initialize well with the boundary near mdw early to mid afternoon,
but with deeply mixed and somewhat breezy conditions in place,
leaning against any lake breeze passage or wind shift for Ord
which is supported by the models. Expect the boundary to
eventually retreat back over mdw with west winds resuming, though
confidence in timing is low.



213 PM CDT

Winds become west overnight and increase to 15-25 kt Monday. Issued
a Small Craft Advisory for gusty winds Monday for points north of
Gary, Indiana. Winds will be 15-25 kt east of Gary, in, but do not
have enough confidence in persistent 20-25 kt winds.

West to southwest winds of 15-25 kt are expected through Wednesday
night as a low over Ontario slowly drifts north to James Bay. High
pressure builds over the plains Tuesday night and then shifts south
of the lake Wednesday night. Winds become north behind a cold front



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743...10 am
Monday to 7 PM Monday.



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