Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 232007
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
307 PM CDT Fri Jun 23 2017

.SHORT TERM...
141 PM CDT

Through Saturday...

KLOT radar getting a breather this afternoon as a cold front has
pushed showers and storms southeast of the area. Comfortable
dewpoints have fallen considerably, with near 50s north to around
60 south with just a few high clouds, and temperatures in the mid
70s to near 80 makes for a fairly pleasant Friday afternoon.
Meanwhile aloft, the upper air pattern features a low over James
Bay, with another low near the Manitoba lakes region. Several
shortwave disturbances ahead of the latter low will continue in
the westerly flow across the plains. The first disturbance will
lead to clouds and some showers upstream this afternoon, but its
arrival later this afternoon/early evening is not favorable for
much in northeast IL or northwest IN more than some light showers
or sprinkles farther west as higher surface pressures will spread
i.

Expect a quiet night with clearing skies. Winds will hold out of
the west as the surface high remains well off to our west, and low
pressure will shift east through the Ohio Valley and towards the
Atlantic. The upper low across Manitoba will spread southeast
through the upper Midwest on Saturday morning, then to the western
Great Lakes late in the day. Fairly cold air aloft for late June
will allow some very steep low level lapse rates to set up. Expect
some later morning to early afternoon cumulus development and
likely some breezy west-northwest winds as the surface pressure
pattern remains the same and decent mixing will occur due to the
steep lapse rates. Forecast soundings do depict a bit of an
inversion around 850 mb or so, which would suggest any showers
that form that would be isolated, shallow and largely void of
thunder. Otherwise, seasonally cool temperatures will top off in
the low to mid 70s, with cool dewpoints dropping into the 40s in
the afternoon.

KMD

&&

.LONG TERM...
259 PM CDT

Saturday night through Friday...

For the longer term forecast period, general trends remain
consistent with previous runs and the short term forecast period
trends.

By Saturday night, the longwave pattern will continue to trend to a
high amplitude, low wavelength, slowly progressive longwave pattern.
broad upper troughing east of the Rockies and strong upper ridging
over the west coast.  The general trend through the weekend and into
early next week will be for well below normal temperatures and low
precipitation chances.  A series of weak shortwaves will drop
through the nwly flow aloft helping to deepen the upper trough
Sunday and Monday and max temperatures will be the lowest of the
period with highs only in the lower 70s.  It is possible that some
locations may only reach the upper 60s.  Highs in the upper 60s to
lower 70s would be 10 to 15 degrees below seasonal normals, which
are in the lower 80s for late June.  With Broad cyclonic flow aloft
persisting across the region through early next week, there will be
periodic chances for scattered showers or a few isolated
thunderstorms, particularly in the afternoon hours.  By Tuesday and
through the remainder of next week, the upper ridge will begin to
progress east, with heights building across the region.  At the
surface, the high pressure that will cover the region through the
weekend and early next week, will shift east, setting up a return
flow of warm/moist air.  A warming trend will begin on Tuesday with
highs increasing to the upper 70s.  The warming trend will continue
through the week, with temps rebounding back to the lower to middle
80s.  The next significant chance for precipitation will come by
Wednesday night into Thursday as low pressure develops over the
northern plains and tracks to the western Great Lakes.  Increasing
warm/moist advection in advance of this system will bring increasing
chances for showers and thunderstorms.

&&

.AVIATION...
For the 18Z TAFs...

Concerns revolve around wind directions around
ORD/MDW. Otherwise conditions will be fairly benign.

The lake influence from the cold front has eased, and ORD/MDW have
settled back to northwest wind direction. Winds at the lake front
do slacken and some form of a lake breeze has formed with
Northerly Island currently NE, but most locations hold the NW wind.
Current high res- guidance keeps the lake breeze out of ORD, and
most guidance keeps it out of MDW as well, though MDW could
briefly jump over to NE this afternoon.

Upper level low pressure across the northern plains will spread
some clouds and showers to NW IL and SW WI, but our sites remain
dry. After clearing later today/tonight, the upper low will push
to the western Great Lakes. Expect some afternoon stratocumulus
clouds, with a low chance of a shower.

Otherwise, high pressure will remain across the plains while low
pressure will shift across the Ohio Valley and to the Atlantic
coast on Saturday. This will maintain modest west winds through
the remainder of the TAF period.

KMD

&&

.MARINE...
307 PM CDT

Low pressure over Quebec will continue to lift northeast while a
trailing cold front pushes through the Ohio Valley. A series of
weak troughs and secondary surges of cooler air will push across
the western Great Lakes region, keeping winds generally
northwesterly through the weekend and into early next week. High
pressure building out of Canada will spread into the northern and
central plains over the weekend and then build over the Great
Lakes early next week. Low pressure will begin to develop over the
northern plains Tuesday night and Wednesday as the high slides to
the east. Increasing sly-swly winds will set up over the lake as
the low pressure and an associated cold front push across the
northern plains to the upper Mississippi Valley Wednesday night,
while the high more slowly moves off the mid Atlantic coast.
Increasing sly-swly winds up to 25-30 kt will set up over Lake
Michigan for Wednesday and Thursday and then diminish and turn
more westerly late next week following the passage of the cold
front.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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