Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230801

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

258 AM CDT

Today and Tonight...

Primary concern in the near term is with the passage of a cold
front across the forecast area today, and convection trends
especially this morning along and ahead of the front.

Early morning surface analysis depicts low pressure over
southeast WI/southern Lake Michigan, with a cold front trailing
to the west and southwest from southern WI and eastern IA into the
Central Plains. This front is progged to push southeast across
the forecast area this morning, clearing the southeast by early
afternoon. Thunderstorms along/ahead of the front will spread
southeast across the area early this morning, and may present a
locally heavy rain and sub-severe gusty wind threat early. Weak
low-level lapse rates and stronger mid-level winds lagging behind
the convective line for the most part should substantially limit
any severe threat. Winds shift to the north-northwest behind the
front, with a gradual filtering of cooler/drier air into the
region. Partly sunny skies by afternoon should still support
afternoon temps around 80, except near the lake in northwest
Indiana where northwest onshore winds are expected. IL shore may
see north-northwest winds turn north-northeast with a bit of a
lake breeze late. Clearing and cooler tonight as temps fall into
the 50`s, and a modest west-northwest breeze.



258 AM CDT

Saturday through Thursday...

Period of cooler weather will be in place across the area this
weekend and into the start of next week. Appears to be a potential
for a few scattered diurnal showers especially Sunday, though
otherwise pleasantly dry with dew points in the in the 40`s and
temps in the 70`s. Warmth and humidity then gradually increase
again mid-week, along with a scattered thunderstorm threat.

Series of mid-level short wave disturbances are progged to slide
across the Upper Midwest/western Lakes region this weekend, as
building upper ridge along the west coast induces amplification of
downstream trough across eastern North America. Diurnally steep
low level lapse rates are evident in forecast soundings, with a
gradual cooling of mid-level temperatures noted late Saturday with
vort digging across the area. Despite fairly dry low levels with
dew points in the mid-upper 40`s, a few isolated-scattered showers
may spread into mainly far northern IL during the late
afternoon/evening hours, especially in GFS depiction of deeper
(but still weak) instability. Similar scenario persists Sunday and
Monday, as additional vort energy digs southeast across the
region. Despite the potential for these isolated-scattered
convective showers, many dry hours are likely, and coverage will
be lower the farther south you go. Highs over the weekend will be
a good 15 degrees or so cooler than the past few days, with 850 mb
temps cooling to +6/7C Saturday and +5/6C Sunday. This would
support high temps only in the low-mid 70`s, with some locations
across far northern IL topping out only around 70 especially where
greater stratocu coverage is likely.

Western CONUS upper ridge eventually builds eastward with time as
flow pattern is progged to become more progressive mid-week. This
results in building heights/subsidence and surface high pressure
developing across the region Tuesday. Building mid-level heights
will support warming surface temps, with highs expected to be back
around the 80 degree mark. Surface ridge moves off to the east of
the area Tuesday night, with a return to southerly winds coming
on Wednesday. GFS and ECMWF both depict a short wave advancing
through the more zonal upper pattern across the Upper Midwest
Wednesday into Thursday, with surface low pressure tracking north
of the Lakes. Warmer and more humid air mass will support an
increasing thunderstorm threat by this time, and into the end of
the week as a trailing cold front slowly sags into the region.



For the 06Z TAFs...

Forecast concerns center around thunderstorms overnight along with
lowering MVFR cigs and then wind direction trends this afternoon.
Surface low pressure over southern Wisconsin is tracking east
tonight with a cool front trailing it to the southwest. Moist low
levels have resulted in lowering MVFR cigs lifting up from the
south and expect cigs to settle in the 1000-1500 ft range
overnight at the terminals. Will need to watch for potential for
some cigs to fall below 1000 ft late tonight. Otherwise bands of
TSRA continue to develop and move eastward from eastern Iowa and
southwest Wisconsin. Timing of arrival of these is a little slower
than previous forecast. Some isolated/scattered development is
possible ahead of this main line and the first half of SHRA
mention and TSRA Tempo groups is meant to cover this where the
latter half of the SHRA/TSRA period covers the bulk of the
activity. May further refine details closer to 07z.

The front will pass this morning and winds will turn northwesterly
and probably even northerly for a time. SHRA/TSRA will end from
the northwest by 12/13z but low cigs will linger but steadily
improve through the morning. Winds will become more west to
northwest through the afternoon. Will need to watch for possible
lake breeze development and passage at ORD/MDW but have kept winds
northwesterly for the afternoon for now.



245 PM CDT

The low over western Manitoba will weaken as it crosses Ontario
tonight and then continues to Quebec Friday. An associated cold
front will sweep across the lake tonight into early Friday
morning with winds becoming west to northwest behind the front.
Westerly winds are expected to continue through early next week.

High pressure should then build in behind the exiting cold front,
spreading across the plains Sunday and the western Great Lakes
early next week, setting up a period of relatively light and
variable winds. As the high shifts to the east and new low
pressure develops over the northern plains, winds will trend more
southerly and strengthen to arnd 15 to 20 kt by Tuesday. A
strengthening southerly pressure gradient could bring winds up to
30 kt by mid week as high pressure remains parked over the sern
CONUS and the trough and associated cold front track east across
the plains.





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