Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241130

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM CDT Sat Jun 24 2017

243 AM CDT

Through tonight...

Early morning water vapor imagery shows a broad
upper trough over much of North America with a large ridge along the
west coast. This general pattern will persist through early next
week. At the surface, a cool frontal boundary has passed through the
local area with cool advection underway. 850 millibar temps will
fall to the 6-7 C range by afternoon. Expect to see a mix of sun and
stratocu from late morning through the afternoon with a breezy west-
northwest wind. Highs should top out in the low to mid 70s. A closer
look at water vapor reveals several closed upper lows with focus
being on one moving south-southeast out of southern
Manitoba/Saskatchewan into North Dakota. This circulation will track
across southern Wisconsin and brush the local area early this
evening. Forecast soundings show minimal elevated instability and
decent lapse rates very early this evening as the circulation
arrives. Will often see a very gradual decline in precip
coverage/intensity with features like this with activity lingering
after sunset. Will carry isolated/scattered shower mention for a few
hours this evening across north central and northeast IL to account
for this and cannot rule out some lingering thunder as well but will
leave thunder mention out for now.



243 AM CDT

Sunday through Friday...

Even cooler 850 millibar air settles in for Sunday keeping highs a
couple degrees cooler over those of today. The large scale upper
trough will still be in place and there may be a greater coverage of
stratocu. A less defined upper wave will approach either late in the
afternoon or early in the evening bringing low chances for showers
and possibly a thunderstorm, especially across the north and east.
Monday will be much the same story. Will need to see how the upper
waves Sunday and Monday evolve as their intensity may be a little
underdone at this range and their track may shift a bit as well.
Overall it`s a pretty similar set up as to what we saw last week and
each day the precipitation coverage was probably a little greater
than what it looked to be a day or two ahead of time. As a result,
may need to tweak pops upward with later updates.

Things start to evolve Tuesday as upper ridging shifts east across
the Plains pushing the big upper trough eastward. Surface high
pressure will pass overhead and low level temps start to recover.
The ridge quickly dampens to a more zonal pattern thanks to a series
of waves that may lead to an active few days in the region for mid
and late week. Medium range guidance shows several opportunities for
showers and thunderstorms into next weekend. Temperatures will
recover into the low/mid 80s starting Wednesday with increased
humidity levels as well.



For the 12Z TAFs...

Only aviation concerns are gusty west-northwest winds at 20-25 kts
today, and the potential for some showers to spread into the area
this evening.

Surface low pressure trough over the northern/eastern Lakes and
high pressure to the west across the Plains will maintain a
modestly tight gradient across the region today. Cool air aloft
will lead to steep low level lapse rates as the day warms,
resulting in winds becoming gusty 20-25 kts by late morning/midday.
Winds will diminish this evening with sunset.

A sharp mid-level disturbance currently over the eastern Dakotas
will dig southeast into the region by this evening, and will
likelydevelop scattered showers across parts of MN, WI, and
northeast IA, which will spread into far northern IL toward
sunset. Guidance depicts a dissipating trend as showers move east
into the Chicago area after dark, but a few may near the
terminals. Have carried a prob30 for RFD where coverage will
likely be greater...though have maintained a dry forecast for now
for the Chicago metro terminals. Will have to monitor
south/eastward extent of precip later today for possible VCSH
mention especially for ORD and DPA.



243 AM CDT

Relatively quiet marine weather is expected across Lake Michigan
through the weekend and into early next week, as weak low pressure
slowly moves from near Lake Superior to the eastern Lakes...and high
pressure builds across the northern and central Plains states. This
will set up modest west-northwest winds across the lake, though
there will be some gustiness especially along the west shore with 20-
25 kt winds during the daytime hours when daytime heating deepens
the mixed layer over adjacent land surfaces. Eventually, the Plains
high will move southeast into the lower Missouri and Ohio Valleys,
with weaker ridging moving east into the western Lakes region by
Tuesday. Attention then turns back to the northern Plains mid-week
where deep low pressure develops and lifts northeast to the northern
Lakes late Wednesday night into Thursday. The deepening of this low
will result in increasing south winds across the lake Wednesday into
Thursday, with 30 kt winds likely and perhaps higher gusts.






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