Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241747

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1247 PM 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 18Z TAFs...

Satellite depicts a broken lower VFR stratocumulus field across
the flying area this afternoon along with gusty west winds. Expect
the cloud field to stick around for a decent portion of the
afternoon, lifting a bit but also clearing out a little from west
to east. The more organized disturbance upstream in MN will slide
across southern WI this evening. Forcing does appear strong enough
even across the Chicago terminals for a broken line of showers,
but moisture is somewhat scant. Confidence high enough for TEMPO
at RFD, and gave some credence to the forcing to carry VCSH at the
Chicago TAFs, but guidance does continue to show a weakening
trend with eastward extent.

Forecast soundings show a near repeat performance tomorrow with
similar steep low level lapse rates suggesting another quick
arrival of a lower VFR stratocumulus field again that lifts some
with daytime heating. Another wave in northwest flow will lead to
a more typical afternoon isolated shower chance. Thunderstorm
potential is still low, but a tad higher than chances tonight.
West winds continue Sunday, likely a tad weaker than today.



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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