FXUS63 KLOT 171735

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 PM CDT Tue Jul 17 2018

308 AM CDT

Through Wednesday night...

Much less humid air mass is filtering into the area and looks to
be a couple days with comfortable humidity, seasonable temps, and
sunshine both today and Wednesday. Winds off the lake both days
will keep lake adjacent areas a bit cooler.

- Izzi


308 AM CDT

Thursday through Monday...

Thursday will feature increasing cloudiness, but still fairly
comfortable humidity and seasonably warm temps. The weather looks
to take a decided turn toward unsettled Thursday night and
potentially through next weekend. Medium range models remain in
good agreement on overall large scale pattern with vigorous (by
July standards) northwest flow shortwave trough closing off a mid-
upper level circulation as it moves into the Upper Mississippi
Valley and Midwest. There remain some non-trivial differences in
timing and placement of key features with this system, but given
the proximity of warm, humid, unstable air and expected stronger
flow aloft associated with the upper low, potential will exist for
some active t-storms in the region late in the week.

Current operational runs of the GFS and ECMWF would tend to favor
areas to the west of our west for threat of more robust convection
Thursday/Thursday night time frame. By Friday, the system looks to
become more vertically stacked and will begin to slowly fill as
our CWA gets into the more favorable position for convection.
There could still be some threat for active storms in the region
Friday despite the weaker shear profiles than Thursday. Cloudiness
and leftover convection could play a role in limiting instability
and magnitude of the threat of stronger storms, but still bears
watching given the strength of the trough.

Upper low is currently progged to evolve into a substantial
longwave trough over the eastern CONUS. Locally, this looks to
result in slightly below average temps and perhaps some threat of
weak diurnal convection depending on the proximity of the mid-
upper level cold pool.

- Izzi


For the 18Z TAFs...

VFR conditions are expected through the period with generally NE
winds and some cirrus and diurnal cumulus. Winds will gust to
around 15 knots through early evening away from RFD.



631 AM CDT

Departing trough east of the lake and high pressure building from
the northwest will support winds becoming more northerly, while
increasing. With the high still well to the northwest, the lake
will remain on the edge of this high where a tighter gradient will
support winds in the 10 to 20 kt range. In this pattern, a few
gusts to 25 kt will also be possible. This will all provide
building waves across the lake, with hazardous waves for small
craft likely across the Indiana nearshore today. Do think these
higher waves will stay mainly over the Indiana nearshore, but will
need to monitor the possibility for slightly stronger winds and
higher waves across the Illinois nearshore today. These hazardous
conditions will likely remain into this evening, but expect winds
to diminish this evening with waves subsiding during this time.



IN...Beach Hazards Statement...INZ001-INZ002 until 10 PM Tuesday.

LM...Small Craft Advisory...IN nearshore waters until 10 PM Tuesday.




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