Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220809

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 AM CDT Wed Mar 22 2017

309 AM CDT

Through Thursday night...

Blustery and fairly cold (especially near and immediately down
wind of the lake) today. Steep low level lapse rates due to the
cold air moving over the relatively milder lake waters is result
in a plume of lake effect stratocumulus. KLOT WSR-88D VWP suggests
that the convective cloud tops only extend up to about 4500ft agl,
meaning convective cloud depth is less than 1500ft and cloud top
tops are around -9C based on late evening ACARS soundings, so it
is unlikely anything will fall out of these lake effect clouds.

The stiff wind off the lake should result in very little temp
recovery along the lakefront with highs probably topping out in
the lower 30s this afternoon. North central IL and far southern
CWA should reach the lower 40s with mid-upper 30s elsewhere.

Developing warm front will lift north across the CWA Thursday,
with the strengthening WAA regime probably enough to kick off some
scattered showers in the afternoon. Warm front should clear the
IL/WI border by early Thursday evening with rising temperatures
expected Thursday night. It now looks like the shower and
thunderstorm threat Thurs night should be mainly confined to areas
north of the warm front across MI and WI.



309 AM CDT

Friday through Tuesday...

The stage looks to be set for a windy and warm day Friday with
little threat of any organized precip in the warm sector. Biggest
question is how extensive cloud cover will be and the impact on
temperatures. If stratus is fairly widespread, then highs will
probably "only" top out in the mid-upper 60s. Should we see some
mixed sunshine, then progged 925mb temps support highs in the low
to mid 70s.

Friday night a couple things will take place, first the warm front
to our north will begin to move back south as a lake enhanced back
door cold front. Timing of the FROPA is tricky, but this time of
year, the lake can be a pretty significant influence giving these
type of fronts a strong push. Behind the front, temps near the
lake will crash into the 40s and there could be some fog near the
lake as well. The other big driver of our wx Friday night will be
the western system gradually moving east toward the area. While
the system will be weakening with time as it becomes vertically
stacked, could be enough lingering instability for some thunder
threat Friday night as precip moves in from the southwest.

Medium range guidance is in pretty good agreement in having this
deep, vertically stacked, quasi-barotropic system slowly
meandering across the region over the weekend. Can`t rule out a
small thunder threat southern CWA Sat afternoon if there is a bit
of sunshine to destabilize things given the colder air aloft. But
overall, our weather will be cloudy and showery, with stiff winds
off the lake Saturday likely keep things quite chilly (40s) along
the lake.

Polar jet is forecast to stay to our north next week, so really
not looking for any significant push of cold air in the wake of
this system early next week. Weaker southern stream wave should
pass to our south Monday afternoon and evening and could bring
some showers into mainly our southern CWA.



For the 06Z TAFs...

Surface high pressure over southwestern Ontario will shift
southeastward towards the eastern Great Lakes by this evening.
As this occurs, east-northeasterly winds will continue to be
favored across the area through the day. While gusts have been up
around 30 KT at the Chicago area terminals over the past few
hours, we expect speeds to ease some towards daybreak. Otherwise,
a deck of MVFR CIGS have moved into northeastern Illinois with the
onshore flow. These CIGS are likely to hang around at times
through the morning hours, before dissipating during the



220 AM CDT

A potent surface high over southwestern Ontario will shift
southeastward towards the eastern Great Lakes by tonight. This
high has been resulting in 30-35 KT northeasterly winds over the
lake for the past several hours. Wind speeds are expected to
gradually ease today as this surface high shifts over the lake
this afternoon. In spite of this, high waves over southern Lake
Michigan will continue to lead to hazardous conditions to small
craft for much of the day.

Winds over the lake will shift southerly tonight as the surface
high shifts over the eastern lakes. However, another cold front
is expected to shift down the lake Friday and Friday night.
Following this frontal passage, another potent surface high will
build over the Upper Great Lakes and southern Canada as a storm
system tracks eastward from the Plains to the southern lakes
region by Saturday night. These two pressure patterns look to
result in a period of strong east-northeasterly winds again on
the lake Friday night through Saturday, especially on the
northern 2/3rds or so of the lake.

Some fog may develop over the southern end of of the lake Friday
night into Saturday as the frontal boundary stalls out near the
southern tip of the lake. Its possible this marine fog could be
dense at times through the weekend, especially off the Illinois
and Indiana shores.



LM...Small Craft Advisory...nearshore waters
     until 4 PM Wednesday.




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