Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221858

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

158 PM CDT

Through Thursday...

Warm advection will kick in tonight as cool high pressure exits to
the eastern Great Lakes region. Mid and high clouds from low
pressure off the CA coast will stream east-southeast over the area
tonight over a flattening ridge across the plains. With the
increasing clouds expect temps to hold a tad warmer than last

The developing warm front will shift northeast across the area
later Thursday and Thursday night. Lower level warm advection with
this front will likely kick off some scattered afternoon showers.
Weak upper ridging and southeasterly flow maintaining drier lower
level conditions will preclude more widespread shower activity.
In spite of cloudier conditions, highs will recover closer to
normal, except away from the lake in northeast IL where onshore
flow remains.



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 18Z TAFs...

Lake effect stratocumulus clouds in the 3000-3500 ft range will
dissipate soon Low level winds will shift to a bit more of an
easterly direction, so the clouds will hang on the longest across
northern Illinois.

High pressure over Lake Michigan will shift to the eastern Great
Lakes region tonight while low pressure will organize in lee of
the Rockies. This will allow winds to shift southeasterly tonight
and strengthen on Thursday. No operationally significant clouds
will enter the picture until Thursday afternoon ahead of a warm
front, though clouds will generally continue to lower through the
period. This feature will also lead to a few scattered rain
showers in the afternoon.



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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