Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232010

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
310 PM CDT Sun Apr 23 2017

213 PM CDT

Through Monday...

Very dry airmass (nearly 50 degree dew point depression at O`Hare
this afternoon) in place over the region will keep northern
Illinois and northwest Indiana sunny and warm through tomorrow.
Surface ridge axis will move across the area this afternoon with
light flow allowing for a lake breeze to push inland and keep
temperatures in the 50s right along the lake front. Meanwhile, a
weak cold front moving south across Lake Michigan this afternoon
will sag into northern Illinois this evening and will help push
temperatures down further inland. This boundary quickly weakens
and lifts back north overnight in response to low pressure moving
east across the Dakotas. A modest southeasterly gradient will be
in place ahead of the low tomorrow which will allow breezy
conditions to develop, especially west of the Chicago Metro.
Onshore flow in Illinois near Lake Michigan will keep temperatures
in the 50s immediately along the lake once again while inland
expect highs in the low to mid 70s.



309 PM CDT

Monday night through Sunday...

The period will begin dry Monday night into Tuesday, as the bulk
of any activity stays situated just to the west. This is due to
approaching longwave trough staying mainly over the western CONUS,
and while any lifting energy and attendant surface reflections
move northeast to the upper Midwest. Mild conditions will continue
as southerly flow ushers in another day of 70 degree weather over
much of the CWA Tuesday. The exception may be over far northeast
Illinois, where winds may turn more off the lake and keep slightly
cooler temps in the 60s. As the large upper level trough moves
further to the east Tuesday night, should see an increase in large
scale ascent over the region. Precip chances do increase during
this time, but it does appear mainly for north central Illinois.
Although some differences among varying guidance with exact
evolution of this large system, most guidance in agreement with
continued increasing chances for precip Wednesday and Wednesday
night. Along with precip chances, thunder chances increase during
this period and will likely need to continue to monitor this
period for possible stronger thunderstorm development. Instability
the uncertain variable at this time, but with this large system
and rather strong forcing expected, would not take much
instability to get at least some isolated stronger thunderstorms.
With increasing moisture content, periods of heavy rainfall may
also be a concern. This system will eject to the northeast on
Thursday, with a lull in any precip activity into Friday. Although
some variability with next period of weather, next weekend does
look active, with periods of showers and thunderstorms once again
becoming more probable.



For the 18Z TAFs...

VFR conditions will prevail throughout the TAF period with shifting
winds comprising the main forecast concerns. Light northerly or at
times variable winds are in place early this afternoon and expect
a lake breeze to push inland across the Chicago terminals mid
afternoon turning winds out of the east. Boundary is visible on
radar and will be able to hone in on timing in the near term as
we get a better fix on its motion. Winds may initially be at or
near 10 kt behind the boundary, but should quickly settle back
under 10 kt the rest of the afternoon. Meanwhile, farther north a
cold front is working south across Lake Michigan and spilling
southwestward across Wisconsin. Models have been in good
agreement showing the stronger winds staying north of the
terminals and winds behind the front are expected to diminish some
as it moves across northern Illinois this evening turning winds
to the northeast. Winds should gradually veer back around to the
southeast tomorrow morning.



223 PM CDT

High pressure situated across the region continues to slowly sag
south, while front pushes down the lake. This front will continue
to move through the remaining areas of the lake this afternoon
into tonight, with winds shifting to more of a northerly
direction. Speeds across the north part of the lake behind this
front have been in the 15 to 25 KT range and these speeds do look
to spread further into the central part of the lake this
afternoon. However, the gradient supporting these 15 to 25 KT
winds will remain mainly to the north. So, these elevated winds
should not move anymore south than the central lake, with wind
speeds over the far southern end of the lake staying lower.
Pattern will then support diminishing winds over much of the lake,
while winds turn more easterly tonight. The exception will be
across the northern tip of the lake, where winds may stay slightly
higher. Easterly winds will then trend more southeast Monday and
Monday night ahead of approaching low pressure from the Plains,
with locations across the lake likely observing speeds of 15 to 25






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