Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240715

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
215 AM CDT Mon Apr 24 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 06Z TAFs...

1230 am...Only forecast concern this period are wind speeds/
potential gusts from mid morning through sunset.

Light northeast winds at the Chicago terminals early this morning
will slowly turn southeasterly through sunrise and then begin to
increase during the morning hours. The gradient will slowly begin
to tighten and prevailing speeds in the 10-15kt range look on
track. Low level mixing will likely allow for some higher gusts...
especially across northwest IL/rfd. But low confidence regarding
how widespread these will become further east. In addition to the
already southeast winds...a lake breeze is possible this afternoon
turning winds more easterly at gyy by early afternoon and possibly
at mdw/ord by mid afternoon. Speeds will diminish some with sunset
and remain southeasterly tonight.

Dry/vfr expected through the period...but some isolated/shallow
ground fog is possible through sunrise this morning in the usual
locations. Otherwise...sct/bkn high clouds. cms



215 am...A weak ridge of high pressure over the southern lakes
region will slowly move southeast today and tonight...while a
stronger area of high pressure remains over northern Ontario. The
gradient is expected to remain tight over far northern Lake
Michigan today and tonight with easterly winds 15-25kts expected.
Over the rest of the lake...east/southeast winds will increase to
15-25kt later tonight as low pressure over the western plains
moves northeast to the upper midwest tonight and dissipates. A
second area of low pressure will develop over the central plains
tonight and move northeast to the western lakes region Wednesday.
This low will then slow as it moves into Ontario Thursday. West/
southwest winds may increase to 30 kt Thursday as this low
departs. cms





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