Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210609

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
109 AM CDT Wed Mar 21 2018

343 PM CDT

Through Wednesday...

The 500 mb low over central Kentucky into north central Tennessee
should slowly slide eastward tonight, before picking up speed as
it moves further east on Wednesday. Any precipitation associated
with this feature should remain to the southeast of the forecast
area. Otherwise, high pressure well north of the area should keep
dry conditions going across the area. High clouds should continue
to linger across the area tonight into Wednesday morning, with an
area of low clouds possibly spreading southwestward into the
Chicago metro area. Drier air should move into the area during the
afternoon, and bring mostly sunny skies. Brisk northeast winds
should subside gradually tonight, with modest north winds
Wednesday. Below normal temperatures are expected during this



343 PM CDT

Wednesday night through Tuesday...

The focus for this period will be the potential for snow Friday
night into Saturday morning.

The end of the work week should be fairly quiet with low
amplitude upper level NW flow in place. Upper heights begin to
rise as an upper level ridge approaches the region. A weak vort
max embedded in the NW flow will pass SW of the area Thu and
shouldn`t do much more than increase the cloud cover over the SWrn
portion of the FA.

A short wave is forecast to under cut the ridge for the end of
the week. This system should pass well south of the area and has
trended south early this week. The GFS and ECMWF have continued
their southward trend with the new 12Z runs and if the GFS
verifies, all but the extreme SWrn portion of the CWA will be dry.
But the ECMWF is farther north and has precip across the entire
CWA. Since temps aloft will be below freezing, p-types will be
modulated based on SFC temps and due to Erly flow bringing in a
steady supply of dry low level air in advance of this system, dew
point depressions should be sizable and wet bulbs are forecast to
be below freezing. This means that precip should quickly
transition to snow late Friday night. The eventual amount of snow
will depend on many things including onset timing...which has been
steadily trending later and later. The slower the system gets
means the less snow potential because more of the precip will
occur during the day Sat when SFC temps will rise above freezing.
Precip should exit the SErn zones Sat evng.

The upper level ridge becomes re-established for the end of the
wknd behind this system with the SFC ridge extending SW into the
Midwest from the high centered across Quebec. Upper flow
transitions to SW flow with the departing ridge as a long wave
trough moves onshore along the West Coast. A short wave will get
ejected from the SWrn US trough for early next week but with
considerable differences in strength, timing and location it is
much too early to be specific other than to say that there should
be precip in the area somewhere in the Mon-Wed time frame. And
with min temps near freezing, the p-type forecast could be tricky.



For the 06Z TAFs...

Primary forecast concern for this period will continue to be
winds. Winds which should remain generally northerly through the
remainder of the night and through the daytime hours tomorrow.
With an inverted trough extending from low pressure over the Upper
Ohio Valley through north-central IL and a weak finer scale ridge
along the west coast of Lake Michigan, wind direction will be a
little difficult to pin down exactly if it will be a little west
of north or a little east of north. At 06z, the dividing line
between NNW and NNE is between ORD/DPA and MDW/GYY. Expect that
there may be some variation to wind direction through the late
morning hours when the low over the Upper Ohio Valley will begin
to merge with the stronger low off of the Mid-Atlantic coast and a
ridge of high pressure begins to build across the Mid Mississippi
Valley. This will, ultimately, lead to a weaker pressure gradient
over nrn IL/IN and set up the possibility for a lake breeze to
develop, or perhaps, just some lake influence, leading to more of
a true NE wind direction in the afternoon. In general, confidence
in wind direction is only low-to-moderate through the morning
hours. Through the late afternoon and evening hours, the ridge
will build east across region, leading to a trend toward lgt/vrbl
winds which should then persist through the overnight hours.

Otherwise, the only other concern is some lingering mvfr cigs for
the remainder of the overnight hours. Latest obs indicate the
low-level clouds scattering, but there could still be brief
periods of mvfr cigs. Otherwise, cloud cover will trend to
operationally insignificant through the morning and then become
skc for tomorrow night.


343 PM CDT

Small Craft Advisory continues into Wednesday evening across the
nearshore waters of Lake Michigan. Gusty northeast winds to 30
knots into tonight will gradually weaken on Wednesday, and back
to the north. High waves will linger into Wednesday afternoon for
areas north of Calumet Harbor, and into the evening for areas to
the east.

May see another period of gusty winds and elevated waves this
weekend, as low pressure passes by to the south of the area.



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

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




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