Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171746

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 PM CDT Fri Mar 17 2017

904 AM CDT

Main update this morning was to adjust pops and trends through
the afternoon. Greater large scale lift has since exited the
area, but anticipate continued weak isentropic ascent through
midday. Greater focus appears to be across far northeast Illinois
and northwest Indiana for the remainder of the morning, supporting
periodic drizzle or light rain. Anticipate drier conditions to be
in place for most locations by early afternoon. Monitoring
possible scattered light precip development later this evening
across much of the CWA as stronger vort max drops southeast
through the region. If it were to occur, at this point expect it
to be all liquid. The arrival of colder air could support the
possibility of some type of brief and light wintry mix though.
However, deeper moisture appears to be a limiting factor at this



306 AM CDT

Through Saturday...

Primary short term concern is with precip today as a clipper
tracks along the Candian border into the northern Lakes region by
tonight. A north-south oriented warm front extending south from
this low should reach the MS River later this morning before
moving across the CWA this afternoon. Ahead of the warm front,
look for light rain and eventually some drizzle to increase in
coverage this morning with this precip likely to continue until
warm FROPA occurs and winds shift to the west. Will likely see
some light fog develop this morning as well as low levels saturate
and CIGS build down just ahead of the warm front.

Ahead of the front, temps likely to just slowly inch upward as
thick cloud cover and precip prevent any quick warm up. Behind the
front temps should warm more quickly, though FROPA over Chicago
and NW IN looks to be mid-late afternoon, limiting the time in
the warm sector during diurnal heating.

Cold front will move through this evening with little threat of
any precip. Cloud cover should be pretty extensive tonight, so
given the relatively weak cold air advection have trended temps a
bit above guidance tonight. Gusty winds will add a bit of a bite
to the air. Quick cold shot likely to linger into Saturday with
clearing expected to be slow to occur and mainly late in the day.



306 AM CDT

Saturday through Thursday...

By Sunday, high aplitude ridging over the Rockies and High Plains
will begin to translate east into our area allowing for a quick
moderating trend. Moderating trend looks short lived as another
northern stream shortwave tracks north of the region Monday with
trailing cold front moving through the CWA Monday afternoon.
Associated shortwave is then forecast to dig into New England by
mid week helping carve out another deep trough over the NE CONUS
with another near 1040mb high settling into the Great Lakes

While our temps will be below normal again Tues/Wed, our area
looks to only get a glancing blow of this shot of arctic air as
the heart of the cold air takes aim on the northeast. Guidance is
in pretty good agreement in keeping this trough somewhat
progressive with a moderating trend in temps expected later in the
week. Details are a bit sketchier later in the week with regards
to precip chances as guidance hints at a western trough moving out
into the central plains late in the week, though there has been
some run-to-run variability, so confidence in the late week
forecast details are a bit below normal.



For the 18Z TAFs...

IFR ceilings still in place early this afternoon, as well as some
vis reduction owing to fog. Both of these will likely persist over
the next couple of hours, but do anticipate an improving trend by
mid to late afternoon in both ceilings and vis. Skies will try to
clear, especially for the RFD area, but pattern will support
additional cloud cover to move back south across the region by
early this evening. Will initially start as VFR, but transition to
MVFR through the overnight hours. Outside of some patchy drizzle
over the next 1-2 hours, precip will be done for the daytime
hours. Continue to monitor the possibility for additional
scattered showery development later this evening. Not too
confident with exact trends, but do think at least isolated
development will be around the terminals. Precip type should
remain all liquid, but could see a wintry mix briefly mixing in
towards later in the night. Confidence is even lower with these
trends at this time.



206 AM CDT

An area of low pressure moving into the Upper Midwest early this
morning, will shift southeastward across Lake Michigan tonight
into early Saturday morning. This pressure system will result in a
period of 20 to 30 KT southeasterly winds over the lake today,
followed by a period of 30 KT northwesterly winds late tonight
into early Saturday. Conditions are likely to be hazardous to
small craft along the Illinois near shore waters early today, due
to the southeasterly winds. A second period of hazardous
conditions to small craft is then expected late tonight into
Saturday for the southern Lake Michigan near shore waters in
association with the strong northwest winds.

A surface ridge axis of high pressure will shift over the lake
late Saturday night into early Sunday. As this occurs, the winds
over the lake will abate and become variable for a short period,
before they become predominately southerly by midday Sunday. Wind
speeds are also likely to increase up around 25 KT, especially
Sunday night as another potent area of low pressure shifts
eastward over the Hudson Bay. An associated cold frontal trough
will extend southward over the Upper Midwest to the Plains Sunday
evening, then shift eastward over the Western Great Lakes region
sometime on Monday. Once this frontal trough moves across the
lake, it appears there will be a decent period of northerly flow
over the lake through Wednesday as another surface high gradually
builds over the Upper Midwest.






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