Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191941

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
241 PM CDT Thu Apr 19 2018

222 PM CDT

Tonight through Friday...

Little in the way of near term weather concerns in this period,
with surface high pressure to spread across the western Great
Lakes region into Friday. Some potential for diurnal radiation fog
does exist late tonight/early Friday morning, primarily across
north central IL.

Early afternoon surface analysis indicates center of 1035 mb
surface high pressure over far southwestern MN along the IA
border. This ridge is progged to drift east to the upper
Mississippi Valley by Friday morning, and toward the western Great
Lakes during the day. This surface high is associated with deep
subsidence downstream of upper ridging building eastward across
the central CONUS, itself downstream of an upper trough and closed
low over the western U.S. As the surface ridge approaches
tonight, mostly clear skies and decreasing north winds can be
expected, with winds becoming light and variable in the vicinity
of the ridge axis over the Mississippi Valley and parts of north
central IL by morning. This will set up favorable conditions for
radiative cooling tonight, especially for portions of far northern
IL west Chicago, with various guidance suggesting shallow diurnal
fog development toward and around sunrise Friday. Have indicated
areas/patchy of fog early Friday morning from the Fox River Valley
westward, and along/north of the I-88 corridor.

Light/variable winds Friday morning will allow for lake breeze
development off Lake Michigan by early afternoon. Otherwise, sunny
skies are expected during the day, with low level thermal fields
supporting highs in the upper 50`s in most areas, except near Lake
Michigan where the onshore lake breeze flow will keep temps
largely in the mid-upper 40`s. A few spots in the west/southwest
parts of the cwa could tag 60.



240 PM CDT

Friday night through Thursday...

Surface high pressure is expected to dominate across much of the
Great Lakes Region through the weekend. Overall, this will help
produce quiet and relatively mild weather through early next week.
A storm system is expected to produce rain into portions of the
Lower and Mid Mississippi Valley late Sunday into Monday. However,
with this system likely to track well south of the area, any
associated rainfall looks to remain out of the area.

Overall, the main story through the weekend will be the warmer
temperatures (near seasonal averages), especially considering we
have experienced such a cold April thus far. While there is likely
to be high cloud cover around, temperatures are still likely to
climb into the upper 50s to low 60s for daytime highs this weekend
and into at least early next week. The main exception to these
warm temperatures will be areas near Lake Michigan. In these
areas, a persistent easterly onshore wind will result in much
cooler conditions, with highs likely remaining in the 40s.

While the storm system tracking to our south on Monday is expected
to have little impact on the area in terms of precipitation, it
does appear that another disturbance may take aim on the area late
Tuesday into Wednesday. This looks to result in our next decent
chance for some light rain showers. Otherwise, mild conditions
into the 50s to around 60 look to continue through the end of the
forecast period.



For the 18Z TAFs...

Only aviation concerns are wind direction a lake breeze expected to
push through ORD/MDW later this afternoon, and the potential for
some patchy MVFR fog west of the Chicago area early Friday morning.

Surface high pressure across western Minnesota and western Iowa
will continue to spread east through the TAF period, in response
to sinking air aloft behind the departing disturbance from
Wednesday. Northerly winds were varying from about 340-020 deg per
ORD/MDW obs, and though guidance not bullish in turning winds more
fully northeast, northerly winds of 340 deg or better usually turn
northeast with cold lake conditions despite stronger NNW gradient.
Thus am expecting an eventual turn to the northeast at ORD/MDW by
late afternoon. Gusts have been slow to develop, though with
temps up to 50 degrees we`ve mixed deep enough for at least some
occasional gusts 15-20 kts.

Winds will become light/variable tonight as surface high slides
overhead. Light winds and clear skies will allow strong
radiational cooling, with the potential for some patchy fog mainly
west of Chicago by Friday morning. Some high res guidance picks up
on this mainly DPA/RFD and further northwest, and have indicated a
tempo for MVFR vis there.



240 PM CDT

Northerly winds will continue to abate over the lake this evening
as high pressure begins to build over the Upper Midwest. This
area of high pressure is expected to remain dominate across much
of the western Great Lakes Region through the weekend. This will
setup light and somewhat variable winds over the lake, with
onshore flow likely developing each afternoon over southern
sections of the lake as lake breeze processes dictate the main
wind direction over the near shore waters.



LM...Small Craft Advisory...Gary to Michigan City IN until 1 AM Friday.

     Small Craft Advisory...Winthrop Harbor to Gary IN until 7 PM




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