Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232044

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
244 PM CST Fri Feb 23 2018

243 PM CST

Through Saturday Night...

For the remainder of this afternoon and into the evening, breezy and
cloudy conditions will persist as the the system that brought some
additional rain overnight moves off to the east.  A zone of height
rises over the region in between an exiting low and high pressure
building across the northern plains to the middle and upper
Mississippi Valley.  As the high builds across the Great Lakes
overnight tonight, a warm front associated with a southern stream
system will set up along the Ohio Valley.  A shallow layer of cold
air will remain in place at the surface across the region, but
aloft, warm, moist advection will set up in advance of the southern
stream system, setting up the potential for some freezing/frozen
precipitation in the morning.  There should be a relatively sharp
cutoff to the precipitation on the northern edge, where the cold air
will be in place.  There is still some uncertainty as to the
progression of p-type as the precipitation shield spreads northward,
but the general trend should be for an area of all snow on the far
northern edge, with a narrow zone of a wintry mix transitioning over
to all rain to the south.  Through the morning hours, both the nrn
edge of the pcpn shield will spread north while the zone of frozen
and freezing pcpn also shifts north, to be quickly followed by the
transition over to all liquid as sfc temperatures rise into the
middle 30s by late morning.

However, of greater concern for Saturday into Saturday night, will
be the additional rainfall moving across areas already impacted by
major flooding.  Latest guidance suggests an area of 0.25 to 0.50
inches of additional rainfall for the period from early Saturday
morning to early Sunday morning.  As of this afternoon, most rivers
across the region, including the Illinois, Des Plaines, Du Page and
Fox Rivers have either crested or will crest soon.  Portions or the
Rock River are not expected to crest until Sunday or Monday as the
crest wave moves downstream.  The Iroquois and Kankakee Rivers and
locations near these rivers will remain a concern through the
weekend and into next week.  The Iroquois River at Chebanse is still
rising as water from upstream on the Iroquois river as well as flow
from Sugar Creek and smaller tributaries.  The Kankakee River in
northwestern Indiana will remain above major flood stage well into
next week as the river is particularly slow to subside there.  The
Kankakee River in Kankakee County will also be slow to subside.
While any additional rainfall will not be welcome at areas already
impacted by significant flooding, since the rainfall will occur over
a 24-30 hour period, it should not contribute to significant rises,
and perhaps delaying the river levels falling.



Sunday through Friday...

153 pm...Primary forecast concerns are the potential for more
rain Wednesday/Thursday and the chance of mixed precip/snow

As low pressure lifts north across the upper lakes region
Sunday...strong/gusty westerly winds will diminish on Sunday but
there could be a period Sunday morning when wind gusts are still
in the 35-40 mph range.

High pressure will move from the central Plains Sunday to the
Ohio Valley Monday with dry conditions expected through Tuesday
evening. With some sunshine expected...temps likely to moderate
back near 50 on Monday and into the 50s on Tuesday. With no snow
pack...its possible these temps could be a bit low but can trend
higher as this time period approaches.

Low pressure is expected to develop over the Plains Tuesday night
into Wednesday and move near northern IL by Thursday morning.
Still considerable differences with the gfs showing a rather
strong low at that time vs. a weaker ecmwf low. Ahead of this
low...showers will be possible Tuesday night into Wednesday with
a period of heavier precip possible Wednesday night into Thursday
morning. While the timing looks similar...the large differences in
the strength of the low make for an uncertain forecast. In
addition...small track changes could make the difference between
all rain or a stripe of accumulating snow...which currently would
be most favored across far northern IL into southern WI. Current
qpf forecasts would suggest the potential for a half inch to an
inch of rain with this system. High pressure will then build
across the region Friday. cms


For the 18Z TAFs...

Cigs/vis gradually improving at the terminals as a secondary cold
front pushes across the region. Expect that conditions should
improve to vfr by arnd 00z.  With the passage of this boundary,
winds are veering to just north of westerly and gusting in the
20-25 kt range. Expect gustiness to persist through the afternoon
as the region remains under a core of pressure rises with low
pressure lifting away from the Upper Great Lakes and high pressure
spreads east across the northern plains.

Main concern for the period will be the chances of a wintry mix of
pcpn spreading from south to north tomorrow morning as a warm
front sets up along the Ohio River and low pressure lifts through
the central plains and the high pressure tracks across the Upper
Great Lakes. Winds will shift to easterly overnight tonight and
tomorrow as the warm front sets up to the south of the terminals.
At the time of pcpn onset, sfc temperatures should be below
freezing, with a warm layer aloft. Pcpn could initially start out
as fzra or fzrasn for a short period tomorrow before changing over
to rain by late morning as warmer gradually air spreads
northward. In the pcpn and easterly flow, cigs may drop to ifr,
especially later in the day.



153 pm...Primary forecast concern remains high end gales and the
potential for storm force gusts Saturday night into Sunday. significant changes to the forecast or reasoning. Low
pressure will develop over the southern Plains Saturday and then
begin to rapidly deepen as it moves north across WI Saturday night
then crossing Lake Superior Sunday morning. Southeasterly gales
are expected to develop over southern Lake Michigan Saturday
evening as the low approaches with southerly gales across the
entire lake early Sunday morning. As the low continues to deepen
over Lake Superior Sunday morning...winds will begin to shift to
the southwest and then west as colder air spreads across the
region. Its during this time period...Sunday morning...that high
end gales are likely and at least some storm force gusts are
possible. If any storms occur...they likely would only last a
short time...perhaps 3-6 hours. The gales will diminish over the
southern part of the lake by Sunday afternoon but gales may
persist across north/central portions of the lake well into
Sunday evening. The current gale watch has this time period
covered well and not planning any headline changes this afternoon.

High pressure will move across the Ohio Valley Sunday night and
Monday as weak low pressure moves across Ontario...maintaining a
west/southwest gradient across the lake. Low pressure may move
across the southern Great Lakes region next Wednesday night into
Thursday. cms


LM...Gale Watch...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...6 PM Saturday to 3 AM Monday.




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