Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151719

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1219 PM CDT Thu Mar 15 2018

247 AM CDT

Through tonight...

In spite of the passage of a weak cold front yesterday evening,
temperatures today are likely to warm well into the 40s (to
possibly near 50 in some areas) again today under mainly sunny
skies. The main exception to this will be areas along the Lake
Michigan shores. In these areas sharply cooler conditions are
expected this afternoon as a lake breeze develops and moves inland
through the late afternoon. Expect this to push temperatures down
into the 30s late in the day near the lake.

Tonight expect mainly clear skies with lows dropping into the 20s.



321 AM CDT

Friday through Wednesday...

The main focus during the long term period continues to focus on
the increasing potential for a wintry mix of snow, sleet and
freezing rain over the area Friday night into early Saturday
morning, then the potential for another round of active weather
early next week.

While conditions will be quiet across the area during the day on
Friday, clouds will be on the increase through the day and
temperatures will likely be several degrees cooler as a
northeasterly wind drags in some cooler air. The main focus for
wintry precipitation over the area is Friday night into early
Saturday morning. During this period, a weakening storm system is
expected to shift from the Plains to the Mid-Mississippi Valley.
As this occurs, model guidance continues to be in agreement that
this will result in the development of precipitation across
portions of the region Friday night as the mid-level disturbance
approaches. The precipitation looks like it will have to fight
drier air near the surface over much of northern IL Friday
evening. As a result, this may delay the start time of
precipitation into northeastern IL until after midnight.

With east-northeasterly surface winds and subfreezing web bulbs
near the surface beneath a melting layer aloft it appears the area
could be in for a mix of freezing rain and sleet, with snow also
possible over far northern IL Friday night. There still remains
questions as to the extent of the freezing rain potential across
my far southern areas (well south of I-80) as surface temperatures
could become marginal, or even remain just above freezing through
the night. If this were to occur, there would be little if any
wintry precipitation in these areas. However, farther north
towards the I-80 corridor, it appears that sleet and freezing rain
is a real possibility. This is certainly not the time of year we
would anticipate significant freezing rain over the area, and this
may end up being the case with this event as well. However, with
surface temperatures progged to be just below freezing around and
north of the I-80 corridor, it does appear probable that at least
some minor freezing rain and snow/sleet accumulation could occur
Friday night. However, any accumulations could end up being
limited to elevated and grassy surfaces due to the possibility of
warmer road temperatures, but uncertainty is high with regards to
the extent of accumulations at this early stage. The threat for
wintry precipitation should end during the morning on Saturday.

After this wintry mix, it appears that the rest of Saturday and
Sunday will be dry. Temperatures also look to moderate into the
lower 50s by Sunday. However, forecast guidance suggests that the
weather pattern could become cool and active again by early next
week. In particular is the potential for a rather stout storm
system to impact the region later Monday into Tuesday. This system
could result in rain possibly changing to snow by Monday night
locally, but this will ultimately depend on the actual track that
system takes. Some of the latest deterministic forecast guidance
has shifted most of the active weather with this storm system to
our south, which would keep much of the area dry during this
period. However, at this early stage I am not convinced that we
are out of the woods yet. I would like to see the trends in the
ensembles and deterministic models continue to point in this
direction for the next day or two before pulling all mention of



For the 18Z TAFs...

Main near term concern is wind shift from northwest to northeast
at Chicago terminals early this afternoon as lake breeze moves

Weak surface high pressure ridge was over northwest IL at midday,
with temps warming through the mid-40`s away from the Lake
Michigan shore. This was allowing a lake breeze to develop and
push inland, with northeast winds already noted in surface obs
east of I-294. Expect this lake breeze boundary to push inland and
through ORD/MDW by around 19Z/2 pm CST.

Winds then are expected to remain northeast or east-northeast
through Friday, as surface ridge moves east tonight and low
pressure develops to the west across the Plains. Some patchy
2500-3000 ft cu/stratocu over the lake may try to push inland
later tonight, with some indication of a high-MVFR layer for a
time early Friday, though confidence is low on any persistent
ceiling at this point.



313 AM CDT

A trough of low pressure and associated frontal boundary will
continue to move through the Great Lakes region today. As this
shifts to the east, high pressure to the northwest will build
southeast today into tonight. With this occurring, expect
northerly winds to be in place today into tonight. Gradient over
the area would support continued winds to 30 kt during this
period. Although stronger winds may stay further to the north
across the north half, do think winds to 30 kt are likely over the
south half and winds of 15 to 25 kt along the nearshore. This
will help waves build across the lake, with waves hazardous for
small craft occurring today for the IN nearshore. Later today,
more north to northeast winds will likely be in place over the IL
shore and think these higher waves and hazardous conditions will
reach this location. So have issued a small craft advisory for the
IL nearshore as well, from this afternoon into Friday morning.
With the high moving across the lake on Friday, winds should



LM...Small Craft Advisory...IN nearshore waters until 10 AM Friday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...4 PM Thursday to
     10 AM Friday.




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