Area Forecast Discussion
Issued by NWS Chicago, IL

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137
FXUS63 KLOT 262000
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
200 PM CST Sun Feb 26 2017

.SHORT TERM...

Through Monday...

200 pm...Strong/gusty southwest winds this afternoon will quickly
diminish with sunset and as the gradient weakens some overnight...
speeds will likely diminish under 10 mph. The gradient tightens
some Monday and forecast soundings suggest southerly winds may
increase a bit more than guidance is suggesting...in the 10-20
mph range. This should be strong enough to prevent a lake breeze
from forming but with temps rising toward 50...some southeast
component along the IL shore seems reasonable and then winds turn
southeasterly by Monday evening. Thus...some cooling near the lake
is possible.

Most of the models show an area of mid clouds developing across
the area late this afternoon into this evening and this appears to
already be developing across southeast IA. Enough instability in
this layer suggests some isolated showers will develop and light
qpf is indicated from guidance this evening. Low levels remain
fairly dry so low confidence regarding how much of this precip may
reach the ground. Enough of a signal to add a chance of sprinkles
for early this evening...mainly across the center part of the
cwa...but trends will need to be monitored for possible isolated
shower mention. cms

&&

.LONG TERM...
338 AM CST

Monday through Saturday...

Conditions begin dry on Monday, with southerly flow in place and
while continued warming occurs ahead of approaching mid level
energy. More defined shortwave trough is forecast to lift
northeast through the central CONUS Monday night into Tuesday,
with associated surface low lifting northeast into Iowa. Fairly
energetic flow and stronger WAA should allow showers to develop
ahead of this system, and so have continued shower chances Monday
night. This precip shield likely exits by Tuesday morning, but as
this low approaches northern Illinois later in the day Tuesday,
will likely see showers blossom across the region once again. With
increasing moisture and instability, do have some concerns for
thunderstorms and have now included mention of thunder. At this
time, this appears to stay confined to the southern CWA and likely
occurring in the afternoon and evening. As this low then departs
to the northeast, larger mid/upper level trough pushing through
the central Plains will allow for additional cyclogenesis to
occur. Additional precip then should develop and quickly lift
northeast across the region. Still some model variability, but am
monitoring for a colder solution to be in place with the arrival
of this precip. If this colder air pushes in quicker as the upper
level trough moves overhead, then could see more snow than
currently forecast. Pattern still appearing active on the backside
of this system, with guidance still indicating a fast moving
system to push southeast through the region Thursday into Thursday
night. With colder air in place, precip would be snow.

Rodriguez

&&

.AVIATION...
For the 18Z TAFs...

1135 am...Only forecast concern this period are winds through
sunset. Low levels will mix fairly deeply through sunset and this
will allow strong gusts into the mid 30kt range to mix down to the
surface. Possible for some isolated higher gusts. Speeds and
gusts should diminish quickly with sunset though some gusts may
persist into the early evening. Speeds will likely drop below
10kts by mid evening. Winds turn more southerly Monday morning and
forecast soundings suggest speeds may increase to 10-15kts with
some higher gusts possible but confidence is low.

Mid clouds this afternoon may lower to 5-6kft this evening as a
weak wave moves across the area. Some of the models suggest there
could be an isolated shower but the low levels remain dry and if
precip were to develop...it would likely be just sprinkles/
flurries so no mention with this forecast. cms

&&

.MARINE...

200 pm...A tight gradient remains across the lake this afternoon
with low pressure over Lake Superior and high pressure over the
Ohio Valley. The gradient will slowly relax this evening with
gales diminishing late this afternoon into early this evening...
ending over the northern portions of the lake last. The gradient
will weaken some Monday morning with a period of light winds
possible. Southerly winds then return Monday shifting southeast
Monday night and increasing as low pressure moves across the
western Great Lakes Tuesday. Winds will shift northerly behind
this low Tuesday night into Wednesday and there remains
uncertainty regarding how strong winds will become. The current
expected gradient and extent of colder air spreading across region
would likely not support gale force winds...but changes to the
track and strengthen of this low may result in stronger winds in
later forecasts. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 UNTIL 6 PM Sunday.

&&

$$

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