Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 262110

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


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 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


309 PM CST

Monday Night through Sunday...

Surface low pressure develops over the Central Plains Monday
night, and moves into the Great Lakes region through Tuesday
night in association with an upper level trough. Broad south-
southwest lower tropospheric flow develops Monday night in
response to pressure/height falls across the Plains, and a lead
short wave lifting across the upper Midwest and northern Great
Lakes. A 45 kt southwesterly low level jet develops by late Monday
night, with associated isentropic ascent and moisture transport
leading to development of showers especially after midnight.
Forecast soundings indicate steep lapse rates above about 600 mb
especially over northwest parts of the cwa, which may aid in
development of a few thunderstorms on the nose of the moistening
layer above the warm frontal inversion. The initial short wave
then lifts northeast of the area during Tuesday morning, with a
decrease in precip coverage and intensity. Shower and thunderstorm
potential should increase again Tuesday afternoon and evening, as
large scale height falls strengthen with the approach of the main
upper trough and surface low and warm front. It should be noted
that differences exist in model handling of the phasing of several
smaller amplitude short waves within the larger scale upper trough
during the Tuesday-Wednesday period, with attendant differences
in strength and track of the primary surface low and surface warm
front. This has implications with far the low-level warm sector
advances northward across the cwa, lending a little lower than
average confidence to details of convective trends/intensity and
temperatures Tuesday afternoon/evening. Most guidance does bring
much of the cwa into the warm sector however, with the surface
warm front either across or north of much of the area by late
tuesday. Proximity and timing suggest some potential for
thunderstorm activity into Tuesday night within the warm sector
and ahead of the cold front overnight. Strong tropospheric wind
fields (145 kt upper jet streak and 40-50 kt mid-level winds) atop
an anomalously warm/moist warm sector for late February certainly
implies the threat for severe thunderstorms, though expected
extensive cloud cover and warm nose above the boundary layer
(around +10 C at 850 mb) may be a limiting factor in developing
near-surface based convection. SPC day 3 outlook does include much
of the southern 2/3 of the cwa in a marginal risk for severe
storms, with higher risk just to our south. Will therefore need to
continue to monitor trends for more detail in severe threat. Even
with cloud cover, surface temps should rise into the upper 50`s
north and into the mid-upper 60`s south, though depending on the
position of the low/warm front may have to watch for a
southeasterly wind component off of the lake for the IL shore
especially north of Chicago Tuesday afternoon.

Model differences continue as the cold front and surface low move
east/northeast of the area Wednesday, still stemming from
different handling of phasing of mid-level energy. This leads to
significant differences with the strength and duration of
deformation precipitation into Wednesday evening, as well as speed
of low level cold air return. Overall, favor using a blend of
global guidance (ECMWF/GFS), cooling of the column enough for a
mix/change over to snow should occur during the afternoon/evening
hours, though with most of the precip ending before changing to

Attention then turns to a clipper-type system which drops quickly
southeast across the region Thursday into Thursday evening. Again,
track and speed details show differences between various
guidance, and a blend has been used. Low level temperatures appear
marginal for snow during the day, especially south, but some mix
of rain/snow or perhaps all snow far north is possible.

A period of quiet weather is expected Friday into the weekend, as
broad upper ridging develops across the central CONUS, with a warm
up expected into Sunday ahead of an upper trough progged develop
eastward across the area Sunday-Monday.



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 would likely be just sprinkles/
flurries so no mention with this forecast. cms



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


LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 UNTIL 6 PM Sunday.




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