Area Forecast Discussion
Issued by NWS Chicago, IL

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899
FXUS63 KLOT 260910
AFDLOT

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

.SHORT TERM...
309 AM CST

Through tonight...

Main forecast concerns are with scattered flurries mainly across
northern Illinois this morning, and gusty southwest winds later
this morning into the afternoon.

Surface ridge axis pushing east through the region early this
morning, with dry conditions and mainly partly cloudy skies in
place. However, mid/high level cloud cover is increasing across
northern Illinois ahead of approaching mid level energy and WAA.
Do think most locations will remain dry today, but am monitoring
weak returns pushing east over Iowa at this time. The bulk of
these returns are not likely reaching the ground given the amount
of drier air currently present in the lower levels. However, some
isolated reports of flurries have been observed across Iowa. As
the approaching mid level wave catches up to the stronger WAA,
while encountering steeper lapse rates, do think some further
blossoming of this precip and lowering ceilings could occur as it
reaches northern Illinois over the next one to two hours. So have
made mention of scattered flurries through mid morning, mainly for
areas north of I-80. At this time only expecting flurries, and so
don`t anticipate any snow accumulation.

Overall cloud trends may be tricky through the day, especially
with some breaks in this progressive mid/high level cloud cover.
Continued partly cloudy to partly sunny skies for this morning,
but do have clearing occurring this afternoon. This will likely
affect high temps for today, but with the advection of warmer
temps, still anticipate temps well into the 40s. While surface
ridge shifts to the east this morning, gradient and flow aloft
really increase throughout today. Thicker cloud initially in place
this morning may limit some daytime mixing, but as this departs
later in the day, should begin to really tap into the stronger
flow aloft. Highest gusts should stay in the 35-40 MPH range, just
below wind advisory criteria today. However, higher speeds at the
top of the mixed layer could reach the surface, providing
slightly higher gusts the currently forecast. Will need to
continue to monitor trends with the potential for stronger speeds
later this morning into the afternoon.

Rodriguez

&&

.LONG TERM...

Sunday night through Saturday...

214 pm...Main forecast concern is a storm system to affect the
region midweek.

The gradient will slowly be relaxing Sunday evening as high
pressure moves off the Carolina coast and low pressure over the
northern lakes moves east. Some gusty southwesterly winds may
persist in the evening but the trend should be steadily down with
sunset. While the gradient should support southerly winds into
Monday it may weaken enough for a lake breeze to develop or for
winds to turn southeasterly on the Illinois shore. Confidence is
low but some onshore shift is possible Monday afternoon.

Low pressure is expected to develop over the plains Monday night
with a warm front lifting north across the area Monday night into
Tuesday morning as this low moves across the western Great Lakes.
A period of showers is possible with this front and winds will
likely turn more easterly north of the front. Just about all the
models suggest this boundary will move across much of the area
Tuesday afternoon with a lull in the precip by midday. A second
low will develop over the south/central plains Tuesday and move
to the southern lakes region Wednesday and this current forecast
is quite a bit different than what most of the models were showing
24 hours ago. As a result...confidence with this solution is low.
As the low approaches...winds will likely shift northerly over
the northern cwa through thermal profiles suggest precip should
remain mainly rain until the colder air spreads into northwest IL
Wednesday afternoon/evening. By that time...the bulk of the qpf
has shifted east so snowfall amounts look minor at this time. But
shifts in this track and timing as well as amount of cold air
spreading in could all result in large changes to this forecast
and uncertainty is high.

The clipper system for Thursday night from 2 days ago...that
shifted well north 1 day ago is now back across the cwa Thursday
night. Temps at this time appear to be cold enough to support all
snow but track as well as the amount of qpf with this system
remain uncertain and changes are likely. Much colder air behind
this clipper appears short lived as warmer air spreads back
across the region next weekend. cms

&&

.AVIATION...
For the 06Z TAFs...

Only real aviation concern is strong winds Sunday from late
morning through the afternoon. Some uncertainty exists regarding
just how strong gusts will get, but if skies clear out some as
forecast, then gusts to around 35kt from the SSW appear likely
midday through mid afternoon. If clouds hang around a bit longer
into the afternoon then gusts will likely be a bitter lower than
indicated in the TAFs.

Izzi

&&

.MARINE...
153 AM CST

The lake will find itself within a tight surface pressure gradient
today resulting in strong southwest winds. Low pressure just to
the north of the Great Lakes will be moving quickly east driving
these southwest winds with temperature profiles appearing
unstable enough to favor gale gusts. Some occasional 40 kts look
likely over the southern part of the lake. A cold front will move
across the northern part of the lake this evening turning winds
west-northwest, but speeds will be on their way down fairly
quickly later this evening.

The active pattern continues into midweek with a pair of low
pressures approaching from the southwest and merging into one
deepening low near Lake Huron by midday Wednesday. There is a high
amount of model spread on how strong the merged low will be and
thus the magnitude of north to northwest wind speeds behind it
over Lake Michigan on Wednesday afternoon and night. Gales are
certainly possible given the pattern and influx of cold air
forecast.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...9 AM
     Sunday TO 6 PM Sunday.

     Gale Warning...LMZ777-LMZ779...8 AM Sunday TO 6 PM Sunday.

&&

$$

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