Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 252343
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
543 PM CST Sat Feb 25 2017

.SHORT TERM...
1159 AM CST

Through Sunday...

Main forecast concerns are ending of light snow showers/flurries
early this afternoon, then very windy conditions Sunday. Could be
approaching wind advisory criteria Sunday afternoon, with gusts 40
mph or higher possible.

Surface low pressure which moved across our area Friday continues
to pull away to the northeast of the Great Lakes early this
afternoon. Mid-level trough/vorticity axis was shifting east of
the area as well per water vapor imagery, which will bring light
snow showers to an end early this afternoon across the eastern
counties of the cwa. A few flurries may linger within cold
advection stratocu deck, though gradual advection of drier air
will tend to thin the cloud deck and allow flurries to gradually
end as well. With thinning of cloud deck, some sunshine will break
through especially across the western cwa, though persistent cold
advection should limit temp rise with highs generally in the mid-
upper 20`s this afternoon with wind chills in the single digits.

Skies clear briefly this evening as weak surface high pressure
ridge approaches, but mid clouds are already pushing east across
western Iowa ahead of a quick-moving mid-level wave which will
move across the area late tonight into Sunday morning. Temps fall
off into the teens/lower 20`s tonight with diminishing winds and
brief clearing, then become more steady overnight as surface winds
turn southwest and cloud deck moves in. Gradient then tightens up
Sunday, with gusty southwest winds developing and spreading warmer
air back into the area. Forecast soundings indicate decreasing
cloudiness in the afternoon especially across the northern half of
the cwa, resulting in deep mixing to nearly 750 mb and tapping
45-50 kt winds. While not all of this momentum will mix to the
surface, the potential for very windy conditions with gusts to
around 40 mph appear likely during the mid-afternoon hours,
perhaps a little lower in the southeast where clouds linger a bit
longer. Could be close to wind advisory criteria (45 mph gusts) so
will have to monitor further guidance trends for clouds/mixing
depths. While guidance highs are generally mid-40`s for Sunday,
progged 925 mb/850 mb temps would support upper 40`s to lower
50`s in some spots under ideal conditions.

Ratzer

&&

.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 00Z TAFs...

Main concern for the period will be very windy conditions
developing Sunday, with gusts of 35-40 kt from the southwest
expected during the afternoon.

As surface low pressure continues to lift northeast Quebec and
high pressure builds across the lower and middle Mississippi
Valley, a moderate wly-wnwly pressure gradient will persist across
the region through the early evening hours. Expect some gusts of
20-25 kt to continue for a couple more hours. As a weak sfc ridge
moves across the region overnight, winds should diminish to less
than 10 kt overnight.  Latest satellite imagery trends indicate
that the lingering mvfr cigs should move east and cigs should
quickly improve to vfr at or shortly after issuance time.

Winds will back to more swly overnight following the passage of
the ridge axis, but will increase significantly again tomorrow
morning. Forecast soundings indicate deep mixing by afternoon,
coincident with loss of a mid-level cloud deck which is expected
to develop overnight. Guidance suggests mixing as deep as
7000-8000 ft by mid/late afternoon, tapping momentum of 40-50 kt
winds aloft. Confidence remains fairly high in winds gusting to
the mid-upper 30 kt range, and potentially a bit higher.
Direction looks to start out around 200 degrees, veering slightly,
to 230 degrees later in the day. Sustained winds and gusts should
begin to diminish at sunset, but expect moderate, gusty winds to
continue into tomorrow evening.

&&

.MARINE...

214 pm...Low pressure over Quebec will continue to move north
tonight with weak ridging moving across Lake Michigan tonight. The
gradient will begin to weaken into this evening and bulk of gale
force winds should diminish this afternoon. Low pressure will move
across the northern lakes Sunday and the gradient will tighten
quickly Sunday morning with southwesterly gale force winds
expected to develop and continue through Sunday afternoon across
the southern part of the lake and into Sunday evening across the
northern part of the lake. Another weak ridge will move across the
lake Sunday night into Monday morning. The next area of low
pressure will move from the central plains to the western lakes on
Tuesday and then a second stronger low will move from the central
plains Tuesday to the southern lakes region Wednesday but
confidence on this solution/track is low. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 9 PM
     Saturday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...8 AM Sunday TO 6 PM Sunday.

     Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 3 AM Sunday.

&&

$$

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