Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250858
AFDLOT

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

.SHORT TERM...
256 AM CST

Through tonight...

Main forecast concerns are with snow showers across the entire
CWA early this morning, and then with a much colder day today.

Upper level low is lifting across the region this morning, with
stout vort lobe swinging around it. At the surface, low pressure
is well northeast of the CWA, but with trough axis on the backside
of this system continuing to push through the area. With these
features in place, have seen precip blossom overnight. Precip did
initially see a brief wintry mix despite a quickly cooling column,
as deeper moisture was lacking. Overall trends would suggest that
the bulk of any precip this morning is all snow. However, the far
southern tier of counties may still observe a brief rain snow mix
before all snow is then quickly observed. Do think this snow will
continue across the area for the next 2-3 hours, before exiting.
Minor accumulations still expected with the highest amounts likely
closer to the IL/WI border, where deformation snow has been more
persistent. However, have seen more showery type snow for
remaining areas in northern Illinois this morning, including the
southern tier of counties that could support some quick minor
accumulations as well. This is especially possible for those areas
from Pontiac east to Watseka and Fowler IN, where better low
level convergence is allowing for slightly stronger snow showers.
Given the recent warm temperatures, the bulk of any accumulation
will be on grassy surfaces. However, lesser used roads may become
snow covered.

Strong CAA still occurring this morning, with temps steadily
falling into the 20s. With temps staying in the 20s for most
locations today and with gusty northwest winds, it will feel
rather chilly today. Back edge of the stratus is expected to reach
the western CWA later this morning. However, pattern would
suggest additional stratocu development throughout the day, and so
have held onto cloud cover a little longer today. Clearing skies
then expected later this evening, but anticipate clouds to return
with the arrival of mid/upper level cloud cover associated with
approaching mid level trough.

Rodriguez

&&

.LONG TERM...

Saturday night through Friday...

204 pm...A ridge of high pressure will move across the area
Saturday night as low pressure moves across the upper Great lakes
Sunday. The gradient will tighten between these two features and
Southerly winds are expected to become strong and gusty Sunday
with gusts as high as 30-35 mph. Models are now in better
agreement with next system and its precip remaining south of the
cwa Sunday night with perhaps a few sprinkles or flurries possible
in the far south. Temps moderate back into the 40s on Sunday and
then back into the 40s Monday...possibly lower 50s in the south
and with the light gradient...a lake breeze is possible Monday
afternoon.

Models also in better agreement with weaker low pressure moving
across the western Great Lakes region Tuesday which then begins to
deepen Tuesday night as its moving away from the area. Only the
Canadian remains the outlier with low pressure developing along
the trailing cold front across the Ohio Valley Wednesday. Trending
toward the Gfs/Ecmwf brings a chance of rain into the area Monday
night that lingers into Tuesday with precip type mainly rain
based on these current trends.

Colder air spreads across the region midweek with a clipper system
still shown by the models for the Thursday/Thursday night time
period...but now further north. Still some potential for snow with
this system but confidence is low. Another shot of colder air is
expected as this system passes east of the area Thursday night.
cms

&&

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

Trends in cigs and vis will remain the primary aviation forecast
concerns through the night, but there will also be some concern
regarding the impacts of a period of light precipitation
overnight. Pcpn has already changed over from freezing drizzle to
snow at RFD and the Chicago area terminals should still see a
short period of light rain/drizzle/freezing drizzle before
changing over to all light snow as colder air filters across the
region. A secondary concern will be wnwly winds becoming gusty at
25-30 kts late tonight and through the day tomorrow..

Surface low pressure is passing across Lake Huron with a sharp
cold front extending southwest to the lower Mississippi Valley and
a sharp warm front extending east through the eastern Great Lakes
region. Winds are backing to nwly with cooler air filtering
across the region. Expect that cigs will gradually improve
through the night as the colder, drier air filters in at the lower
levels.

As the surface low continues to pull away from the area, the
upper trough axis will pass across the area late tonight into
early tomorrow morning. An area of light snow is overspreading
ncntrl IL, with some drizzle and freezing drizzle immediately
preceding the snow. The pcpn should change over to snow at the
Chicago area terminals shortly after midnight. This is expected to
produce a period of light snow through tomorrow morning. Snow
should taper off to flurries and end by mid-late morning. Winds
will be the main concern through the day tomorrow, with 290-300
direction and sustained speeds 15-20 kts gusting to 25-30 kts.

&&

.MARINE...
143 AM CST

The pattern remains active over the Great Lakes this weekend.
Departing low pressure from the central lakes this morning will
continue cold advection and at least occasional northwest gales
are likely across the north and central, while more marginal
south. Weak ridging tonight will ease the winds only temporarily,
as a quick moving low over the northern lakes on Sunday will
result in stout southwest winds. The air mass looks inherently
cool enough going into Sunday to prevent an inversion and likely
support southwest gales during the day Sunday, especially central
and south. Some near gale force gusts are possible in the Illinois
and Indiana nearshore on Sunday.

The next chance for higher wind speeds comes Wednesday behind a
northeastward moving low pressure which passes during or near
Tuesday night/early Wednesday. Still a decent amount of spread in
our computer model guidance on this system, but this could
support some near gales, as well as more of a due north wind
direction.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Gale Warning...LMZ777-LMZ779...6 AM Saturday TO 3 PM Saturday.

     Small Craft Advisory...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 9 PM
     Saturday.

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

&&

$$

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