Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250743

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

236 PM CST

Tonight through Saturday...

Period of light accumulating snow overnight, colder temperatures
and increasingly gusty west-northwest winds are main forecast
concerns through the short term.

Surface low pressure was analyzed along the south shore of Lake
Michigan near Michigan City Indiana at 19Z/1 pm CST, with an
occluding cold front trailing into far east-central IL. Large
temperature differential noted across the strong baroclinic zone
associated with this system, ranging from the mid-upper 30`s over
far northern IL to the mid-60`s across the far southeast WFO LOT
cwa counties. Extensive low stratus, drizzle and fog have
persisted on the cold side of the surface warm front and low
track, while showers and thunderstorms were developing ahead of
the occluded front from near Danville to Benton Harbor MI. These
showers and storms will move quickly east of the cwa in the next
1-2 hours, with any severe threat expected to be mainly east of
our forecast area as well.

Attention then shifts to the parent upper trough, and a band of
deformation precipitation to our west across IA/MN and northwest
WI. The mid-level trough will weaken/open up as it propagates east
across the forecast area tonight into early Saturday morning, with
the attendant deformation precipitation band also expected to
become more diffuse as the lower-middle level baroclinic zone
weakens. Patchy drizzle within the dry slot currently across the
area will give way to light snow as the deformation band moves
east, beginning in north central IL by mid-late evening and across
the Chicago area closer to midnight. Light snow may mix briefly
with rain at onset, with relatively warm low-levels, but should
turn over to light snow within 1-2 hours. Deepest saturation will
remain across WI and far northern IL, mainly north of I-88, where
perhaps an inch or so of accumulation is possible. Gradual
weakening of deformation band should result in lower amounts (less
than an inch) farther east into northern parts of Chicagoland,
and little/no accumulation is expected along/south of I-80. Snow
will end from west to east early Saturday morning, likely clearing
far eastern parts of the forecast area by 8-9 am CST.

Winds will shift to the west-northwest this evening as the low
lifts out to the northeast, with blustery conditions anticipated
in the lingering tight surface pressure gradient. Winds will
gradually increase to 20-30 mph by Saturday morning. This will
provide persistent advection of colder air from the upper Midwest,
with temperatures cooling off into the mid-upper 20`s overnight
and only rising modestly a few degrees Saturday. While a few peeks
of sun are possible mainly west Saturday afternoon, skies will
otherwise be mostly cloudy.




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

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.


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

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.


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



LM...Gale Warning...LMZ777-LMZ779...6 AM Saturday TO 3 PM Saturday.


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




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