Area Forecast Discussion
Issued by NWS Chicago, IL

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161
FXUS63 KLOT 250013
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
613 PM CST Fri Feb 24 2017

.SHORT TERM...
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.

Ratzer

&&

.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 00Z TAFs...

Trends in cigs and vis will remain the primary aviation forecast
concerns through the evening, but then focus will shift focus to
the impacts of a period of light snow late tonight into early
Saturday morning. A secondary concern will be wnwly winds
becoming gusty at 25-30 kts.

Surface low pressure is passing northeast through central lower
Michigan with a sharp cold front extending southwest to the lower
Mississippi Valley and a warm front extending east through the
eastern Great Lakes region. Winds are backing to nwly with cooler
air filtering across the region. Not much in the way of drying is
expected through the night, preventing much in the way of
improvement in the near term, cigs hovering between ifr and ocnly
lifr. Guidance still shows a little variation in cig heights over
the next few hours, but expect that cigs should gradually lift
overnight to prevailing lower end mvfr through the early morning
hours as winds shift more wnwly.

As the surface low continues to pull away from the area, the
upper trough axis will pass across the area late tonight into
early Saturday morning. An area of precipitation currently from
eastern Iowa into western Wisconsin will move east across the
area during the late evening and overnight hours. This is expected
to produce a period of light snow through tomorrow morning.
Precipitation will likely begin as a mix of rain/snow, before
transitioning to all snow after an hour or two. IFR vsby is
expected with some minor accums mainly on grass areas. 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...

204 pm...Low pressure over southwest lower Michigan this
afternoon will move northeast tonight reaching Quebec on Saturday
as it slowly deepens. Northeasterly gales on the northern part of
Lake Michigan will diminish this evening...then as colder air
spreads across the region tonight...combined with a tightening
pressure gradient...northwesterly gales are expected to develop
and continue through early Saturday afternoon. A ridge of high
pressure will move across the southern lakes region and the Ohio
Valley Saturday night as low pressure moves across the upper lakes
Sunday. The gradient will quickly tighten between these two
features and southerly winds may increase to low end gales Sunday
morning and continue through Sunday afternoon. Winds will shift
westerly Sunday night behind a cold front and then become
southerly Monday into Tuesday with another area of low pressure
expected to move across the northern lakes region Tuesday night or
Wednesday. cms

&&

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

     Small Craft Advisory...LMZ741-LMZ742-LMZ743...MIDNIGHT Saturday
     TO 9 PM Saturday.

     Small Craft Advisory...LMZ744-LMZ745...MIDNIGHT Saturday TO 3 AM
     Sunday.

     Small Craft Advisory...NEARSHORE WINTHROP TO WILMETTE HARBORS UNTIL 9 PM Saturday.

&&

$$

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