Area Forecast Discussion
Issued by NWS Chicago, IL

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056
FXUS63 KLOT 242036
AFDLOT

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

.MESOSCALE DISCUSSION...
154 PM CST

Closely monitoring cold frontal passage across northern Illinois
and soon northwest Indiana. At 145PM CST, the cold front
stretches from eastern Will County south through eastern Kankakee
County and just east of Rantoul and the fast eastward progression
of the front should clear it of our eastern counties by roughly
3pm or shortly thereafter. Visible satellite imagery indicating
some bumpiness to the clouds immediately along and ahead of the
front. Veered surface winds across the area are resulting in long
straight hodographs across the warm sector with very strong
effective shear noted with roughly 75kt winds at 500mb. Despite
the strong and concerning kinematic environment, marginal low
level lapse rates and a moderate CIN has inhibited convective
growth thus far. SPC meso analysis indicates some erosion of the
CAP immediately ahead of the front, and very steep mid level lapse
rates are resulting in several hundred joules of CAPE. If storms
are able to tap into the better instability there is potential for
supercell development, possibly splitting supercells given the
straight hodographs. Given abundant cloud cover in the warm sector
and the fast motion of the front, probability of anything in the
local CWA is lowering by the minute, but bears close watching for
the next couple hours.

Deubelbeiss

&&

.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 18Z TAFs...

Trends in LIFR cigs and IFR vsbys are the primary aviation
forecast concern in the near term, eventually shifting focus to a
period of light snow late tonight into early Saturday morning with
west-northwest winds becoming gusty 25-30 kts.

Surface low pressure was analyzed just south of the terminals at
midday, along a sharp frontal zone which runs from north of KPNT
to just south of KGYY. North of the front/low track, very moist
low level air mass was trapped beneath the strong frontal
inversion, with cool northeast surface winds and patchy drizzle
helping to maintain LIFR cigs of 300-400 ft and vsbys around 2SM
across the Chicago terminals. Surface low is expected to track
just south of KGYY over the next few hours, with low level winds
backing to north-northwest by late afternoon. Not much in the way
of drying is expected, preventing much in the way of improvement
in the near term. Guidance shows a little variation, perhaps 100-200
ft in cig height over the next few hours, though eventually should
see some slow improvement as winds shift more west-northwest by
early this evening when improvement to IFR cig and MVFR vis is
expected.

The surface low pulls away from the area this evening, while the
upper trough axis approaches and eventually passes across the area
late tonight into early Saturday morning. Deformation precipitation
currently from western Iowa into northwest Wisconsin will move
east across the area during the late evening/midnight time frame,
and is expected to produce a period of light snow through 7-8 am.
Precipitation will likely begin as a mix of rain/snow, before
transitioning to all snow after an hour or two. IFR vsby expected
with some minor accums mainly on grass areas. Snow should taper
off to flurries and end during the mid-late morning. Winds will be
the main concern beyond Saturday morning, with 290-300 direction
and sustained speeds 15-20 kts gusting 25-30 kts.

Ratzer

&&

.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...Dense Fog Advisory...LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 UNTIL 3 PM Friday.

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

     Small Craft Advisory...NEARSHORE WINTHROP TO WILMETTE HARBORS UNTIL 3 PM Friday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...MIDNIGHT
     Saturday TO 9 PM Saturday.

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

&&

$$

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