Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 131945

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
245 PM CDT Tue Mar 13 2018

245 PM CDT

Through Wednesday...

A narrow band of moderate to heavy snow squalls will continue to
impacts northeast IL through the early evening hours. A localized
convergence band is forcing the snow squalls along with an upper
level disturbance. As the upper level disturbance shifts south
through this afternoon, the convergence band will also weaken.

Visibilities will be severely reduced under the snow squalls with
less than a mile to near white-out conditions expected impacting
travel. A quick inch of snow may accumulate under the heaviest snow
bands. The snow will mainly accumulate on elevated and grassy
surfaces, but slushy conditions could develop on pavement.

While most locations will be dry beginning this evening, lake effect
snow may linger over far northeast Porter County. Lake effect snow
will shift east of the forecast area as winds back to northwest
after midnight. Up to an inch of additional snowfall is possible.

Quieter conditions are forecast for Wednesday as high pressure
passes over the Gulf states and a surface low drops southeast over
Ontario.  Expecting a mix of sun and clouds with southwest winds
gusting to around 25 MPH at times. High temps will be warmer in the



213 PM CDT

Wednesday night through Tuesday...

Amplified upper ridge will translate from the Great Plains midweek
to the upper Mississippi Valley through the latter half of the
week. Baroclinic zone settles just to our south through the latter
half of the week with the local area under the influence of high
pressure centered over the Canadian Prairies with ridging
extending into the midwest. This will keep the area dry but on the
cool side with temperatures running slightly below normal. North
flow will further suppress temperatures near the lake front due
to onshore flow.

A strong shortwave is progged to undercut a closed upper low
meandering over the Pacific Northwest late in the week and is
progged to eject over the Central Plains late Thursday/early
Friday. Surface low will deepen in response and track east
across the mid Mississippi Valley by early Saturday. Isentropic
ascent increases ahead of these features with precipitation
overspreading central Illinois Friday night and Saturday morning.
Latest suite of guidance in reasonable agreement through this
point with the northern edge of the precip shield staying at or
just south of the southern tier of counties in the CWA. This looks
to be the best chance for any precipitation nearby through the
weekend, but most of the CWA will be dry.

Given the overall pattern characterized by blocky/amplified upper
flow with closed off features, model agreement becomes quite poor
by early next week. Guidance tries to break down the block with
additional shortwaves moving across the region. Given model
differences and their propensity to break down these patterns too
quickly, confidence in the details is low, but for now will carry
chance PoPs Monday through Tuesday to account for these



For the 18Z TAFs...

Scattered snow showers will impact the eastern terminals through
this afternoon. ORD, MDW, and DPA will likely see the most snow
and lowest visibilities under the heavier snow showers. Forcing
responsible for the moderate to at times heavy snow will shift
south and the more enhance snow showers will shift south of the
terminals. Visibilities have been dropping to 1 mile or less
under the heavier snow squalls, so I included a tempo for IFR
vsby. Brief periods of MVFR cigs are also possible, but the
majority of this afternoon will be VFR.

RFD will likely only see flurries through this afternoon with no
accumulation expected.

North to NNE winds will back through the night becoming southwest
by Wednesday morning. Dry, VFR conditions are expected Wednesday.



213 PM CDT

The Great Lakes fall between a strong low off the east coast and
high pressure over the upper Mississippi Valley resulting in a
moderate northerly flow across Lake Michigan. NNW winds will
continue to gust to around 30 kt then will begin to diminish
overnight as the ridge axis begins to move over the region. A
small craft advisory remains in effect for the Illinois nearshore
zones through late this evening and for the Indiana zones into
Wednesday morning where waves will persist longer. Winds back to
the southwest Wednesday behind the ridge axis and ahead of low
pressure over the Canadian Prairies. A trailing cold front will
move down the lake Wednesday night with winds veering back to
northerly through the remainder of the week. BMD


LM...Small Craft Advisory...IN nearshore waters until 10 AM

     Small Craft Advisory...IL nearshore waters until 4 AM Wednesday.




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