Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 091916

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
116 PM CST Sat Dec 9 2017


111 PM CST

Through Sunday...

A convergent band of lake effect snow is moving into northwest
Indiana. Latest observational trends upstream across Lake Michigan
follow the conceptual model that the focus of heavy lake effect
will generally shift east of the area through the afternoon. RAP
boundary layer convergence fields have captured the evolution
fairly well, and it keeps at least a few hour period into early
evening of strong convergence across LaPorte county eastward.
There is still some lingering though weaker convergence in
northern Porter county, and the strong signal is still awfully
close to northeast Porter such that we do not plan any changes to
the winter storm warning for lake effect snow and blowing snow.
And the winds in northeast Porter and southwest LaPorte are
howling, with gusts of 38-45 kt being reported right at the
shoreline. Rapid changes in visibility are expected, though the
strong winds may preclude accumulations from getting out of hand
and keep the snow progressive, at least in our area.

Deep low pressure over southeast Lake Michigan will shift east
and weaken through the afternoon and more so this evening. The
gradient will be strong enough to maintain wind gusts near 30-35
mph through the afternoon even away from northwest Indiana. And
the lake snow belt can continue to expect at least occasional
moderate to briefly heavy snow shower bands into early evening.

Elsewhere, lower clouds will fade this evening. High pressure
will pass well south of the area tonight while a secondary low
pressure trough will develop from the upper Midwest into the
plains. Winds will shift back to southwesterly overnight ahead of
this low that will dive southeast ahead of another sheared vort
max in northwest flow. Model guidance paints just some mid level
clouds and scattered lower clouds. But, the mid level forcing and
warm advection ahead of this low may be able to offset somewhat
drier lower levels for a few flurries overnight with this
feature. The southwest winds will allow temps to bounce back into
the 30s Sunday. The trough axis will pass through in the early
afternoon and a send a weak cold front through, again with most
likely just an increase in clouds.



349 AM CST

Sunday through Friday...

Active weather pattern is expected to persist across the region
through much of the coming week, with a series of waves digging
through northwest flow into large scale eastern CONUS upper
trough. Model guidance does suggest a break down of the western
CONUS upper ridge toward the end of the week, which would flatten
the upper flow pattern somewhat and lead to a period of somewhat
milder weather next weekend.

In the meantime however, northwest flow will persist across the
region. While several minor amplitude mid-level disturbances will
likely ripple through the flow at times, guidance is in pretty
good agreement in depicting a stronger, more amplified short wave
digging out of central Canada and into the Midwest late Sunday
night into Monday. Models develop light qpf into mainly the
northern parts of the cwa by Monday morning, though forecast
soundings indicate only brief periods of better deep saturation,
somewhat similar to the current (Saturday morning) system. Still,
would expect some potential for light accumulating snow early
Monday, especially across far north/northeast parts of the
forecast area. A strong cold front then pushes through Monday
afternoon, with a slug of arctic air spreading into the area
Monday night and Tuesday. 850 mb temps around -16C, and blustery
winds look to support temps only in the low-mid 20`s, and wind
chills in the single digits on Tuesday behind the departing low
and cold front, on what looks to be the coldest day of the week.
While models do not indicate precip into Tuesday, deep adiabatic
lapse rates with the strong cold advection may very well support
some snow showers or flurries.

The cold moderates a bit Wednesday however, as another wave and
associated surface low dig into the region, with a warm front
moving across the area and bringing another chance of some light
snow. By Friday however, upper flow begins to flatten, and the
next low in the active pattern is progged to pass across the
northern Lakes. Temps in the 40`s may be in the cards by the



For the 18Z TAFs...

Gusty northwest winds and MVFR clouds are the concern in the near
term, otherwise concerns for the terminals will remain fairly low
following the afternoon period today. Deep low pressure over
southeast Lake Michigan will shift east and weaken through the
afternoon. The gradient will be strong enough to maintain wind
gusts in the 20s through the afternoon with a 320-340 direction.
In the 22z-0z time frame speeds will relax some to ease 22L
departure concerns for ORD. There could still be a few flurries
across NE IL, but the main snow band will be across northwest

MVFR clouds will fade this evening. High pressure will pass well
south of the area tonight while a secondary low pressure trough
will develop from Lake Superior southeast to Iowa. Winds will
shift back to southwesterly overnight ahead of the weak low with
just some mid level clouds and scattered lower clouds, and maybe
a few flurries overnight. The trough axis will pass through in the
early afternoon, and winds will head back to the northwest with a
lowering VFR deck in place.



317 AM CST

Gale warning is in effect for the southern 2/3 of Lake Michigan
from 6 am this morning into this evening.

Low pressure was moving across central Lake Michigan early this
morning, in association with a deep mid-level disturbance tracking
across the region. Winds will shift to the north-northwest and
quickly increase to gale force across much of the lake this
morning, as the low moves off to the east. Strong pressure rises
and a pressure gradient enhanced by mild lake waters may aid in
producing a period of 45 kt gales by midday/afternoon. Winds will
decrease quickly this evening as the low pulls away and a weak
ridge of high pressure spreads east across the lake overnight.

An active weather pattern will persist into next week, with a
series of low pressure troughs and cold fronts moving through the
region. A weak trough is expected to move across the lake Sunday
afternoon, with a stronger low moving through Monday. This low is
expected to continue to deepen as it moves east of the lake Monday
night, with strong pressure rises and a push of cold arctic air
leading to another period of northwest gales Monday night into
Tuesday. Winds eventually diminish by Wednesday as the next high
pressure ridge moves in, but with another low and potentially
gusty northwest winds again Thursday.



IN...Winter Storm Warning...INZ002 until 9 PM Saturday.

     Lakeshore Flood Advisory...INZ001-INZ002 until midnight Sunday.

LM...Gale Warning...IL nearshore waters until 6 PM Saturday.

     Gale Warning...LMZ743-LMZ744-LMZ745-LMZ777-LMZ779 until 9 PM




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