Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 250114
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
714 PM CST Tue Jan 24 2017

.UPDATE...
705 PM CST

This initial band of precipitation has largely been a rain/sleet
mix, with some snow likely mixed in as well, especially along the
Wisconsin border. The sleet should be relatively short lived and
confined to the heavier precipitation elements, with the lighter
echoes turning to more of a rain profile.

The next batch of precipitation will return overnight, with the
peak period of forcing occurring between 1 am and 5 am as the
coupled northward moving warm front and upper forcing is best co-
located. This looks to be rain south as warmer air wraps in, with
snow along the Wisconsin border, especially in the period of
heavier precipitation which could really only be a few hour
window, with some rain/snow/sleet mix in between, again with the
heavier echoes more of the wintry type.

KMD

&&

.SHORT TERM...
319 PM CST

Tonight through Wednesday night...

Surface low pressure over the Central Plains will lift northeast
to near the IL/WI border through midday Wednesday, in association
with a deepening upper level low. Models have trended deeper,
slower and a bit farther south with the track of the surface
circulation since yesterday, which increases the concern for mixed
rain/snow or a period of just wet snow across far northern IL
tonight before the column warms enough for those locales to change
over to rain early Wednesday morning. Initial isentropic ascent in
warm advection ahead of this system is leading to development of
some banded mid-level returns across the area, with wet-bulb
cooling in dry lower levels of the column resulting in some
isolated light rain/sleet/snow reaching the surface across north
central Illinois. Forecast soundings depict these large temp-dew
point spreads and sub-freezing wet bulbs between 900-700 mb,
supportive of some mixed p-types as precipitation develops this
evening. While strong warm advection eventually warms the column
over the central/southern parts of the cwa enough for mainly rain
through tonight, strong ascent and dynamic cooling of the column
may keep areas north of the I-88 corridor (and especially farther
north toward the IL/WI border) wet snow or a rain/snow mix.
Despite meager snow/liquid ratios and relatively warm surface
temps (mid-30`s), may very well see some wet accums perhaps up to
an inch or so overnight. Isolated thunderstorms are also possible
overnight as steep mid-level lapse rates spread across the
northern parts of the forecast area ahead of the approaching
upper trough. Temperatures edge up Wednesday morning as the
surface moves across the area and the occluding cold front moves
through. Southern counties of the cwa will flirt with 50 degrees,
while even near the WI border the mercury approaches 40.

Colder air then spreads back in from the northwest late Wednesday
and Wednesday night as the system begins to move off to the east.
Forecast soundings depict the column cooling enough to begin to
mix rain/snow across the far north/northwest counties early
Wednesday evening, eventually changing to light wet snow across
the cwa overnight with the (by then) weakening trailing upper
trough axis. Additional light accums generally less than an inch
are expected area-wide during the night.

Temps have been suppressed by persistent cloud cover today, and
will hold in the mid-upper 30`s in most areas tonight, before
warming into the 39-49 range from north to south Wednesday.

Ratzer

&&

.LONG TERM...
319 PM CST

Thursday through Tuesday...

Upper pattern shifts to deep positive-tilt trough from eastern
Canada to the Midwest in the wake of the strong mid-week low
pressure system. This keeps the Great Lakes region in an area of
northwest winds, cyclonic flow, and a series of reinforcing short
waves which maintain temperatures slightly below normal through
the weekend. Flurries and occasional light snow showers can be
expected on and off through this period, with perhaps the best
chance of more organized light snow/snow showers Saturday night as
a deeper short wave digs across the area from the north.
Otherwise, hard to pinpoint timing of snow shower periods at this
distance.

Upper ridging eventually develops across the central CONUS Monday
and shifts east across the area into Tuesday ahead of another
northwest flow trough. A bit of a warm up appears in store in
advance of this next system, but it may be short-lived per
extended guidance.

Ratzer

&&

.AVIATION...
For the 00Z TAFs...

549 pm...Multiple forecast concerns this period including the
potential for mixed precip this evening and overnight as well as
thunderstorm potential overnight. Additional concerns are cigs/vis
Wednesday and rain changing to snow Wednesday evening.

Low pressure over eastern Kansas will move to northern Illinois by
early Wednesday afternoon. Easterly winds will increase into the
10-15kt range by later this evening with gusts developing
overnight. Winds will gradually turn more southeasterly Wednesday
morning and then shift to the southwest Wednesday afternoon.
Specific direction/timing changes will need further refinement as
the track of the low approaches. With a track further south...
winds may end up more variable Wednesday morning. Winds will
shift more to the northwest Wednesday evening as speeds/gusts
increase.

Three waves of precip to affect the terminals this period. The
first extends from near the Quad Cities southeast to cmi. As this
lifts north this evening...there will likely be a period of rain
that may mix with snow or sleet. The further north this precip
extends...including into rfd...the more of a mix or prevailing
snow may develop. Precip duration with this first wave should be
short...1-2 hours.

Second wave of precip looks to develop over the terminals after
midnight as a stronger upper wave moves across the area. This will
likely produce convective precip that may result in at latest
isolated thunderstorms. Current vicinity thunder mention seems
reasonable but tempo thunder mention may be needed with later
forecasts. With precip rates expected to be briefly moderate...its
possible precip type may become mixed with snow/sleet with the
heavier precip. Duration again looks to be short...1-2 hours.
Further northwest at rfd...precip may persist longer and remain
mainly snow for a few hours. Once this wave lifts north of the
area...light showers or drizzle will be possible into Wednesday
morning.

Third wave of precip will arrive later Wednesday afternoon and
continue into Wednesday evening. Precip type should initially be
rain but as colder air spreads into the area...precip will likely
mix with and then change to light snow.

Mvfr cigs 1-2kft this evening are expected to lift to high mvfr by
late evening after the first wave of precip and many scatter out
briefly before ifr cigs redevelop after midnight. Cigs will
continue to lower overnight into Wednesday morning as the surface
low arrives and cigs lower to lifr and possibly vlifr in some
areas...dependent on the track of the low. Vis may also drop to
1sm or less in some areas across northwest IL. Confidence is low
regarding how far to the southeast across the terminals vis/cigs
will lower. When winds shift to the southwest...some improvement
in vis/cigs should occur by early/mid afternoon. cms

&&

.MARINE...
222 PM CST

Relatively lighter winds in place across the lake this afternoon
as high pressure builds across the region. Dense fog had still
been present over the north half of the lake earlier this morning
and so had extended the dense fog advisory during that time.
However, recent obs would suggest that this dense fog is no longer
present. Have decided to cancel the dense fog advisory, but do
still think there might be some isolated areas of fog and will
continue to monitor trends this afternoon/evening. The lighter
winds in place will begin to increase later this evening/overnight
as low pressure over the central Plains tracks northeast towards
the southern end of the lake. Winds and waves do approach
hazardous conditions for small craft along the nearshore later
tonight into Wednesday morning, but they should stay just below
before they lighten with passage of the low. On the backside of
this system on Wednesday, winds could once again approach small
craft advisory criteria. However, confidence is low at this time.
Another period of concern for higher winds across the lake will be
Wednesday night into Thursday. Do feel fairly confident for higher
winds across the nearshore, but do think speeds could be too low
over the open waters during this time. Once again confidence is
low, but some guidance is showing the possibility for gales over
the south half. Have not included that in the forecast but will
continue to monitor this potential as well.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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