Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151804

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1204 PM CST Sun Jan 15 2017

237 AM CST

Through Monday night...

Going forecast remains largely on track with the biggest change
being slowing the onset of the wintry precip. Strong Canadian high
over the area will move off to the east today as a storm system
organizes over the southern plains. As is typical with closed
off upper system, model guidance has been gradually trending a bit
slower each run, and it now appears that precip will hold off
until after midnight tonight.

Dry low levels will need to be overcome via top-down saturation
late tonight into Monday morning and its possible a lot of the
initial band of precip will be lost to virga, especially eastern
CWA. Typically, top-down saturation events like this start out as
sleet once precip does penetrate the dry layer, though as column
saturates and gradually warms would look for precip to transition
to light freezing rain showers. Temperatures should be slowly
climbing Monday with the rain vs freezing rain line likely
steadily moving north through the morning, likely making it to the
WI border by early afternoon. Moderately confident that a winter
wx advisory will eventually be needed, particularly northern and
western CWA, but given we`re looking at mainly a 3rd period event
will allow day shift to get another look at things and hash out
details like which counties to include and timing.

Precip looks to continue Monday night as system closes in on the
area. Guidance generally a bit slower weakening the system
compared with previous runs, so given the fairly strong forcing
and model depiction of a couple hundred j/kg of MUCAPE, thanks
largely to colder mid levels, have opted to introduce a slight
chance of thunder Monday night. Bigger story Monday night in
addition to the showers, could end up being the areas of fog,
potentially dense, that develop as higher dewpoints surge north
into the area ahead of the sfc low.



237 AM CST

Tuesday through Saturday...

Showers look to have ended by Tuesday morning, but a weaker
upstream short wave could be enough to squeeze out a couple light
showers Tuesday and certainly should keep the cloud cover around.
Temps Tuesday into Wed look to remain modestly above average for
what is climatologically the coldest time of the year.

By late in the week and through the weekend, medium range models
develop a strong/high amplitude northern stream upper level ridge
over eastern NOAM. ECMWF and GFS have flip flopped in their
handling of southern stream shortwave potentially affecting the
area in the Friday time frame bringing rain and keeping temps
"only" solidly above normal rather than WAY above normal. Until
medium range guidance is more consistent with each other and run
to run, no changes made to blended model solution for late week
into the weekend. Any day (Fri-Sun) that is dry without fog or
low stratus being a big factor could see highs soar well into the
50s, if rain, stratus, or fog are prominent then highs may "only"
reach into the upper 40s/low 50s. Normal highs are near 30 and
lows in the teens, so temps during this period should generally be
20F+ above average during the day and >25F above average at night.



For the 18Z TAFs...

Quiet weather and primarily VFR conditions will prevail through
the remainder of the day with lgt/vrbl winds and increasing mid
and high cloud cover. The main forecast concern will be centered
around an of precipitation lifting across the region Monday
morning and associated precipitation type issues.

High pressure over the area today is resulting in calm to ocnly
light and variable winds, but expected to trend to a more ssely
over night tonight.

Low pressure is expected to lift from the south high plains to ern
Kansas by tomorrow morning. The trend will be for the atmosphere
to gradually saturate with cigs lowering to mvfr by early tomorrow
morning. Expect that cigs will continue to lower through the
morning especially as pcpn potential increases. The latest
guidance continues to suggest a slightly slower pcpn trend,
spreading from southwest to northeast and not beginning at the
terminals around daybreak. Latest model forecast soundings are
trending toward a very shallow sub-freezing layer at the surface
with a warm layer aloft. The depth of the surface based freezing
layer is looking shallower that previous guidance had suggested,
which would, in turn, limit the sleet potential. Also, the warm
layer aloft with temps above freezing is already in place that
temps within the warm layer should increase with time, so do not
expect much potential for snow at pcpn onset and will go with fzra
as the predominant precipitation type. The chances for some sleet
to mix in with the freezing rain certainly is not out of the
question, but is appearing less likely.

Precipitation coverage and associated cig/vis impacts will become
more of a concern for the morning with cigs expected to lower to
ifr through the morning and continue to lower through the day. The
window of opportunity for freezing pcpn is also looking to be a
little shorter with the warmer air at the sfc possibly coming in
quite quickly. Currently would anticipate that the fzra should
transition to all liquid by arnd 18z, but there is the potential
for the change over to be a little earlier if the warm air can
move in quicker.

As the sfc low track newd to nrn MO by tomorrow afternoon,
cigs/vis should continue to lower, with lifr cigs possible in the
afternoon. Looking a little past the TAF period, there is a
developing potential for dense fog to move in tomorrow night as
the warm, moist air at the sfc continues to stream into the


304 AM CST

High pressure is centered over the Upper Midwest and western Great
Lakes early this morning resulting in light flow across lake
Michigan. The high will shift to the eastern Great Lakes today and
then to New England on Monday with southwest flow increasing
behind the high. Winds are expected to peak in the 20 to 25 kt
range across the north half of Lake Michigan late this afternoon
through tonight. Winds should begin backing to the southeast and
increasing from the south late tonight into Monday as low pressure
lifts across the plains. The low is expected to reach Lake
Michigan early Tuesday with winds turning westerly behind the low
through the day Tuesday and continuing Wednesday, again peaking
around 25 kt.






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