Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152040

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
240 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 PM CST

Monday night through Sunday...

The surface low of the mature mid-latitude cyclone is agreed upon
within guidance to pass directly over the CWA overnight Monday
night as the negatively tilted upper wave lifts northeast. With
still a closed upper feature and the continuous slowing trend in
guidance with this system, it could still further slow, but the
main theme for Monday night should remain consistent...and that is
wet. Precipitable water of around an inch, which is in the top
percentile for mid-January, is forecast to nose into the area
just ahead of the low in a zone of strong upper ascent Monday
evening. An area of elevated instability is predicted to encroach
ahead of the quasi dry slot and low-level warm front, so continue
the mention a chance of thunder for most of the area. Rainfall
amounts from Monday through Monday night are forecast to be one
half to one inch across the area. As the low center moves
overhead, fog and drizzle become more likely overnight and very
early Tuesday morning. There is potential for this to be dense.

Temperatures will warm overnight Monday night to around 50
degrees in the far south within the system warm sector, while
hanging in the mid-upper 30s in the far north. As winds turn west
behind the low passage, temperatures will drop some, before
recovering slightly or holding steady on Tuesday. There still
should be plenty of cloud cover and some spotty rain showers
remain possible. Guidance is struggling a bit with handling a
northern stream wave Tuesday afternoon and night, and how it
interacts with the preceding system, but global guidance indicates
there could be at least scattered shower coverage in northern
areas during Tuesday evening. With the column cooling, can`t rule
out some brief wet snow if enough saturation and lift exists in
the mid-levels.

The pattern remains advertised to become quite amplified and
blocked going forward, with the CWA under anomalously high
heights and 850mb temperatures for the time of year by late week.
A closed low is forecast to lift across the Plains states Thursday
and have introduced some precip chances mainly Thursday night and
Friday, but again a slowing, more westward trend, may be noted
with that system too. Temperatures look to moderate to well above
normal values whether clouds or not given the projected 925mb
temperatures and lack of snow cover here and to our south.
Nonetheless, clouds still look to be key on just how far above
normal. Continue introducing some 50s in the south on Friday and
areawide on Saturday, which looking back at local climatology and
CIPS analogs is certainly within reason.



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


237 PM CST

High pressure over the general lakes region will depart east
Monday as low pressure approaches from the Plains states. The
pressure gradient should sharpen quite a bit over the
southern/central parts of the lake Monday evening, and there
could be 30 kt easterly gusts in response. Waves should build to
potentially small craft advisory criteria along the northern
Illinois shore. As the low moves east of the lake on Tuesday
afternoon, winds will become west-northwest, although there is not
a particularly strong push of cold advection, so speeds should
remain in check. A prolonged period of southerly winds are then
expected, with some southeast component at times. No period stands
out for real strong wind speeds Wednesday through Saturday.






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