Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152045

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

244 PM CST

Through Monday...

Quiet weather conditions will prevail through the remainder of the
day and into the evening with lgt/vrbl winds and increasing mid and
high cloud cover. The main forecast concern will be centered around
the time on onset of precipitation and precipitation moving into
the area tonight.

This afternoon, strong high pressure extends from southern Quebec
through the Great Lakes to the middle Mississippi valley.
Temperatures this afternoon have been able to climb into the lower
to middle 30s under cloudy skies with calm to lgt/vrbl winds.

Weather concerns will quickly shift tonight to the developing
winter weather system developing over the plains. For the
afternoon forecast update, have made a few changes to the going
forecast with regard to timing of onset of pcpn as well as pcpn
types. As is usually the case with deep closed lows dropping into
the desert southwest, the system has trended slower with every
model run, up to and including this morning`s runs. Latest thermal
and moisture profiles suggest that the predominant p-type, at
least initially, should be freezing rain, with the chance for some
snow mixing in on the nrn fringe of the pcpn shield and some
sleet possibly mixing in farther south. Temperature profiles
already indicate a warm, over +1to +2 C layer over areas where the
sfc temp remains at or below 32 F. Going into the evening, any
areas that had been above 32 F will quickly cool of and sfc temps
will be below 32 F before the onset of pcpn overnight tonight.
have introduced slight chance to low chance PoPs into the far swrn
portions of the CWA shortly before midnight, but any pcpn that
manages to reach the ground before midnight should be light,
perhaps even only trace amounts. As the system gradually lifts
northeast overnight, pcpn will begin to spread from southwest to
northeast across the CWA, reaching the Rockford area by arnd 08-
09z and then the Chicago metro area by arnd 12Z. The window of
opportunity for the potential for the greatest ice accumulation
from freezing rain will a relatively short period durg the early
morning hours. Will hoist a Winter Weather Advisory, which will
cover the morning rush hours. Again, while there could be some
snow or sleet mixing with the freezing rain for short periods, the
main wither weather concern will be the freezing rain. As for
duration, the latest guidance continues to bring the warmer sfc
air into the area relatively quickly, with the changeover from
freezing to liquid pcpn occurring durg the late morning hours.
However, given the strength of the warm, moist advection in
advance of the sfc low, there will be the potential for the pcpn
to change over to all liquid a little earlier than currently
outlooked in the Winter Weather Advisory. Currently anticipate
that temps will reach 34-35 F by arnd 14z over the far south
portions of the CWA, with the entire CWA reaching the middle 40s
to around 40 F by 18z. Highs should reach onto the lower 40s over
the far south this afternoon. Since the strong high will be slow
to move out, light esely flow and sfc ridging will still poke into
nrn IL/srn WI into the afternoon, so locations toward the WI
border will only hit the middle to upper 30s.


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.



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ019-ILZ020-ILZ021...5 AM Monday TO NOON




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