Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161603

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1003 AM CST Mon Jan 16 2017

947 AM CST

Rain continues mainly along and north of I-80. Air temperatures
have risen to above freezing mainly south of I-88. No additional
ice accumulation is expected south of I-88. As such, removed Cook,
DuPage, Kendall, and counties south from the Winter Weather
Advisory. Any ice on the ground in those counties should begin to
melt as temperatures continue to climb. North of I-88, freezing
rain and minor ice accumulation will continue until noon when
temperatures rise above freezing.

Looking further south, visibilities south of I-80 are mainly below
one mile. Reports from various towns and observations suggest
that widespread dense fog is not occurring so will continue to
hold off on a dense fog advisory. Guidance continues to suggest
that dense fog may form late this afternoon into this evening, but
only have medium confidence in dense fog occurring. Models may be
confusing low cloud heights for dense fog and will continue
analyze for the afternoon forecast.



215 AM CST

Through Tuesday...

Concerns center on expansion of mixed precipitation this morning,
then a transition to some moderate to locally heavy rainfall
later today and tonight, along with the potential for at least a
period of dense fog.

Ascent has and will continue to increase through the morning hours
from a lead shortwave ahead of the deep upper low across the
southern plains coupled with increased warm advection/isentropic
lift in the strong southwest flow. The forcing is not super strong
and with a dry layer in place in the lower level generally lighter
precipitation is expected initially during the surface subfreezing
conditions, but still enough to create concerns. High res guidance
does show an uptick in lift and precip as an elevated warm front
will move through this morning, thus expect locally heavier
elements once saturation occurs which we are seeing on radar and
enhancements on IR. Temps and dewpoints have crept upward, and
thermally cloud cover has allowed temperatures to remain somewhat
elevated this morning, but there is a corridor of subfreezing
temperatures (with 20s dewpoints) such that the concern for
freezing rain exists, especially north of I-80 (and more so north
of I-88) and northeast of I-55 along a line northwest of Morris
to ORD to Waukegan (where it is still only 24). Reports along and
north of this line suggest untreated surfaces quickly get a light
glaze, even under the lighter echoes. This does include the city
of Chicago, though at least of the next few hours rain will be
light away from north central Illinois. Southeast of here temps
are largely at or above freezing and forcing is weaker thus
concerns are lower, but still dewpoints are below freezing so a
narrow window of concern still. Amdar soundings early this morning
show a sufficient warm layer that would suggest FZRA is the prime
concern, with possibly some wet bulbing for a period of sleet
especially along and north of I-88 before saturation suggests
FZRA/RA to dominate. Surface flow will remain ese today as the
surface low remains positioned to the southwest of the area and
thus a rapid warmup/moisture surge is not anticipated during the
daytime hours which initially will limit surface warming , but
continued lower level southerly flow will eventually allow all
areas to warm above freezing. Current advisories seem reasonable
with up to one tenth of an inch of ice or so in the aforementioned
corridor and even some light sleet accums, with the much lighter
accums are expected farther southeast where more marginal surface
temps exist and weaker forcing is expected, but as mentioned a
light glaze would be a concern. We do not anticipate any changes,
though will have to watch the Will County and Lake/Porter areas
for any possible expansion.

Heavier precip rates can be expected later today and tonight as the
upper low over OK/KS will move northeastward across the lower
Great Lakes region. Ascent increases markedly due to the
strengthening of the low level jet and approach of the left exit
region of the upper jet. Precipitable water values (indicators of
available moisture) are on the high side. Scattered showers can be
expected in the afternoon, with more widespread coverage this
evening as more organized forcing will arrive. With the approach
of the upper low and cooler temps aloft, high level lapse rates
(500-300 mb) steepen enough to suggest some potent showers with
even a few thunderstorms possible with the more significant
moisture surge this evening. The upper low will kick east of the
area overnight into early Tuesday. The system`s cold front will
keep the threat of showers in late tonight into early Tuesday
though possibly with less intensity then the evening activity.

With a surface warm front lingering in place ahead of and with
the surface low passage, forecast soundings depict the low level
moisture (already in place) to our south to remain trapped below a
strengthening thermal inversion as the just above surface warm
(and moist) air surges northward. Expect a quick lowering of
clouds this morning and likely decreasing visibilities. This
concern will be focused south of I-80 this afternoon where lighter
precip will be occurring, and dense fog is suggested by high res
guidance. This will shift northward later this evening and
overnight. Some of this will be mitigated in periods of heavier
rain and thus hints in model guidance of at least some brief
improvement during the evening hours, but then potentially
redeveloping overnight with the arrival of the low.

Tuesday afternoon the northern stream shortwave will pass through.
Limited moisture suggests lighter showers with no more than
scattered coverage. Thermal profiles get close to maybe a rain
snow mix late in the day if there is even enough saturation into
the ice nucleation layer which at this point is in question.



215 AM CST

Tuesday night through Sunday...

Surface high pressure will build in Tuesday night into Wednesday.
There could be some lingering low clouds into Wednesday, though
increasing subsidence behind the departing northern stream wave
would appear to help squash the moisture layer. Dry conditions
will be in place for Weds/most of Thurs.

Another slow moving cut off low over the southwestern United
States (currently off the Baja CA coast). Southwest flow in this
pattern will support above normal conditions, but this seems like
a moist pattern where at least occasional cloudiness, possibly
more often than not, will rule, especially after Thursday. No
precipitation type concerns exist through the period, but precip
chances do return later Thursday into Friday as this next cutoff
low finally will lift northeast with another chance for decent
rainfall. The pattern will remain unsettled as additional lows
will spread northeastward, though we certainly remain on the mild
to warm side, especially with dewpoints forecast to be in the
upper 30s to mid 40s late in the week which are quite a bit warmer
than actually air temperature normals, thus we are still on tap
for well above normal temperatures, though did not stray from a
model blend as our warming will be limited given high model RH
progs thru the weekend, suggesting cloud cover and occasional
rain with additional weak lows headed our way in SW flow.
Eventually cooler air will filter in late in the weekend or early
next week.



For the 12Z TAFs...

Complex weather forecast on tap today as area of low pressure
lifts across the region. Out ahead of the low expect a wintry
mix this morning eventually transitioning to rain. Ceilings and
visibility are expected to trend down through the day with LIFR
likely and VLIFR a concern.

Early this morning, initial band of precipitation has set up
across far northern Illinois and has produced light freezing
rain and some sleet for much of the area, including ORD and RFD.
In the near term, expect the threat for a wintry mix will continue
for the next few hours. Temperatures are hovering right around
freezing, however, dew points are in the mid to upper 20s
indicating that temperatures could dip slightly as precipitation
occurs. MDW has been an exception, holding onto slightly warmer
temperatures (around 34) all night and missed out on much of the
precipitation from the initial band that set up just to its north.
Confidence in any prevailing icy conditions at MDW is low despite
the threat being non-zero, and opted to keep out of the forecast
in the meantime.

Low pressure is centered over the Central Plains early this
morning and is expected to track across northern Illinois late
this evening and overnight. Early morning observations across
Kansas and Missouri immediately ahead of the low are likely a
preview of what is to occur locally as we head into the afternoon
and evening hours. Upstream obs show ceilings of 002 to 005
across much of the aforementioned area and guidance is in good
agreement on these lower ceilings developing across northern
Illinois. Anticipate IFR ceilings in place by mid to late this
morning, then LIFR conditions are expected by early to mid
afternoon. Confidence is high in the trends and heights, but
medium in timing. Have lower confidence in specifics in the
visibility trends through the day, but most models continue to
advertise widespread IFR much of the day, while a number bring
visibility to a quarter mile or less.

Finally, as the low lifts across the Mississippi Valley mid
evening, a moderately strong low level jet will transport ample
moisture and some instability across the local area which could
result in some isolated to widely scattered embedded
thunderstorms. Will maintain a Prob30 for TSRA threat at the
terminals for mid to late this evening.



312 AM CST

A modest southwesterly gradient is in place across northern Lake
Michigan early this morning between a departing ridge over New
England and low pressure over Hudson Bay. Winds will peak around
25 kt over northern Lake Michigan this morning but should
gradually diminish through the day as the two features shift
eastward. Attention shifts to the southwest as low pressure
approaches from the Central Plains today. East to southeast flow
is expected to strengthen into the 15 to 25 kt range across
southern Lake Michigan this afternoon into the evening and
gradually spread north across the lake overnight. The low is
expected to track over southern Lake Michigan Tuesday morning with
winds turning west and eventually southwesterly Tuesday night into



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     UNTIL NOON Monday.




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