Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160913

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

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 06Z TAFs...

Multiple forecast concerns continue with a period of freezing rain
Monday morning and then cigs/vis lowering by Monday afternoon and
likely to vlifr Monday night. Isolated thunderstorms are also
possible Monday evening. significant changes from the 00Z tafs. Short term
guidance is in fair agreement with precip developing across the
terminals in the 10z-12z timeframe with precip becoming prevailing
after 12z. Precip may briefly mix with some sleet as it begins but
precip type should be mainly freezing rain until temps warm up for
precip to change to all liquid rain by mid morning. However...
temps currently hovering around freezing at mdw/gyy. Temps may
dip a few degrees as precip begins but freezing rain duration may
end up being shorter than currently advertised at gyy and mdw and
trends will need to be monitored. Periods of light rain/showers
will continue through Monday afternoon and then an area of low
pressure will move across the area Monday night. As this low
approaches... periods of moderate/heavy rain may develop Monday
evening along with a chance of thunderstorms. Duration of thunder
at any one location may be rather short...but added prob mention
for thunder with this forecast during mid Monday evening. Rain is
expected to taper off after midnight into early Tuesday morning.

Some patchy fog is possible overnight and mvfr vis is expected as
precip develops but vis is expected to gradually lower to ifr
Monday afternoon and eventually lower to 1sm or less Monday
evening and especially Monday night as the low pressure arrives.
Confidence in both timing and specific visibilities is only medium
and changes should be expected with later forecasts.

Cigs will lower through mvfr with the precip arrival and into ifr
Monday morning and into lifr Monday afternoon. Confidence on these
trends is high but tweaks to timing can be expected. Cigs will
likely drop to vlifr later Monday evening.

Light easterly winds will slowly increase Monday morning and into
the 10-15kt range Monday afternoon which will continue into Monday
evening. Some gusts up to 20kts may develop by Monday evening.
Wind speeds will diminish Monday night becoming light and variable
as the low moves across the area. cms


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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