Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250013

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
613 PM CST Fri Jan 24 2020

315 PM CST

Through Saturday night...

The forecast expectations for the evolution of the moderate to at
times heavy snow tonight into Saturday remain largely unchanged.
Despite the close proximity in time to now, there remains fairly
large uncertainty on the precise placement of the most sustained
snow and thus the highest values which could be pushing to over
six inches in isolated locations by Saturday morning. Confidence
is highest in the center to western parts of the current Advisory
to start the earliest in snow and maintain the longest into
Saturday morning with efficient rates. The eastern parts certainly
will have Advisory criteria impacts, it`s just uncertain how
quick they will onset. We expect the poorest conditions to be the
most expansive later this evening through overnight and into
early-mid Saturday morning. With gradually tapering snowfall
rates for the Saturday afternoon time, impacts should ease and
possibly quickly, but difficult to say. Only change to Winter
Weather Advisory was to back up start time for parts of the
central and south metro to 8 p.m.

The latest hand surface analysis places a broad surface low across
the southern forecast area and into central Illinois, meandering
its way slowly north-northeast. This will soon take better shape
over northwest Indiana and into north central Indiana this
evening as an upper jet maximum rounds the pesky closed upper low
over southern Illinois. With light winds into early evening and
low temperature-dew point spreads north of the warm front,
visibility in fog will be around a quarter mile in places
along/north of I-80. An uptick in rain showers should continue
across northwest Indiana into northeast Illinois through late
afternoon and that should abate any persistent sub quarter mile
visibility, but at least occasionally being down to that level is
expected into the commute. The Dense Fog Advisory was
expanded/extended northeast across the metro for the commute.

GOES-16 water vapor imagery has a magnificent presentation of the
closed upper low as well as the jet streak rounding its eastern
side over the Ohio River Valley. This jet maximum will continue
to blossom precipitation from northern Indiana into northeast and
far northern Illinois into early this evening. This precipitation
will be fed by a moist conveyor belt, also clearly evident on
satellite imagery into far northern Illinois, as well as a
developing trough of warm air aloft (TROWAL) and associated steep
lapse rates aloft. With the juxtapose of upper level forcing and a
convergent focus in the lower-mid levels across the northern CWA,
precipitation should increase in rate fairly quickly. As such,
dynamic cooling will support probable changeover to snow not that
long after onset. The challenge is precisely where this developing
deformation sets up residence. Have felt that the low-level and
even mid-level trends being a tad further west than earlier
indications point toward the western/central portions of the
Advisory area early this evening seeing that rapid onset. The RAP
and HRRR have been consistent with this today. For the Chicago
area there likely will be some rain/snow mix even shortly after
sundown, but low-level wet bulb profiles and being southeast of
the deeper convergence point toward less potential of going fully
snow early this evening, and thus less evening impacts.

While guidance varies on nitty gritty details such as placement,
the snow area`s morphology through tonight and even into Saturday
morning is pretty well agreed upon. Not surprisingly given the
placement with respect to the upper low, it is expected to pivot
over part of the area...most likely north central Illinois...and
then expand and creep southeastward. This makes conceptual sense
in the deformation and also with column winds turning
northwesterly. Within this snowfall area, 7.5+ C/km lapse rates
above 650 mb and a deeper dendritic growth zone support more
efficiency in rates to overcome the marginal surface wet bulb
values and shortly thereafter accumulate quicker. Because of the
slow movement of the upper low, this zone of snow should continue
over part if not a good portion of the CWA into at least early
Saturday morning. Coming out of the nighttime period, snow should
be able to continue accumulating on most surfaces even in the
southern and eastern CWA, so impacts during that time due to rate
and total accumulation should match that of the worst conditions
of overnight. The overall persistent depth of the system and
broad nature of the TROWAL look like they would support snow into
the rest of the day Saturday, but it should become more streaky,
and possibly more snow showers as a mode, as the system occludes
and drier air pivots in.

The big impacting factors with this will be the duration and
rates, especially where this pivots, again most likely over a good
portion of the Advisory area from the western Chicago suburbs on
westward. Totals of 4+ inches in that area that we have forecast
are supported by much of the guidance, just different placement
and orientation. Isolated amounts over six inches given the
attributes are certainly possible, even with the broadly lower
snow ratios (8:1-10:1). On the eastern edge of that over the
Chicago metro, there could easily be a very sharp cutoff in
amounts, depending on the behavior of the snow area. Also to note,
that Saturday morning, especially early morning, should be having
notable impacts continuing during that time even possibly beyond
the Advisory area. With marginal temperatures and the snow area
becoming more limited in coverage, that should support more
marginal impacts during the P.M. hours. It seems like the Advisory
could be able to be peeled away in parts earlier than its end
time, but no changes made at present.



315 PM CST

Saturday night through Thursday...

At the start of the period Saturday evening, our slow moving upper
level low is expected to be centered somewhere over lower
Michigan, before it shifts eastward over New England during the
day Sunday. Because of its slow movement, some lingering light
snow will be possible over the area Saturday evening. We will also
have to keep an eye on the potently for any light drizzle or
freezing drizzle Saturday night as drier air moves over the area
aloft. I certainly do not have a degree of confidence that there
will be much in the way of freezing drizzle Saturday night,
especially since surface temperatures are likely to continue to
hover right around 32. For this reason, I have kept the mention
out of the forecast.

We could also see another light batch of light snow or flurries on
Sunday as a weak mid level disturbance tries to drop southward
over the area on the far western periphery of the upper low.
However, given some model disagreements on this feature, I have
opted to keep this small mention out of the forecast as well,
since it would likely not cause much of any impacts. Otherwise,
expect temperatures Sunday to remain in the low to mid 30s under
mainly cloudy skies.

A period of dry weather looks on tap early next week, but another
weak weather disturbance could produce another period of light
rain and snow over the area around midweek. But with no real good
push of colder air with these Pacific systems, expect
temperatures to remain on the warm side of average, with highs
mainly in the 30s.

Later in the week, there is the potential to have a bit more
active weather pattern across the region as additional
disturbances of Pacific origin take aim on the area. As a result,
more inclement weather, with some rain and snow chances will be
possible over the area at the end of the current forecast period.



For the 00Z TAFs...

Low ceilings/visibility and moderate to briefly heavy snow are all
concerns through the TAF period, especially over the next 12-18

Ceilings are lifting and visibility is increasing early this evening
as steady -RA has spread over northern IL. This precip will begin to
switch over to snow by mid-evening, with all sites at SN by 04-05Z.
Once the switch occurs at ORD/RFD/DPA, the SN will fall moderate to
briefly heavy for several hours. Precip trends at MDW and GYY are
less certain as the precip shield pivots over northeast IL under the
upper-level low. The southern edge of the shield will bisect the
Chicago metro, and may even stall between ORD/MDW. If this occurs,
precip rates at MDW will decrease and ceilings/vis may lower again
for a period. However, the precip shield should rotate back into MDW
with steady moderate SN overnight.

As the low begins to shift east late tonight, a deformation snow
band will likely extend northwest from the low across all of the
TAFs sites. Snowfall rates under this band could be moderate at
times through the morning hours before intensity diminishes through
the afternoon. Ceilings will only gradually increase through the day
Saturday, and will likely remain IFR.



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ010-ILZ011-ILZ012-ILZ019 until midnight Sunday.

     Dense Fog Advisory...ILZ004-ILZ005-ILZ006-ILZ011-ILZ012-ILZ013-
     ILZ014-ILZ020-ILZ021-ILZ022 until 9 PM Friday.

     Winter Weather Advisory...ILZ013-ILZ014-ILZ020-ILZ021-ILZ022
     until midnight Sunday.

LM...Dense Fog Advisory...nearshore waters
     until 9 PM Friday.



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