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Area Forecast Discussion
Issued by NWS Chicago, IL
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FXUS63 KLOT 010459
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1059 PM CST Tue Jan 31 2017

.UPDATE...
916 PM CST

Monitoring a couple of areas of snow showers drifting southeast
across the region. One area, will primarily stay to the south of
the CWA but still possibly affect the far southern area. Several
waves of energy are helping to provide support, but do think low
level dry air will limit much of any snow across the far southern
CWA tonight. However, do think a flurry or two will be possible
and so have included mention after midnight tonight. Main area of
concern is up to the north over south central WI where a line of
moderate to heavy snow showers are dropping south southeast. This
line, supported by approaching energy, is also developing along
surface trough axis where rather steep low level lapse rates are
currently in place. If current trends were to continue, time of
arrival would put them into far northern McHenry and Lake IL
counties in the 5-6z time frame. Although I do think these snow
showers will reach the northern portions of McHenry and Lake
counties, it should be more of the less defined and more isolated
showery development on the current west/southwest part of this
line. The heaviest should stay just to the north in Wisconsin, and
then shift more east with the stronger forcing. Did increase pops
in this location and have added some minor snow accum below one
inch. Will continue to monitor trends here this hour for any
possible updates.

Rodriguez

&&

.SHORT TERM...
109 PM CST

Through Wednesday...

Low pressure is moving through the eastern Great Lakes this
afternoon and high pressure builds across the central plains and
into the Ohio Valley.  A strong westerly pressure gradient over the
region will keep gusty wly-wnwly winds to 25-30 mph until early
evening. Gustiness should subside arnd sunset. Mostly cloudy skies
will persist through the night and tomorrow as the region remains
under fast nwly flow aloft. A series of weak shortwaves rippling
through the cyclonic northwest flow aloft will bring periodic
chances for some shra/shsn into the evening, but any weak forcing
from the cyclonic flow aloft will weaken and pcpn chances will
quickly diminish through the evening. Even with the passage of a
cold front last night, temperatures had remained steady overnight
and the temperature trend has been flat through the day.
Temperatures have been a bit higher than normal, but the brisk
winds have kept wind chill reading in the middle to upper 20s, so
conditions have felt more like a typical late January day. A
secondary cold front will drop south through the upper Mississippi
Valley tonight and push through the region tomorrow. The boundary
should be relatively diffuse, with winds gradually turning nwly.
The front should push into nrn IL tomorrow afternoon, allowing
temperatures across the northern tier counties to drop off through
the afternoon, with highs near the Wisconsin border only in the
lower 30s. Locations generally south of the I-88 corridor should
have more opportunity to warm through the day, with highs in the
middle to upper 30s. With a dry air mass expected to be in place
tomorrow, no pcpn is expected with the front.

&&

.LONG TERM...
226 PM CST

Wednesday night through Tuesday...

Slightly cooler than normal temperatures, but much cooler than
the past several weeks, will prevail for the end of the workweek.
A lobe of the polar vortex will rotate across Hudson Bay and
eastern Canada Wednesday Night through Friday, allowing for
northwest flow to be reinforced across the local area and 850mb
temperatures dipping to -15C (about the 10th-15th percentile for
this time of year). The surface air mass will be modified
considerably from upstream however, due in large part to the lack
of immediate snow cover. The challenge with temperatures,
particularly lows at night, will be if clouds can clear or not. On
Thursday and Friday nights, there could be mid/high cloud cover
from waves passing to our south. If there are not thick clouds,
both of those nights could have single digits in outlying areas
with early morning wind chills around zero. Confidence is low on
sky cover and temperature specifics though. Highs both days look
to be in the mid 20s with higher confidence.

The upper flow pattern transitions to quasi-zonal during the
weekend with a low amplitude wave forecast to traverse across the
Upper Midwest Saturday afternoon and night. There is a
disagreement on how much deep moisture return there is, with the
31.12 GFS quite a bit more aggressive on its moisture flux than
the EC. If precipitation can materialize within a forecast broad
zone of upper/mid-level ascent, forecast profiles mainly favor a
light snow in our area. Stronger forcing is presently forecast
over Wisconsin.

A deep upper low is forecast to develop off of the Pacific
Northwest coast this weekend, and global guidance has been
indicating a vigorous (160kt+) jet to round the base of this into
the beginning of next week. Both the current 31.12 runs of the GFS
and EC indicate this quickly developing a negative-tilted upper
trough with rapid surface cyclogenesis in the Midwest Monday night
into Tuesday. Despite the recent global guidance agreement on
this, confidence in the key features is quite low at this point,
especially with the blocked pattern forecast in the northeast
Pacific that holds the aforementioned upper low in the place. The
pattern has been active in general though, and ensemble support is
there for a surface low to head northeast across the middle of
the country that could have a broad variety of impacting weather.
It just is too far out though to speak to any details and get an
idea on which way guidance will shift (and it very likely will).
For now have gone fairly close to blended guidance temperatures
and winds, with decently high precipitation chances (rain at this
juncture) for day seven.

MTF

&&

.AVIATION...
For the 06Z TAFs...

The main concerns during the period will be with
regards to the threat of a period of snow showers
across the terminals Wednesday.

West-northwest winds will continue through the period
as low pressure moves into Quebec and high pressure
builds in across the Northern Plains. While VFR CIGS
will likely continue across the area for the next few
hours, some MVFR CIGS may develop over the area
overnight into Wednesday morning.

A weak surface frontal trough is expected to gradually
sag southward across northern Illinois on Wednesday,
and there are some concerns that this could act as a
a focus for snow showers by Wednesday afternoon given
the likelihood for some steep lower-level lapse rates.
While some of the lingering snow showers over southern
Wisconsin could briefly impact the terminals by
morning, it appears that there will be a slightly
better chance for some scattered snow showers by early
Wednesday afternoon. There are still uncertainties
with the extend of these snow showers, but the
potential is there for at least a few short periods of
reduced visibilities in snow. Given these uncertainties,
and the questions of areal coverage of the snow showers,
we have opted to only mention a prob30 at this time.

KJB

&&

.MARINE...

Persistent low pressure over sern Canada and and high pressure
building across the middle Mississippi Valley into the Ohio Valley
will keep brisk, generally westerly winds over Lake Michigan
through Thursday or Thursday night. While gales are not expected
through this period, the wnwly fetch could bring periods of small
craft advisory conditions, especially for the Indiana nearshore
waters. The center of the high is expected to build south and east
later this week, with the high centered over the Ohio Valley by
Saturday morning, bringing lighter winds to the lake into the
weekend. Meanwhile, as the high is continues to shift east, to
the mid atlantic coast, low pressure will develop over the
plains, bringing increasing southerly winds to the lake on
Saturday. An associated cold front is expected to sweep across the
lake Sunday.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 5 PM Thursday.

&&

$$

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