Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190229
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
929 PM CDT Wed Apr 18 2018

.UPDATE...
929 PM CDT

Continue to see a mix of drizzle and light snow across the area
this evening with sfc temps between 32 and 34F. There has been
somefreezing drizzle where temps are at freezing, though thus far
IDOT is only reporting some snow/ice accums on roads up in
Winnebago and Ogle counties. Series of ACARS soundings out of MDW
this evening do show cloud top temps hovering in the -10 to -14C
range, which explains the drizzle and struggle to switch over to
all light snow as clouds border on the necessary temp range for
ice nuclei needed for snow. Forecast soundings and GOES IR imagery
continue to support the notion that cloud tops will cool further
and should support a change over to all snow eventually. The more
robust radar echos over NC IL are all snow and would anticipate
that area to spread gradually southeast with a change over to all
snow taking place area-wide late this evening.

Even with very marginal sfc temps at or a hair above freezing, it
is possible that there could be some minor accums on roads and
slick spots developing into Lake and Mchenry counties, so even
though total snow accumulations will probably end up less than an
inch, plan to let the winter wx adv ride until scheduled
expiration time at 06z. Some potential for slick spots to develop
outside of the advisory area due to patchy freezing drizzle, but
with trends suggesting a transition to snow soon and current
reports of no icing on roads yet, not planning to expand the
advisory. Will continue to closely monitor conditions over the
next several hours.

Precipitation still looks to wind down in the 06-09z time frame.

- Izzi

&&

.SHORT TERM...
252 PM CDT

Through Thursday...

A fairly compact and slow-moving mid-level low over eastern Iowa
this afternoon will drift across northern Illinois this evening as
an associated surface low tracks from southern Illinois to central
Ohio.

Much of the CWA is currently under an elevated dry slot/EML that has
removed ice from the cloud layer. A few sites are beginning to
report drizzle under a lowering cloud deck, but minimal
precipitation is expected over the next few hours.

As the mid-level low passes overhead, deeper moisture into the mid-
levels will activate ice in the column, allowing the precip to
change over to snow. An expanding shield of light precip over
eastern IA is currently changing to snow, supporting this idea as
the low shifts eastward. The transition looks to occur around 21Z
for Rockford, 23-00Z for Chicago, and 02Z for Valparaiso. QPF of 0.1-
0.2" this evening supports 1-2.5" of wet snow primarily north of I-
80, with the highest amounts along the WI state line. Some hi-res
guidance is indicating a slight enhancement of QPF within about 10
miles of Lake MI as a result of some lake enhancement and increased
convergence from a lingering surface trough over northern Indiana.
This would support the potential for 2" from Gary to Chicago, and up
to 3" near Waukegan.

Did not make any changes to the winter weather headlines at this
time. Though snowfall amounts should remain sub-advisory, did not
see it prudent to cancel the advisories as slushy conditions on area
roadways are still expected this evening.

The snow will quickly diminish after midnight as the mid-level low
exits to the east. Overcast skies will likely linger to around
daybreak Thursday before scattering out into an isolated cumulus
field during the day.

Kluber

&&

.LONG TERM...
217 PM CDT

Thursday night through Wednesday...

Quiet and warmer weather is expected during the period. Surface
high pressure is expected to dominate across the western Great
Lakes Region through the weekend. This will result in more sun and
gradually warming temperatures into the 60s this weekend into
early next week. Unfortunately, a persistent easterly onshore wind
looks to occur through the upcoming weekend. So, while areas well
inland will experience temperatures into the 60s, this will not
be the case closer to Lake Michigan. In these areas, expect much
cooler conditions, mainly in the 40s.

The period looks dry until possibly late Tuesday or Wednesday of
next week. During this period the next weather disturbance could
produce some light rain across the area. However, this far out
confidence on timing is quite low.

KJB

&&

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

DZ will transition to -SN this evening with continued IFR
conditions. Brief periods of moderate snow could knock VSBY down
to LIFR at times. Snow will end not too long after midnight with
CIGS likely to lift to MVFR before scattering out Thursday
morning. NNW winds expected Thursday, gusty at times, but given
the direction would anticipate a lake breeze to develop early in
the afternoon over NW IN then building west into NE IL by mid-late
afternoon. Guidance is not very keen on lake breeze development,
which does lower confidence, but given climo and expected wind
direction opted to introduce a lake breeze into MDW/ORD/GYY
Thursday afternoon.

- Izzi

&&

.MARINE...
217 PM CDT

A fast moving area of low pressure will shift eastward south of
the lake into this evening. As this occurs northerly winds to
increase across the lake tonight. Winds to 30 KT are likely,
though a period of low end gale force gusts to 35 kt is also
possible overnight. This strong winds will abate during the day on
Thursday as surface high pressure begins to build from the
northern Plains into the Upper Midwest. This area of high pressure
is expected to remain dominate across much of the western Great
Lakes Region through the weekend. This will setup light and
somewhat variable winds over the lake, with onshore flow likely
developing each afternoon as lake breeze processes dictate the
main wind direction over the near shore waters.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008
     until 1 AM Thursday.

IN...None.
LM...Small Craft Advisory...Winthrop Harbor to Gary IN until 7 PM
     Thursday.

     Small Craft Advisory...Gary to Michigan City IN until 1 AM Friday.

&&

$$

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