Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181953
AFDLOT

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

.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 18Z TAFs...

Complicated precip forecast over the next 12 hours with a compact
mid-level low drifting across the area. Ongoing -SHRA with some PL
will shift NE of all sites over the next hour or two. The loss of
mid-level moisture behind the -SHRA will promote DZ and lowering
ceiling to IFR levels. Precip will then change over to -SN from NW
to SE this afternoon into the evening, with the change already
underway at RFD. Approximate change at ORD will occur at 23Z, with
a chance for as early as 22Z. MDW will follow ORD by about one
hour. Steady light snow is then expected for about a 7-9 hour
window thereafter. A broken MFR cloud deck will linger after the
snow ends before scattering out Thursday morning.

Kluber

&&

.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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