Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210832

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
332 AM CDT Sun May 21 2017

332 AM CDT

Through tonight...

Main forecast concerns/challenges this forecast period are with
dense fog as well as showers and an isolated thunderstorm early
this morning.

Areas of dense fog still in place across northern Illinois at
this time. However, coverage of this fog is lowering as frontal
boundary pushes through northern Illinois, ushering in a drier air
mass. Will be able to cancel the dense fog advisory from west to
east over the next couple of hours as any remaining dense fog
diminishes. An expanding precip shield has been the trend across
northern Illinois over the last couple of hours, with locations
from just east of RFD southeast to Kankakee observing precip at
this time. This is a result of large scale ascent still in place,
as the upper level center remains just to the west. With limited
instability in place, thunder has been isolated and will likely
continue to be isolated as instability shifts further to the east
early this morning. Not expecting any severe weather this morning,
but any stronger development will be able to produce brief heavy
downpours. Precip will likely exit to the northeast by the 11-12z
time frame this morning, as surface trough and front slide to the
east. Anticipate dry conditions for the remainder of the period.
There is a clearing line behind the front, currently pushing into
northwest Illinois. Although narrow, most locations will see a
brief window of clear/sunny skies this morning. With cyclonic flow
in place as the upper level system remains in place, will see
cloud cover spread back east this morning into midday with cloudy
or at least partly sunny skies returning.



332 AM CDT

Monday through Saturday...

Much of the weather this week will be influenced by the same
upper level system, with this system or at least a piece of this
upper level system remaining across the region. With initial sunny
skies in place Monday, will likely see mid/high could spreading
back over the area throughout the day. This will be as a stronger
piece of energy rounds the base of this system, and associated
surface reflection shift east southeast across the region. Monday
morning will be dry, but precip chances will increase in the
afternoon and Monday night as this system pushes overhead.
Instability does not appear overly great at this time, but there
may be enough for isolated storms. Forcing really increases on
Tuesday, with the highest chances for more widespread showers and
thunderstorms expected during this time. This system really closes
itself off, and although coverage of any precip not expected to
be as high Tuesday night into Wednesday, cloud cover and scattered
showery development is still likely to remain. This system should
finally exit to the east by Thursday but pattern appears to
remain active with yet another system possibly approaching on
Friday. Temps this week start off milder with 70s expected Monday.
As this upper level system settles back in temps in the 60s, and
quite possibly cooler in some locations, are expected Tuesday and



For the 06Z TAFs...

Moist low level air mass and light winds allowing some fog to
develop early this morning. OVC high deck should limit
radiational cooling and hopefully should keep widespread dense fog
from developing, however anticipating VSBY to drop to IFR at
times, and probably LIFR in favored fog spots like DPA. Occluded
front will sweep across the terminals overnight shifting winds to
the west and scouring out the fog and lower clouds. After a brief
window of clear skies, look for a lower end VFR deck behind the
front to linger through Sunday night. Winds will shift to the
west-southwest behind the front and likely increase and become
gusty Sunday morning after sunrise. Gustiness likely to abate
gradually Sunday evening.

- Izzi


259 AM CDT

Occluded front will sweep across the lake this morning shifting
winds to the southwest. Fairly stable conditions this afternoon
should keep wind somewhat decoupled over the open waters with a
somewhat lighter southerly wind expected when compared to the
stronger southwesterly offshore winds near the southern/western
nearshore. Stability decreases some tonight as cooler air filters
in, so expected stronger winds to develop over the open waters
tonight, probably peaking in the 25-30kt range for gusts after
midnight. Generally west/southwesterly winds likely until cold
front moves across the lake Tuesday shifting winds to the north in
its wake. Medium range models are showing some variability in
solutions with respect to the midweek storm system, but does look
like a potential for a period of fairly stout northerly winds in
the Tuesday night through Wednesday night time frame.

- Izzi


IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011 until 4
     AM Sunday.

     Dense Fog Advisory...ILZ005-ILZ006-ILZ012-ILZ013-ILZ014 until 6
     AM Sunday.

LM...Dense Fog Advisory...Winthrop Harbor to Northerly Is. until 6 AM Sunday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...noon Sunday to 9 PM Sunday.




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