Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171725

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1225 PM CDT Sat Mar 17 2018

301 AM CDT

Through Sunday...

Have seen the whole gamut of precipitation types across the area
overnight including here at the office. With one upper wave moving
to the east have been seeing the main precip shield begin to
decrease in coverage and intensity with the northward push
starting to slow. Expect that areas seeing precip through the rest
of the overnight will see a mix of sleet, rain with some snow and
since temps are hovering around freezing there will be areas of
freezing rain depending on the surface. May see the northern
fringes turn more to some light snow. Mixed precipitation will be
ending by around mid morning. With the mix of precip types and
continued chance for some icing will maintain the Winter Weather
Advisory until 8 am CDT for most areas but may remove the far
southwest counties early as precip coverage will probably be
minimal. Water vapor imagery shows the next upper low weakening as
it tracks eastward over eastern Iowa and northern Missouri. The
attendant surface low is also weakening. An area of
scattered/showery coverage of precipitation lingers closer to the
upper low but will also be decreasing in coverage with time. As a
result, while most areas dry out into mid/late morning far
southern areas will probably see some lingering showers through
early afternoon as the weakening system passes. Whatever
precipitation remains into early afternoon will be rain as
temperatures warm into the mid 40s while skies clear from north to
south. Clear skies and light winds will allow temperatures to
drop off into the 20s tonight.

With the departure of the weakening upper low, upper ridging will
spread into the plains and Mississippi River Valley Sunday
bringing a mild day with highs in the 50s for most areas except
near Lake Michigan. Surface high pressure will be overhead setting
the stage for a lake breeze to push inland. Lakefront temps may
reach the upper 40s early before falling back into the lower 40s
for the afternoon.



301 AM CDT

Monday through Saturday...

Water vapor imagery shows a broad upper trough over the Pacific
Northwest with several embedded waves noted around the periphery.
One such wave is approaching the northern California coast and is
expected to traverse the southern flank of the main upper low and
eject eastward across the southern Rockies Sunday night. This will
attempt to disrupt the upper ridge over the Plains. Model
guidance has had difficulty handling this thus far but still
continues to track this system across the Ohio Valley by Tuesday.
The mid level baroclinic zone tightens with the approach of the
low which may bring some precipitation to far southern sections of
the forecast area by Monday afternoon. Will continue with chances
for rain in the far south for now. Given the interaction that
needs to occur as the wave moves onshore and points east suspect
that additional changes may occur in the guidance. Otherwise
Monday will see temps range from the lower 40s north to around 50
south but a gusty easterly wind will keep the lakefront and
adjacent inland areas in the upper 30s and lower 40s. May see wind
gusts to around 30 mph provided the low track doesn`t shift too

The ridge will be breaking down in the wake of the Monday system as
the remains of the large western upper low meanders eastward into
the mid/upper Mississippi River Valley. Another round of colder
air will spread in for Tuesday and Wednesday. Will need to monitor
some minimal precip chances Tuesday night and Wednesday but the
energy associated with the trough will be diminishing. The airmass
will be cold enough to support some lake effect snow shower
potential by Tuesday night but at this point forecast equilibrium
levels look pretty low to support much activity. Trajectories
would allow convergence to develop along the Illinois shoreline
but given this is mid next week and parameters are marginal will
keep mention out of the forecast for now. Expect highs in the
upper 30s and lower 40s with coolest readings near the lake.

Another ridge builds across the plains Thursday and moves east
toward the area through Friday. Active flow into the west coast
may lead to lee trough development across the high plains which
may bring a system to the area Friday night into Saturday. Do
expect a warming trend from Thursday onward.



For the 18Z TAFs...

No major aviation concerns this period, with precipitation having
ended early this morning, and dry northeast winds providing VFR
conditions with clearing skies through this afternoon.

Surface low pressure continues to weaken across the lower Ohio
Valley early this afternoon, with precipitation now limited to
areas mainly south of the I-80 corridor across northern IL. Breezy
northeast winds north of the low track are pulling drier air into
the region, with low clouds eroding and even mid/high clouds
expected to clear out this afternoon. Sustained wind speeds
between 10-15 kts will slowly ease by late afternoon, with winds
becoming light/variable or calm tonight as high pressure builds in
from the northwest. Clear skies and light winds may allow for some
shallow fog development late tonight, though areas south of the
terminals appear more favored, with higher surface dew points
lingering from today`s precip/cloud cover.

Light winds and sunny skies Sunday should allow for easterly lake
breeze development into ORD/MDW by mid-afternoon. Guidance looks a
little slow/late with arrival at the terminals just based on
expected strong temperature discontinuity and weak gradient in the
ridge axis. Have picked 20Z-ish for wind shift, though will likely
need a little tweaking as afternoon approaches.



224 AM CDT

Marine concerns focus on some periods of stiffer northeast winds,
especially Monday into Tuesday.

East-northeast winds this morning across the southern part of the
lake will turn more northerly in the afternoon and ease in speed.
High pressure will move across the southern part of the lake
tonight while low pressure drops southeast across Lake Huron. This
low will shift winds across northern and central parts of the lake
from westerly overnight to northerly by Sunday morning.

The north to northeast winds will increase going into Sunday night
as high pressure builds across Ontario and low pressure takes
shape across the central Plains. As this low tracks east on Monday,
the pressure gradient will increase across the lake, driving
northeast winds to 30 kt and possibly occasional gales across the
south and Illinois and Indiana nearshores. Waves to 8-12 ft are
appearing likely for these nearshores by later Monday into Monday

A northeast to north flow over the lake is expected to continue
much of the rest of the week.






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