Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171849

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

148 PM CDT

Through Sunday...

Lingering light precipitation will continue to dwindle south of
I-80 this afternoon, otherwise clearing skies and diminishing
winds will be the trends into tonight. Sunshine and temps in the
50`s expected for Sunday, through light winds will allow a lake
breeze to develop and drop temps into the lower 40`s near the lake
in the afternoon.

Weakening surface low pressure was over the lower Ohio Valley of
western Kentucky early this afternoon, associated with the upper
level low which was shearing/dampening eastward across the region.
Some patchy lingering light precipitation persisted over the
southern half of the forecast area within the shearing deformation
axis north of the mid-level vort center, with some reported mixed
rain/sleet/snow per mPING, though surface temps in the mid-30`s
and dew point temps above the freezing mark indicate freezing
precip is not a concern. High-res guidance remains consistent in
continuing to weaken/dissipate this precip over the next several
hours, ending before evening as the mid-level wave continues to
drift off to the east-southeast. Farther north, dry northeasterly
low-mid level flow has helped to clear clouds across far northern
IL, generally along and north of I-88. This clearing will continue
to work south through the remainder of the afternoon and early
evening hours, leading to mainly clear skies overnight.

Strong mid-level height rises develop this evening as the short
wave tracks away from the region and upper ridge moves east from
the Plains. Subsidence results in surface high pressure drifting
across the region into Sunday, allowing our current blustery
northeast winds to diminish to light/variable flow tonight and
Sunday. May be some patchy shallow fog development across southern
parts overnight of the cwa where clouds clear late and dew points
remain a bit higher. Sunshine Sunday, combined with light winds,
should make for pleasant early-Spring weather. 925 mb temps of
+4/+5C should allow for afternoon highs in the low-mid 50`s,
though the presence of the weak pressure gradient within the
surface ridge axis will result in lake breeze development off Lake
Michigan by early-mid afternoon, which will cool area near the
lake quickly back into the 40`s. Overall however, not a bad end to
the weekend.



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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