Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172024

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
324 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.



300 PM CDT

Sunday night through Saturday...

For those hoping for a warmer weather after what will be a
pleasant Sunday away from the lake, unfortunately the upcoming
work week will be a cool one. This will be due to persistent
northeastern North America/North Atlantic blocking (-NAO) keeping
broad mid-upper troughing locked in over the region and Canadian
high pressure anchored to our north. The high pressure will result
in prolonged and occasionally gusty northeast surface flow, which
will keep lakeshore areas particularly chilly and in the 30s to
possibly around 40 much of the time. The blocking pattern does
now appear likely to result in surface low track far enough south
to keep precipitation from the Monday-Tuesday system mostly south
of the CWA, with much better ensemble agreement than yesterday.
Can`t yet rule out a slight adjustment back north, but if any
light precip occurs, it will likely be well south of I-80.

As mentioned above, the main sensible weather effect of the
described surface pattern Monday-Tuesday will be northeast winds
gusting up to 25 to 35 mph, strongest near the lake (particularly
on Monday-Monday night). Dry conditions appear likely to persist
influenced by surface high pressure Wednesday-Thursday. A strong
short-wave will eject out of the next western trough late in the
work week, with a robust surface low likely taking shape over the
High Plains. The blocky pattern will modulate the track of this
surface low as well as gradually weakening it eastward over next

Current consensus of the operational and much of the ensemble
members are for the low to be forced over or south of the CWA.
There`s overall solid agreement in a storm system affecting the
region, however as is to be expected 6-7 days out, there`s plenty
of uncertainty with specifics, including timing. With steady
precipitation expected on Saturday, lowered high temperatures to
upper 30s-mid 40s, which could certainly be generous considering
most raw guidance is cooler than this. In fact, depending on the
exact track and strength of this likely storm system, thermal
profiles in portions of the CWA could be supportive of wet snow
even during the day on Saturday. Stay tuned and expect plenty of



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.



324 PM CDT

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. The low pressure track is appearing to
be far enough south to preclude gales over the southern half of
the lake and nearshore waters, but can`t rule gales out yet. Winds
will gradually ease on Tuesday, but given the prolonged northeast
flow, expect hazardous waves for small craft to persist into or
possibly through Tuesday evening.

A northeast to north flow over the lake is expected to continue
much of the rest of the week. The next stronger storm system could
track close enough south of the lake to bring 30+ kt winds Friday
night into Saturday.






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