Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180613

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
113 AM CDT Sun Mar 18 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 06Z TAFs...

A lake breeze will develop this afternoon and provide a wind shift
to northeasterly at Chicago area airports. Confidence is high in
speeds after the wind shift being 9-12 kt. As for the forecast
wind shift specific hour of 21Z, confidence is medium in that
timing. Winds look to diminish this evening, then re-increase from
the northeast to 10-15 kt by daybreak Monday.



113 AM CDT

A weak cold front will settle over the northern part of the lake
this afternoon, turning winds to the north-northeast. Otherwise
high pressure will prevail across the southern part of the lake
with an afternoon lake breeze in the nearshores.

This week will be dominated by northeasterly flow, with general
high pressure to the north and low pressure to the south on
Monday-Tuesday and likely again late in the week. The first bout
of stronger northeast winds will take shape late tonight and
through Monday. Northeasterly wind gusts look to peak Monday
night, with the highest gusts of around 30 kt favored over the
south including Illinois and Indiana nearshores. This will support
high wave action and Small Craft Advisory criteria that will
continue through at least Tuesday. North-northeast winds continue
but less in speed midweek, before an increase in northeast or
east winds is favored again late in the week.






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