Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180502

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1202 AM CDT Fri May 18 2018

310 PM CDT

Through Friday...

A bit of a tricky forecast regarding precip trends late tonight
into Friday. Prior to that, most of the night will be quiet under
a cirrus canopy, with cool conditions near Lake Michigan where
temperatures lakeside have only been in the 50s this afternoon due
to steady onshore winds. The same steady onshore winds should slow
the temperature fall near the lake tonight, with upper 40s to
lower 50s lows there, coolest in far northeast Illinois. Further
inland, where the warmest readings occurred into the upper 70s-mid
80s, it will be a pleasant evening, followed by a mild overnight
in mid-upper 50s.

Turning to the threat for showers lifting north-northwestward
toward and across portions of the area late tonight into Friday,
this will be driven by upper level low pressure seen on W/V
imagery from Missouri to the lower MS Valley. The upper low will
gradually pivot east-northeastward into Friday. Convection
associated with it currently stretches from southern MO to the OH
Valley. It appears that convectively enhanced vorticity maxima on
the northern edge of the ULL provide the forcing to gradually
spread showers north-northwestward. However, timing will be
unfavorable and into an environment with weak lapse rates. Model
guidance, including CAMs are in poor agreement in coverage of
showers and how quickly and far they spread northward during the
day on Friday. Some spotty showers could make into the far
southern CWA counties in the pre-dawn hours.

Given the uncertainty of how exactly things will evolve keeping
confidence on lower side, kept PoPs in chance range and gradually
spread them north on Friday, with at least areas north of I-88
having a decent chance to stay dry through the afternoon. Even if
precip makes it farther north, it should be light and spotty as
precip shield will also be fighting dry east-northeast low level
flow. Can`t completely rule out isolated embedded thunder with
meager CAPE shown on soundings, though again weak lapse rates and
very minimal instability supports keeping thunder out of the

Temperatures on Friday are also a bit tricky with cloud cover (and
any precip) effects, as well as steady onshore flow possibly
keeping lakeshore fairly chilly again. Highs should reach the 70s
well inland of the lake, with around 70/low 70s southeast where
confidence is highest in showers. Considering how temperatures
behaved lakeside today, kept immediate shore highs in IL in the
50s, gradually warming to 60s several miles inland. Finally, depth
of mixing could also play a role in how breezy the east-northeast
winds are on Friday. With upper low having a weak surface
reflection encroaching upon high pressure over the Great Lakes, it
could get fairly windy, with gusts up to 25 mph forecast, and
potential for 30 mph+ gusts if mixing is deep enough.



310 PM CDT

Friday Night through Thursday...

Friday night through Monday: A slowly-evolving mid/upper-level
pattern across the central CONUS during this time will result in
periodic chances of scattered showers and possibly some
thunderstorms. An initial upper-level low stretching from MO to MS
will meander northward up the MS River Valley through Saturday.

Indications are that a vorticity max evident on water vapor across
far western NC this afternoon will be the impetus for precip chances
Friday evening as it merges with the northward moving upper-low.
Isolated to scattered showers will be possible through Friday night.
However, a majority of the night should remain dry for most areas as
upper-forcing remains somewhat disorganized.

Isolated showers may linger well through the morning and possibly
into the early afternoon Saturday as the upper-low shifts eastward.
Again, coverage should be fairly minimal, so some areas will likely
remain dry through the day.

A second upper-low with a corresponding stronger surface will drift
eastward from eastern CO Saturday afternoon to northern IL Monday
afternoon and the eastern Great Lakes Tuesday morning. A dissipating
MCS on a veering LLJ late Saturday night into Sunday morning may
result in a period of showers and some thunderstorms. Then, as the
weakening upper-low and dissipating surface low track across the
western Great Lakes, expect periodic chances of showers a some
thunderstorms Sunday afternoon into Monday.

Tuesday through Thursday: Dry conditions are expected through this
period as upper ridging slowly builds eastward across the western
Great Lakes.



For the 06Z TAFs...

Dry and VFR conditions expected for much of today, while northeast
winds continue. Expect these northeast winds to increase later
this morning into the afternoon, with gusts in the low 20kt range
still anticipated. Have continued the same precip timing/coverage
later today, with scattered showers expected to move overhead
this afternoon into the evening. Scattered showers will likely
continue for much of the evening, and possibly linger through the
overnight hours. Will need to monitor ceiling trends later this
evening into tonight, as lower ceilings than currently forecast
are possible. Confidence is still low, but with a period of low
end MVFR and even IFR possible at times tonight.



310 PM CDT

High pressure will shift over southeastern Canada and the Upper
Great Lakes region tonight into Friday while a trough develops
across the central and northern Plains. NE winds to 25 knots will be
possible for much of the lake during this time. The high pressure
will then retreat to eastern Canada by Saturday night. A series of
weak low pressure systems will cross the southern Great Lakes
Saturday afternoon through Monday before high pressure gradually
builds across the western Great Lakes beginning Tuesday. During this
period, winds will remain generally N/NE/E at under 20 knots.



LM...Small Craft Advisory...IL nearshore waters until 10 PM Friday.




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