Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242024
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
324 PM CDT Wed May 24 2017

.SHORT TERM...
215 PM CDT

Through Thursday...

Sharp upper level trough with pinwheeling lows will continue to
impact the region in the near term bringing overcast skies
areawide and scattered showers. Water vapor imagery this
afternoon reveals two upper lows embedded within a broader
longwave trough centered over the Mississippi Valley. A western
low is digging south across Missouri into Arkansas while another
low is lifting north across the Ohio Valley. The western low
resulted in some scattered showers over the western CWA earlier
today but is otherwise exiting the area with mid level height
rises building across the mid and upper Mississippi Valley. More
widespread precipitation is in place with the eastern low and some
of the showery activity with this low will impact areas mainly
east of the I-55 corridor through the remainder of the afternoon
and evening. There is some very modest instability over
northwestern Indiana, 100-200 J/kg MLCAPE which may result in an
isolated embedded thunderstorm or two, but overall lightning
potential is low for the CWA.

The western upper low is progged to dig into the deep south late
this evening and overnight which will help kick the eastern low
farther east out of the CWA. This will result in diminishing
precip chances through the late evening and overnight hours.
Lingering low level cyclonic flow through mid afternoon Thursday
will likely keep much of the CWA locked under cloud cover, but
ridging building into our west mid to late in the afternoon should
result in some clearing west and perhaps a few peeks of sunshine
east. Temperatures are expected to be several degrees warmer
tomorrow, with highs in the upper 60s to around 70 inland, but
only in the 50s and low 60s along the lake front.

Deubelbeiss

&&

.LONG TERM...
318 PM CDT

Thursday night through Wednesday...

There are not many periods without a mention of rain from late in
the week through next Wednesday, but the window from late Thursday
into early Friday appears to be one exception.  Other breaks appear
likely early Saturday and again Sunday night.  This is not to say
the other periods are likely to see widespread rainfall, but chances
increase for most afternoons in response to diurnal heating, the
ongoing presence of longwave cyclonic energy aloft, and the timing
of shortwaves moving across the area.

The axis of a surface and upper level ridge positioned over the
Plains this afternoon will push into the western Lakes by early
Friday but by midday Friday already appears to be east of the area.
Deterministic guidance from the ECMWF/GEM/GFS solutions all show an
embedded shortwave over the area by this time in the more zonal flow
upstream from the departing ridge.  The surface reflection of this
upper shortwave is handled quite differently by the different
models, but all suggest a wave will develop along a baroclinic zone
running south of the area. Given model differences it is difficult
to pin down specifics, but generally the better chances of showers
and thunderstorms would be focused across the southern half of the
forecast area Friday afternoon and evening.

This baroclinic zone drops south and allows a weak ridge to move
into the area early Saturday, but then the next surface wave
develops upstream and lifts toward the area late Saturday along with
the next upper shortwave.  Models again show poor agreement with the
handling of these features, so we have not included more than chance
POPs for Saturday afternoon and evening.

By late in the weekend the next upper low drops from central Canada
into the Great Lakes and remains overhead through late Tuesday.
Chance POPs persist each day in this period, especially in the
afternoons and evenings, and then drop to slight chances by
Wednesday as the upper low begins to lift off to the east.

Lenning

&&

.AVIATION...
For the 18Z TAFs...

Low pressure over the Ohio Valley this afternoon will meander over
Indiana and Ohio over the next 24 hours with a modest northerly
breeze and primarily MVFR conditions in place across northern
Illinois and northwest Indiana. Ceilings will pose the primary
forecast challenge this afternoon. Ceilings are expected to hold
steady or improve slightly this afternoon, though erring on the
side of persistence in the meantime as confidence in the details
is low. There is some lingering patchy IFR this morning (at RFD
and GYY), though anticipate that should lift to low MVFR over the
next couple hours. There is potential that ceilings could lift to
VFR for several hours mid to late this afternoon into the evening,
but confidence is too low to include in the TAF at this time.
Showers are expected to increase in coverage across northern
Indiana this afternoon and eventually into southwest Michigan late
afternoon into the evening. The best coverage of showers is
expected to stay east of the terminals, but cannot rule out some
of the showers rotating west across the terminals this evening.
Overnight, expect ceilings to lower back to low MVFR with IFR
again a possibility.

BMD

&&

.MARINE...
318 PM CDT

Low pressure over Kentucky today will move into Ohio by Thursday
and then quickly off the New England coast by Friday. This low
will support northerly flow across the lake through late
Thursday. Winds will be lighter and more variable Friday and
Saturday as a weak ridge builds over the lake. Winds eventually
increase from the southwest later in the weekend as the next low
pressure system moves slowly across Ontario Saturday and toward
James Bay by Monday afternoon.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ014 until 7 PM Thursday.

IN...Beach Hazards Statement...INZ001-INZ002 until 7 PM Thursday.

LM...Small Craft Advisory...IL nearshore waters until 10 AM Thursday.

     Small Craft Advisory...IN nearshore waters until 4 PM Thursday.

&&

$$

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