Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220518

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1218 AM CDT Mon May 22 2017

911 PM CDT

Although a few light echoes popped up on radar earlier this
evening and some locations reported a period of sprinkles, this
activity now appears to have cleared the area. Overcast skies
remain but the back edge of the cloud shield, seen on satellite
over Iowa, is moving this direction and at its present pace should
allow Monday morning to dawn mostly sunny. Given the clearing
trends, overnight lows may have to be monitored in case they dip a
bit cooler.



127 PM CDT

Through Monday...

Deep upper level low pressure across the upper midwest will
continue to stratocumulus deck across the region through the
evening and early overnight before diminishing. With cold
advection that will continue through the night and the lingering cloud
cover, expect temperatures at this point to largely hold steady
or even fall some through the afternoon. At the surface, high
pressure across the southern plains will shift to the lower
Mississippi Valley tonight while low pressure lingers across the
northern Great Lakes. This will maintain an occasionally gusty SW
wind through Monday especially during the daytime hours, though
cannot rule out occasional gusts overnight.

Clouds should thin by daybreak Monday which like today should
start off on the sunnier side. Several waves will rotate around
the back side of the low beginning Monday which will increase
clouds and bring the chance for showers and even a few
thunderstorms Monday afternoon. Confidence is high that there will
be at least scattered rain showers in northeast Illinois, and
possibly thunderstorms during this time frame, but uncertainty on
how much coverage precludes likely pops just yet. The
west/southwest wind on Monday will be a warmer wind then today,
thus expect temps to climb above today`s lower 60s readings.



328 PM CDT

Monday night through Sunday...

A fairly active pattern will remain in place into Memorial Day
weekend with multiple opportunities for periodic showers and at
times thunderstorms. The mid week period will be marked by cooler
than normal temperatures for late May as a large upper low
traverses the region, followed by a return closer to seasonable
temperatures Friday into the holiday weekend.

On Monday evening and night, weak broad surface low embedded in an
elongated trough stretching across the Great Lakes will be near or
just northwest of the area. Multiple mid-level shortwaves in
southwest flow aloft will keep a threat for scattered showers
going. Thunder potential should be largely diurnally driven and
focused into Illinois so should wane later in the evening and
especially overnight.

Looking ahead into Tuesday, the closed mid-upper low will move
slowly southward along with associated surface low. The surface
wind pattern will be tricky and based off position of the surface
low which is exhibiting variance in operational guidance. This
will affect temperature trends in the afternoon, especially near
Lake Michigan. Temperatures will range from the lower to mid 60s
northwest of I-55 and upper 60s to around 70 southeast, though
again an earlier onset of onshore winds shown on some guidance
will result in temperatures several degrees cooler near Lake

More importantly, short-wave energy rotating around the
upper low will keep forcing going for shower activity with an
assist from lower-level convergence. With the cold mid-level
temperatures associated with the upper low over the area featuring
freezing levels below 10kft AGL and providing minor instability,
isolated/slight chance thunder potential is evident. This setup
may be one to watch for small hail/graupel given such low freezing
levels. Moisture levels will not be all that high/around 1" PWAT
values/but relatively slow storm motions could result in decent
downpours, particularly in any thunderstorms. Since instability
will already be limited to begin with, isolated thunder potential
will again likely be diurnally driven and unlikely to linger into
Tuesday evening.

The track and intensity of the upper level low into Wednesday and
Thursday will drive the mid-week pattern and our sensible weather.
Model guidance often struggles with these features, decreasing
confidence in trends. Among the 12z guidance, GFS and GEFS are on
the deeper and farther west side of the envelope for surface low
associated with the upper low, though ECMWF and its ensembles did
trend a bit westward and stronger vs. 00z suite. The difference in
handling of the synoptic system will dictate whether the weather
is merely showery and cool (especially lakeside) or downright raw
with steady rain and strong north winds.

These trends will affect Wednesday night into Thursday as well and
how quickly the cloudy showery and cool conditions exit east.
Short-wave ridging looks to slide east on Friday to return
seasonable temperatures, but looks to be accompanied by a threat
for showers and possibly thunder from a ridge rider disturbance.
There is plenty of uncertainty regarding the extent of
precipitation potential over the Memorial Day weekend, so do not
cancel outdoor plans yet. Temperatures appear they will be
seasonable in the 70s, though likely affected by cloud and shower
coverage, with Saturday currently favored to have lake cooling.



For the 06Z TAFs...

VFR ceilings still in place but clearing line is steadily pushing
east across the area at this time. Expect this clearing line to
reach the terminals in the 7-8z time frame, with clear skies then
likely through much of the morning. Anticipate only mid/high
clouds for much of the day, until the next chances for precip
arrive later this afternoon. While VFR ceilings return this
afternoon, expect scattered precip to develop around the
terminals as well. Have not made any big changes to precip timing
in the TAF including the possibility for thunder. Will monitor
trends this morning, but at this time, conditions don`t look
overly favorable for any widespread thunderstorm development
across the terminals. Scattered showers will likely linger into
the evening. Monitoring for possible lowering of ceilings to MVFR
later tonight, but at this time, setup would favor continued VFR
ceilings. West southwest winds in place with some sporadic gusts
still occurring. Do think gusts will diminish and speeds lower as
this clearing line moves overhead early this morning. Then
anticipate speeds/gusts to increase once again later today.



350 PM CDT

A cold front will continue to push across the lake through this
evening, with southwest to west-southwest winds generally up to
25 kt through mid day Monday. Winds will turn southerly on Monday
over the open lake due to new low pressure forming over the Upper
MS Valley and stable air mass. The surface low will meander
gradually southeastward through Wednesday morning while slowly
deepening. The low could then further deepen and retrograde
northward on Wednesday afternoon into Wednesday night. The track
and strength of the low is uncertain and will affect the strength
of northerly winds on its backside into Thursday. The potential
exists for stronger northerly winds than current forecast (up to
25 kt) with a deeper farther west low and vice versa with a
weaker farther east low pressure. The northerly winds will
gradually give way to close out the week as weak high pressure
ridging moves over the region, with generally lighter speeds and
east to southeast direction for southern and central portions of
the lake.



LM...Small Craft Advisory...IN nearshore waters until 4 AM Monday.




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