Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192009

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CDT Sat May 19 2018

245 PM CDT

Through Sunday...

Main concerns center around shower and thunder chances from late
evening through tomorrow afternoon followed by temp trends for
Sunday. A complex pattern is in place across the region with water
vapor showing a closed upper low over western Nebraska with a
series of waves moving northeast out ahead of it. At the surface,
one surface low moving northeast across lower Michigan and another
weaker low across central Iowa aligned along a cold frontal
boundary which is moving southward toward the area. Late afternoon
cloud cover will scatter/clear from west to east allowing cloud
covered areas to see a rebound in temps, minus lakeshore areas
which are now seeing onshore flow and will be held into the 50s.

The next feature to watch will be the low over central Iowa which, in
conjunction with an upper wave, is generating a band of
precipitation. This will continue to drift northward and likely
have additional development northeastward as the upper wave
continues its progress. This may lead to far west and northwest
sections of the forecast area seeing some shower activity toward
mid evening. These features will continue to the east and
northeast and am expecting the bulk of new late evening activity
to remain north of the forecast area into the early overnight.

The aforementioned cold frontal boundary will sag into the area this
evening and stall out as another surface low moves into Missouri.
The low level jet will increase into Illinois overnight and may
see additional scattered shower development as it does so with the
frontal boundary just to the south. Will also need to watch
convection currently developing into eastern Kansas as this is
expected to develop into an organized complex which will move
northeast. The upper flow overhead is becoming more southwesterly
with 850-300mb thickness becoming oriented southwesterly as a
result suggesting that any complex would move toward the local
area. Instability will likely be the limiting factor as highest
values are expected to remain south of the area. Thus, the
expectation is that any primary complex would work in this
direction but the strong/severe thunderstorm potential would stay
southwest of the area but elevated/weaker convection would be able
to work into the forecast area late tonight and linger into first
thing Sunday morning. Thunder occurrence is favored for southern
sections roughly south of Interstate 80 but cannot completely rule
out something further north. The overall severe threat is limited
as mentioned, but not zero. If some better organized elevated
cells can develop then some hail threat would exist, especially
across the south closer to the front.

Sunday`s forecast remains a challenge. Do expect lingering
showers/isolated thunder through mid morning, but this may be
mainly confined to northwest Indiana given the potential for a
weakening complex to be exiting. Otherwise, am not all that
optimistic about organized precipitation from late morning through
the afternoon at this point. The frontal boundary looks to stay
south of the area leaving stratus in place through at least early
afternoon if not all day. Some drizzle or spotty light rain will
be possible with the stratus. East to northeast winds will keep
highs in the upper 50s and lower 60s for areas along and north of
Interstate 80, with lower 50s at the lakefront. Areas further
south should see 70s, though may be a little optimistic with highs
in the far south.



305 PM CDT

Sunday night through Saturday...

Sunday night through Monday evening: The forecast becomes rather
complicated by Sunday evening as it will be highly defendant on
how showers/convection unfold tonight into Sunday morning.
Decaying convection that develops across MO early Sunday evening
should spread northeast through the night, reaching the CWA by
around daybreak Monday. With rather modest elevated CAPE this far
north, am expecting only some embedded thunder within a decaying
precip shield. However, the lack of any appreciable convection
across northern and central Illinois tonight may allow for the
instability gradient to remain a bit farther north for Sunday
night. Much of the activity early Monday morning will exit the
area by late Monday morning. Some support aloft from a passing
mid-level trough will allow for small chances of showers during
the afternoon and evening.

Late Monday night through Thursday: Mid-level ridging will
gradually build across the central CONUS during this time, with
multiple bouts of convection riding along the northern periphery.
There is a lack of agreement among guidance regarding the
amplitude and persistence of this ridge, thus translating into
uncertainty in precip potential across the CWA. Indications at
this time are that the ridge should suppress any precip/convection
before reaching the CWA. If any precip were to reach the CWA, it
would mostly likely brush far northern portions Wednesday
afternoon into Thursday. Otherwise, expect seasonably warm
conditions inland with cooler reading near the lake.

Friday through Saturday: Significant model differences begin to
develop by this point as a Pacific NW trough attempts to capture a
closed low over the southwest U.S., then erodes the aforementioned
ridge. Given this significant uncertainty, have low confidence in
the large-scale pattern and thus the local weather by late next
weekend. With that said, there is some agreement that the Pacific NW
trough and at least some energy from the closed low will cross the
western Great Lakes region Friday night through Saturday and result
in a chance of showers and some thunderstorms.



For the 18Z TAFs...

Cigs/vsby continue to steadily improve early this afternoon as low
pressure pulls off to the northeast across Michigan. Expect that
cigs will further improve to lower end VFR mid/late afternoon with
scattering from the west starting at RFD in the next few hours
continuing eastward through late afternoon. Wind direction will be
tricky near Lake Michigan with westerly winds currently prevailing
at the terminals. Expect lake influence to take over at GYY early
this afternoon turning winds NNE and then later this afternoon at
ORD/MDW. May need to tweak timing of shift in the TAFs.

Conditions deteriorate late this evening and especially tonight as
a boundary settles across the area and a series of upper features
bring shra and possibly some isolated tsra chances. Northeast
winds will increase with low cigs redeveloping with IFR or even
LIFR looking more likely. Chances for showers will increase later
this evening with a better chance overnight. Coverage may be
somewhat scattered but a better organized area of rain may
approach from the southwest late tonight/toward daybreak. This has
a better chance of containing tsra but at this point the bulk of
tsra should remain south of the terminals. Organized precipitation
looks to exit by late morning through some spotty light rain or
drizzle may linger into the afternoon. May be a little optimistic
with ceiling/vsby improvement into the afternoon with the 18z TAF
so will need to keep an eye on trends. Overall confidence in
forecast after about 16z Sunday is on the low side in terms of
cigs/vsby/tsra potential.



305 PM CDT

High pressure will build across the northern Great Lakes through
Sunday, then expand across the entire Great Lakes Region through
Tuesday. Meanwhile, a low pressure trough across the southern
Great Lakes this afternoon will drift southward to the Ohio River
Valley by Monday. After a brief surge of northerly winds to 25
knots tonight into Sunday morning, expect a period of N to NE
winds under 20 knots through Wednesday. The high pressure will
shift to the eastern Great Lakes by Thursday as low pressure forms
over the northern and central high Plains, resulting in winds
becoming more southerly by late Thursday.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...midnight Sunday to
     9 PM Sunday.

     Small Craft Advisory...LMZ743-LMZ744...6 AM Sunday to 9 PM




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