Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191946

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
246 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.



211 AM CDT

Sunday through Friday...

Active weather looks to be in store for portions of the region if
not the local area to close out the weekend and continue into at
least Tuesday. Broad surface high will build across the Great
Lakes Sunday into Monday and gradually spread east over New
England by Tuesday. Through this time frame, several rounds of
convection appear likely for portions of the Great Plains and
Midwest on the southern periphery of the high.

A disorganized mid-level low is progged to lift across the
central Plains on Sunday while a weak surface reflection develops
across portions of northern Missouri during the day and drifts
east into Central Illinois Sunday evening. Thunderstorms are
expected to develop in advance of these features on Sunday. Will
have to closely monitor the position of the warm front which will
likely be influenced by the evolution of convection from Saturday
night. Given the overall weak synoptic setup, it`s quite possible
the effective warm front is shunted much farther south by
tonight`sconvection which will limit the severe threat for the
local CWA on Sunday. Still, the latest GFS indicates moderately
strong MLCAPE of around 1500 J/kg will lift into the southern tier
of counties which coupled with around 40kt of deep layer shear
would result in some threat for severe storms near the warm front.

Expect a large temperature gradient across the CWA if the warm front
is able to lift into the area. Northern portions of the CWA with
northeast flow off of Lake Michigan under the influence of the high
will experience chilly conditions with highs only in the upper 50s
to mid 60s. South of the front, expect temperatures into the 80s.

Aforementioned surface low will move from central Illinois into
Indiana on Monday. Models continue to hint at additional chances for
showers and thunderstorms for the local forecast area near the low,
though wouldn`t be surprised to see this threat also shift south.
Temperatures remain on the cooler side again Monday for much of
the area, topping out 60s and low 70s north with onshore flow
through the day. Far southern counties may again tag 80 depending
on the low track.

Upper ridge will build across the central Plains and Mississippi
Valley Tuesday into Wednesday while at the surface, high pressure
takes hold over the western Great Lakes. Expect primarily dry
weather through this period, though showers and thunderstorms won`t
be too far off to our west. Also expect a gradual warming trend
through midweek with widespread 70s Tuesday eventually reaching
widespread 80s Thursday and Friday. Upper ridge is progged to buckle
late in the week with increased precip chances overspreading the
region once more.



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.



315 AM CDT

Two separate areas of low pressure will continue to move through
the great lakes region. In this pattern, winds have seen a slight
diminishing trend, with speeds mainly staying in the 10 to 20 kt
range for most of the night. The trend for the remainder of the
morning into midday and early afternoon will be for continued
diminishing speeds. This will quickly change late this afternoon
into tonight with the arrival of low pressure to the northwest.
This low and front will shift south down the lake later this
evening into the overnight hours, with strengthening northerly
winds expected over much of the lake. Expect speeds to generally
stay in the 15 to 25 kt range, however, do think there will be
periods of speeds approaching 30 kt at times tonight. Across the
nearshore, this northerly shift and increase is expected late
tonight into early Sunday morning. Although speeds will increase
over the IN nearshore, at this time, it is looking as if the
strongest winds will be observed over IL nearshore. These
increasing winds and likely building waves could bring a period of
hazardous conditions for small craft late tonight into Sunday,
especially across the IL nearshore.






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