Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202003

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
303 PM CDT Wed Jun 20 2018

302 PM CDT

Through Thursday...

Main forecast concerns are with periodic showers and
thunderstorms through much of the period, a low chance for a few
stronger storms south of I-80 early this evening and again
Thursday afternoon, and the potential for heavy rainfall which
could lead to localized flooding.

In the near term, only isolated showery development occurring
across the CWA with most locations staying dry. Temps have been
slow to rise given the extent of the clouds in place, however,
will likely continue to see more holes in the clouds for the
remainder of the afternoon into early evening with some warming
still likely this afternoon. Across the southern CWA, but mainly
more into west central IL, have seen slightly higher coverage of
precip with even some thunder present. This area of precip is
slowly developing along surface trough axis and lingering
boundary. Although this development has a decent chance of
clipping the far southern CWA, its coverage/intensity should
diminish some as it moves east.

Main focus later this afternoon into this evening will be with
the upstream development currently across northern MO and eastern
IA. As the large upper level slowly spins towards the area this
evening, will see continued stream of mid level energy move
through the region. This will assist further thunderstorm
development along surface low/trough and boundary which extends
through the far southwest portions of the CWA. In this location,
heating has supported increasing MLCAPE and steepening low level
lapse rates. Although this instability axis is situated just to
the west, trends suggest this will shift further east this
afternoon into the early evening. Steering flow and increasing
instability will allow this upstream development to move into
locations along the I-39 corridor closer to 00Z this evening, and
then spread into remaining areas in northern IL and northwest IN
through the remainder of the evening. While the higher threat of
severe storms is just to the west/southwest, do think an isolated
stronger storm or two will be possible as it reaches the I-39
corridor, with high winds the main threat. East of this location,
think the threat of any stronger storms would quickly diminish.
Given that PWATS close to 2 inches remaining in place across the
region, heavy rainfall and localized flooding will be possible
with any storm.

This upper low will continue to slowly push east later tonight,
with waves of showers and thunderstorms likely to move overhead.
Higher chances for precip are across north central IL, however,
most locations in northern IL and northwest IN do have a chance of
observing periodic showers tonight. Additional mid level
energy/additional forcing arrive late tonight into early Thursday
morning, with most guidance indicating an increase in coverage and
intensity of this precip will be likely. It`s during this time
that lightning will likely become more prevalent, along with
increasing precip rates. High PWAT air will remain in place, and
think another period of heavy rainfall is possible during this
time. Will need to monitor how much rain falls this evening, as
already saturated grounds and likely additional rain Thursday
morning could provide an increasing flooding threat on Thursday.

Thursday morning showers and storms will lift across the region
through midday. Scattered showers and thunderstorms may persist
into early afternoon, however, coverage will likely lower. Will
need to monitor Thursday afternoon for another period of storms
with increasing coverage and intensity. Boundary will still be in
place across central IL and IN, and if clearing occurs in this
location, high moisture content air with instability axis likely
in place could help support strong to severe storms Thursday
afternoon south of I-80. Strong winds and heavy rainfall will once
be the main threats.



256 PM CDT

Thursday night through Wednesday...

Thursday night through Friday: A stacked closed low over northeast
MO Thursday evening will drift E/NE to near Indianapolis by late
Friday evening. A low to mid-level band of frontogenesis/deformation
is expected to extend from western Ohio to eastern Iowa Thursday
into Friday as the low pivots south of the CWA. The combination of
PWAT values of 1.5-1.75" and a notable frontogenesis circulation
should allow for multiple bands of heavy rain to slowly lift
northward across the CWA Thursday night through Friday. If any band
were to remain relatively stationary for several hours over areas
that have received heavy rain within the past several days, minor
localized flooding will be possible. The best chance for any rain
bands to become nearly stationary will be late Thursday night and
Friday morning as the low pivots NE from southern IL to IN.
Additionally, enough instability should exist during this time to
support some embedded thunder in the more widespread rain bands,

With substantial cloud cover expected for Friday, daytime highs will
likely reach only the mid to upper 60s north to mid 70s south.
Meanwhile, NE winds of 10 to 20 mph along the IL shore of Lake
Michigan is expected to produce a high swim risk through most of the
day Friday, with a moderate swim risk lingering well into Friday

Saturday: The closed low will continue NE to near Detroit by
Saturday afternoon. Much of the area should remain dry through the
day on Saturday. But while appreciable upper forcing will have
shifted east of the CWA by this time, continued weakly cyclonic flow
combined with plenty of low-level moisture will be supportive of
widely isolated light showers across the area. Cloud cover looks to
be fairly abundant during at least the morning Saturday. If a deeper
low-cloud deck persists into the afternoon, Saturday may also end up
being unseasonably cool with max temps struggling to rise much into
the 70s.

Sunday: Long-wave troughing shifting southeast from northern Canada
will brush the CWA to the NE as a convectively-induced shortwave
tracks eastward across southern IL. Uncertainty in the strength of
the shortwave as well as the timing in which the longwave picks up
the shortwave reduces confidence on precip potential for Sunday. At
this time, an expected later arrival of both on Sunday supports at
least a chance of showers with a few thunderstorms during the
afternoon and early evening Sunday. Strong to severe storms are not
expected given a lack of appreciable instability and shear.

Sunday night through Wednesday: High pressure passing NE of the area
will bring a period of dry conditions under generally NE winds
Sunday night and Monday. Guidance then begins to diverge on the
handling of deep troughing shifting east from the Rockies Tuesday
and Wednesday. While at least some chance of rain exists from late
Monday night through Wednesday night depending on how quick the
trough shifts east, consensus guidance supports a more focused
window for rain chances Tuesday evening into Wednesday evening.



For the 18Z TAFs...

Expect an active period with showers and some thunderstorms
appearing likely across all the sites. In the near term, dry
conditions in place with isolated showery development continue to
develop away from the terminals. Expect this general pattern to
remain, however, think precip chances will increase later tonight
into Thursday as an upper level system approaches the area.
Periodic showers then likely through the end of the period with
some thunder potential tonight, but with the highest chances
arriving Thursday morning. MVFR ceilings will likely improve to
VFR this afternoon, but with MVFR ceilings returning later tonight
into Thursday morning.



256 PM CDT

A nearly stationary frontal boundary will remain stretched west to
east across central Illinois and Indiana through Thursday. A low
over the central Great Plains today will meander around the
central Great Plains and middle Mississippi Valley through
Thursday, then drift from Missouri to the eastern Great Lakes
Thursday night through Saturday night. Expect a persistent period
of 15-25 knot N to NE winds through the weekend. However, a period
of winds to 30 knots are possible Thursday night into Friday
morning across the south half of Lake Michigan. High pressure will
then build across the Great Lakes region Sunday night through
Monday, bringing a period of NE winds on Monday.






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