Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190545

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1245 AM CDT Mon Jun 19 2017

801 PM CDT

Updated to include an isolated light shower mention mainly over
northeast IL/southern Lake Michigan next 1-2 hours, as a couple of
sprinkles light/showers have developed from closed-cell stratocu
field recently. Area of better cu development appears to be
correlated beneath tail of mid-level short wave rotating across NE
IL at this time and slight cooling of temps around 650-700 mb.
HRRR and RAP soundings suggest weak instability will diminish by
02-03Z making for a fairly small window for any isolated showers,
thus have updated to include a minimal 15 percent just through

No other forecast changes anticipated at this time.



228 PM CDT

Through Monday...

Stratocumulus clouds in the cold advection regime have filled in behind
upper level low pressure to our northeast. West winds will
continue gusting to 25-30 kt as a fairly tight pressure gradient
sits over northeast IL with a fairly deep summer time low over
upper Michigan. There are a few showers upstream, but the cu
field will be fairly shallow and coverage should be low.

Clouds thin tonight and west winds will ease, but expect a
similar situation tomorrow though with a bit lighter west wind as
the main surface low will shift farther northeast across Ontario.
We remain in northwest flow behind upper level low pressure, a
pattern that favors diurnally driven showers, but better shower
coverage potential is expected Monday as several models depict a
modest wave in the flow. A few thunderstorms are also possible,
with these showers forming as early as mid to late morning.
Temperatures look to fall slightly cooler than today`s, with
readings generally topping out in the mid 70s.



232 PM CDT

Monday night through Sunday...

Northwest flow still expected to be in place at the start of the
period, with broad upper level trough across the eastern half of
the CONUS. In this pattern, will see continued stream of mid level
energy move overhead with guidance trying to pick up on some more
energetic waves to move over the area. Some variability, but the
first stronger wave looks to arrive sometime Tuesday morning. This
could provide a more solid area of showers for the area, however,
looks to quickly depart to the east during the day. Instability
not appearing to be too high at this time but with the possible
stronger forcing, have continued mention of slight chance thunder.
CAA will be a persistent feature during this time, with near
normal temps expected Tuesday.

Upper level trough begins to shift to the east by mid week with
more zonal flow returning to the region. In this setup, region
would be favored for mid level energy to stream overhead, which
guidance is indicating into the end of the work week. Periods of
showers and thunderstorms will be once again possible, with more
widespread coverage returning by late Wednesday into Wednesday
night. This will be ahead of a developing trough of low pressure
and frontal boundary, with guidance indicating will remain across
the region Thursday into early Friday. This would make sense given
there does not appear to be any real big support to help kick
this out of the region. With this in place, additional
thunderstorm chances are possible during this time.



For the 06Z TAFs...

Overall, expecting VFR conditions and west winds. Gusts to around
20 kt are possible from the late morning through the afternoon.

An upper level wave will likely force showers and thunderstorms
this afternoon as it moves over the region. Expecting scattered
coverage from about 16Z/17Z through the late afternoon. Therefore,
all TAF sites should expect ocnl showers and thunderstorms this
afternoon. Some models hint that coverage may diminish after
21Z/22Z so left the PROB30s for -TSRA as is in the TAFs. I want to
see how models handle the showers as far as coverage and timing
before possibly changing the PROB30s to TEMPOs for -TSRA.

Convection diminishes in the evening with west winds and VFR
conditions continuing tonight.



232 PM CDT

As low pressure just to the north of the lake has lifted north,
it has continued to strengthen. With this occurring, winds of 15
to 25 KT and even up to 30 KT are likely in place mainly over the
north half of the lake. Winds over the south half have approached
the 15 to 25 KT range, but do think winds have generally stay
confined to 10 to 20 KT. As this low lifts further to the
northeast tonight, expect winds to diminish across the entire
lake. Included in this diminishing trends are the current
hazardous conditions occurring over the nearshore. Westerly winds
in place on Wednesday, but with speeds expected to be lower over
the entire lake.






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