Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161731

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1231 PM CDT Wed May 16 2018

211 AM CDT

Through Friday..

The stagnant pattern aloft will work in our favor for several
days as an upper trough across the Missouri valley will slowly
migrate to the Ohio valley later in the week thanks a weak rex
block high across the lower Great Lakes. Therefore, surface high
pressure across the region today, and only slowly retreating to
the northeast on Thursday. This pattern will feature plentiful
sunshine and mild temperatures, but with onshore flow, cooler
conditions will continue at the lakeshore. Expect more higher
cloud cover east of I-57 as the low the south inches closer into

The upper low will congeal a bit more late Thursday
night into Friday, then as the blocking high to the north makes
some minor headway east across Canada, the higher clouds
associated with the low will make their way into east central
Illinois and northwest Indiana. We have the potential to get quite
more cloud cover than advertised with the low even into the
northeast Illinois, but the suite of guidance still keeps the bulk
of the precipitation associated with the low out of NE IL/NW IN.
We will need to hang onto some low chances of showers to account
for this.



211 AM CDT

Saturday through Tuesday...

Upper ridging should now keep most of the daylight hours dry on
Saturday, with decent agreement from mid range guidance. The
approach of low pressure will bring several rounds of showers and
maybe a few thunderstorms Saturday night into Sunday. The first
would come as the low level jet ramps up in the evening. The main
upper trough axis will still be sliding through with cold front
on Sunday. There are some timing differences with the front on
Sunday, and the upper wave is not all that strong, but with a
surface low maintaining its intensity expected to move through the
area during peak heating, both the GFS/EC build decent surface
based instability in the warm sector. Stronger shear is displaced
a bit farther north, but it appears a favorable pattern for
potential stronger storm development somewhere in the region. All
just depends on how quickly the front approaches.

High pressure will quickly return to the Great Lakes region to
begin next week, with several days of seasonal and dry weather.



For the 18Z TAFs...

VFR conditions with light winds under 10kts are expected through
much of the forecast period as a surface ridge remains in place
across northern IL through tonight. Winds should be calm or light
and variable tonight. NE winds will resume while nearing or
reaching 10kts Thursday afternoon.



308 AM CDT

Surface high pressure will remain in place across southern Lake
Michigan today, before weakening tonight. Overall, this will
result in light and variable winds over the south half of the lake
today and tonight, but on the north end of the lake expect southwesterly
winds of 10 to 20 kt today. A cold front will shift south over
the lake late tonight into Thursday morning as a weak area of low
pressure tracks eastward into Quebec later today. In the wake of
this front, northerly winds will set up on the lake for Thursday,
before shifting east-northeasterly Thursday night into Friday as
another area of high pressure builds across southeastern Canada
and the Upper Great Lakes. Waves could build to near 4 feet over
the southern and western shores of the lake Thursday night with
this period of east-northeasterly winds, so a small craft advisory
could be needed into early Friday.






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