Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190525

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1225 AM CDT Sat May 19 2018

253 PM CDT

Through Saturday...

An upper-level low over southeast MO and associated weak surface low
over western KY/TN will continue to drift N/NNE through tonight,
reaching northwest IN by Saturday morning. Troughing over the
northern Plains will then shift the low eastward away from the area
through the day Saturday.

Several batches of showers are expected into Saturday morning as
several pockets of enhanced vorticity rotate around the upper-low. A
more compact vort max currently crossing the CWA will bring
scattered showers over the next couple hours. Overall coverage
should remain scattered this evening as the upper-low nears. When
the low begins to pivot eastward overnight, guidance is indicating
an increase in coverage of showers across the east half of the CWA
in response to increased deformation on the western flank of the

Showers will gradually end from west to east on Saturday as the low
departs. Low clouds should gradually lift into a broken strato-
cumulus deck through the afternoon. Confidence is low on whether
clouds will break up considerably in the afternoon, so kept max
temps in the mid 70s north to near 80 south. If clouds do break up
faster than anticipated, max temps will likely be several degrees



253 PM CDT

Saturday night through Friday...

There are two primary features of interest as we head into the
weekend.  First is the upper low centered near St. Louis this
afternoon, and the second is the low pressure system developing in
the lee of the Rockies and lifting into the Plains.  The first
system should be exiting the area by late Saturday before the next
one begins to make its influence felt locally.  Under weak upper
ridging this would provide a break in precipitation from Saturday
afternoon or evening through Sunday morning, and perhaps only
through the predawn hours Sunday for western portions of the
forecast area.

For only being 54 hours out, deterministic models are in fairly poor
agreement regarding the position and evolution of the sfc low
moving out of the Plains into the Missouri Valley on Sunday.  By 18z
Sunday the depicted positions of the low range from Omaha to
St. Louis.  What they do agree a bit better on is the position of a
moisture and temperature gradient somewhere in the US-24 to I-80
corridor.  This would support a wide range of forecast highs for
Sunday, ranging from the mid 50s along the lake in northern portions
of the area, to around 80 farther south.

This also would provide a focus for additional shower and
thunderstorm development during the day Sunday and continuing into
Sunday night.  Given the warm and moist airmass along this boundary
as well as decent deep layer speed and directional shear, and
despite the ridgy mid-level heights referenced in SPC`s Day 3
Outlook this morning, the marginal risk of severe storms for
southern portions of the area appear reasonable.  Farther north, dry
northeasterly flow could inhibit saturation at lower levels early in
the day, but shortwaves moving through the mid-level ridge, as well
as uncertainty regarding the position of the nearby baroclinic zone,
warrant the mention of at least chance POPs areawide Sunday
afternoon and night.

Models continue to disagree heading into next week, with the GFS and
GEM showing deeper development aloft and the lifting of the surface
low across MO into IA on Monday.  Meanwhile the NAM kicks the low
well east of the area and the ECM somewhat splits the difference.
Given these uncertainties, will maintain a chance of showers into
Monday with a bit lower chance of thunder.  Will also maintain a
considerable north to south temperature gradient that could be even
more pronounced than presently depicted.

Overall trends in all four models are for surface and upper level
ridging to become more established midweek as the slow moving low
finally makes its way clear of the area.  Based on this, will keep
the area mostly dry through the remainder of the week with at most
slight chance POPs along with seasonable to slightly above
average temperatures.



For the 06Z TAFs...

Low ceilings, periodic showers, and reduced vis are expected to
continue this morning. Latest radar imagery showing scattered
showers still developing over the area, which will likely persist
this morning. This rain and fog will reduce vis, possibly in the
1-2sm range. Lowest ceilings falling as low as 400-500ft are
located around ORD, while the other sites remain IFR or low end
MVFR. Should see a trend downward toward ceilings similar to ORD
in the near term, that will last through mid morning. An east
northeast wind will remain in place this morning but as low
pressure moves through the region, will see winds briefly go more
north. A northwest wind is then expected soon there after, but
will monitor sites near the lake where an additional shift to the
northeast could be observed quicker than forecast. Have continued
previous wind forecast, but will monitor a possible sooner shift
over to the northeast later today. A front is expected to push
south through the area later tonight, that will increase the
northeast winds and bring additional chances for showers.



304 PM CDT

Weak ridging over the lake will give way to a low pressure system
lifting north through the Mississippi Valley overnight. The low
then slides east of the area on Saturday leaving a relatively weak
gradient and allowing for the increasing influence of local
thermal effects Saturday afternoon. The next area of high pressure
moves from the northern Plains toward the lake by Sunday, briefly
supporting a tighter gradient and stronger north winds Saturday
night. The ridge then parks over the north half of the lake into
next week and supports easterly flow across the south half.






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