Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200725

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
225 AM CDT Sun May 20 2018

220 AM CDT

Through tonight...

Early this morning, high pressure is building into the Upper Midwest
and has helped push cooler air south into northern Illinois. A cold
front has surged down the lake moving across northeastern Illinois
early today where temperatures have fallen into the 40s. Southwest
flow aloft overriding this colder air is kicking off a few scattered
showers this morning over portions of north central Illinois. Expect
this to continue across the northern tier of counties in Illinois
into southern Wisconsin through the morning. Temperatures will
remain on the cooler side today topping out in the 50s and low 60s
north of I-80 and eventually warming into the 70s south.

A pair of shortwave troughs are expected to lift across the region
later today bringing additional chances for thunderstorms. The lead
wave lifting from the southern Plains will move across central
Illinois early to mid this afternoon. There remains relatively poor
model agreement on location and coverage of any potential storms,
but the best chances will likely be confined to areas south of the
Illinois/Kankakee River valleys this afternoon, with a decent
likelihood it stays south of the CWA altogether. Modest instability
coupled with moderately strong shear across the southern counties in
the CWA would support a conditional severe threat if storms were to
be in the area.

A second wave lifts across the region overnight. Given the fairly
modest forcing for ascent and unfavorable diurnal timing of the
wave, opted to lower PoPs from going forecast for tonight. Think
that widely scattered showers/isolated thunderstorms that form
upstream may be able to persist for a while across the CWA
overnight, but likely in a weakening state.



220 AM CDT

Monday through Saturday...

Showers and a few thunderstorms are possible again on Monday. GFS
and ECM are in good agreement with right entrance region to upper
jet translating overhead coincident with a damping 500mb vort max.
Soundings show very modest instability over northern Illinois and
northwest Indiana, though enough to support at least a low end
thunder threat. Temperatures continue to run on the cooler side
Monday under the influence of the broad surface ridge over the
Great Lakes. Onshore flow will hold highs in the 50s and low 60s
near the lake while inland 60s and 70s are expected.

Tuesday through Thursday, upper low is expected to close off
across SoCal while downstream amplification of upper ridge is
expected across the mid section of the country. Lee side of the
upper ridge begins to build across the western Great Lakes Tuesday
while surface high pressure builds into portions of the Upper
Midwest and western Great Lakes. This provides the best chance for
a couple days of primarily dry weather, though guidance does show
convection all around the CWA, and couldn`t rule out a stray
shower moving into the area. Expect some sunshine on Tuesday,
though would also expect cirrus from upstream convection to stream
overhead at times. Soundings show more mid/high level moisture on
Wednesday as upstream convection moves even closer to the CWA.
Despite increasing cloud cover, guidance does indicate we will see
a gradual warming trend as the upper ridge builds overhead. By
Wednesday expect widespread upper 70s with some 80s possible, with
the exception again being areas along the lake which will be held
in the 60s. Warming trend should continue into Thursday as the
500mb ridge axis passes overhead. Convection won`t be too far off
to our west by this time, focused along the Mississippi Valley.

Friday and Saturday, buckle develops within the upper flow pattern
and shower and thunderstorm chances return in earnest to the
forecast area. Global models in reasonable agreement showing a broad
surface low developing over the Canadian Prairies and Upper
Midwest Friday and shifting east across the Great Lakes over the
weekend. Initial warm advection wing brings precip chances on
Friday and a trailing cold front brings additional chances on



For the 06Z TAFs...

Surface trough/front working its way down the lake at this time,
and will move into northern IL and northwest IN within the next
hour. With its arrival, an increase to the northeast wind with
gusts will be observed. Also, expect ceilings to fall to IFR and
briefly LIFR for a time early this morning, along with fog and
some vis reduction likely in the 2-4sm range. Upstream trends are
indicating that the lower conditions should be only for a small
window, with MVFR conditions then returning through the morning
and for much of today. Precip trends still somewhat of a challenge
today, however, do expect highest chances for showers and
isolated thunder to be across the RFD area in the near term with
scattered showers then possible for the remaining terminals.
Conditions will likely be dry later today, but precip chances
return later tonight.



305 PM CDT

High pressure will build across the northern Great Lakes through
Sunday, then expand across the entire Great Lakes Region through
Tuesday. Meanwhile, a low pressure trough across the southern
Great Lakes this afternoon will drift southward to the Ohio River
Valley by Monday. After a brief surge of northerly winds to 25
knots tonight into Sunday morning, expect a period of N to NE
winds under 20 knots through Wednesday. The high pressure will
shift to the eastern Great Lakes by Thursday as low pressure forms
over the northern and central high Plains, resulting in winds
becoming more southerly by late Thursday.



LM...Small Craft Advisory...IL nearshore waters until 9 PM Sunday.

     Small Craft Advisory...LMZ743-LMZ744...6 AM Sunday to 9 PM




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