Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201749

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1249 PM CDT Sun May 20 2018

1009 AM CDT

An area of showers with embedded thunder in response to a mid-
level jet max and possibly a subtle short-wave has been rather
persistent this morning across the far northern CWA. Model
guidance continues to struggle with this evolution, but the 12Z
suite appears to have a somewhat better handle on it. Expectation
is for showers to continue to drift eastward and possibly expand
in coverage farther south to near I-80 into early afternoon. The
12Z DVN RAOB showed nearly 800J/kg of MuCAPE that, while
diminishing with eastward extent, will continue to support an
isolated thunder threat. Precip looks to gradually end by late
afternoon. Additionally, with the extensive cloud cover and
blustery NE winds, have dropped max temps a degree or two for
northern portions of the CWA.



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 18Z TAFs...

IFR ceilings should gradually lift into the low to mid-MVFR range
through this afternoon in response to diurnal heating. However,
lower ceilings may persist a bit longer into the afternoon at
ORD/DPA with increased moisture off Lake Michigan. Ceilings are
expected to lower into IFR levels and potentially LIFR levels by
the early overnight hours. Vsby may also drop as DZ likely forms.
A shield of SHRA will then spread across all sites from the SW
beginning around daybreak Monday. Confidence was too low to
include any mention of TS in the TAF at this point, but the
leading edge of the SHRA may have an isolated embedded TS or two.
IFR ceilings should then gradually lift into MVFR levels late
morning into the afternoon.



248 AM CDT

A surface trough/front has pushed south through much of the lake
early this morning, with the northerly winds observing an increase
to the 15 to 25 kt range over much of the lake. Although winds up
to 30 kt are not expected, a few gusts approaching these speeds
may still occur early this morning. While northerly winds will be
present today across the lake, for the nearshore waters, highest
speeds this morning should remain mainly along the IL nearshore
with speeds and waves slightly lower over the IN nearshore. Winds
are then expected to turn more to the east tonight, while speeds
and waves lower.



LM...Small Craft Advisory...Winthrop Harbor to Burns Harbor until
     9 PM Sunday.




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