Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231924

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
224 PM CDT Sun Apr 23 2017

213 PM CDT

Through Monday...

Very dry airmass (nearly 50 degree dew point depression at O`Hare
this afternoon) in place over the region will keep northern
Illinois and northwest Indiana sunny and warm through tomorrow.
Surface ridge axis will move across the area this afternoon with
light flow allowing for a lake breeze to push inland and keep
temperatures in the 50s right along the lake front. Meanwhile, a
weak cold front moving south across Lake Michigan this afternoon
will sag into northern Illinois this evening and will help push
temperatures down further inland. This boundary quickly weakens
and lifts back north overnight in response to low pressure moving
east across the Dakotas. A modest southeasterly gradient will be
in place ahead of the low tomorrow which will allow breezy
conditions to develop, especially west of the Chicago Metro.
Onshore flow in Illinois near Lake Michigan will keep temperatures
in the 50s immediately along the lake once again while inland
expect highs in the low to mid 70s.



303 AM CDT

Monday through Sunday...

Dewpoints remain very low into the extended, which will limit
precipitation impacts through at least Tuesday evening, but a
significant pattern change will take place with a series of low
pressure systems tracking from the west toward our region, while a
big area of high pressure will take shape across the southeast

Decent model agreement on the low for Tuesday night from the
southern plains midday Tuesday and shifting into central
Wisconsin by daybreak Wednesday, which will bring our first chance
for rain after midnight. There is some weak elevated instability
to warrant keeping a very low t-storm mention, but not a great
thunder setup. Guidance is in better agreement with the cold
frontal timing for Wednesday, bringing it through the heart of the
area in the afternoon. There is decent strengthening of an upper
level jet, actually several jet segments during this time with a
southern and northern stream system, and therefore leading to a
strengthening of a series of lows into one across southern or
central lower Michigan. These forcing mechanisms suggest an uptick
in shower activity, and with a little better instability this also
warrants a little higher chance t-storm mention. Surface based
instability may be hard to come by given the cloudy pattern.

The cold front will push through the area Wednesday night. There
could be some lingering lighter rain as the upper trough axis will
still be shifting through. Weak high pressure will build in on
Thursday which should dry things out, but some lingering clouds
and onshore flow/possible lake breeze will make for cooler
conditions Thursday. The cold front will get shoved well south to
the Ohio valley during this time. With high pressure stretched
out across the northern Great lakes later Thursday into Friday,
much of the region will remain in a cool onshore flow pattern,
possibly with lingering clouds. While there still remain
significant model differences regarding the eventual evolution
and progression of a deep upper low across the west. Elevated
showers and storms can be expected at times during the weekend
with the GFS/GEM having a bit more precip with a potential first
round Friday afternoon. There is better agreement with the EC in
at a least a portion of the sloped warm frontal boundary lifting
through Friday night into Saturday morning where a better chance
of showers and maybe a storm would result. These same models and
the GEM show the main upper low not clearing the region into late
Sunday if not early Monday. Therefore Saturday night into Sunday
is another window where rain is expected, with a slightly better
chance of thunderstorms as well. Model blend pops are bit wishy-
washy given the inconsistencies in timing from the extended
guidance, so tried at least nudge them a bit more in periods of
concern. Expect additional model shifting given the evolution of
deep upper lows can certainly give the models fits at this



For the 18Z TAFs...

VFR conditions will prevail throughout the TAF period with shifting
winds comprising the main forecast concerns. Light northerly or at
times variable winds are in place early this afternoon and expect
a lake breeze to push inland across the Chicago terminals mid
afternoon turning winds out of the east. Boundary is visible on
radar and will be able to hone in on timing in the near term as
we get a better fix on its motion. Winds may initially be at or
near 10 kt behind the boundary, but should quickly settle back
under 10 kt the rest of the afternoon. Meanwhile, farther north a
cold front is working south across Lake Michigan and spilling
southwestward across Wisconsin. Models have been in good
agreement showing the stronger winds staying north of the
terminals and winds behind the front are expected to diminish some
as it moves across northern Illinois this evening turning winds
to the northeast. Winds should gradually veer back around to the
southeast tomorrow morning.



223 PM CDT

High pressure situated across the region continues to slowly sag
south, while front pushes down the lake. This front will continue
to move through the remaining areas of the lake this afternoon
into tonight, with winds shifting to more of a northerly
direction. Speeds across the north part of the lake behind this
front have been in the 15 to 25 KT range and these speeds do look
to spread further into the central part of the lake this
afternoon. However, the gradient supporting these 15 to 25 KT
winds will remain mainly to the north. So, these elevated winds
should not move anymore south than the central lake, with wind
speeds over the far southern end of the lake staying lower.
Pattern will then support diminishing winds over much of the lake,
while winds turn more easterly tonight. The exception will be
across the northern tip of the lake, where winds may stay slightly
higher. Easterly winds will then trend more southeast Monday and
Monday night ahead of approaching low pressure from the Plains,
with locations across the lake likely observing speeds of 15 to 25






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