Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251732

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1232 PM CDT Thu May 25 2017

307 AM CDT

Through Friday...

Trough and upper low responsible for the cool and unsettled
weather this week is finally moving out of the area. Should see
cloudiness gradually decreasing through the day as heights rise
and subsidence and mixing gradually take their toll. The addition
of some sunshine and heart of the cold pool moving east should
allow temps to rebound nicely toward, if not into, the lower 70s
today. Gradient favors north winds which should allow the lake to
quickly exert its influence and pop a lake breeze with temps
stead/slowly falling through the 50s near the lake this afternoon.

As our current upper low moves out into the Atlantic Maritimes the
upper flow across the CONUS should quickly flatten out, becoming
zonal for a time Friday into Saturday. The zonal flow means very
low amplitude waves which guidance continue to struggle with
leading to lower than average forecast confidence wrt precip
chances. The NAM and GFS continue to bring a shortwave through
Friday afternoon with associated chances of showers and perhaps a
storm, though both models have backed off on QPF compared to runs
last night. The ECMWF hints at a weak ripple, but QPF in the ECMWF
is minimal. Maintained chance pops for Friday, but thinking that
if any precip that does occur wouldn`t be particularly impactful
or long lasting.

- Izzi


307 AM CDT

Friday Night through Wednesday...

Over the weekend, look for a transition from a zonal flow to
another long wave trough and associated closed low digging south
into the Great Lakes. One potentially more significant shortwave
looks to get squeezed eastward through the zonal flow ahead of the
digging trough on Saturday. This should result in significant
amount of convection near and just north of the primary baroclinic
zone which is forecast to be well to our south. There is a high
likelihood that one of more MCVs could become the dominant
players in precip placement Saturday afternoon through Saturday
night, which could ultimately keep the majority of the precip to
our south. Assuming the MVCs don`t overwhelm the synoptic
shortwave and cut off northward moisture transport into our area,
then one of our better precip threats this weekend looks to be Sat
night as synoptic s/w passes across the region.

Sunday looks like to be dry and mild with probably enough of a
westerly gradient to preclude any lake cooling on the IL side. As
it looks now, Monday should start dry, but with upper low and
associated cold pool aloft digging south into the area it looks
like a decent shot of diurnally driven convection in the area
Monday afternoon into the early evening. Doesn`t look like a
wash-out, but Monday afternoon looks like it could be the weekend
day with the best chance of daytime rainfall.

Because of differences in models in handling convectively enhanced
features, resulting in different timing and tracks of shortwaves,
blended model guidance paints pops over the area much of the
weekend. In the morning weather story, plan to message the weekend
forecast in a manner that stressed the majority of the weekend
with be dry despite some the threat of rain looming most of the

Heading into Tuesday and Wednesday look for a continuation of
somewhat below average temperatures and some small chances of
afternoon/evening showers and perhaps a storm as chilly upper
trough looks to remain anchored over the region.

- Izzi


For the 18Z TAFs...

Stratus clouds associated with low pressure over the IN/OH border
continue to slowly lift and scatter out through the afternoon today.
Visible satellite imagery shows decent clearing over Lake Michigan
which should expand over land east to west. Expect the trend of VFR
to spread across the terminals in the next couple hours. There is a
weak lake breeze visible via surface observation as well as
terminal doppler radar. As a result, a slight shift to NNE winds
near 10kt will occur 20-21z today. As low pressure departs further
to the east this evening, improving conditions will result. High
pressure overhead tonight will lead to variable winds, but there
is not much of a fog threat as dewpoint temps remain well below
surface temps. There is a small chance for rain or thunderstorms
Friday afternoon/evening which will be updated in future



311 AM CDT

Low pressure continues to be situated to the southeast of the
lake this morning, while northerly winds are occurring over much
of the lake. Although slightly lower over the north half half,
speeds are generally in the 15 to 25 KT range. Do also think there
are likely gusts to 30 KT also still occurring, mainly over the
south half. These persistent stronger winds have allowed waves to
build across the south half, with conditions hazardous for small
craft in place. Expect these similar winds to persist for most of
the morning, even possible gusts up to 30 KT. Hazardous conditions
will also continue, and likely persist slightly longer today and
this evening. Then a diminishing trend is expected this afternoon
as this low finally departs to the east and a weaker pattern over
the lake settles in. This diminishing trend will then continue
tonight into Friday as high pressure moves across the region.



IL...Beach Hazards Statement...ILZ006-ILZ014 until 7 PM Thursday.

IN...Beach Hazards Statement...INZ001-INZ002 until 10 PM Thursday.

LM...Small Craft Advisory...IL nearshore waters until 4 PM Thursday.

     Small Craft Advisory...IN nearshore waters until 10 PM Thursday.




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