Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251941

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

238 PM CDT

Through Friday...

The 19z experimental GOES-R imagery shows clearing skies
associated with a lake breeze across the WI/IL/IN shorelines of
Lake Michigan this afternoon. Thickness of the cloud cover is also
decreasing as the bases lift. Some peeks of sunshine will then be
likely through the evening with a scattering trend of the clouds.
Clouds are also taking on a more cumulus-nature as compared to
the stratus observed across the area earlier today. Churning low
pressure over the Ohio Valley will push further east through the
evening, allowing weak high pressure to influence the region. This
will bring lighter winds as the pressure gradient relaxes.
Moisture content remains quite low, but there will still be a
small chance for patchy fog in outlying rural areas tonight. This
is dependent on current cloud cover clearing out to allow for
radiational cooling and fog formation.

The 500mb pattern shows a departing trough in the eastern CONUS
as a ridge builds in from the Central Plains. This will allow for
warming temperatures the next couple days, and the flow will
become zonal for Friday. Satellite imagery is showing a southern
Pacific jet that continues to pump in moisture to the Southwest
US. A weak embedded wave near NM/TX will quickly shoot northeast
to help spark some showers for Friday. This wave will work in
association with developing low pressure and a warm front. There
is a marked increase in H85 theta-E and total totals index as
warmer and unstable air works into the region from the southwest.
While model guidance is quite variable with the probabilities for
convection or rain showers Friday, there remains a chance for
development from afternoon through evening.



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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