Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210525

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1225 AM CDT Sun May 21 2017

714 PM CDT

Thunderstorm activity is moving across Benton and far northern
Jasper and southeast Porter. Jasper/Porter activity has been
weakening but still shows good instantaneous rainfall rates.
Activity in Benton is likely in the warm sector but may still be
very close to the warm front so will need to continue to monitor
it for new rotation. Should see activity exit by 8 pm CDT or so.

To the west, next wave of energy is kicking off some scattered
storms from east central IA to far west central IL and eastern MO.
May see an uptick in coverage into the mid evening with this
activity working across the area late this evening and early



236 PM CDT

Through Sunday...

The weekend began cool, windy, and wet, and hopefully we will have
an improving trend to some degree. Concern is on any additional
shower or thunderstorm development late this afternoon and
possibly overnight.

Mature low pressure continues to spin across the plains well to
our west this afternoon. The leading band of rain and embedded
thunderstorms has also lifted north. A warm frontal boundary is
draped across Central Illinois with the main mixed layer
instability positioned to the south and east of this front, most
active across far southeast IL and more so east of there. Several
lobes of vorticity will spread northeast ahead of the low late
this afternoon and again overnight. The first wave appears to
trigger isolated to scattered showers and possibly thunderstorms
late this afternoon/early evening, with the focus being along and
east of I-57 and mainly across northwest Indiana. This fits the
satellite picture of little activity farther east and flatter
cumulus fields. Severe chances do appear on the low side.

Farther upstream closer to the low where the cloud field is
developing but not super organized is another wave that will move
through our area overnight. Model guidance again suggests showers
and some embedded thunder remains possible late tonight into the
early morning hours as there is still some elevated instability,
though it is somewhat weak. Hi-res guidance paints a better chance
for areas closer to the low, i.e. Northeast Illinois to have
better precip chances overnight with this wave. With the moist
air moving north and continued cool northeast winds, there could
be some fog closer to the lake this evening and early overnight
hours which could get briefly dense.

The system`s dry slot will pass through into Sunday morning, and
we will be left with increasingly gusty southwest winds. After
brief morning clearing, expect a stratocumulus cloud field to fill
in for a good portion of the day. In spite of a cold frontal
passage, temperatures north will actually be warmer than today
with the end of cool northeast flow.



215 PM CDT

Sunday night through Saturday...

A high amplitude flow pattern featuring an upper low that will
likely cut off over the region will lead to a bit of a roller
coaster in temperatures and bring a period of unsettled weather
through the week. The upper trough currently driving our weather
will have lifted into the upper midwest with the downstream upper
ridge amplifying over the eastern seaboard and into Quebec.
Surface high pressure will pass south of the local area Sunday
night and Monday keeping a steady southwesterly low level flow in
place and bringing warmer temps for Monday. Highs will be in the
lower 70s. Upper level energy works down the western flank of the
upper trough over the upper midwest later Monday bringing
increased ascent to the local area into the evening. The upper
trough amplifies and gradually pivots into the area from the
northwest by Wednesday morning before cutting off as it shifts
over and east of the area into Thursday. This will result in
colder temps Tuesday and especially Wednesday with intermittent
chances for showers and some thunder starting later Monday and
continuing through Wednesday. Temps will moderate as things dry
out Thursday but a wind off the lake will likely keep lakeshore
areas cooler both Thursday and Friday. An upper ridge is expected
to move into the region late in the week but there are mixed
signals on how quickly it will do so and how amplified it will be.
Higher amplitude would likely result in a lower chance for
showers/thunder as that activity would be focused further north
while lower amplitude would result in a better chance for waves to
ripple through the flow bringing rain/thunder chances with it. As
long as this overall pattern plays out then would expect a mild
weekend with some chance for showers/thunder but it`s too early
for any kind of detail.



For the 06Z TAFs...

Moist low level air mass and light winds allowing some fog to
develop early this morning. OVC high deck should limit
radiational cooling and hopefully should keep widespread dense fog
from developing, however anticipating VSBY to drop to IFR at
times, and probably LIFR in favored fog spots like DPA. Occluded
front will sweep across the terminals overnight shifting winds to
the west and scouring out the fog and lower clouds. After a brief
window of clear skies, look for a lower end VFR deck behind the
front to linger through Sunday night. Winds will shift to the
west-southwest behind the front and likely increase and become
gusty Sunday morning after sunrise. Gustiness likely to abate
gradually Sunday evening.

- Izzi


215 PM CDT

Low pressure will move from Iowa to northwest Wisconsin
by late Sunday morning and then across central Lake Superior
Sunday night. East to southeast winds will turn southwesterly
starting Sunday morning as a cool front passes, with northern
areas of the lake hanging onto the easterly winds longest.
Southwesterly winds will then persist into Monday night as the
surface low stalls out. An upper level system will then enter the
region into Tuesday and move slowly eastward. Surface low pressure
will develop over Lake Michigan later Tuesday before drifting east
into Wednesday. Wind direction will likely become variable as
this occurs but become steadier from the north and northeast
Wednesday and Thursday. The system exits by Friday allowing for a
weaker northerly flow which will likely turn onshore during the
afternoon. South or southwest winds look to set up Saturday.



LM...Dense Fog Advisory...Winthrop Harbor to Northerly Is. until 6 AM Sunday.




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