Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231748

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1248 PM CDT Sun Jul 23 2017

1248 PM CDT

Compact and digging upper trough over the upper great lakes will
continue southeast tonight. Height falls on the back side of the
trough will provide enough forcing ahead of a cold frontal
boundary to trigger scattered showers and thunderstorms during the
peak heating period this afternoon/early evening. Surface
convergence is weaker, possibly aided by the lake breeze, thus
scattered coverage seems appropriate. The parameter space of
instability/shear of approaching around 1000 j/kg and 30kt
respectively, with hail growth and downdraft cape at respectable
levels, places a marginal risk of damaging winds and large hail,
consistent with the SPC outlook.

The thunderstorm concerns will end quickly this evening as these
combined factors all shift southeast with the arrival of the
cooler and drier airmass behind the frontal boundary and
increased surface pressure ahead of large high pressure across
the upper midwest.

Behind the front in MN and across lake superior there is an area
of lower clouds. Some of these clouds will likely move southward
behind The front later tonight into early monday in our area as
well and Linger a bit in the morning. Breezy northeast winds will
also be in place through the remainder of the day providing some
natural air conditioning to the region after the recent muggy
period, with 70s for highs most areas.


226 AM CDT

Monday through Saturday...

The first and likely last parts of this coming week look much
quieter and less muggy than much of what July has provided so far.
During midweek, particularly sometime later Wednesday through the
day Thursday, a cold front will bring a chance for storms from
northwest to southeast across the area.

Northeast winds on Monday will steer in cooler boundary layer air,
with 850mb temperatures of 12C-13C. This will keep highs in the
mid to upper 70s in most areas, while onshore flow and possibly
areas of morning low clouds provide lower 70s along the immediate
lake shore. Tuesday will see less synoptic wind speed allowing
for a lake breeze push. Temperatures inland will warm back up to
around or a little above 80. More sun should be seen on Tuesday
than Monday.

The upper ridge, while dampened from last week, will attempt to
build into the area later Tuesday into early Wednesday, before a
strong upper wave across Canada cuts into it. An associated
surface cold will be moving southeast with quite a bit of warming
expected Wednesday across our area. If current frontal timing
stands and not until Wednesday night/early Thursday, think
forecast highs on Wednesday could be a tad underdone, with
possibly low 90s more prevalent. High dew point air, not tough to
come by in late July in the Corn Belt, will return ahead of the
front, assisting in modest to high instability. While frontal
speed cannot be latched onto with a high degree of certainty at
3-4 days out, guidance is in fair agreement with the boundary a
ways north of the CWA during peak heating Wednesday. So the storm
chances presently peak Wednesday night and early Thursday, which
may keep the severe weather threat more limited. There could be
somewhat of a heavy rain threat with this despite it being cold
frontal driven, as the steering winds become unidirectional to the
boundary as it and its associated high PWAT plume pass. Also some
anafrontal rain characteristics of the front indicated on
guidance as it slowly creep south on Thursday. Something we will
have to keep an eye on, but 2-3 days of drying ahead of time will
be well received.

Beyond we look to repeat Monday and Tuesday`s weather later
Thursday into the start of next weekend per recent runs of global



For the 18Z TAFs...

Scattered showers and thunderstorms this afternoon are the main
concern for the tafs. We are seeing development across WI, and we
continue to work on destabilization across IL ahead of an upper
level shortwave/. The window will be fairly narrow for any one
location. Convergence ahead of the upper wave is somewhat weak,
Though could be aided by any lake breeze this afternoon. Therefore
scattered coverage seems appropriate. Some gusty/erratic winds or
even some hail are possible. The lake breeze will also push
inland, but confidence on how quickly it reaches ORD/MDW is not
super high given any potential modulation due to convection.
Eventually winds will settle NNE.

Behind the front in MN and across Lake Superior there was an area
of Mvfr/local ifr conditions this morning. Some MVFR ceilings may
filter southward behind the front later tonight into early monday
in our area as well and linger a bit in the morning. Breezy
northeast Winds will also be in place through the remainder of the
taf period.



145 AM CDT

Broad low pressure and an associated diffuse and mainly light wind
field will prevail over the lake this morning. Isolated showers or
storms may continue over the lake during today associated with
this low. As the low pressure shifts southeast, an associated cold
front will progress south this afternoon and evening, turning
winds from westerly to northerly. This front/wind shift should
reach the far southern tip of the lake by midnight.

On Monday, the northerly winds look to occasionally gust over 20
kt over the open water and southern nearshores. This fetch will
build waves to at least close (3-5 ft) to Small Craft Advisory
criteria in the Illinois and Indiana nearshore beaches late
tonight and throughout Monday.

The next similar oriented cold front will move southward across
the lake on or near Wednesday evening. This could have some gusty
storms in advance of and along its passage. Behind the passage,
another tight pressure gradient looks to drive northerly winds and
potential Small Craft Advisory criteria waves, and maybe even
winds, for Illinois and Indiana on Thursday.






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