Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231811

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
111 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.


111 PM CDT

Monday night through Sunday...

Model guidance is in pretty good agreement that the upcoming week
into next weekend will generally be much drier, both in terms of
precipitation amounts and dewpoints.

Tuesday should feature sunshine and seasonably warm temps and
comfortable humidity levels with an afternoon lake breeze knocking
temps lakeside down a bit. Temps and humidity will begin to creep
back upward Wednesday as southerly flow becomes established in
advance of the next shortwave and associated cold front.

Generally pretty good agreement between the GFS/GEM/ECMWF with
respect to timing of midweek cold frontal passage, which looks to
be in the later Wed night into Thursday morning time frame. Fairly
stout trough with plenty of moisture around would suggest a still
respectable chance of showers/storms despite the unfavorable
frontal timing. If timing of front slows in later model runs, then
chances for convection would likely end up being a bit higher.
Regardless of the timing, all indications are that this will be a
progressive trough and front, not one that looks overly concerning
at this distance for a major rain event that would have a big
impact on already swollen rivers.

In the wake of that front, all indications are that next weekend
will feature some really fantastic mid-summer weather with fairly
seasonable temps (maybe a bit below average), afternoon lake
breezes, comfortable humidity, and sunshine.

- Izzi


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.



111 PM CDT

Fairly typical summer pattern over the lake with high pressure
resulting in primarily land/lake breeze circulation winds Tuesday
before the high moves east Tuesday night allowing for a period of
more defined southerly winds midweek. Cold front will move down the
lake Wednesday night with a shower and thunderstorm potential
followed by a wind shift to northerly in the wake of the front.
Does look like a period of fresh northerlies Thursday into Thursday
evening before high pressure settles into the western Lakes region
allowing for a return to primarily a land/lake breeze regime next

- Izzi





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