Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260521

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1221 AM CDT Mon Jun 26 2017

802 PM CDT

An upper wave pivoting east-southeastward across Wisconsin
continues to generate showers which are tracking southeast across
southeast Wisconsin. Some of this activity may clip extreme
northern McHenry and northern Lake Co. IL before 10 pm CDT
resulting in a sprinkle or brief shower. A few additional cells
have popped up over southeastern Lake Michigan and have moved into
LaPorte County. Could see another shower or two develop in this
area and potentially clip northern Porter Co. in the next hour or
two but the chance is minimal. Otherwise have made minor tweaks to
temp trends with a cool night in store. A few spots could drop
into the upper 40s but expect the majority of the area to remain
at 50 or better, with mid to upper 50s in the Chicago metro.



224 PM CDT

Through Monday...

Little change necessary for the going forecast for the short term
forecast period.  Main concerns focus on unseasonably low
temperatures and diurnal cloud cover/pcpn trends.

Broad upper troughing remains in place, over the eastern 2/3 of the
CONUS with high amplitude ridging over the west coast.  A series of
weak impulses will continue to drop through the nwly flow aloft, out
of the upper Mississippi Valley and across the Great Lakes region.
Broad cyclonic flow aloft will help generate increasing diurnal cu
coverage for the late morning through early evening hours.  Under
the cyclonic curvature aloft, daytime warming coupled with
unseasonably cold air aloft will generate scattered showers with
isold thunderstorms possible durg the afternoon hours invof the
short wave energy dropping through the nwly flow aloft.  The pattern
has remained very slowly progressive, so conditions for tomorrow
will largely be similar to today, with chances for sct shra/isold
tsra from late morning through early evening.  For the overnight
hours, sky cover should become mostly clear.  Temperatures today
should top out in the lower 70s.  As short wave energy dropping
around the swrn periphery of the upper trough should help increase
the amplitude of the upper trough, with heights lowering a little
tomorrow.  With general cold advection through a deep layer and
lowering heights, temperatures tomorrow should be a couple degrees
lower, with highs in the upper 60s to lower 70s.  As cloud cover and
winds diminish overnight, temperatures tonight should drop into the
upper 40s to lower 50s, except for the urban heat island of the
Chicago Metro area, where temperatures should only drop into the
upper 50s.


249 PM CDT

Monday night through Sunday...

Upper level heights will build Monday night as low pressure that
has brought our cooler and occasionally through briefly showery
weather will shift to the eastern third of the country. Surface
high pressure, which has been parked over the center of the CONUS
will shift across Central IL early Tuesday and to the eastern Ohio
Valley later in the day. Model time heights depict a largely cloud
free day, save for some possible high clouds, and with warm
advection behind the high, temperatures will rebound back a little
bit closer, though still a tad shy of normal for late June.

Dampening upper level ridging will keep Tuesday night dry, but
warm/moist advection will ensue of ahead of low pressure to the
west, zonal and active flow aloft, while the Gulf of Mexico
moisture will open up a bit more. Overnight convection across the
plains will be driven by a strong low level jet and a lower
amplitude shortwave. This wave and an associated cold front will
near our region late in the day and Wednesday night. Models are in
fairly good agreement with the pattern with some modest
disagreement in timing, leading to a likely period of showers and
thunderstorms. Severe chances are not zero given the strength of
the low to mid level wind fields, but the timing of the activity
does suggest a possible weakening trend with some chances
lingering for heavy rainfall, especially in our western areas.

There looks to be a lull Thursday morning as this wave will shift
east. The frontal boundary will stall out a bit somewhere in the
region as low pressure to the north will promote a continued
warm/moist feed from the southwest. This will continue a warm and
somewhat muggy pattern, with occasional chances for showers and
thunderstorms ahead of an upper low across the Canadian prairies
that will slowly shift to the upper Midwest and eventually to the
western Great Lakes this weekend. The best chances Thursday appear
along and south of our area which is where the frontal boundary
may linger. It is along and south of this axis that will stand a
severe threat as instability increase ahead of the upper trough.
Our best chances of precipitation would be Thursday night. There
are additional chances on Friday ahead of this system`s cold
front. Temperatures will hold around normals in the lower to mid
80s, likely tempered by at least occasional cloudiness and
dampened during wet periods. Saturday does appear to be a drier
period as a more benign surface pressure pattern becomes a bit
more evident. As mentioned in the overnight shift discussion, the
overall weather pattern supports milder conditions with occasional
bouts of storms, but upstream activity will play a role in our
local impacts.



For the 06Z TAFs...

Main aviation forecast concerns are with gusty west winds again
Monday, with a gradual decrease in speed and an eventual wind
shift to the northeast expected toward evening. Scattered showers
or an isolated thunderstorm also possible late afternoon/early

Forecast area remains between low pressure over the northern Great
Lakes and high pressure across the Plains. The modestly tight
pressure gradient between these two features will again support
gusty westerly winds after sunrise Monday, with gusts to around 25
kts likely especially during the morning/midday period. Gradient
looks to weaken a bit later in the day, with a gradual shift to
more north-northwest winds late as a mid-level disturbance digs
southeast across the region. Guidance continues to hint at the
potential for a wind shift to northeast by early evening, in
response to a weak surface frontal trough, perhaps enhanced by
outflow from showers/isolated storms across WI and Lake Michigan,
and subtle lake breeze push. High-res guidance also depicts
showers along/ahead of this wind shift, with a weakening trend
expected as instability is rather weak. Have fairly high
confidence there will be showers near the terminals by 22-00Z,
though lower confidence in coverage. Have indicated VCSH in
terminals at this time. Forecast soundings depict less than 200
J/kg of MLCAPE, and while isolated thunder is possible, think TS
coverage may be fairly low.

Winds are expected to become light westerly again later Monday
evening as surface high pressure approaches from the west.



249 PM CDT

A cold front extending from low pressure across the northern
Great Lakes region will push southeast over Lake Michigan tonight.
The stagnant pattern of high pressure to the west and low pressure
to the east will then hold for one more day with a slight uptick
in the west northwest winds, reaching near 25 kt. High pressure
across the plains will shift south of Lake Michigan on Tuesday,
which will ultimately shift winds to southerly through the day,
and winds will largely hold a south component through late
Thursday night. The high will shift to the eastern Ohio Valley
late Tuesday and to the Atlantic basin Wednesday. Meanwhile, a
decent (995 mb) low across the northern plains will shift to
northern WI by early Thursday. A fairly tight pressure gradient
later Wednesday into early Thursday may bring winds close to 30
kt. The low will weaken over northern Lake Michigan ahead of a
cold front that will move over the lake Friday night. Winds will
generally settle to more of a west-northwest at this point.



IN...Beach Hazards Statement...INZ002 until 4 AM Tuesday.




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