Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230545

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1245 AM CDT Fri Jun 23 2017

915 PM CDT

Main concern is isolated risk for strong to severe thunderstorms
tonight. Forecast is largely in good shape, with no significant
changes needed to the grids. The surface cold front is fairly
diffuse across Wisconsin this evening. Some scattered shower
activity with thus far no lightning observed has broken out north
of I-88 in Illinois. While the strongest mid and upper level and
jet forcing lags to the north the more widespread thunderstorm
activity from central IA to central and southern WI is tied to the
cold front and a subtle shortwave noted on W/V imagery.

The shortwave and front will gradually drag this higher coverage
of thunderstorms southward tonight. Current thermodynamic
environment features moderate instability and fairly weak deep
layer wind shear. As the higher coverage of thunderstorms arrive
tonight and sag south through the night, SBCIN and MLCIN should
gradually wane despite warm and humid air mass ahead of the front
with nocturnal stabilization, while MUCAPE will remain in
1500-2000 j/kg range. A modest increase in winds aloft
accompanying the shortwave will bump up deep layer shear a bit,
though still fairly marginal. The moderate instability and
marginal to moderate shear environment will be supportive of
isolated stronger thunderstorms. Had coordinated with SPC to lower
severe risk to Marginal/Isolated. Main risks from any
strong/severe storms will be wind and hail. In addition, high PWAT
air mass (near 2") will make the more intense storms very
efficient rain producers, so some ponding/localized flooding can`t
be ruled out.



225 PM CDT

Through Friday...

Concerns center around isolated thunderstorm development this
afternoon, and then with a line of storms late this evening and

Inhibition gradually continues to erode from the south generally
driven by low level moisture advection soundings. AMDAR soundings
still depict some decent capping thanks to fairly warm
temperatures in the 800-700 mb layer, at least closer to Chicago
The HRRR now has backed off a bit on the coverage for the
afternoon and early evening, consistent with very little/weak
convergence in place and a lack of a larger scale forcing
mechanism. But there is some weak upper forcing pushing in from
the south, thus we maintain an isolated shower and storm mention
for the afternoon/early evening. Shear profiles are still fairly
weak during this period thus severe threat is low.

We will hang onto instability through the evening ahead of the
cold front currently draped across the northern plains/upper
midwest. Stronger upper level winds will approach the area along
the southern fringes of the upper low across Manitoba. Clusters of
storms will continue to develop along and ahead of the front. Any
storms that area able to get going this evening ahead of the front
will form in an axis of 1000+ J/Kg of MLCAPE and shear in excess
of 30 kt, which keep the threat of hail or locally damaging winds,
mainly across north central IL this evening, especially if
anything forms upstream and develops a cold pool while pushing
ahead into the continued unstable airmass. The cold front will
pass through the Chicago area after midnight, with the line of
storms passing southeast with it. There are some waves in the
southwest flow aloft that may initially slow the progression of
the front before it gets shoved southeastward late tonight, and
may prevent a solid line, but confidence is pretty high in many
areas seeing some activity. Moisture pooling ahead of the front
would suggest at least some brief heavy downpours in this
scenario, while the more significant flood threat is largely just
to our north/northwest.

Quite the airmass change will filter air on Friday, with much
drier air across the upper Midwest (note the 40s Tds in MN). These
will not get here that quickly but eventually will get in here
this weekend. Still humidity levels will be much lower, and temps
look to top off in the upper 70s to near 80. It may be a bit
cooler along the IN shore. Showers and storms look to still be
ongoing tomorrow morning, but guidance is in good agreement of
getting the front through in the afternoon.


238 PM CDT

Friday night through Thursday...

For the longer term forecast period, general trends remain
consistent with previous runs and the short term forecast period

By Friday night, the longwave pattern will be trending toward a
higher amplitude with building ridging over the west coast and broad
troughing over the ern 2/3 of the CONUS.  The broad upper trough
will help contribute to a period of below normal temperatures for
the weekend through early next week.  The local area will be under
fast nwly flow aloft, with a series of nrn stream shortwaves
dropping through the nwly flow, sharpening the upper trough and
generating sct showers.  Chances for thunderstorms will be low,
though not out of the question for the afternoon hours.  The highest
chance for any pcpn through the weekend will likely be Sunday
afternoon into Sunday evening when a stronger shortwave is expected
to drop through the upper Mississippi Valley and across the upper
Great Lakes.  There are some strength differences noted in the
longer range guidance, with the ECMWF trending stronger than the
GFS/GEM, though all indicate similar timing.  So, will carry chance
PoPs for the nrn portions of the CWA with some scattered
thunderstorms possible.  Any pcpn should be scattered, so the day
should not be a washout.  Temperatures for the weekend through early
next week will be below normal, with highs only in the lower 70s
through Monday. With high pressure associated with a dry continental
polar air mass sliding out of Canada and across the cntrl Plains and
Mississippi Valley, dewpoints will also be anomalously low for late
June and should only be in the upper 40s to around 50 F.

By Tuesday, the upper ridge is still expected to build east across
the Rockies and out over the cntrl CONUS while sfc high pressure
shifts to the east coast.  Rising heights aloft and increasingly sly
flow at the sfc will set up a return flow of warm/moist air with
temps reaching the upper 70s for Tuesday and the lower 80s for
midweek.  Dewpoints should also be on the rise back into the upper
50s to lower 60s.


For the 06Z TAFs...

Forecast concerns center around thunderstorms overnight along with
lowering MVFR cigs and then wind direction trends this afternoon.
Surface low pressure over southern Wisconsin is tracking east
tonight with a cool front trailing it to the southwest. Moist low
levels have resulted in lowering MVFR cigs lifting up from the
south and expect cigs to settle in the 1000-1500 ft range
overnight at the terminals. Will need to watch for potential for
some cigs to fall below 1000 ft late tonight. Otherwise bands of
TSRA continue to devleop and move eastward from eastern Iowa and
southwest Wisconsin. Timing of arrival of these is a little slower
than previous forecast. Some isolated/scattered development is
possible ahead of this main line and the first half of SHRA
mention and TSRA Tempo groups is meant to cover this where the
latter half of the SHRA/TSRA period covers the bulk of the
activity. May further refine details closer to 07z.

The front will pass this morning and winds will turn northwesterly
and probably even northerly for a time. SHRA/TSRA will end from
the northwest by 12/13z but low cigs will linger but steadily
improve through the morning. Winds will become more west to
northwest through the afternoon. Will need to watch for possible
lake breeze development and passage at ORD/MDW but have kept winds
northwesterly for the afternoon for now.



245 PM CDT

The low over western Manitoba will weaken as it crosses Ontario
tonight and then continues to Quebec Friday. An associated cold
front will sweep across the lake tonight into early Friday
morning with winds becoming west to northwest behind the front.
Westerly winds are expected to continue through early next week.

High pressure should then build in behind the exiting cold front,
spreading across the plains Sunday and the western Great Lakes
early next week, setting up a period of relatively light and
variable winds. As the high shifts to the east and new low
pressure develops over the northern plains, winds will trend more
southerly and strengthen to arnd 15 to 20 kt by Tuesday. A
strengthening southerly pressure gradient could bring winds up to
30 kt by mid week as high pressure remains parked over the sern
CONUS and the trough and associated cold front track east across
the plains.





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