Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211949

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
249 PM CDT Wed Jun 21 2017

1039 AM CDT

Through Tonight...

Main forecast concern this update remains rain and thunderstorm
chances this afternoon and tonight.

Clusters of showers and storms continue to propagate east across IA
and into western IL this morning in an area of good warm air
advection/isentropic lift.  With low level jet helping to feed showers
and storms weakening and remnants moving into a much less
favorable and dryer airmass further east over the local area...
expect convection to weaken considerably as it approaches the
western counties (Lee/Ogle/LaSalle, etc.) of the forecast area in
the next few hours. Still with that... clouds will certainly
increase along with some sprinkles and showers still holding
together... and even an isolated storm. Initial cluster of
rain/storms tracking more ESE and should impact LaSalle-Streator-
Pontiac corridor while second cluster tracks further north
impacting Dixon-Oregon and eventually DeKalb corridor later this

Expect coverage to be scattered at best by the time the remnants
reach the local area through the afternoon.  Focus then shifts to
development of additional showers/thunderstorms tonight with area of
WAA/isentropic lift shifting further east ahead of returning warm
front over central IL and low pressure over the central plains.
Placement of this convection remains challenging as better chances
appear associated with re-intensifying low level jet later this
evening and north of our area. However can`t rule out an isolated-
scattered shower or storm... mainly north sections of the forecast
area ... although CAMs trending dryer this time period with focus
shifting farther north later.

Ed F


240 PM CDT

Thursday night through Wednesday...

For the longer term forecast period, general trends remain
consistent with previous runs and the short term forecast period
with the cold front sagging south through srn WI/ern IA.  Expect an
area of showers and thunderstorms with the boundary.  Latest
guidance suggests pwats approaching 2 inches pooling along the
boundary which would signal the potential for heavy rainfall with
thunderstorms through Thursday night.  With flow generally parallel
to the front above the boundary layer, there will be the potential
for echo training as the general area of thunderstorms slowly sag to
the south.  With the high pwats, echo training and slow progression
of the boundary, there will be the potential for locally heavy
rainfall and possible flooding.  The models are in relatively good
agreement on pushing the boundary south and east of the CWA by
Friday morning, with the possibility for some lingering showers over
nwrn IN.  Otherwise, pcpn should end by late morning Friday and
conditions then drying out through Friday night.  High pressure will
quickly build across the middle Mississippi Valley and across the
upper midwest Friday afternoon and through Friday night.  Following
the passage of the cold front, max temperatures for Friday should be
at least 10 degrees lower than Thursday, topping out around 80 F.

The cooler, drier air will gradually filer in Friday night with
dewpoints dropping back into the lower 50s.  With the cooler/drier
air overspreading the region, highs for Saturday will drop to the
lower to middle 70s.  The cooling trend will continue through the
weekend and into early next week as broad long wave troughing
develops east of the Rockies and upper ridging builds over the west
coast.  A series of shortwaves will drop out of the upper
Mississippi Valley and across the midwest through the weekend and
into early next week, helping to amplify the upper trough.  With the
individual shortwaves dropping through the nwly flow aloft there will
be periodic chances for some showers or a few afternoon
thunderstorms, but given the typical timing and intensity
differences among the models for the weak ripples through the nwly
flow aloft, will limit any PoPs to lower chance to slight chance
levels.  By midweek, the longer range guidance is still suggesting
some progression to the longwave pattern, with the upper ridge
moving east across the central CONUS.  Also, the sfc high pressure
will push to the east with lee troughing developing over the nrn
plains.  Increasing heights aloft and strengthening slwy flow in the
lower levels will lead to a return flow of moisture and increasing
temperatures, with highs in the low to middle 80s and dewpoints arnd
60 F.


For the 18Z TAFs...

Main concern for terminals in the next 24 hours are timing and
risk of thunderstorms tonight. Light winds will continue this
afternoon and evening with SE flow increasing slightly. High and
mid level cLouds will thicken through the afternoon in advance of
rain to the west moving into a dryer/unfavorable environment
overhead and decaying. May see a few sprinkles survive all the
way east to the airports toward late afternoon. Better chances
for rain come tonight however uncertainty remains high as latest
model trends suggesting rain and storms staying mainly north of
the area in region of better forcing. Best chances albeit minimal
for rain/storms looks to be during the 04z-06z timeframe

Thursday shaping up to be a breezy gusty day (southwest winds G22) with
high clouds. Rain and storm chances gradually increase but expect
better chances to hold off till evening or later.

Ed F


245 PM CDT

High pressure over the lake is shifting to the east this
afternoon, with generally light and variable winds giving way to
increasingly southerly flow developing overnight tonight. Low
pressure tracking east across south-central Canada will drag a
cold front across the lake Thursday night and then push south of
the lake Friday. Winds may increase to 15-25 kt in southwest flow
ahead of the front. High pressure will quickly build in behind the
front but building across the middle Mississippi to the Ohio
Valley, setting up an extended period of northwest winds, which
should persist through the weekend. A surface ridge axis will then
cross the lake early next week, bringing a period of light and
variable winds to the lake for Tuesday into Tuesday night.





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