Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211957

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

257 PM CDT

Through Thursday...

Forecast concerns the next 24 hours remain rain and thunderstorm
chances. Heat and humidity also return Thursday with gusty southwest
winds during the afternoon.

Decaying showers left over from clusters of showers and thunderstorms
across Iowa earlier today just working their way into the
forecast area from the west. Low dew point air combined with very
dry low levels per forecast sounding data continue to present a
very unfavorable environment for rain to move into... hence expect
downward trend in coverage and intensity to continue. Yet with
every wave of rain... the atmospheric column moistens more. Hence
expect scattered showers into the evening mainly fox river valley
and points west... with isolated showers or sprinkles possibly
surviving into northeast Illinois and northwest indiana as well.
Can`t rule out an isolated thunderstorm given modest lapse rates.

Expect another good round of convection to develop later this
evening and overnight associated with intensifying low level jet
in pattern of decent warm air advection/isentropic lift.  Best
forcing appears to remain to the north however.  Short range
ensemble and CAMs suggest best window for scattered rain/storms in
IL (mainly north of I-80) to come in the 03-05z timeframe north
central IL and 04z- 06z northeast IL with any activity that does
develop then shifting east and north.

Southerly flow increases Thursday ahead of a cold front to drop
southeast across the upper Great Lakes to central plains. Look for
this front to become progressive being pushed along by a vigorous
upper level short wave trof dropping into MN by late in the day
and a surface 1024 high pressure area building in to the Rockies
and high plains behind the wave. May see a stray storm pop
Thursday afternoon in the unstable airmass but if cap holds which
seems more probable showers and storms will hold off until along
and ahead of the cold front Thursday evening and overnight.

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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