Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210715

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

302 PM CDT

Through Wednesday...

Main concern for the remainder of the afternoon going into the
evening hours continues to be scattered thunderstorms developing
ahead of an upper level short wave trough in the northwest flow
aloft.  Airmass destabilizing given both solar heating and cold
pocket aloft advecting SE to be aided by larger scale ascent ahead
of the trough.  Series of weak surface boundaries also enabling low
level convergence zones to help focus new storm development per
meso discussion above. Initial outflow shows up well on ORD
terminal doppler pacing ahead of line lead of showers currently
over the south metro continuing to move SSE. Next boundary being
aided by lake breeze dropping south-southwest across the metro
with temps dropping back north burbs and north shore with this
trend continuing south.

Expect convection to continue to fire in vicinity of boundaries and
surface front into northern IL associated with the axis on the nose
of the upper trough - see details in meso discussion above. Steep
low-mid level lapse rates combined with low-mid level shear
supportive of isolated severe threat per SPC MCD with perhaps a
few cases of damaging wind gusts possible late this
afternoon/early evening mainly along/south of I-90. Expect
convection to continue to move southeastward through the early
evening with activity also winding down due to loss of diurnal
heating toward 8 pm and onwards.

Expect skies to then clear overnight with pleasantly cool overnight
low temps as weak surface high pressure builds into the area. Temps
rebound quickly Wednesday for a warm day with the exception of areas
near Lake Michigan due to a lake breeze kicking in by late morning
in the weak synoptic flow becoming a weak E-SE gradient flow.

Ed F


206 PM CDT

Wednesday night through Tuesday...

Once again, little change necessary for the extended forecast period
as the larger scale pattern transitions to a fast, zonal flow
pattern over the nrn CONUS through Friday and then becoming more
highly amplified through the weekend and into early next week.

A period of showers and thunderstorms is likely Wednesday night as a
long wave upper trough crosses the upper Mississippi Valley and the
Great Lakes region.  An associated cold front should sag south
through the region. The combination of the cold front and any
lingering boundaries from earlier convection along with upper level
height falls associated with the passing upper level wave should act
as a focus for convective activity as the lower level environment
remains moderately unstable and moist with sfc dewpoints increasing
into the middle 60s overnight.  Even the larger scale model guidance
maintains CAPE values up to 1000 j/kg over nrn IL/IN through the
overnight hours.  Forecast soundings through the overnight hours
indicate a veering profile through the lower levels which would keep
severe thunderstorm potential across nrn IL/IN and as is indicated
in the latest Day 3 SPC convective outlook for a slight risk of
severe weather.  The models indicate that the front should stall out
across nrn IL/IN through much of Thursday, keeping chances for
shra/tsra across the region through Thursday night before high
pressure building out of the nrn plains and Upper Mississippi Valley
finally forces the front south of the CWA.

Forecast uncertainty increases into the weekend.  By late Friday
into saturday, the pattern will begin to shift from fast/zonal aloft
to more highly amplified, with another trough dropping out of the
upper Mississippi Valley and into the western Great Lakes.  The
various longer range models begin to show quite a spread in timing
and strength solutions with this wave.  So, will carry generally
lower chance to slight chance PoPs through the weekend.  By early
next week, the trend should be toward broad upper troughing east of
the Rockies and increased ridging over the west coast and Great
Basin.  This will set up a period of nwly flow aloft which should
lead to a relative cooling trend, toward more seasonable
temperatures following the relatively warm conditions of the short
term forecast period.  As the first long wave though digs over the
upper midwest and is reinforced by additional short wave energy
helps to deepen the trough, temperatures through the weekend should
be below normal, with highs only in the lower to middle 70s.  Into
early next week, the pattern will become more progressive with upper
ridging building east across the Great Basin and the Rockies as
broad troughing gradually moves across the ern CONUS.  Height rises
in advance of the ridge will help bring a warming trend with temps
returning back to more seasonable levels with highs returning to the
lower 80s next week.


For the 06Z TAFs...

Light winds are expected tonight. GYY continues to report fog, and
may see a few other areas report shallow ground fog. Thinking DPA
also has a chance of fog developing, but do not have enough
confidence in fog to put it in the TAF at this time.

East winds continue today, and winds veer to southeast through the
late afternoon and evening. I have low confidence in precip timing
and coverage tonight into Thursday morning. Decided to leave the
PROB30 as is and not include a PROB30 for GYY as precip should
stay to the north. Winds then become south to SSW Wednesday night.



214 AM CDT

High pressure over the lake will shift east this afternoon. Light
winds become southerly this evening. Meanwhile, low pressure shifts
east across southern Canada. May see a brief period of 15-25 kt
especially over the southern half of the lake late tonight into
Thursday morning as the pressure gradient tightens between the
departing high and Canadian low.  Will have borderline small craft
advisory conditions Thursday, but not confident enough to put a
small craft advisory out at this time. Will have 15-25 kt winds in
the nearshore forecast though.

Guidance suggests a secondary surface low may form over Lake
Michigan along the Canadian low`s cold front Thursday evening. One
model features 30+ kt winds in response to the low while other models
are not as aggressive. Decided to cap wind speeds at 15-25 kt.
Southwest winds become northwest immediately after the front, but
then back to westerly Friday.  A prolonged period of west winds is
then forecast as the remnants of Tropical Storm Cindy travel east
over the Tennessee River Valley through the end of the week.






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