Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202002

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
302 PM CDT Tue Jun 20 2017

258 PM CDT

An upper level trough continues to swing southeastward across
Wisconsin and generating one area of convection across southern
Wisconsin and another band across the local forecast area. A
fairly complex surface set up is in place. An outflow boundary
stretches from just south of Rochelle to Joliet to Lansing to just
north of Valparaiso with convection positioned to the north of
it. A lake breeze boundary is charging inland from eastern McHenry
Co. to central Cook Co. bringing in a push of stable air. Some
instability remains between the main outflow and the lake boundary
keeping storms going with while areas to the south continue to
see a greater degree of instability. Visible satellite does show
some agitated cumulus south of the outflow in the unstable air
and this would be the likely place for new convection to pop in
the next hour. Otherwise, there is also existing convection
working southeast from eastern Iowa that is increasing in coverage
and will likely continue into the western forecast area. Some of
this is right along the boundary while the rest of it is out

Looking ahead, the main concerns will be along and south of the
outflow boundary where the greatest instability is found and the
boundary may offer some additional impetus for storm
intensification. Low level lapse rates are steep with good temp-
dewpoint spreads supporting a high wind threat. There is not an
abundance of CAPE but what there is is well focused in the lower
parts of the sounding. While mid level air is warmer than
yesterday the steep lapse rates up to around 600 mb and abundance
of instability above the freezing level support large hail
potential. Shear in this layer is also strong so rotating
updrafts are possible further increasing the severe threat.
Overall, sustained severe threat is pretty low but a few storms
may pulse above severe limits.

Next area of concern is the area between the main outflow and the
charging lake boundary. The lake boundary will reach existing
convection shortly and may help it intensify. Air behind the lake
boundary will quickly stabilize with temps into the 60s.



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 18Z TAFs...

Line of showers dropping across the metro causing winds to shift
north - 360 at 8-9 kt... expect winds to increase in the hour
following to 11-12 kt and occasionally G 17-18 kt. Have resolved
this in the TAFs. Still looking for scattered thunderstorms to
then develop mid afternoon with best window for any thunder at the
TAF sites coming between 3-5 pm local time. Upper level
disturbance and surface cold front move south of the area after
6-630 pm and expect with that any rain/storm chances at the
terminals to end by that time. Winds remain a challenging forecast
behind the convection but expect NNW to NE synoptic flow to
establish itself during the evening with winds dropping below 10
kt by 00z as well.

Ed F


215 PM CDT

As high pressure builds over Lake Michigan tonight, winds should
trend toward light and variable overnight. High pressure will
remain the dominant feature for the western Great Lakes through
Wednesday. As the high Shifts to the east on Thursday, winds will
become more southerly to southwesterly and increase to 20-25 kt.
weak low pressure slowly moving through south-central Canada will
eventually drag a weak cold front slowly across the lake, bringing
a period of thunderstorms to the lake with the front. This front
should finally push south of the lake by Friday while another high
builds across the northern plains to the upper Mississippi Valley.
An extended period of north winds is possible for the open waters
for the weekend with lake breezes and onshore flow likely for
the nearshore waters.





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