Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 282006

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
306 PM CDT THU JUL 28 2016

306 PM CDT

THIS AFTERNOON THROUGH TONIGHT...A very complex pattern has set up
across nrn IL/IN this afternoon.  A quasi-stationary front extends
from near Rockford through the Chicago metro area and a cold front
extends from near Freeport through to Macomb.  Another
complicating factor is a strong outflow boundary extending from
Aurora to Joliet to Lansing, moving to the southwest.  While there
is little activity along the actual cold front and the outflow
boundary, as of 245PM CDT, there is a solid line of convection along
a pre-frontal trough extending from ern McHenry County through
southern LaSalle County. The quasi-stationary boundary will continue
to be a focus for thunderstorm development until the cold front can
push through the region. The factor which will control to the end
time of the convection is whether there will be any additional
development along the actual cold front, which currently is not
exhibiting any development. Currently, satellite imagery is
suggesting an area of subsidence behind the leading pre-frontal
trough, so have generally followed the back edge of the convection
associated with the pre-frontal trough as the ending time for
showers and thunderstorms. The latest timing suggests that the
organized lines of TS could be ending along the I-39 corridor btwn
20-21Z and btwn 23-00z along the I-55 corridor.  The lines of TS
would then push east of the nwrn IN counties of the CWA.  However,
some of the short range, high res guidance is suggesting the
potential for TS to fire along the outflow boundary as it pushes
south of the I-80 corridor.  While the thunderstorm activity has
been sub-severe so far, there is the potential for isolated severe
thunderstorms, with strong winds being the main severe threat.  The
storms have also been very efficient rain producers in spite of
pwats less than 1.5 inches.  At this point, any flooding potential
is marginal as thunderstorms are becoming more progressive.  Thus
far, urban and small stream advisories have covered any flooding
potential.  However, flash flooding is certainly possible is there
is any significant echo training or if individual cells become more

Another fly in the ointment as far as shra/tsra is concerned is the
upper level shortwave initially over swrn WI.  Latest radar mosaic
shows a broad circulation associated with this shortwave and it
could still provide enough forcing for scattered shra/tsra through
the night.  This would largely impact the northern portions of the
CWA, so have gone with high chance PoPs over the nrn portions of the
CWA and low chance PoPs over the south. The strongest forcing should
be north of the WI border, so feel that chance PoPs/SCT coverage
wording will suffice for now.




Friday through Thursday...

235 pm...Forecast concerns include thunderstorm chances Friday
and Saturday and again next week along with a return of
heat/humidity next week.

Weak low pressure over the southern Great Lakes region will remain
nearly stationary through Saturday morning and then begin to lift
northeast across the eastern lakes region Saturday night into
Sunday. A frontal boundary will slowly sag into far northern IL on
Friday and perhaps a little further into the cwa Saturday. Expect
similar conditions to today on Friday and Saturday with scattered
thunderstorm development in the late morning...continuing through
the afternoon and dissipating in the evening. Best precip chances
may end up sagging south some on Saturday with the location of the
front. Main threat from these storms will be heavy rain and
localized flooding with expected slow movement. By Sunday...a weak
ridge of high pressure will move across the area with an afternoon
lake breeze. While most of the models suggest Sunday/Sunday night
will be isolated shower or thunderstorm is possible along
the lake breeze but confidence is too low to add mention just yet.

Slightly cooler air will spread into the region Friday and persist
into Saturday. This combined with expected mostly cloudy skies and
scattered precip will likely hold highs both days in the upper 70s
to around 80...perhaps a bit warmer if sunshine persists for a few
hours. Temperatures rebound back into the lower/mid 80s Sunday/

The upper ridge begins to build back into the region Monday night
through Tuesday night next week. This period looks unsettled with
increasing chances for thunderstorms. Still some timing
uncertainty but Monday looks mainly dry right now with the focus
on Monday night or Tuesday. Heavy rain will certainly be possible
wherever thunderstorms develop. Maintained temps generally in the
mid 80s Tuesday...which could be a tad low but quite a bit of
uncertainty from this distance. While the edge of the ridge will
be nearby... it appears Wednesday may end up mainly dry and
potentially hot with highs in the lower 90s. A cold front will
arrive Thursday or Thursday night...bringing another chance of
thunderstorms and cooler weather into next weekend. Timing of this
front could lead to Thursday being dry and possibly hot/humid. cms


For the 18Z TAFs...

Main concern for the afternoon into early evening will focus on TS
potential and associated wind disruptions. On the larger scale, a
weak cold front extends from swrn WI through nrn IL/IN. Scattered
TS have already developed along this corridor and expect activity
to continue through the afternoon, with the main focus invof this
boundary. Prevailing wind direction should be nely north of the
boundary, impacting all TAF sites, but thunderstorms are producing
outflow boundarys all over the area, so wind direction will likely
be quite variable, with ocnl gusts to arnd 25kt or possibly
higher. A weak mid level impulse is expected to cross the region
late this afternoon should allow the area of TS over the RFD are
to become more progressive, with TS ending at RFD arnd 00-01z and
a couple hours later at the Chicago area terminals.

Storms should dissipate quickly through the evening with continued
northeast winds and generally prevailing VFR conditions. Guidance
hints at lower cigs early Friday morning, but do not have enough
confidence to mention low cigs in the TAFs at this time. There is
a greater chance for MVFR vis at RFD/GYY/DPA since those locations
should see residual wet ground combined with high dewpoints and
light and variable winds.


314 AM CDT

The cold front over central Lake Michigan will continue south and
clear the southern end of the lake this afternoon.  Showers and
storms will form along the front.  A weak low over IL will shift
east through tonight.  Northeast winds increase to 15-25 kt late
this afternoon, and waves build to 3 to 5 ft north of Wilmette
Harbor. Waves should remain below small craft criteria so will not
be issuing a small craft advisory.  Winds diminish late tonight into
Friday morning as the low shifts east. However, northeast to north
winds increase to 15-25 kt Friday afternoon and evening as the high
pushes south over the northern Great Lakes.  The high spreads over
the lake this weekend with winds becoming light and variable under
the high. While the western edge of the high will stay over the lake
through mid next week, the center of the high will shift east. Winds
will become south Monday.






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