Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 281935

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

300 AM CDT

Through Friday...

A mid level shortwave is dropping southeast across the area
this morning leading to scattered showers, with some additional
forcing for upstream activity being provided by an W-E oriented
cold front across southern Wisconsin then extending back into
Central Iowa, where the front is more N-S oriented. Better
coverage from upper level divergence/jet dynamics is found both
north and south of the area. Stabilization has also limited
coverage in NE IL/NW IN.

This leading shortwave will continue its eastward movement though
will down and dampen today, but broad upper troughing will at
least support some weak lift with additional shortwave energy in
northwest flow arriving late this afternoon. Meanwhile, The cold
front will approach far northeast Illinois (and adjacent Lake
Michigan) by daybreak, and then continue inland with onshore flow
today. Meanwhile, the western portion of the frontal boundary,
then get a bit better push southward tonight. This front will
likely be the focus for any redevelopment of showers and
thunderstorms today, with appears to largely be south and west of
Chicago. Decent heating should occur ahead of the stabilizing
influence of the front and incoming shortwave to contribute to
scattered showers and thunderstorms, with the favored area closer
to the arriving shortwave to our west, but with the broad lift in
place will need to maintain some chance pops elsewhere, especially
in the morning and the afternoon. Weak mid level lapse rates and
weak shear suggest that these will be more garden variety showers
and storms, though with the slow storm motion, and seasonally high
PWATs moderate to brief heavy rainfall can always be a concern..
And while not a perfect pattern for this, strong low level lapse
rates and moist low level conditions (Td`s in the low to mid 70s
near PNT) with a slow moving upper wave moving overhead could
support a few funnel clouds in the coming days.

This pattern again favors a diurnal weakening in shower activity
in the late evening and overnight, though cannot go dry given the
multitude of small scale waves embedded in the flow. The next more
notable shortwave in the broad upper trough is currently over
Montana will again approach our area on Friday. Expect another go-
around with an increase in slow moving afternoon showers and




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.






WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.