Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 310514

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1214 AM CDT SUN JUL 31 2016

820 PM CDT

For evening update:

Relatively minor changes to going forecast primarily to
remove/lower pops across southern half of the cwa with diurnal
decay of shower/thunderstorm activity. Also tweaked cloud cover a
bit based on satellite trends.

Slow-moving upper trough continues to propagate east across the
region this evening, with weakening/sheared mid-level vort axis
sliding to the east. ILX/DVN 00z RAOBS continue to show weakly
unstable thermodynamic profiles with MLCAPE`s of 1400-1600 J/kg,
though negligible deep shear and weakening upper support helped
to minimize thunderstorm coverage this afternoon, and diurnal
cooling of the boundary has allowed significant decrease in
convective coverage early this evening. Still a few cu towers
noted in last visible satellite images in the Sterling (SQI) and
Peru (VYS) areas, as well as some activity along a lingering
boundary southeast of Rensselaer (RZL) and have kept an isolated
less than 20 percent mention for the next couple of hours.
Otherwise, low clouds noted spreading inland from Lake Michigan
across far northeast IL this evening which should keep those areas
partly/mostly cloudy. More clearing farther inland will likely
support development of patchy fog overnight, especially south of
I-80 where temp-dew point spreads are only a few degrees and rain
fell this afternoon. Going forecast has this well in hand, and
other than changes to pops/wx and minor sky tweaks, no changes
necessary at this time.

Updated grids/zfp available.




Tonight and Sunday...

250 pm...Weak frontal boundary continues to sag south across the
southern cwa this afternoon with convection continuing to develop
mainly south of I-80. This activity will slowly move south through
the afternoon and begin to weaken by early evening with isolated
showers through mid/late evening possible. Further north...only
expecting a brief light shower and adjusted hourly pop/wx for
these trends. Main threat remains very heavy rain/localized
flooding with slowly moving storms as well as development over the
same areas.

Cloud cover will likely scatter some tonight especially over
northern portions of the area. With light winds this could lead to
some patchy fog development but drier air will also be advecting
into northern areas. Think the best chance of fog development will
be across the southern cwa...perhaps into central IL but low
confidence regarding how low visibility may drop.

As the surface low slowly lifts northeast across the eastern lakes
tonight into cover wraps back around the western
lakes and may spread into the area Sunday morning along with the
chance of a few showers. Quite a bit of uncertainty regarding how
far west precip chances will extend and maintained dry forecast
both overnight/Sunday morning and again Sunday afternoon. High
pressure will build south across the western lakes Sunday into
Sunday night and expect a lake breeze to form by early/mid
afternoon. As this moves inland it may allow a few showers to
develop. Anything that develops would be isolated and short-lived.

With the expectation that clouds will at least partially clear
tonight...guidance lows are in the upper 50s across much of the
northwest cwa so low temps tonight will need to monitored and at
least partly cloudy skies Sunday...airmass begins to warm with
highs likely back to the lower 80s...cooler near the lake. cms



Sunday night through Saturday...

250 pm...High pressure over the lakes region Sunday night will
slowly move east/southeast on Monday as the upper ridge begins to
build back into the western lakes region. There seems to be some
consensus with convection firing Sunday night from IA southeast
into eastern MO/far western IL. This activity would likely
diminish before reaching the area but maintained low chance pops
in the southwest Monday morning. Focus for northern IL and
northwest Indiana continues to be later Monday night into Tuesday
morning. Ecmwf has been very consistent with thunderstorms during
this time period for several runs though now is centered more into
WI with the convection. Bumped pops up to likely for early Tuesday
morning with these trends. Precipitable water values quickly
increase to 2 inches Monday night into Tuesday so if these trends
pan out...heavy rain will be possible.

Still quite a bit of uncertainty Tuesday afternoon into Wednesday
which will be dependent on precip trends Monday night into
Tuesday. There could be additional periods of thunderstorms either
Tuesday afternoon or Tuesday night but low confidence in timing/
location. These could shift winds off the lake and make high temp
forecast Tuesday and Wednesday tricky. But precip chances appear
to decrease Wednesday with perhaps dry weather Wednesday night
into Thursday as the upper ridge builds across the area. If this
were to pan out...highs Wednesday could rebound to around 90 and
lower to perhaps mid 90s Thursday. Dewpoints during this time
likely to surge back into at least the lower/mid 70s with heat
index readings upper 90s/near 100 Wednesday and 100-105 Thursday
possible based on current advertised temps/dewpoints.

A cold front is expected to move across the area Thursday night
into Friday morning based on current timing and push south of the
area Friday night into Saturday. Still some timing differences
with how fast this front clears the southern cwa so current low
chance pops into next weekend could be too high as this time
period approaches. cms


For the 06Z TAFs...

Concerns for the Chicago area TAFs today are MVFR cigs through a
good part of today, including potentially around 1,000 ft at and
in the couple hours just after daybreak. There is a small chance
of a shower primarily in the morning through early afternoon, but
any should be brief and light.

A high pressure ridge is anchored from Wisconsin southward into
north central Illinois and will move little through today. Patches
of MVFR clouds should continues across the area through daybreak
especially across northeast Illinois and feel TEMPO represents
that well. More moisture on northeast winds as well as some rare
summertime lake instability will begin to move in from the lake
near daybreak. This should support broken clouds through at least
mid afternoon and profiles favor MVFR, with confidence medium-high
in this. A period of near or even IFR (700-900 ft) is possible
mainly in the 11Z-15Z window, with low confidence in this.

Outlying airports may experience some shallow IFR fog early this
morning in the light variable winds, with confidence fairly high
that visibility will remain 5SM or higher at ORD and MDW.
Northeast wind speeds should increase to around 8-10 kt at Chicago
area sites during the afternoon.



224 PM CDT

Persistent, generally nely winds  between low pressure over northern
Ohio and high pressure over the Upper Great Lakes has driven winds
up to 15 to 25 kt.  The strongest winds have been over the nern
portions of the lake.  The slightly weaker winds over the south end
of the lake have generated winds to 3 to 5 ft, with the highest
waves over the Illinois nearshore waters.  Conditions have remained
just below small craft advisory criteria, but the winds and waves
have been sufficient to create a potential for a high risk of strong
rip currents to the IL shoreline.  With a shorter fetch, winds and
waves over the Indiana shore, have carried a moderate risk of rip
currents there.  Winds and waves are expected to diminish through
the remainder of the weekend, with winds varying between easterly
and northerly.  This should keep 2-4 ft waves over the far south end
of the lake, while waves diminish to 1-2 ft Sunday night over the
north end of the lake.  By Monday, the high will shift east to Nova
Scotia while the low moves off the New England coast and a new low
move out of the Rockies and across the Northern Plains.  This will
set up sely-sly gradient over the lake by Monday night.  Generally
sly winds will persist over the lake through midweek.


IL...Beach Hazards Statement...ILZ006-ILZ014 UNTIL 4 AM Sunday.




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