Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300534

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1234 AM CDT SAT JUL 30 2016

934 PM CDT

Things have quieted down this evening. We opted to transition the
flash flood warning for the western suburbs to an areal flood
warning valid into early Saturday afternoon. This time might be a
bit long, but due to 3 to 4 with isolated higher amounts falling
in this area, especially over southern Dupage and portions of
Kane county, numerous roads were still reported to be impassible
this evening. Although some scattered showers are still possible
overnight, the threat of heavy rain has ended for the night.
Another area hit particularly hard this afternoon was southern
Cook county and extreme northwestern Lake county in Indiana. This
area also experienced 3 to 4 inch amounts. The flash flood warning
for this area ends just before 11 pm this evening. This will also
likely be replaced with an areal flood warning into Saturday.

Water vapor imagery this evening continues to indicate a slow
moving mid-level perturbation over northern Illinois. This
feature will likely continue to support some widely scattered
showers overnight, though most areas will likely be dry for most
of the night. The weather pattern changes little on Saturday,
therefore additional slow moving scattered thunderstorms are
possible again Saturday afternoon.

The other concern tonight, like last night will be fog
development, especially over the southern half or so of the area.
Some dense fog is also possible late.




Through Saturday Night...

236 pm...Thunderstorms developed along the weak cold frontal
boundary earlier this afternoon and have been training over the
same areas producing torrential rain and flooding. An outflow
boundary moving south of the current activity should continue to
provide the focus for additional thunderstorm development through
early this evening. Short term/Hi res guidance has all been in
fair agreement with this activity weakening and dissipating this
evening. The main threat from these storms will continue to be
heavy rain and flash flooding but isolated strong wind gusts will
also be possible.

Models show a weak wave moving across the region overnight into
Saturday morning which may allow additional showers/thunderstorms
to develop overnight toward morning. Confidence is fairly low
regarding coverage and location...which could end up mainly north
of the cwa and over the lake but maintained low chance pops thru
the night and trends will need to be monitored later this evening.

The front will slowly sag south across the region tonight into
Saturday and similar to this morning...there will likely be areas
of fog...possibly dense across the southern cwa. Added patchy fog
away from the metro area overnight but trends will again need to
be monitored this evening and overnight.

Weak low pressure over the southern lakes region will slowly begin
to move east Saturday and Saturday night...but additional
thunderstorm development is expected by late Saturday morning and
should continue through Saturday evening...similar to today and
Thursday. Location of thunderstorm development is uncertain due
to possible ongoing precip Saturday morning but focus may end up
being over the southern cwa by Saturday afternoon. Activity should
dissipate Saturday evening with mainly dry conditions Saturday
night along with the potential for fog...especially over the
southern cwa. cms



Sunday through Friday...

300 pm...High pressure will be building south across the lakes
region Sunday into Sunday night. This will allow a lake breeze to
develop Sunday afternoon which may allow isolated showers or
thunderstorms to develop during the afternoon. Confidence however
remains low and coverage could end up being quite isolated. As
this time period approaches...some slight chance mention will
likely be needed but opted to remain dry with this forecast and
see if future runs continue with these trends. This high appears
to depart more slowly and much of Monday continues to look dry
with any activity holding off until Monday evening or more likely
Monday night into Tuesday. Quite a bit of difference between the
gfs/ecmwf regarding precip coverage and amounts. Maintained chance
pops for now but quite a bit of uncertainty during this time
period. Uncertainty continues mid/late week with how fast the
upper ridge builds into the area...winds off the lake and how fast
an approaching cold front arrives from late Thursday afternoon
through Friday morning. cms


For the 06Z TAFs...

Once again the challenges are potential for low clouds near
daybreak, mainly MVFR though a small chance for IFR, and scattered
to numerous showers and some storms late this morning through the
afternoon. There also will be isolated showers through mid-

A challenging pattern continues with a slow moving, somewhat broad
upper low over the area. Sporadic, widely isolated showers have
been seen on radar early this morning and would expect that to
continue with several high-resolution models indicating a
concentration in far northeast Illinois within the 08Z-13Z
period. While some of these may be overdone and confidence is not
particularly high at all, there are some observational trends to
indicate this so have included shra in the 06Z ORD TAF in the pre-
dawn hours.

Otherwise the main threat for showers will return again late this
morning and afternoon. There are less ingredients in place than
yesterday but still plenty for at least scattered showers and
some thunder. The focus today for initialization of showers/storms
and better coverage is thought to be a rough line just south of
Rockford to just south of Midway during 16Z-17Z, and then slowly
evolving southward. Confidence in this is medium, and just a
little difference in placement could impact the airports more
significantly or quite a bit less and not even worth a VCSH.

Northeast low-level flow continues through the TAF period with
again the possibility of any nearby showers/storms influence the
wind direction. There is potential for fog Saturday night into
early Sunday morning as well and will assess that further for 12Z




434 pm...Weak low pressure over the southern lakes region will
gradually move east Saturday and Saturday night while high
pressure over the northern lakes builds south Sunday into Sunday
night. A period of east/northeast winds 15-25kts is expected
tonight over the southern portions of the lake. These winds may
diminish a bit Saturday morning but then increase again and turn
more northerly Saturday afternoon. Northerly winds will continue
Sunday with winds eventually becoming light southeast by Monday
morning as the high settles across the lake. Winds will turn more
southerly Monday into Tuesday as a large trough of low pressure
develops over the plains. cms


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

IN...Beach Hazards Statement...INZ001 UNTIL 4 AM Saturday.




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