Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 292134
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
434 PM CDT FRI JUL 29 2016

.SHORT TERM...

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

&&

.LONG TERM...

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

&&

.AVIATION...
For the 18Z TAFs...

The concern for the next few hours with the TAFs will center on
scattered showers and thunderstorms rapidly developing across the
region.

At issuance time, development is largely driven by diurnal heating
in a moist, unstable environment. There is no strong focusing
mechanism across nrn IL/IN so activity invof the terminals should
remain scattered. So, have gone with the prevailing VCTS with a 4
hour TEMPO group through 22z. With time, expect that the TS may
focus more to the south of the terminals where old outflow
boundaries may help focus activity there and tsra/shra potential
may diminish with time. The 4 hour TEMPO group may end up a bit
pesimistically long, but confidence is too low to narrow the
timing any more.

A series of mid level impulses moving across the region will keep
some shower potential across the area through the remainder of the
late afternoon hours, through the night and, possibly even into
tomorrow morning. This shra activity should also be scattered,
with no significant sfc focus, so will maintain the VCSH from the
going forecast.

Winds may be a bit problematic through the afternoon. On the
larger scale, winds should remain generally northeasterly at arnd
10kt, but there will likely be some random direction gustiness
with outflow from the thunderstorms.

There will be potential for lower cigs and visibility again
tonight into Saturday morning, but should be less prevalent than
last night since winds might remain a bit stronger and not
necessarily light and variable. However, given the low confidence
in the reduced cig/vis potential, will maintain the going trends.

&&

.MARINE...

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

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ014 UNTIL 4 AM Sunday.

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

LM...NONE.
&&

$$

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