Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 301749

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1249 PM CDT SAT JUL 30 2016


1051 am...Morning update...Weak frontal boundary/trough axis
extends east/west across the center of the cwa mainly along the
I-80 corridor late this morning. Isolated showers continue to form
and quickly weaken across southwest parts of the area. Short term/
high res guidance is in fair agreement with development southwest
of a Rockford to Gary line early this afternoon and then slowly
drifting south/southwest through the afternoon with an overall
weakening trend by early evening. Can`t completely rule out
activity in the I-88/I-80 area of the metro area and maintained
high chance/low likely pops there thru mid afternoon but may be
able to better refine pops as activity emerges in the next few
hours. Main threat from these storms will be very heavy rain and
localized flash flooding...especially if anything develops over
areas that received heavy rain the past few days. Clouds appear
to remain mainly bkn/ovc which will likely keep highs in the
mid/upper 70s...lower 80s in areas that break out into sunshine
which most likely would be just the northern areas. cms


328 AM CDT

Through tonight...

Main forecast concerns/challenges are with shower/thunderstorm
trends later this morning into mid afternoon, with heavy rainfall
and localized flooding possible once again.

Am concerned this morning that the area will see a repeat of slow
moving thunderstorms later this morning into the afternoon, with
heavy rainfall and localized flooding. Similar pattern is expected
to remain in place today with upper level trough still slowly
moving through the region, with surface trough axis still situated
over the area while a secondary surface focus also still remains
in place. Outside of a few stray and brief showers, the majority
of the CWA is dry early this morning and think this dry period
will likely persist over the next several hours. Don`t think the
CWA will observe blossoming showers and thunderstorms until more
towards mid morning, when current speed max/short wave energy
currently pushing into Iowa reaches northern Illinois. Have this
additional forcing timed into northern ILLinois by around the mid
morning time frame, and this is consistent with most short term
and hires guidance indicating an increase in precip coverage
across northern Illinois.

With high dewpoint air in place, higher PWAT axis overhead, and
with instability axis still situated through the CWA, feel a
similar situation with scattered thunderstorms developing across
the CWA will occur today. Have increased pops today with likely
pops in place by mid to late morning over a majority of north
central Illinois and northwest Indiana, and continue these higher
pops through late in the afternoon. Timing of highest coverage and
intensity of thunderstorms will be slightly sooner given the
quicker arrival of the additional ascent. Guidance continues to
vary to the placement of this precip axis today, but do feel that
this axis could be very near where the heaviest rainfall occurred
yesterday or just slightly to the southwest. A slightly further
southwest shift would make sense given the current surface
pattern. Although somewhat lower confidence with exact location of
the heaviest rainfall, best thoughts right now off of current
surface pattern, would suggest the corridor extending from areas
near/around Amboy to Manhattan could see the higher amounts.
Guidance still once again bouncing around today, and its possible
that locations in Kane, DuPage, and Cook county that observed the
highest amounts yesterday could see additional heavy rainfall
today. Had contemplated some type of flood headline for today but
with the lower confidence on placement, have opted not to issue a
headline at this time. With departing ascent later this evening,
should see precip weaken/diminish through the evening with dry
conditions then expected through the overnight hours.

Regarding the Beach Hazards Statement...The persistent northeast
winds today bring another day of near 4 ft waves for the northeast
Illinois shore, though conditions are likely to be more marginal
for rip currents. Waves heights have been around 3.5 to 4 ft at
the Wilmette buoy since yesterday. A slight drop is probable late
this morning and early afternoon, but a slight rise might follow
this up as the pressure gradient tightens and the wind direction
aligns better with the lake fetch. So will waves will remain
marginal, feel better keeping the Beach Hazards Statement for rip
currents going at this time, and allowing day shift to see trends.



328 AM CDT

Sunday through Friday...

The end to the weekend will remain generally dry. Guidance
continues to bounce around on the idea for some possible isolated
showers/thunderstorms in the afternoon across the area. However,
given lower confidence, have decided to keep the forecast dry at
this time. Although some cloud cover expected Sunday, shouldn`t be
enough to keep temps from rising to the low 80s. The beginning of
the work week will see a start to a warming and more humid trend
through much of the week with high temps slightly warmer each day
through Thursday. Increasing dewpoints during this time are
expected to provide hot/humid conditions, especially Wednesday and
Thursday. However, this pattern will also support increasing
chances for thunderstorms. Monday will likely remain dry for most
areas but do think higher potential will be Monday night into
Tuesday, when guidance is indicating the arrival of more energetic



For the 18Z TAFs...

two immediate concerns for the terminals early this afternoon will
revolve around pcpn chances and MVFR cigs.

MVFR cigs have been more persistent than previously anticipated,
but with continued nely flow off of Lake Michigan, there has been
little change in the pattern that would cause cigs to improve very
quickly other than diurnal warming. So, have trended a bit more
pessimistic with the cig trend, though would anticipate that
bases should lifr to vfr levels through the late afternoon and
into the evening hours. There is some concern that additional
lower level moisture may wrap around the slow moving upper low
that is sitting over the region and could bring a return of mvfr
cigs late tonight. Little guidance is suggesting lower cigs other
than the typically too moist NAM, so will keep VFR cigs for now.

As for showers and storms, latest trends suggest that the main
focus for convective development will be along an old outflow
boundary extending roughly from KVYS to KRZL which has been
interacting with other old SW-NE oriented boundaries well south
of the terminals. So, have dropped all mention of pcpn at all
sites with the exception of KGYY which may be closer to boundary
interaction. Also, latest obs indicate a weak convergence axis
over nwrn IN which could also act as a focus for convection.

Winds should remain ENELY through evening on the larger scale as
a sfc trough/weak cold front extends from weak low pressure over
nrn OH through cntrl IL. The pressure gradient should weaken
overnight and winds will go lgt/vrbl through tomorrow morning. The
sfc low should slowly move east and while the sfc trough gradually
sags south and winds set up more NNELY.


200 AM CDT

Persistent low pressure to the south of Lake Michigan will begin
to slowly move east this afternoon into tonight. Overall east-
northeast to northeast winds will continue over the lake today and
this evening. Waves have been consistently around 3.5 ft at
Wilmette since yesterday. While a slight drop will probably occur
late this morning into early afternoon, a rise back to around 4 ft
should be seen later into the day into evening across the Illinois
nearshore. Conditions look to remain below Small Craft Advisory
criteria though. A high pressure ridge will move across the lake
late Sunday Night into Monday before southerly flow returns to the
lake. This should persist through at least early Thursday. Some
15 to 25 kt winds may be seen ahead of a cold front later in the



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




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