Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 301125

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

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 12Z TAFs...

Today`s concerns with the airport forecasts once again revolve
around late morning and afternoon showers and storms. The
confidence in showers and some storms developing around ORD and
MDW is fairly high, however the confidence in specific timing and
duration of these is low. In addition, there is concern for some
brief IFR cigs early this morning at MDW.

As for showers and storms, similar to yesterday is expected except
1.) an earlier initiation time and 2.) less duration in close
proximity to the airports. Ongoing showers and a few storms over
the southern part of the lake and adjacent areas early this
morning will likely continue to weaken as they evolve south. New
showers may develop off the outflow of this but low confidence in
this. Around 16Z, confidence increases that the primary
convection will develop for the day. The storms should slowly
develop southward/southwestward. We continue to focus the primary
window of impact 16Z-20Z, though it likely will be a narrower
window, hence the two hour tempo in-between that window.

Northeast winds will be dominant today, but again like yesterday
any nearby showers and storms could produce outflow that
temporarily changes the wind direction. Winds will diminish in
speed after sunset and then begin to back to the north. Speeds
should be light overnight and it is possible some shallow fog
develops, mainly at outlying airports.



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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