Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 290759

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
259 AM CDT MON AUG 29 2016


For evening update:

Little change to going forecast this evening...with most focus on
adjusting near term pops/weather for ending of showers and
thunderstorms across the waterlogged southern cwa.

Thunderstorms have greatly diminished in coverage across the
region with loss of diurnal instability. 00z DVN sounding exhibits
very moist profile with weak veering wind profile, depicting
persistent weak warm advection flow/weak ascent potential for the
overnight hours. Low level baroclinic zone stretches from southern
Iowa into central Illinois, and while isolated shower or
thunderstorm development can`t be ruled out across the southern
cwa overnight, very weak flow generally less than 10 kts suggests
little in the way of focus for organized activity. Moist ground
conditions and fairly small temp-dew point depressions combined
with weak low level wind field suggest potential for areas of fog,
especially later tonight as convectively produced cloud cover
thins out.

As for flooding across the south, have combined earlier flash
flood warnings into a couple of aerial flood warnings which are
valid through mid-day Monday.



345 PM CDT

Through Tonight...

Latest radar imagery depicting showers and thunderstorms
continuing at this hour, with the intensity of most of the storms
having since lowered within this past hour. However, some more
robust development is still occurring near areas of greater
instability, near Lee and Dekalb, and in Livingston counties.
Although gusty winds will still be possible, think any severe
threat is low at this time. Torrential rainfall is still ongoing
though, with high rainfall rates resulting in locations easily
receiving several inches of rainfall within a very short time. So,
additional heavy rainfall and flooding will be the main threats
over the next several hours. Still anticipating an overall
diminishing trend this evening, but it is possible for some
lingering development into the early evening. Lack of precip
tonight with likely clearing will result in fog tonight into early
Monday morning, some of which could be dense. With the higher
dewpoint air in place, think most locations have the same chances
of observing the more dense fog. However, locations with added
moisture from the precip today will likely see a greater



320 PM CDT

Monday through Sunday...

The continued challenges of the past few days will continue Monday
and Tuesday, and possibly even Wednesday in the southern CWA. The
high precipitable water swath of 1.75+ inches will very gradually
spread back north on Monday as subtle return flow begins. Today
only subtle convergence in very moist air was able to blossom
storms as convective temps were reached by late morning. Think
that could be possible again Monday, and high-res models indicate
low-level streamlines with convergence draped from the Quad
Cities toward LaSalle-Peru to Gibson City. Challenging to say
where exactly that may be due to some influence of this afternoons well as just how active the boundary may be due to
such a light flow regime with general surface/upper high
pressure. But given the air mass it will not take much to spark
isolated/scattered storms. So have PoPs mainly in the western and
southern CWA. The conditional flooding threat will be there once
again due to the high rate efficiency and slow storm motion under
very weak 20 kt 3-6km flow.

A cold front will drop southward on Tuesday as influenced by a
northern stream jet developing back to the south. Upper lift in
advance of this jet will support better upper forcing than we had
today or will have Monday. The greater convergence looks to be
just north of the area in the afternoon, but again a modest to
possibly highly unstable air mass should be in place over the
region. So have chances for storms during the day and night as the
boundary shifts into the region. A localized severe wind threat
may exist as well as a continued flooding risk with any
training/slow moving storms. The chance for storms will continue
across the south on Wednesday ahead of the front, at least
depending on how quickly the front moves through.

Cooler than normal air will move in behind the front for
Wednesday with north-northeast winds gusting to 25 to possibly 30
mph along the lake front. This looks to probably bring favorable
conditions for rip currents on Wednesday possibly into at least
early Thursday.



For the 06Z TAFs...

Expect some patchy fog overnight tonight. Latest guidance is not
hitting fog as hard with the high clouds moving through the main
terminals. Areas south stand a better chance for fog where heavier
rain fell and high clouds, at least initially are less.

Concern will shift to the slow moving showers and thunderstorms
that will tend to remain south of the terminals overnight, but
then additional shortwave energy across Iowa will glide
northeast-east along the back side of the upper ridge axis and
right through Northeast Illinois today. Expect that this will
trigger additional slow moving showers and thunderstorms today as
the airmass destabilizes. Current forecast would suggest impact
only for RFD and possibly GYY, but will need to monitor the
north and eastward progression today especially with any outflow
boundaries to act as subtle triggers closer to ORD/MDW.



259 AM CDT

A few days of quiet conditions across Lake Michigan will occur
today and most of Tuesday as high pressure slowly departs east.
Low pressure will continue east from northern Saskatchewan today
to James Bay Tuesday, and then east through Quebec through mid
week. This low will send a cold front to northern Lake Michigan
Tuesday morning. As the low continues east, high pressure will
strengthen across north central Canada mid week, with its
associated ridge axis extending south across the western Great
Lakes. The initial cold front will reach southern Lake Michigan by
daybreak Wednesday, and as the high strengthens a secondary surge
of cooler air later Wednesday into Thursday will maintain a
seasonably strong north-northeast wind across the lake. With a 2
day period of NNE winds Weds/Th with winds reaching 25 to possibly
30 kt at times, expect waves to build across southern Lake
Michigan. Small craft advisory conditions can be expected
Wednesday and into Thursday. The high will then shift east Friday
and Saturday.






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