Area Forecast Discussion
Issued by NWS Chicago, IL

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504
FXUS63 KLOT 161855
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
155 PM CDT Thu Aug 16 2018

.SHORT TERM...
154 PM CDT

Through Tonight...

Slow moving showers and thunderstorms continue to develop this
afternoon, with greater coverage closer to the stronger upper
trough across the plains. While there is no lighting in our area
yet, weaker shortwave forcing ahead of this low combined with
1000-1500 J/Kg of MLCAPE and moist low- levels suggests scattered
showers and some storms will continue and maybe expand a bit.
Greater coverage is still anticipated over Wisconsin and in Iowa.
Shear is low, and thus severe weather is

As the main upper low over Iowa and a lead wave ahead of this
move over the area this evening into overnight, festering or
redeveloping showers and some storms are probable, though low
confidence on what coverage would be. The mid-level jet associated
with the primary upper low will rotate eastward across central
Illinois later this evening into overnight, and this is where
there is better potential for some organized storms and wet
microburst potential, though cannot rule it out into parts of the
western and southern CWA.

Guidance is not hitting fog and low clouds too hard, but with the
weak wind field there may be pockets of it around tonight also
given the moist conditions and weak wind field.

KMD/MTF

&&

.LONG TERM...
314 AM CDT

Friday night through Wednesday...

Second upper trough finally pulls away from the region Friday
night, with the surface trough/weak cold front pushing south of
the cwa overnight. Weak surface high pressure then builds into the
region into the weekend, though northwest flow aloft maintain
neutral height trends. Some guidance depicts some isolated qpf in
a couple of spots both Saturday and Sunday afternoons, though
forecast soundings suggest little/no instability with weak capping
and thus not inclined to include pops for either day at this
time. Temps look to be about average for mid-August, with highs
both days in the low-mid 80s, and modest lake cooling with onshore
winds.

Models continue to depict another upper trough digging across the
Plains by Sunday night, and increasing precip potential as the
wave amplifies across the area through Monday. The deepening
surface low then tracks off to the northeast Tuesday as additional
energy digs into the deepening upper trough, with cooler and
drier air spreading in on north-northwest low level flow. After a
lingering chance of some showers Tuesday, cool and dry weather
takes hold mid-week.

Ratzer

&&

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

Concerns for the Chicago area airports through Friday are:

- Scattered showers and thunderstorms in the region from mid-
  afternoon through the evening and possibly the overnight

- East-northeast winds increasing to 7-9 kt this afternoon and
  then becoming light and variable again this evening

- Increasing potential for MVFR ceilings and or visibility
  overnight into Friday morning, with possible IFR conditions

Morning stratus has lifted/filled into cumulus across the area
early this afternoon, with some of this agitated, including near
the lake breeze which is advancing over ORD and MDW. Would expect
there to be scattered showers and storms to develop with peak
heating by mid-afternoon, with the greatest coverage over Iowa,
Wisconsin, and into far northern Illinois. For the rest of
northern Illinois, coverage may be more isolated. As the influence
of an upper low over Iowa early this afternoon approaches and
moves over the area this evening into overnight, the coverage of
showers is likely to maintain itself, along with some thunder. All
in all, low confidence to say if any of the Chicago area airports
will be impacted directly. RFD has slightly higher confidence and
with that have included a TEMPO for thunder there.

As the upper low moves over the area tonight, there are again
likely to be some showers. This may disrupt the potential for
stratus and fog tonight, or at least make it less ideal than last
night/this morning. Still, there is potential for IFR, especially
at RFD that could linger into Friday morning.

MTF

&&

.MARINE...
240 AM CDT

A baggy trough and weak gradient will remain in place across the
Midwest and southern Great Lakes region today resulting in light
flow over southern Lake Michigan and fresh northeast flow across
the north. A northerly breeze will overspread the entire lake by
tonight as low pressure consolidates over the eastern lakes
tonight. North flow is expected to persist into the day Saturday.
High pressure will build across the region Saturday night into
Sunday with southerly flow returning Sunday night into Monday. A
cold front is expected to move down the lake Monday night and is
expected to usher in strong northerly winds by Tuesday. Winds
could be near gale force through the day and are expected to
diminish overnight.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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