Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 240634

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
134 AM CDT Thu Aug 24 2017

249 PM CDT

Through Thursday...

Main forecast concerns/challenges are with scattered showers late
tonight into Thursday morning, the majority of which will occur
near the lake in far northeast Illinois and northwest Indiana.

In the near term, relatively quiet conditions with below normal
temps in place while high pressure is situated just to the west.
Partly cloudy to at most partly sunny skies across the area this
afternoon will see a diminishing trend through the remainder of
the afternoon with only a slight rise in temps expected over the
next couple of hours. Lake breeze has developed today and is
situated from Cook county into areas in northwest Indiana. While
gradient relaxes this afternoon, should see some westward movement
of this boundary. Then anticipate mainly clear skies tonight,
with cool temps once again anticipated, dropping to the 50s.

Challenge this period will be with coverage of possible scattered
showers late tonight into early Thursday morning. Main focus for
this development will be tied to the lake, as cooler air over the
lake along with southward moving surface trough will provide a
setup for lake effect shower development across areas near the
lake in northeast Illinois and northwest Indiana. Setup appears to
become more favorable late tonight into early Thursday morning,
especially as a more defined/stronger shortwave drops southeast
through the region. This should support some blossoming of any
convective showers already present, with any development likely
continuing through midday and early afternoon Thursday. Did
increase pops slightly but with some uncertainty as to exact
placement, duration, and coverage, have limited to chance pops. In
this setup, a chance of waterspouts will be possible and have
continued mention in the forecast.

While showers will mainly be confined more towards areas near the
lake, continue to monitor for at least scattered sprinkles for
remaining areas in northern Illinois and northwest Indiana. Low
confidence of this but some guidance is showing the possibility
for the current precip shield ahead of the main shortwave of
interest currently in far northwest MN to hold together as it
pushes through WI late tonight. Currently think that if it were
hold together, that primarily sprinkles would be possible.
However, will need to monitor for a more solid area of rain to
possibly move through portions of the CWA Thursday morning.



318 PM CDT

Thursday night through Wednesday...

Primary focus is on unseasonably cool temperatures Thursday night
into early Friday and then the return of precipitation potential
over the weekend into next work week. Stout upper trough axis will
be exiting east Thursday night, but sprawling surface high over
the Great Lakes region, dry air mass underneath and mostly clear
skies until near dawn will yield favorable radiational cooling
conditions. Expect unseasonably chilly lows in the upper 40s to
lower 50s outside Chicago and mid to upper 50s in and near the
city. Some patchy fog is possible as air temperatures fall to the
dew points.

Short-wave impulses over the Plains could kick off convection to
our west and northwest Friday through Saturday, but the dry
surface high and northwest flow from troughing over the eastern
CONUS will likely shear this out and keep precipitation largely at
bay. While mid and high clouds will likely be the main effect on
sensible weather, can`t completely rule out isolated showers into
parts of north central and far northern Illinois on Saturday.
Daytime temperatures will remain below seasonable for late August
in the 70s, but quite comfortable.

From Saturday night into Sunday and Monday, models are keying on
short-wave energy over the northern Plains merging with a trough
dropping south toward the northern Lakes, with a weak elongated
surface trough reflection. The trough then looks to close off into
an upper low that will meander slowly southward and eventually
eject eastward and weaken into an open wave by mid week. This
will present our next opportunity for showers and possibly
isolated embedded thunder with some weak elevated instability
advecting eastward.

However, the main uncertainty is tied to how quickly the surface
high moves east and the upper low and weak surface trough move
toward our area. At this time, have higher chance category PoPs
focused on Sunday night and Monday, but confidence is low. Closed
upper lows are typically quite challenging for guidance to handle.
Most recent suite of global guidance keeps Harvey/Harvey`s
remnants well south of the region through mid next week, but it`s
possible some of the moisture from it could get entrained into
the system that may affect our area depending on how the pattern
evolves at the mid and upper levels. Temperatures in the 70s by
day will continue to be the rule, near to slightly below normal
for the end of August.



For the 06Z TAFs...

Main concerns will be small chances for showers late overnight
into Thursday morning, and timing of stronger NE winds on

Clear skies and light winds have set up across the region as high
pressure builds at the surface. A few isolated locations may see
some shallow ground fog, but some cloud cover spreading into the
area durg the early morning hours should help limit fog potential.

A mid level wave, resposible for the increasing cloud cover, is
also generating some showers over sern MN and swrn WI. The latest
guidance suggests that this wave will continue to drop southeast
and could bring some isold/sct light showers to nrn IL/IN invof
the terminals. However, the low levels are relatively dry and any
forcing is from aloft, so the chances of wetting rain at the
terminals is low as coverage should be relatively sparse. However,
will maintain the VCSH in the TAFs, though operational impact
would be minimal at best.

The greater concern will be timing of the wind direction shift to
nely and speed of winds following the wind shift. Have been
following a convergence boundary over srn Lake Michigan and as the
boundary slowly moves south down the lake, winds should gradually
shift to nely. Latest guidance suggests that the shift to nely
should occur arnd 18z and that sustained speeds should be 10 to
15 kt, which should then persist through the afternoon. Confidence
in the wind shift timing and wind speed foolowing the wind shift
is moderate at this time.

Otherwise, other than the low chance for showers later this
morning, conditions should be quiet at the terminals.


342 PM CDT

Waves hazardous for small craft over the Indiana nearshore will
subside early this evening. The main marine focus then shifts to
the chance for waterspouts late tonight through Thursday morning
on the southern half of the lake, including the Illinois and
Indiana nearshore. A surge of unseasonably cool air will move
south down the lake tonight accompanied by steady north winds,
with an upper disturbance kicking off scattered showers. Given the
unseasonably cool air over the warm lake waters, parameters
support waterspout potential through about mid day. The moderate
northerly winds could build waves back into the 3 to 4 foot range
again on the south half on Thursday.

High pressure will spread over the western Great Lakes
late Thursday and linger through Saturday, resulting in lighter
and likely variable at times winds. The next period of concern is
on Sunday night into Monday. High pressure will shift east on
Sunday, yielding steady southeast winds. A weak surface trough
will then approach from the west, strengthening the east-
southeast winds Sunday night into Monday with the potential for
gusts up to 25 kt and possibly as high as 30 kt on the south half
depending on the track of the trough.






WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.