953
FXUS63 KLOT 181126
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
626 AM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Today and tomorrow will be similar to the past few days with
  sunny skies, highs in the upper 80s, light winds, and daily
  lake breezes.

- A few showers and storms may move into northwestern Illinois
  Thursday night.

- Friday through the weekend will be noticeably more humid.
  Chances for widespread rain return this weekend.

&&

.DISCUSSION...
Issued at 327 AM CDT Wed Sep 18 2024

Through Tuesday:

Patches of dense fog continue to meander through northern IL
near the Fox and Kankakee River Valleys within a westward-
traveling moisture axis. A Dense Fog Advisory is in effect until
800 AM for Lake (IL), McHenry, Kane, and DeKalb counties where
webcams, ASOS/AWOS observations, and nighttime microphysics
satellite imagery shows a solid "slug" of dense fog. Fog (in
the Dense Fog Advisory and elsewhere) will erode quickly after
sunrise.

National infrared satellite imagery depicts an upper-level trough
digging along the Pacific Coast upstream of an upper-level shortwave
lifting into the northern Plains. Meanwhile, an upper-level low
comprised of the remnants of two tropical systems continues to
slowly meander through the Carolinas. Our area remains locked in a
relative stagnant zone of the atmosphere between all features of
interest. Just to illustrate the banality of our current pattern,
the mean of all sampled winds from the surface to 200mb (basically
the depth of the troposphere) from the past 9 RAOBS from ILX (7 AM
Saturday through 7 PM Monday) is just shy of 12kt. This has been
quite the slow pattern. With little change expected in the synoptic
pattern over the next 24 to 48 hours, the weather today and tomorrow
will be nearly the same as the past few days with sunny skies, highs
in the mid to upper 80s, and daily lake breezes. The one departure
today from the past few will be passing high clouds this evening and
overnight, though they should clear in time for sunrise on Thursday.

Thursday night into Friday, showers and storms should develop along
and ahead a weakening cold front across parts of Iowa and Wisconsin.
With the moisture axis feeding the convection lagging behind and
supportive upper-level forcing departing further into Canada, the
showers and storms may struggle to make much inroads into Illinois.
Blended NBM guidance offers 30 to 50% chances for showers and storms
before daybreak Friday focused along and west of I-39, which seems
fair.

After daybreak on Friday, low-level moisture characterized by
surface dew points in the mid to upper 60s will finally slosh into
our area as winds turn westerly. (Another gee whiz fact...The last
time Chicago O`Hare logged a wind with a westerly component greater
than 5 knots appears to have been on September 11. Again, that
speaks to the persistence of the pattern this week). Anyway, the air
will feel noticeably heavier by Friday afternoon, especially when
combined with highs in the mid to upper 80s. While forecast
soundings depict minimal capping by mid-afternoon, a dearth of large-
scale forcing mechanisms suggests that development of any shower or
storm Friday afternoon would be isolated at best, consistent with a
10 to 20% chance.

This weekend, ensemble model guidance continues to favor a return of
chances for welcome rainfall seemingly associated with an
approaching upper-level shortwave emanating from the Plains.
However, exactly how and when is not exactly discernible at this
point given ensemble clustering exhibiting several forecast
scenarios related to phasing opportunities with upper-level
shortwaves parading along the jet stream near the US/Canadian
border. At this point, blended NBM chances for thunderstorms ranging
from 20 to 40% seems fair from Sunday onward keeping in mind they
will change as time goes on.

Borchardt

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 626 AM CDT Wed Sep 18 2024

Forecast concerns include...

Areas of fog early this morning.
Lake breeze this afternoon.
Chance of fog late tonight.

Areas of fog extend just west of the Chicago terminals this
morning and will continue for a few more hours before lifting.
Conditions will likely improve quickly at DPA as the fog drifts
west, but may lower at RFD. Only medium confidence for how low
the vis may drop at RFD, prior to quickly improving and trends
will need to be monitored. There will be another chance of fog
late tonight into early Thursday morning, but confidence remains
low and no mention in any of the current tafs. If fog does
develop, it may be in similar areas as it currently is this
morning.

Light/calm winds to start may end up becoming southerly or even
south/southwest by mid/late morning, with speeds 4-7kts. A lake
breeze will then move inland, shifting winds east/northeast for
the Chicago terminals with speeds to 10kts. Timing of the lake
breeze may need some refinement with later forecasts. Winds will
diminish under 5kts this evening and likely become light and
variable again overnight. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 8 AM CDT this morning for ILZ005-
     ILZ006-ILZ011-ILZ012.

IN...None.
LM...None.

&&

$$

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