287
FXUS63 KLOT 180526
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1226 AM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Very warm to hot and dry by day through Thursday.

- Widely scattered showers and thunderstorms possible late
  Thursday night-early Friday mainly west of the Fox Valley.

- Very warm and a bit more humid through the weekend,
  potentially followed by a better chance of showers and
  thunderstorms late Sunday into Monday.

&&

.DISCUSSION...
Issued at 251 PM CDT Tue Sep 17 2024

Through Thursday:

Tonight will be quiet and relatively cool again (upper 40s-mid
50s outside of Chicago) underneath expansive surface high
pressure. As overnight temperatures over interior northern
Illinois drop to near or below their afternoon dew points
(crossover values), some patchy ground fog may develop and then
erode quickly after sunrise. Can`t rule out some spotty dense
fog over far northeast Illinois, and will let the evening shift
assess this potential.

Wednesday and Thursday will essentially be more of the same of
what we`ve seen lately, and stepping a bit warmer as thermal
ridging aloft sloshes back eastward. Forecast highs are in the
mid to upper 80s away from Lake Michigan on Wednesday (upper
70s-around 80F near the lake) and more solidly upper 80s to
locally near 90F on Thursday (around 80F near the lake).
Afternoon dew points will mix out a bit less each day (vs.
today), though still quite dry with afternoon RHs in the 20-35%
range inland.

Thursday Night through Tuesday:

A glancing blow of forcing from a short-wave trough lifting into
the upper MS Valley and northern Great Lakes will induce
strengthening warm and moist advection of an initially parched
and strongly capped column. Contingent upon sufficient mid-level
moistening for the erosion of MUCIN, widely scattered elevated
convection (showers and storms) may develop over the west and
northwest CWA overnight and track eastward on a general
weakening/decaying trend as the instability drops off rapidly to
the east. 30-40% PoPs are generally progged for areas west of
the Fox Valley (northwest 1/3 or so of the CWA). Most if not all
the overnight activity (regardless of exact coverage) should
fade out prior to sunrise, though can`t rule out a lingering
isolated shower or thunderstorm in the early morning.

A weak cold front will move across the area on Friday afternoon,
with height rises and dew points likely to mix out given poor
trajectories for moisture return and burgeoning drought, suspect
that we`ll stay strongly capped, so it will by and large be a
dry cold front passage. Can`t completely rule out an isolated PM
shower or thunderstorm over northeast Illinois (15-20% PoPs).
Friday will be yet another summer-like day with highs in the
upper 80s away from the lake.

In the wake of the cold front passage, weak high pressure will
build back in through Saturday night, which combined with
neutral mid-level height tendencies and weak flow aloft, should
keep any meaningful precip. chances primarily well to our west
and southwest. Saturday will not be quite as warm, but still
well above normal (highs in mid-locally upper 80s) away from the
lake, along with higher humidity levels than we`ve had of late.

There are signs that the seemingly never ending warm and dry
pattern may transition to a more active one in time for the
start of astronomical fall (autumn equinox 7:43 AM CDT Sunday).
However, whether we can make up some of the lost ground with the
substantial rainfall deficits since late August remains to be
seen. Given the uncertainty in pattern evolution out in the
Sunday-Tuesday period, lower end PoPs in the 20-30% range seem
reasonable for now. Temperatures will also trend cooler than
they have been, but likely still above normal for late September.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1226 AM CDT Wed Sep 18 2024

Forecast concerns include...

Patchy fog early this morning and again Thursday morning.
Lake breeze this afternoon.

Patchy fog will be possible early this morning, dissipating
quickly after sunrise. It will likely favor the usual more rural
locations but fog already being reported at GYY and is possible
at DPA as well. Trends will need to be monitored for possible
fog at ORD and RFD. Fog will be possible again late tonight
into Thursday morning, likely in the same areas as this morning,
though confidence is too low to include mention with this
forecast.

Light/calm winds early this morning will become light southeast
by mid/late morning and then a lake breeze will move inland this
afternoon, turning winds east/northeast to 10kts for the
Chicago terminals. Winds will become light and variable or calm
again this evening into early Thursday morning. cms

&&

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

&&

$$

Visit us at weather.gov/chicago