710
FXUS63 KLOT 180154
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
854 PM CDT Tue Sep 17 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.

&&

.UPDATE...
Issued at 854 PM CDT Tue Sep 17 2024

Minor update to overnight forecast to expand mention of patchy
fog across parts of northeast IL. Excellent viewing conditions
are expected this evening for the partial lunar eclipse,
however, which reaches its peak around 945 pm CDT.

Forecast area remains in western periphery of surface high
pressure centered over the northeastern U.S. This setup will
provide clear skies and light east winds becoming light and
variable overnight. This, in turn, will allow favorable
radiational cooling conditions, allowing temperatures to fall
into the lower 50s away from the core of the Chicago urban heat
island and into the upper 40s in a few of our typically cooler
spots. This cooling, combined with slightly higher dew points
spreading inland from Lake Michigan, will likely support the
development of patchy ground fog overnight. Areas most likely to
see patchy fog are mainly in a corridor from the north and
northwest suburbs, southward through and just west of the Fox
River Valley where the higher dew points will be juxtaposed
with the better cooling way from the city.

Ratzer

&&

.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 /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 630 PM CDT Tue Sep 17 2024

No significant aviation weather concerns exist through much of
the period. Light east-northeast winds will become light/variable
after sunset tonight, before becoming easterly once again on
Wednesday. Skies will be generally cloudless, though some
shallow fog is possible toward sunrise away from the warmer
metro sites. Various model guidance is generally highlighting
the fog potential in areas from the north through west Chicago
suburbs and the Fox River Valley (KDPA area). Both ground and
atmosphere are fairly dry, though this also sets up good
radiational cooling conditions which may allow for some patchy
ground fog to develop. At this time, confidence is too low for
inclusion in TAFs, but will continue to monitor visibility
trends overnight. Any fog that may develop would burn off
quickly within an hour or two of sunrise.

Ratzer

&&

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

&&

$$

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