793
FXUS63 KLOT 162340
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 PM CDT Thu May 16 2024

.KEY MESSAGES...

- Scattered gusty thunderstorm threat continues into early
  evening. Severe weather threat remains low.

- Patchy to areas of dense fog may develop overnight, especially
  northwest of I-55.

- Summer-like warmth inland of Lake Michigan this weekend and
  primarily dry, though can`t rule out an isolated afternoon
  shower or thunderstorm (10-20% chance or less) either day.

- Active Monday-Tuesday: Scattered thunderstorms possible Monday
  and waves of thunderstorms on Tuesday through Tuesday evening,
  with an attendant threat for strong/severe storms and flooding.

&&

.DISCUSSION...
Issued at 256 PM CDT Thu May 16 2024

Through Friday night:

Scattered showers and thunderstorms will continue this afternoon,
though the coverage is expected to wane with sunset early this
evening. While occasional cloud-to-ground lightning and brief
heavy downpours can be expected with these storms, modest mid-
level lapse rates and deep layer shear are expected curtail the
severe weather threat. However, some instances of gusty winds (up
to 45 mph) and small hail will continue to be possible with the
strongest pulse-type storms. While the coverage of storms will
drop significantly with sunset, a low chance (20-30%) for a few
showers and storms will linger this evening. This is mainly the
case across parts of northern/northwestern IL in close proximity
to a weakening surface frontal boundary, and in advance of
another approaching mid-level impulse from western IA. For this
reason, I have held on to some low end POPs this evening, before
drying things out overnight.

During the overnight hours, low-level moisture is expected to
remain in place amidst decreasing cloud cover and light surface
winds (under 5 mph). This setup looks favorable for
overnight/early morning fog development. Conceptually, it`s easy
to envision pockets of dense fog (with visibility less than one
quarter of a mile) developing by daybreak, particularly in
areas where wet vegetation remains.

Any early Friday morning fog will then quickly erode during the
mid-morning hours, giving way to partly cloudy afternoon skies.
High temperatures near 80 will make for a spectacular and warm
May day. The exception will be along the Lake Michigan shoreline
as a lake breeze surges inland and results in cooler weather
during the afternoon hours. The day is largely looking to be
precipitation free for a majority of the area, especially for
areas along and north of I-80. However, farther south, another
weak disturbance and surface boundary could again foster a few
isolated to widely scattered afternoon showers and storms. There
is a 20-30% chance for some of these across areas well south of
I-80 Friday afternoon into the early evening. Dry and quiet
weather is then anticipated for Friday night.

KJB


Saturday through Thursday:

Unseasonably warm conditions are in store inland of Lake Michigan
through the weekend. Progged 925 mb temps support highs in the
lower to mid 80s both Saturday and Sunday, albeit with dew points
only in the mid 50s to around 60F keeping humidity levels and heat
indices in check. Onshore flow through the day on Saturday will
keep locations near Lake Michigan in the upper 60s to lower 70s.
Then on Sunday, a backdoor cold front slipping south and lake
breeze enhancement point toward similar high temps near the
lake.

Weak flow aloft and neutral to rising heights fitting a summer-
like pattern will combine with very dry mid-level air
(substantial dry air entrainment into incipient updrafts) to
greatly limit the threat for any showers and thunderstorms. Only
counter to the negating factors above will be the likelihood of
little to no capping. Thus, can`t rule out a widely isolated
afternoon shower or thunderstorm. On Saturday, a convergence zone
near and south of a Pontiac to Valparaiso line may provide just
enough low level lift for kick off sparse convection where 15-20%
PoPs are indicated. Turning to Sunday, the backdoor front and
inland lake-enhanced push of the front may trigger a field of
robust Cumulus. More prohibitively dry air aloft could preclude
any actual convection, thus for now only have silent PoPs around
10%. That said, a few models are showing splats of QPF, so we
may need to entertain some slight chance (~20%) PoPs in later
updates.

A warm front will surge north of the area on Monday, putting us in
the more humid warm sector amidst highs potentially well into the
80s. The magnitude of large scale forcing is somewhat
questionable, though there`s enough of a signal across the
guidance for chance (30-50%) PoPs across northern Illinois Monday
afternoon, increasing to 40-60% Monday evening and overnight.
Marginal deep layer shear looks to be a limiting factor for a more
appreciable severe threat, though wouldn`t be surprised at a
level 1 of 5 (marginal risk) threat for parts of the area. In
addition, PWATs up near or upwards of 1.5" (150-200%) could
support localized ponding/flooding.

The most "interesting" day next week convectively speaking
continues to be Tuesday. Another more substantial short-wave
trough and associated cyclogenesis over the Plains (tracking
northeastward) introduce the prospect for a more synoptically
classic set-up for thunderstorms, severe weather, and flooding
over the region. As is typical at this range of the forecast,
there`s plenty of uncertainty in the evolution of key features
and mesoscale influences of multiple potential waves of
convection. At this juncture, felt comfortable indicating
"showers and thunderstorms likely" in the official forecast and
recommend keeping an eye on this period as it draws closer. The
systems cold front will likely sweep across the area sometime
Tuesday night or early Wednesday and result in a quieter and
more seasonable period through Thursday (current day 7).

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 619 PM CDT Thu May 16 2024

Key Aviation Messages:

- Isolated to widely scattered TSRA this evening, primarily for
  the Chicago metro TAFs.

- Light/variable winds overnight into Friday morning, with
  potential for fog/low stratus development especially west of
  Chicago.

- Winds remain light/variable Friday, though a lake breeze
  should bring east-northeast winds to KORD/KMDW/KGYY by
  afternoon.

Weak surface low pressure was over the upper Peninsula early
this evening, with a weak cold front pushing slowly east across
the Mississippi River into western IL. Ahead of the front,
isolated to widely-scattered showers and thunderstorms were
occurring from near KMKE to southwest of KDKB and just west of
KMQB. These showers/storms were moving to the east at around 15
mph, and will continue to push east into northeast IL this
evening. Current radar trends suggest activity southwest of KDKB
will pass south of KORD/KMDW through 02-03Z, with an eventual
weakening/dissipating trend with sunset and the loss of diurnal
instability. Have maintained a VCTS mention for the Chicago
terminals, except for KGYY where storms may dissipate before
approaching the field.

The cold front is forecast to wash out across the area tonight,
with light/variable winds and little scouring of low-level
moisture. This will likely set the stage for some patchy
fog/stratus development after midnight, with IFR/LIFR conditions
possible. Some guidance hits this pretty hard, though confidence
is low enough to limit IFR mention to KDPA/KRFD for now, where
there is a bit better model agreement with the foot print. Will
continue to monitor for development into tonight for possible
lower conditions needed at the major Chicago terminals.

Any fog/stratus should burn off by mid-morning Friday, with VFR
conditions then expected. Winds will remain somewhat light and
variable within a weak pressure gradient, which should support
an east-northeast lake breeze to develop for KORD/KMDW/KGYY by
early afternoon. Can`t rule out some lingering stratus/fog over
the lake which could attempt to push inland, but would think it
would be limited to areas closer to the shore with inland
locations remaining VFR.

Ratzer

&&

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

&&

$$

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