072
FXUS63 KLOT 170544
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1244 AM CDT Fri May 17 2024

.KEY MESSAGES...

- 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.

&&

.UPDATE...
Issued at 851 PM CDT Thu May 16 2024

One lonesome thunderstorm managed to trek across a good portion
of the CWA this evening until just recently dying out over
Grundy County shortly after 8 PM. This storm was leftover from a
line of activity that developed earlier this afternoon along a
cold front which had outrun the boundary. The weak cold front
has now just about stalled over northwest Illinois and just
grazes our northwestern CWA. This front looks to basically
dissolve over the next several hours and shouldn`t play much of
a role in our weather from here. Latest RAP data indicates some
MLCAPE still resides over the CWA ahead of this front and a few
pieces of guidance suggest a couple of additional showers may
still try to develop late this evening. However, this will be
difficult to achieve with the cap reinforcing for the evening
in addition to a big lack of forcing and low-mid level dry
advection. That being said, it looks like the remainder of the
night should be primarily dry.

In other news, nearly calm winds and clearing skies should
promote fog development tonight, some of which could be dense.
Dewpoint temperatures this evening sit in the upper 50s to
lower 60s where temperatures are forecast to fall into the
middle 50s to near 60 degrees overnight. This looks like a
favorable setup for widespread fog development including
possible areas of dense fog, especially over parts of the CWA
that saw rain today. Any fog that develops late tonight should
stick around past daybreak possibly impacting portions of the
Friday morning commute.

Doom

&&

.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 /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1244 AM CDT Fri May 17 2024

Key Aviation Impacts for the current TAF period:

- Potential for lower cigs and vis development early Friday
  morning

- Lake breeze provides northeast winds Friday afternoon for
  terminals closer to the shoreline

With sufficient low level moisture present and southerly winds
becoming light and variable, there is a chance for reduced vis
and even some lower cigs Friday morning. Dense fog is not
currently expected, but vis could reach 3 to 4 SM for a brief
window between 09Z and 13Z, though potentially lower vis in
more rural areas away from the lake.

After day break, there should be enough mixing to scour out any
lingering low clouds. VFR conditions are expected with a lake
breeze developing in the afternoon. As temperatures increase in
the afternoon, there is a slight chance for showers to develop
closer to the front exiting to the east. While there could be a
small window for showers near KGYY, for now it was kept out of
the TAF.

DK

&&

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

&&

$$

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