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647
FXUS63 KLOT 122201
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
501 PM CDT Wed Jun 12 2024

.KEY MESSAGES...

- Weakening showers/storms may move into the northern portion of
  the CWA late tonight or early tomorrow morning.

- Level 2 to 3 out 5 risk for severe weather Thursday afternoon/evening
  for large hail and strong winds. There is also a localized
  flooding threat.

- Hot and humid conditions likely to develop early next week,
  particularly on Monday.

- Periodic chances for showers and storms next week as well.

&&

.UPDATE...
-- Changed Discussion --
Issued at 501 PM CDT Wed Jun 12 2024

We are monitoring a small complex of storms moving ESE across
southwest Wisconsin that has showed some increase in strength
late this afternoon. This is primarily in response to a long-
lived MCV emanating from storms in the northern Great Plains
early this morning. The downstream environment remains only
slightly capped, with gradual erosion of this cap into the
northwest CWA as surface dew points increase from the upper 40s
into the mid to upper 50s. The trajectory of the MCV closer to
the IL line with time combined with an only slightly less
favorable kinematic profile suggests thunderstorm activity could
survive as far south as a line from Rockford to Chicago in the
6-9pm window. The primary threat would be strong gusty winds,
perhaps locally damaging, owing to a significant DCAPE reservoir
nearing 1500 J/kg.

Kluber
-- End Changed Discussion --

&&

.DISCUSSION...
Issued at 313 PM CDT Wed Jun 12 2024

Through Thursday Night:

It will continue to be hot (but with thankfully tolerable dew
points/humidity levels) through the remainder of the day with
temperatures holding in the upper 80s to lower 90s until near
sunset. The lake breeze hugging the Illinois shore will get
pushed back offshore and result in rising temps there into this
evening.

Zonal northwest flow across the northern Plains has already
allowed a cluster of storms to get going. As these storms
potentially approach the far northwestern and far northern CWA
this evening, they will be in a weakening phase with isolated
showers and perhaps an embedded rumble of thunder. Another round
of storms will likely form later tonight and advance southeast
through MN and WI towards the northwestern and far northern CWA.
Dry low to mid-level air and a capping inversion across the
area will result in these storms weakening as they reach the
CWA.

Although unlikely, there is a chance that a few of these storms
both tonight and into early tomorrow morning could pose a
strong to locally severe wind threat if outflow is able to
maintain itself (best chance far NW CWA). Outflow may maintain
long enough for a breezy/variable wind shift as far southeast as
the Chicago metro around dawn. Kept PoPs in the 15-35% range
through the late morning with the best chance for showers and
storms around sunrise Thursday morning across the northwest 1/3
to 1/4 of the CWA.

Thursday afternoon, models are in good agreement that
southwesterly flow will allow moisture to build into the area
with dew points in the upper 60s (more aggressive models reach
the low 70s). With temperatures rising to around 90 degrees,
MLCAPE values in the 2000-3000 J/kg range are expected. A
surface cold front will be moving southeast towards the CWA
Thursday afternoon. Initially weak surface convergence, a cap,
and weak upper-level support will inhibit storms early in the
afternoon, however, as frontal convergence increases and a
modestly strengthened mid-level jet moves overhead, storms will
become more likely through the late afternoon.

The large CAPE values and 35-45 kts of shear will cause storms
to become supercellular in nature early on. Steep mid-level
lapse rates (7-8 degrees C/km) will present a threat for large
to locally destructive (2"+ diameter) hail with large DCAPE
values (>1000 J/kg) bringing a threat for strong to severe winds
(70-80 mph gusts possible). These storms will also pose a
localized flooding threat with heavy rainfall rates and PWATs
forecasted to be around or potentially over 2 inches. The threat
for severe weather has warranted a level 3 out of 5 risk for
the southwestern portion of the CWA (where confidence in higher
coverage of severe weather is highest) and a level 2 out of 5
risk for the remainder of the CWA from SPC.

Depending how the convection plays out Thursday morning could
impact how things unfold Thursday afternoon/evening. If
showers/storms hold on through the late morning across the area,
then temperatures will take longer to rebound and instability
will struggle to build early in the afternoon. This stability
would limit storm initiation chances until later Thursday
allowing the cold front to advance further south and thus moving
the highest threat for storms and severe weather further south
as well.

However, if showers dissipate before they reach the CWA, then
temperatures and instability will be able to build quickly and
storms may be able to form further north during the afternoon.
The 12Z HRRR seems to be playing more into this scenario, which
would be a "worst case" scenario for this setup, featuring
convective initiation and the anticipated level 3 type threats
currently farther southwest to likely include more of the
Chicago metro. This will be monitored closely.

Initial supercell development will likely grow upscale into
clusters or a more linear mode as the evening progresses. Once
the convection moves out of the CWA Thursday night, things will
quiet down into Friday morning.

Carothers/Castro


Friday through Wednesday:

An upper trough is expected to be pivoting through the Great
Lakes on Friday as broad upper-level ridging begins to establish
across the southwestern CONUS. The position of the upper trough
will advect in cooler air into the region resulting in warm but
comfortable temperatures and humidity levels for Friday and
Saturday with readings in the low to mid-80s. Though, daily lake
breezes will likely keep temperatures cooler (in the 70s) for
locations closer to Lake Michigan. Otherwise, dry conditions are
expected during this period courtesy of the aforementioned
ridge.

Heading into Sunday, the aforementioned ridge is forecast to
shift into the Mid-Atlantic region as a large upper-level trough
establishes over the northwest CONUS. As this occurs southwest
flow is expected to develop across the Midwest resulting in
increasing heat and humidity through at least the early portion
of next week. While guidance is in good agreement on the upper-
level pattern, there continues to be a lot of variance on how
far east the ridge will drift. Therefore, the exact magnitude of
heat remains uncertain but there is a notable signal amongst
both deterministic and ensemble guidance that high temperatures
in the 90s are likely with overnight lows possibly in the 70s
Sunday through Tuesday.

Additionally, there is also the threat for showers and
thunderstorms Sunday and Monday which may further add a failure
mode to the magnitude of the aforementioned heat. If the ridge
does establish further east then our area would be in the
favored area for shortwave troughs to pivot through and generate
at least scattered showers and thunderstorms and in turn keep
high temperatures lower. However, if the ridge remains further
west then showers and storms could miss our area resulting in
highs over performing from the current forecast and possibly
even nearing record values (upper 90s) particularly on Monday.
Given these uncertainties I have decided to maintain the 20%
POPs offered by the NBM for Sunday and Monday, but did lower
high temperatures into the low to mid-90s to account for the
cooler temperature potential with storms. Obviously this will be
a period to watch especially for those with heat sensitivities
so stay tuned.

Guidance is hinting that cold front may try to push through the
region during the Wednesday into Thursday timeframe which looks
to bring a better potential for showers and storms in addition
to a break from the heat. Though, details on when and where this
possible front will track remains uncertain at this range
therefore stay tuned.

Yack

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1249 PM CDT Wed Jun 12 2024

The primary aviation weather concerns through the 18Z TAF period
include:

* A potential for a few gusty showers early Thursday morning

* Showers and thunderstorms Thursday evening

SW winds near or under 10 kt will continue through the day.
Overnight, sustained winds are expected to be predominantly less
than 10 kt. However, occasional gusts will be possible throughout
the night.

A decaying system will likely bring showers to RFD late tonight into
early Thursday. There remains a good deal of uncertainty regarding
whether the showers make it to the Chicago metro early Thursday
morning. These showers will have the potential to produce gusty
outflow winds which could cause wind direction to get rather erratic
while they remain nearby.

Breezy SW winds can be expected for the better part of Thursday.
Late in the afternoon, a system of showers and thunderstorms will
approach from the north. Confidence is low on coverage and timing,
but most model guidance suggests we`ll have at least pockets of
precip in the metro by 22Z. Similarly to the morning showers, these
storms could pack a good punch of outflow wind which could result in
sporadic wind shifts. This potential will continue beyond the
current 30-hr TAF period.

Doom

&&

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

&&

$$

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