421
FXUS63 KLOT 130532
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1232 AM CDT Thu Jun 13 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.

&&

.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 /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1232 AM CDT Thu Jun 13 2024

Forecast concerns include...

Scattered strong/severe thunderstorms this afternoon.
Gusty southwest winds later this morning/early afternoon.
Possible east winds after storms.

Various areas of thunderstorms nearby this morning. Storms over
southeast Lake Michigan will move away from the terminals while
storms over eastern IA will move southeast into west central IL
and remain southwest of the terminals. But the area of storms
across eastern MN into central WI will continue moving southeast
and is expected to slowly weaken and dissipate. However, if
this area of convection does not dissipate fully, showers may
reach into far northern IL and RFD by daybreak this morning.

The focus will then shift to new thunderstorm development which
is expected to develop just north or potentially right over the
terminals this afternoon. There is still some uncertainty for
timing and location and there is a small chance thunderstorms
will develop south of the terminals. However, the consensus
among the latest guidance is that thunderstorms will develop
over northern IL ahead of an approaching cold front and then
push to the southeast this evening with some isolated thunder
potential remaining through sunset. Opted to trim tempo mention
to just 2 hours, which may still need some refinement as trends
emerge and then include vicinity mention for a longer time
period. Any of the thunderstorms that form today may become
severe with locally strong/erratic winds/gusts. Ifr/lifr cigs
and vis will be possible along with very heavy rain.

Southwest winds will slowly increase early this morning with
some gusts possible by daybreak. Then gusts into the 20kt range
are expected through early afternoon. Winds will turn westerly
ahead of the front and then possibly northerly though will be
impacted by thunderstorms. Models are now showing an easterly
wind for a few hours behind the storms, that settles into a
light northerly wind for the rest of tonight. cms

&&

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

&&

$$

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