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403
FXUS63 KLOT 132255
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
555 PM CDT Thu Jun 13 2024

.KEY MESSAGES...

- Several rounds of severe thunderstorms are possible are likely
  this afternoon and evening, especially along and south of
  I-80.

- Hot and humid conditions early next week.

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

&&

.UPDATE...
-- Changed Discussion --
Issued at 555 PM CDT Thu Jun 13 2024

The initial round of convection that developed across the south
half of the Chicago metro earlier this afternoon has sagged
southeast across the Kankakee River Valley. These storms
continue to feed into an increasingly narrow corridor of higher
MLCAPE as low-level moisture profiles become less favorable with
southeast extent. With low-level flow and deep-layer shear
vectors also becoming generally parallel to existing low-level
confluent axes, expectations are that thunderstorms will
maintain pulse-like characteristics. The strongest cores may
produce large hail, while an antecedent DCAPE reservoir supports
the potential for damaging wind.

Eyes then turn to a cluster of impressive severe storm cores
around the IL/IA/MO tri-state region and its downstream
evolution toward the southern CWA in the next few hours. By the
time any upscale MCS growth occurs in the vicinity of the
southern CWA, the southward-moving cold front (analyzed almost
just north of and parallel to I-80 as of 5pm) will have shifted
south to roughly a Pontiac to Rensselaer line. The downstream
environment into our area will remain favorable for maintenance
of strong to severe storm clusters with primarily damaging wind
swaths in the 8-11pm window.

Also of note is a west to east axis of 800-650 hPa moisture
across northern Iowa ahead of the main broad mid-level trough.
Marginal mid-level lapse rates to around 6.5C/km could support
some a corridor of isolated to scattered showers and storms
south of I-88 and especially into the IL/Kankakee River Valleys
from 9pm-midnight. Steep lapse rates below the LPL/cloud base
could support gusty sub-severe winds with any more formidable
cores.

Kluber
-- End Changed Discussion --

&&

.DISCUSSION...
Issued at 236 PM CDT Thu Jun 13 2024

Through Friday Night:

Recent satellite imagery depicts an axis of agitated cumulus
clouds extending from waukegan to Davenport within a broad zone
of low-level confluence across the Lower Great Lakes. Lingering
capping has thus far kept a lid on early attempts for sustained
convection. However, continued surface heating (temperatures
rising toward 90F), low-level moistening/pooling via
southwesterly flow and evapotranspiration (dew points rising
toward 70F), and mid-level cooling/moistening via evaporation of
convective turrets into the base of the cap should eventually
win the battle and lead to explosive thunderstorm development
within the next few hours. Extrapolating the position of the
axis of agitated cumulus clouds, such explosive development is
poised to occur between the I-88 and I-80 corridor sometime
between 2 and 4 PM. With MLCAPE >2500 J/kg (including some 1000
J/kg in the hail growth layer) and convective-layer shear >45kt
(largely focused from 2-6 km), sustained thunderstorms will be
poised to become supercells with a threat for destructive hail
locally greater than 2" in diameter through the afternoon. With
time, clustering of cells may lead to an increasing damaging
wind threat including localized gusts as high as 75 mph.
Coverage of thunderstorms this afternoon remains an item of
lower than average confidence, with anything from just an
isolated cell or two to literally an entire line of supercells
plausible.

Meanwhile, a second area of explosive thunderstorm development
is expected in southern Iowa this afternoon near an area of
focused low-level convergence along a cold front (e.g. the Level
3/5 threat area in the SPC outlook). Upscale growth into one or
more clusters appears plausible as they move east-southeastward
into western and central Illinois this evening. If coverage of
thunderstorms this afternoon is relatively sparse, incoming
convection from Iowa may provide a "round 2" of sorts along and
south of I-80 where the receiver of MLCAPE >2500 J/kg remains.
Similar to this afternoon, any storm this evening will carry a
threat for destructive hail >2" and, if clusters can become
established, winds >75 mph. Moreover, if a west-to-east axis of
thunderstorms were to materialize within a mesoscale zone, PWATS
>2" would certainly support efficient downpours with a threat
for localized flash flooding. The WPC threat level 2/5 area for
flash flooding highlights the area well. In all, "round 2" may
occur from 7 PM to as late as 3 AM.

Tomorrow will be decidedly quiet with seasonable temperatures
(highs in the low to mid 80s) and a northwesterly breeze. A lake
breeze should surge inland during the afternoon leading to
cooling temperatures along the lakeshore.

Borchardt


Saturday and beyond:

An upper-level ridge is expected to begin to drift into the
southern CONUS on Saturday which will continue to support dry
conditions. While winds will begin to turn southerly on
Saturday, the strongest warm advection is expected to be west of
our area which will likely keep temperatures near more
seasonable readings in the low to mid- 80s. Though, a lake
breeze is forecast to develop and keep highs in the 70s near
Lake Michigan, particularly the IL lakeshore.

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 and advect in warmer and much more
humid air. While guidance remains in decent agreement on the
upper-level pattern, there continues to be uncertainty on how
far east the ridge will drift on Sunday into the early part of
next week. Therefore, the exact magnitude of heat remains a bit
of a question mark. Regardless, there continues to be a strong
signal (around 70-80% chance) for highs Sunday through at least
Tuesday to be within the low to mid-90 range with the potential
for readings to overperform, especially on Monday. Couple these
temperatures with the forecasted increase in humidity (dew
points in the mid to upper 60s to near 70) and there is a
growing threat for heat indices to top out in the upper 90s to
near 100 degrees early next week.

Depending on where exactly the aforementioned ridge sets up
will also dictate whether or not the troughing over the western
CONUS will be able to eject shortwaves into northern IL and
generate periods scattered showers and thunderstorms. Given the
previously mentioned uncertainty in the ridge position, the
confidence on whether or not we will see any storms remains on
the lower side. That said, any days that do see showers and
storms may also see highs verify cooler than forecast which
would in turn lower the threat for heat related concerns. To
encompass this potential in the forecast I have maintained a 20%
chance for showers and thunderstorms for most of the area
Sunday through the middle of next week, but specifics on timing
and coverage will likely need to be adjusted with future
forecasts. Though, latest guidance is eyeballing the late
Saturday night into Sunday timeframe as a favored period for
showers and storms.

As for when this period of heat and humidity will break,
guidance continues to hint at a cold front moving towards the
region towards the later portion of next week. However, timing
on when exactly the front, and associated relief, will move
through continues to vary amongst the guidance. Couple this with
the fact that guidance typically struggles with blocking
patterns and confidence to say when a break in the heat will
occur remains low at this time. Therefore, we recommend keeping
updated on the forecast as we head into next week for any
changes.

Yack

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1235 PM CDT Thu Jun 13 2024

Forecast concerns for the terminals...

- Scattered to widespread thunderstorms this afternoon and
  evening, some storms could be severe

- Winds become northerly behind a cold front this evening

- Lake breeze to move through the Chicago terminals Friday
  afternoon

Breezy west-southwest winds have developed at the terminals,
especially the Chicago ones, this morning as the atmosphere has
mixed into a low-level jet overhead. While this jet is gradually
diminishing, gusts in the lower 20 kt range will likely persist
through early afternoon.

At the same time, an upper disturbance and associated cold front
will continue to move towards northern IL from WI which will aid
in developing scattered to widespread thunderstorms this
afternoon and evening. While confidence is high that storms will
develop, the confidence on exact timing and coverage over the
terminals, especially at RFD, is lower. Therefore, I have
maintained the TEMPOs and their respective timings which seem
reasonable based on latest guidance trends. However, I did
extend the VCTS mentions at MDW and GYY an hour later to account
for the potential for the front and associated storms to stall
near I-80 this evening. Additionally, there is also the threat
that any storms this afternoon and evening could become severe
with the threats of large hail (possibly in excess of 2 inches),
damaging winds (possibly in excess of 60 kts), and locally
heavy rain which will result in reduced visibilities.

Storms will gradually come to an end from north to south late
this evening as the front moves through. As a result winds will
become northerly (possibly varying between 350 and 010) with
speeds in the 6 to 8 kt range overnight and persist into Friday
morning. Though a lake breeze is expected to develop early
Friday afternoon and push through the Chicago terminals between
18z and 20z. Otherwise, expect mainly VFR conditions outside of
storms with gradually scattering clouds overnight into Friday.

Yack

&&

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

&&

$$

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