588
FXUS63 KLOT 131827
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
127 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.

- Heat and humidity will return next week, as well as chances
  for showers and thunderstorms.

&&

.DISCUSSION...
Issued at 127 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...

Coming Soon...

NWS Chicago

&&

.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.

&&

$$

Visit us at weather.gov/chicago