968
FXUS63 KLOT 081917
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
217 PM CDT Sat Jun 8 2024

.KEY MESSAGES...

- A cold front will move through the area this afternoon and
  evening with a band of showers and a few thunderstorms.
  Widespread severe weather is not expected.

- Breezy northwest winds and seasonable highs are expected on
  Sunday, followed by breezy northeast winds and seasonably cool
  conditions on Monday.

- A gradual warming trend is expected from Tuesday onward with a
  return of above-average temperatures by the end of the week.

&&

.DISCUSSION...
Issued at 216 PM CDT Sat Jun 8 2024

Today and Tonight:

Recent surface, radar, and satellite observations
depict a low pressure system centered near Clinton, Iowa along a
surface cold front extending from southern Wisconsin into eastern
Iowa and central Missouri. A narrow tongue of low-level moisture
characterized by surface dew points in the low to mid 60s continues
to slide into northwestern Illinois ahead of the front and just to
the south of subtle low-amplitude upper-level shortwave propagating
within a seasonably strong upper-level jet across the central Great
Lakes. Pockets of clearing across northern Illinois combined with
increasing low-level moisture is allowing for the development of
shallow low-level instability, with the most-recent RAP analysis
indicating around 500 J/kg of MLCAPE (50-75 J/kg of which in the
lowest 3 km).

Over the next few hours, coverage of ongoing showers near the
Wisconsin state line will increase and pivot southeastward over
northern Illinois within the shallow but destabilizing boundary
layer. A few thunderstorms remain possible amongst otherwise
scattered showers within the southeastward-shifting zone. With
strong low- to mid-level shear (thanks to a stark shift in wind
direction along the front and a close proximity of the upper-
level jet to our north), any sustained updrafts will have the
opportunity to develop transient low- to mid-level rotation and
become a low-topped supercell. As a result, we cannot rule out a
report of a funnel cloud or even brief tornado this afternoon,
particularly near the best overlap of low-level instability and
near-surface vorticity along the front. Extrapolating the
positioning of the low-level moisture axis and arrival timing of
the front, such a threat for funnel clouds should materialize
near I-80 between 3 and 6 PM. Now, this is not an environment
that appears supportive of damaging tornadoes, and the most
likely scenario is that a band of showers with an embedded
lightning strikes moves through without any fuss. Nevertheless,
move indoors if you see a funnel cloud or otherwise hear
thunder. The axis of showers and embedded thunderstorms will
move southeastward into northwestern Indiana this evening.

Behind the front, an increase in northerly winds will cause
waves on Lake Michigan to quickly build to 2-4 feet. While not
an ideal beach day, be prepared for quickly changing conditions
if out on the water. Overnight lows tonight will range from the
low to mid 50s.

Sunday and Monday

As upper-level cyclonic flow gradually moves through the Great Lakes
Sunday into Monday, several embedded short-wavelength troughs
will move overhead. As a result, Sunday will be marked by
seasonable temperatures in the mid to upper 70s and breezy
northwest winds. Winds will turn onshore on Monday, leading to a
seasonably chilly day with highs in the lower 60s lakeside to
lower 70s inland. While our forecast is dry both days, it
wouldn`t take much to squeeze out a few sprinkles during both
afternoons as low-level lapse rates steepen and allow for
appreciable depth to cumulus clouds. Also, onshore winds will
cause choppy waves and dangerous swimming conditions across all
Illinois and Indiana Lake Michigan beaches on Monday.

Tuesday Onward:

A gradual warming trend is expected Tuesday onward. Ensemble model
guidance continues to advertise summer-like (and above-average)
temperatures returning by the end of the week, following by
increasing humidity levels (and chances for thunderstorms) sometime
during the 3rd week of the month.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1256 PM CDT Sat Jun 8 2024

Forecast concerns for the terminals...

- Scattered showers through early evening with associated MVFR
  ceilings and visibilities

- Low confidence threat for isolated thunderstorms (around 20%)
  this afternoon

- Gusty northeast winds behind cold front late afternoon

- Breezy northwest winds Sunday afternoon

Scattered showers will continue to develop ahead of the cold
front as it drifts southward out of Wisconsin this afternoon.
While most of these showers will be of the lighter variety, a
few may become more robust and result in MVFR (if not high-end
IFR) visibilities. Additionally, ceilings will lower to MVFR
with the showers before conditions begin to improve behind the
cold front this evening.

There also continues to be a threat for isolated thunderstorms
this afternoon as well. However, confidence on coverage and
placement of thunderstorms remains low due to uncertainties on
available instability ahead of the front. Therefore, have
decided to forego a formal thunder mention in the TAFs with this
issuance but did modify the TEMPO groups to encompass the
expected window any storms would occur. If storms do develop we
will tactically amend and include thunder within these
aforementioned TEMPO groups as necessary. Showers and any storms
will come to a conclusion behind the front this evening with dry
for the rest of the period.

A period of gusty northeast winds is expected to develop behind
the cold front this afternoon with gusts topping out in the
lower 20 kt range. While the anticipated timing of the wind
shift at the terminals has not changed, the timing was shifted
back an hour in the TAFs to account for the aforementioned TEMPO
groups. That said, expect the northeast wind shift to occur +/-
one hour of the noted time in the TAFs. Regardless, the gusty
winds will ease this evening as directions become northwesterly
overnight. However, wind speeds will increase Sunday morning
with gusts in the 20-25 kt range expected Sunday afternoon.

Yack

&&

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

&&

$$

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