337
FXUS63 KLOT 071730
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 PM CDT Mon Jul 7 2025

.KEY MESSAGES...

- Dangerous swimming conditions at Lake Michigan beaches through
  this afternoon.

- Low chance (20-40%) for showers and thunderstorms Tuesday
  afternoon into Tuesday night.

- Better chances (40-60%) for showers and thunderstorms Friday
  night and Saturday.

&&

.DISCUSSION...
Issued at 150 AM CDT Mon Jul 7 2025

Through Tuesday Night:

Northerly winds gusting into the 25 mph range will maintain
waves in the 4 to 7 foot range and dangerous swimming
conditions at Lake Michigan beaches through this afternoon.
Winds will slowly diminish later this morning in this afternoon
with waves then slowly subsiding into this evening. Its possible
the beach hazard statement may need to be extended, depending
on how fast waves subside.

There may be an isolated rain shower across far northwest IN
this morning. Otherwise, dry weather is expected into Tuesday
morning. The HRRR/RAP are showing at least scattered convection
across the area Tuesday afternoon/evening with much of the rest
of the guidance keeping this activity either south of the local
area, or perhaps just into the southern cwa. Convective trends
on Tuesday will be dependent on how trends emerge over the next
18-30 hours to our west and confidence is low for the HRRR/RAP
solution. Blended pops are now in the 20-40% range for this time
period, and that seems reasonable for now. Its also possible
that if there is convection in the local area, it may be later,
into Tuesday night.

Cloud cover is expected to slowly scatter out from west to east
later this morning into this afternoon and if mostly sunny skies
materialize, high temps will likely reach the lower/mid 80s for
areas away from Lake Michigan. Highs in the 70s may also persist
downwind of Lake Michigan into northwest IN. Lows generally in
the lower 60s tonight, warmer in the immediate Chicago metro
area, then highs in the mid/upper 80s Tuesday, cooler near the
lake with an expected lake breeze. cms


Wednesday through Sunday:

Toward the middle of the week, an upper-level trough
originating from the Gulf of Alaska will dive toward the
northwestern United States. At the same time, a building
southwestern US monsoonal ridge will pick up a cut-off low
pressure system meandering off the California coast and "kick"
it northeastward toward the northern Rockies. The aggregate
effect will be the development of an expansive ridge across far
northeastern North America, albeit of the "dirty" variety owing
to a remnant shortwave trough trapped over southern Ontario.
This is all to say, the overarching pattern over the Great Lakes
during the middle of the week will be fairly benign with weak
flow (30kt or less) within the column from the surface to
tropopause.

On Wednesday, a warm and sunny start in conjunction with
remnant low- level moisture should allow for a healthy cumulus
field to develop by mid-afternoon. With forecast soundings
depicting minimal capping, would have to think isolated to
perhaps scattered storms will flare during peak heating perhaps
tied to any remnant outflow boundaries or the lake breeze where
low- level convergence will be maximized. With that said,
Wednesday hardly looks like a washout (chances for storms are
only 20-30% areawide). Thursday looks like a carbon copy of
Wednesday, albeit with less moisture suggesting a lower coverage
of afternoon showers and storms (call it a 10 to 20% chance at
any given location). Both days will feature highs in the mid to
upper 80s inland and in the upper 70s lakeside.

Thursday night and into the weekend, the aforementioned
California shortwave and Pacific trough will approach the Great
Lakes. While the ensemble envelope on how both features will
interact (if at all) is fairly broad at this range, the overall
pattern appears supportive of episodic thunderstorm events in
the general Midwest region to close the week.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1230 PM CDT Mon Jul 7 2025

Forecast concern for the terminals...

- Scattered showers and thunderstorms (20-30% chance) Tuesday,
  mainly in the afternoon at the Chicago terminals.


A surface high will continue to move into the area this
afternoon resulting in mostly clear skies and light (5-10 kt)
east-northeast winds. Though directions will become more
variable overnight as the high moves overhead before winds
settle into a southwest direction on Tuesday.

A thunderstorm complex is forecast to develop across the north-
central Plains this evening and then traverse towards northern
IL by Tuesday morning. Given that the environment in northern IL
should be dry and somewhat stable, the complex should dissipate
prior to reaching the terminals. Though a stray shower cannot
be ruled out at RFD hence a PROB30 has been added. That said,
additional showers and storms may redevelop Tuesday afternoon
across northeast IL and northwest IN as the remnant disturbance
pivots overhead. While moisture and instability look sufficient
for storms, the modest forcing makes the coverage more
uncertain especially with westward extent. Therefore, have
decided to introduce a PROB30 at the Chicago terminals for this
period.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 7 PM CDT this evening for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement until 9 PM CDT this evening for INZ001-
     INZ002.

LM...Small Craft Advisory until 3 PM CDT this afternoon for the IN
     nearshore waters.

&&

$$

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