782
FXUS63 KLOT 210808
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
308 AM CDT Thu Aug 21 2025

.KEY MESSAGES...

- Choppy waves and dangerous rip currents will continue at Lake
  Michigan beaches through midday/early this afternoon.

- Patchy fog (perhaps dense in spots) may develop well outside
  of Chicago late tonight and linger into Friday morning.

- Otherwise, little to no impactful weather is expected through
  the upcoming weekend and early to mid next week. An early
  Fall temperature preview remains in store Sunday-Tuesday.

&&

.DISCUSSION...
Issued at 308 PM CDT Thu Aug 21 2025

Through Friday:

A surface high pressure system is centered over the Great Lakes
early this morning leading to another night of relatively
tranquil conditions. Nighttime microphysics imagery depicts
mostly clear skies across much of our area, save for a large
patch of low stratus extending from upstate New York all the way
to west-central Illinois. A separate lobe of stratus is
spreading across Lower Michigan, as well. The combination of
light surface winds, mostly clear skies, and residually moist
soils have allowed for surface dew point depressions to once
again collapse. As a result, do think patches of shallow fog
will once again develop this morning, though with less coverage
compared to last owing to some 10kt or so of flow atop the near-
surface stable layer.

Toward daybreak, northeasterly steering flow along the southern
side of the high pressure system will guide the aforementioned
lobe of stratus in Lower Michigan into northeastern Illinois and
the rest of northwestern Indiana. Stratus should then hang
around through early afternoon before boundary layer mixing
encourages holes to develop. Upward mixing into the low-level
northeasterly steering flow will induce similarly northeasterly
surface winds, leading to a fairly healthy lake shadow in
temperatures. Expect highs to range from lower 70s lakeside to
around 80 west of I-39 and near US-24.

Tonight, the center of the surface high will drift southward
causing low-level winds to become nearly calm. Depending on
whether pesky stratus redevelops or not, there will be a shot
for dense fog to develop overnight given otherwise ideal
conditions for radiational cooling. Will introduce patchy fog
into the forecast for all but urban areas and let the day shift
take another look at stratus trends to determine whether to hit
the fog mention harder. Overnight lows will range from the upper
50s to lower 60s.

With the center of the surface high still directly overhead on
Friday, tranquil (or simply splendid) weather is in store with
mostly sunny skies, light winds, and highs around 80 degrees.
Even with a modest land/lake temperature differential, do expect
a lake breeze to ooze inland during the afternoon leading to
temperatures in the low to mid 70s lakeside.

One final note - opted to extend the Beach Hazards Statement
along the southwestern beaches of Lake Michigan through 1 PM
owing to lingering swell. Buoy observations of wave heights in
the 4 to 5 foot range are running a hair higher than model
guidance. Continued easterly wind, even if weakening, should
cause wave heights to only slowly subside throughout the day.

Borchardt


Friday Night through Wednesday:

A strong upper low and its associated surface low will track from
Manitoba into western Ontario on Friday night. The cold front
trailing from the surface low will be approaching the area from
the northwest. Upstream across the northern Plains, upper
Midwest, and the northern Lakes, sufficient moisture, MUCAPE, and
respectable large scale forcing on the southern flank of the upper
low will likely result in an at least loosely organized band of
convection developing by the evening. As the showers and storms
press southeast, they`ll encounter a sharp drop-off in MUCAPE
locally at a diurnally unfavorable time for convective
maintenance. Thus, the thinking remains similar for the
possibility of a few decaying showers and perhaps an embedded
thunderstorm surviving into far northern Illinois during the
pre-dawn hours of Saturday.

Into Saturday morning, can`t rule out spotty high based sprinkles
out of the mid-level debris overcast. The cold front will be
trailing the remnants of overnight convection a bit, but there
still appears to be key missing ingredients for much in the way
of renewed shower and thunderstorm development immediately ahead
of the front early Saturday afternoon. Most notably, the guidance
strongly favors winds veering to west and even west-northwest
ahead of the front, which will greatly curtail low-level frontal
convergence. Blocked off moisture trajectories coming out of
surface high pressure Friday night will also limit destabilization
to an extent. Finally, very strong mid-level dry air advection is
forecast (700-500 mb RH down to 10% or less for northwest 2/3 of
the CWA by midday Saturday).

Likely owing to these above factors, despite some modest
instability and little capping evident, the simulated lightning
output from the 00z ECMWF depicts an absence of lightning flash
density across our area. Can`t rule out a short window with a ~15%
chance of a storm southeast of I-55 (ie. 1-3pm) if a strong
enough updraft can overcome dry air entrainment. Given the
distinct possibility of just showers with no storms ahead of the
front (if any showers develop), sufficient deep layer shear for
storm organization is less concerning at this time. All in all,
our preference for Saturday`s cold front is non NAM guidance,
which points to a primarily dry fro-pa for most of the area.
Temperatures will be pleasant for late August, in the upper 70s
for lower 80s, with modest humidity levels. Saturday night will be
clear and seasonably cool in the wake of the cold front.

The previously mentioned upper low will carve out deep mid-upper
level troughing near James Bay (south end of Hudson Bay) early
Sunday through early to mid next work week. 850 mb temps of +6 to
+9 Celsius through this time will set the stage for breezy west-
northwest winds Sunday-Monday, highs in the upper 60s to mid 70s,
dew points in the 40s and 50s, and overnight lows in the upper 40s
(a few localized mid 40s?) to lower 50s outside of Chicago! Some
of the guidance members with cooler temps aloft in recent forecast
cycles would entail even cooler highs than in the official
forecast. In addition, an early season lake effect rain (and even
thunderstorm) episode may unfold primarily to our east and
northeast over western Lower Michigan and north central Indiana
Sunday through Monday night. Some lake effect showers may sneak
into Porter County Indiana at times dependent upon low-level
flow/boundary layer convergence orientation shifting more
northerly.

Below normal, but comfortable/pleasant, temperatures should
persist through at least mid next week before readings return back
to near to slightly above seasonal norms heading into Labor Day
Weekend. Also importantly, primarily dry conditions should prevail
through next work week, giving more time to dry out from the
recent heavy rainfall.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1230 AM CDT Thu Aug 21 2025

No significant concerns through the current TAF period.

Aside from a chance RFD and DPA occasionally dip to MVFR VSBY
early this morning and patchy brief MVFR CIGs near the lake,
primarily VFR conditions are expected. Light northerly to
calm/VRB winds in the pre-dawn hours of this morning will
increase to around 10 kt from the northeast after sunrise this
morning and then speeds will diminish again with sunset.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 1 PM CDT this afternoon for
     ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement until 1 PM CDT this afternoon for
     INZ001-INZ002.

LM...Small Craft Advisory until 7 AM CDT this morning for the IL
     and IN nearshore waters.

&&

$$

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