455
FXUS63 KLOT 281648
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1148 AM CDT Thu Aug 28 2025

.KEY MESSAGES...

- Widely scattered showers and a isolated T-storms will end this
  morning, before another round this afternoon.

- Another period of dangerous rip currents and swimming
  conditions expected for Lake Michigan beaches late Thursday
  into Friday morning.

&&

.UPDATE...
Issued at 1148 AM CDT Thu Aug 28 2025

Forecast thinking remains similar in terms of shower development
near and behind the cold front this afternoon. Strongly suspect
that recent runs of the HRRR have been suffering from the
summer-season long bias of overly mixing out dew points,
resulting in little to no simulated shower activity. Leaned
toward the CAMs with a more realistic depiction of dew points
vs. current obs and expectations this afternoon as the front
spreads inland. For this reason, kept PoPs in the 20-35% range
(isolated to scattered showers).

Regarding the potential for any embedded thunderstorms, despite
the cold 500 mb temps for late August, as noted in the short
term discussion below, there are key limiting factors. Looking
at forecast soundings from the more favored CAM solutions this
morning, such as the 3km NAMnest, even these cast doubt on the
ability for updrafts to be sufficiently tall (cold) long enough
for electrification. Rising 500 mb heights (along with implying
weakening large scale forcing) will result in gradual warming
at that level, with more pronounced warming around 450 mb.
Already meager lapse rates farther down below 600 mb are
forecast to be further weakened by a developing inversion around
650 mb. Finally, strong mid-level dry air advection will
increase the likelihood of deleterious dry air entrainment for
incipient updrafts.

These additional limiting factors would likely make it tough
for sustained updrafts to reach much above 20kft AGL. Freezing
levels are not all that low (~12kft AGL), so unless tops of the
low-topped convection can persistently reach up around 30kft,
thunderstorm coverage may very well be isolated at most. As
such, for the gridded forecast, capped thunderstorm mention at
slight chance (vs. isolated to scattered shower coverage). The
caveat of a sneaky mini-supercell threat unfolding as detailed
in the discussion below still applies, but the chance of that
occurring seems rather low.

Aside from the above thinking on the convective forecast,
dangerous swimming conditions on southern Lake Michigan this
afternoon through Friday afternoon are on track. We did move up
the start time of the Beach Hazards Statement for Illinois
beaches to 1pm this afternoon to account for an earlier
deterioration than down at the Indiana beaches (4pm start there).

Castro

&&

.DISCUSSION...
Issued at 315 AM CDT Thu Aug 28 2025

Through Friday:

Scattered showers and a couple of isolated thunderstorms are
moving across the area early this morning in response shortwave
trough combined with warm air advection associated with a
modest westerly low level jet. This activity is expected to push
east of our CWA early this morning with a break in the precip
mid-late morning into the early afternoon along with some
partial clearing.

Cold front currently over the Upper Great Lakes will continue
pushing south into our northern CWA and near the lake early this
afternoon, likely clearing our southern CWA by early this
evening. The break in the precip and clearing should allow temps
to warm into the 70s ahead of the front midday, though temps
will fall a bit in the wake of the front, particularly closer
to the lake. Well inland, where the front isn`t expected to
arrive until late this afternoon or early this evening, highs
should be able to make a run at 80 degrees.

Dewpoints across the region early this morning are mostly in the
upper 50s with some lower 60s over eastern IA. Guidance is in
good agreement depicting an axis of dewpoints in the lower 60s,
if not mid 60s, pooling ahead of the front today. Mid level
temps are unseasonably cold, generally -13C to -15C, especially
across our eastern CWA. These cold mid level temps juxtaposed
over top the 60F+ dewpoints should result in SBCAPE of 500-1000
J/kg ahead of the front. Forecast wind shear profiles are
actually quite favorable for severe weather this afternoon,
with long straight hodographs. If any sustained deep convection
does occur this afternoon, wouldn`t be surprising to see some
supercellular structures and perhaps even a threat of some hail.
This would be contingent on there being thunderstorms this
afternoon and there are some limiting factors to discuss...

The biggest limiting factor today is the depth of the forcing
which will be pretty shallow with the cold front. In fact, the
primary 500mb shortwave trough axis is progged to pass south of
the area prior to the sfc cold front. Forecast soundings aren`t
showing much subsidence in the wake of the mid level trough,
but it is possible that there could be a "CAPE robber"
subsidence inversion ahead of the front even though guidance
isn`t depicting one. Even without that, the cold front will be
fairly shallow and likely moving at a good clip, so the front
may struggle to push parcels to the LFC or result in updrafts
quickly being undercut by the front. Given these limiting
factors, not planning to advertise any type of concerning
convective threat today, but today is one of those days to
monitor trends for the potential for a sneaky mini-supercell
threat with any sustained thunderstorms that do develop.

Brisk northerly winds down the full fetch of the lake will
resulting in building waves and hazardous swim conditions
developing again later this afternoon and continuing into at
least Friday morning. Current beach hazard statement runs
through late Friday afternoon, which could be a bit on the long
side, but not uncommon for waves to hold on longer than
forecast, so no changes planned going headlines.

- Izzi


Friday Night through Wednesday:

High pressure will remain nearly stationary across the eastern
Great Lakes over the weekend. This will allow for dry conditions
for the local area with a slow warming trend back toward normal
temps, though they`ll likely still be a few degrees below
normal. The dry conditions are likely to continue Monday into
Tuesday though there is a bit more uncertainty by Tuesday and
especially Tuesday night as a large upper trough and strong
cold front is expected to move across the area midweek. This
will bring the next chance of showers and perhaps a few
thunderstorms. Another cool airmass is than expected to settle
across the region for the end of next week. cms

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 637 AM CDT Thu Aug 28 2025

Forecast concerns include...

Showers ending this morning.
Chance of showers/isolated thunderstorm this afternoon.
Wind shift to northeast this afternoon.
Possible mvfr cigs this afternoon/evening.

Scattered showers will continue moving southeast and away from
the terminals over the next few hours. While an isolated
thunderstorm remains possible, expected coverage too low to
continue with thunder mention.

A cold front will move across the area this afternoon when there
will be another chance for showers and possibly an isolated
thunderstorm. Coverage this afternoon appears to be low and
current prob shower mention looks on track for now.

Southwest winds will slowly turn northwest by late morning and
may become north/northwest for a short time before shifting to
the northeast as the cold front moves across the area. Timing is
still a bit uncertain with the latest guidance suggesting the
wind shift could be as late as 20z-21z for ORD/MDW. Opted to
maintain the current 19z timing until trends emerge but some
timing tweaks are likely with later updates. Northeast winds
will then continue for the rest of the period.

Few/sct mvfr level clouds will be possible this morning but
there will be a better chance for mvfr cigs this afternoon into
this evening for the Chicago terminals after winds turn
northeast off the lake. Confidence is low for prevailing mvfr
cigs and have maintained scattered mention for now.

There will also be a chance of fog overnight into early Friday
morning, mainly west and south of the Chicago terminals,
possibly at RFD but confidence for fog is also low and no
mention with this forecast. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Friday afternoon for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement from 4 PM CDT this afternoon through
     Friday afternoon for INZ001-INZ002.

LM...Small Craft Advisory from 4 PM this afternoon to 1 PM CDT
     Friday for the IL nearshore waters.

     Small Craft Advisory from 4 PM this afternoon to 10 AM CDT
     Friday for the IN nearshore waters.

&&

$$

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