078
FXUS63 KLOT 242050 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
350 PM CDT Tue Sep 24 2024

.KEY MESSAGES...

- Scattered heavier showers near the lake into early evening,
  though threat for localized urban flooding has diminished.

- Patchy fog may develop tonight and become locally dense into
  early Wednesday.

- Chances for periodic showers return Friday night into early
  next week, but confidence in specifics remains low.

&&

.DISCUSSION...
Issued at 300 PM CDT Tue Sep 24 2024

Through Wednesday Night:

A partially lake-induced or enhanced MCV earlier resulted in
focused pockets of heavy rain and localized flooding into
portions of the Chicago metro through the early afternoon. With
efficient warm-rain processes ongoing, the lingering shower
activity focused east of the I-355 corridor will continue to
produce occasional downpours over the next couple of hours. With
this being said, thankfully, the feared more pronounced lake
induced meso-low developed over Lake Michigan, which will in all
likelihood keep any additional torrential rates safely off to
the east.

Lake effect showers should eventually get going tonight into
Wednesday morning for portions of northwest Indiana (possibly
brushing the Chicago shore), though we remain skeptical of the
more bullish HRRR depictions and suspect it`s overdoing
instability. The official forecast held onto chance (~30%) PoPs
near the lake in Porter County given the current forecast wind
directions and orientation of lake enhanced convergence. Spotty
shower activity could then expand inland into northwest Indiana
Wednesday before dissipating by or during the afternoon.

Aside from the precip. trends, have some concern for fog
development overnight, especially where clouds are able to
sufficiently clear (favoring interior northern IL). It isn`t a
very favorable fog setup due to moderate northerly flow aloft
above the inversion. However, given the return of rain over the
past few days and light surface winds tonight, will need to
monitor for patchy ground fog development, some of which may be
locally dense. Any fog should erode quickly Wednesday morning.

The rest of Wednesday and Wednesday night will be quiet, with
seasonably mild temperatures (low-mid 70s highs) Wednesday
afternoon and a relatively cool Wednesday night. Expecting dew
points to mix out Wednesday afternoon, and with light/calm winds
and clear skies Wednesday night, this should set up favorable
radiational cooling conditions. Forecast lows are in the upper
40s to lower 50s outside of Chicago and mid 50s to around 60F in
and near the city. Some concern exists for fog development
Wednesday night as well, though will let the midnight shift take
a look at this potential.

Castro

Thursday through Tuesday:

A complex evolution to the synoptic weather pattern continues
to limit forecast confidence for the long term forecast period,
although some trends and clustering across the guidance/ensemble
suite have been noted today. The latest multi-model consensus seems
to suggest that much--if not all of--our forecast area may see
little/no precipitation through at least into Friday evening.

By Thursday, a trough will have been pinched off from the jet
stream and become a closed upper-level low centered over or just
east of the Ozarks. To its north, the axis of a high-amplitude
ridge will be encroaching upon the western Great Lakes, while to
its southeast, soon-to-be Hurricane Helene will be on final
approach to making landfall along the Florida Gulf Coast. With
the influence of the ridge in our region on Thursday, we will,
in all likelihood, be treated to dry conditions and near to
slightly above normal temperatures with highs in the mid to
perhaps upper 70s (coolest near the lake).

After making landfall, Helene will encounter the upper low and
likely engage in a Fujiwhara-like interaction that will see the
tropical cyclone rapidly get torn apart with its remnants then
getting slung anticyclonically around the low and eventually
absorbed into the low itself. While there is still quite a bit
of spread exhibited in the latest suite of deterministic and
ensemble forecast guidance regarding exactly how this interaction
will play out, there seems to be growing support for most or
all of our forecast area remaining dry on Friday due to either
1.) Helene having a slower forward motion and making landfall
later and/or 2.) the continued presence of the ridge suppressing
precipitation associated with both cyclones to the south.

There`s enough of a QPF signal in the Grand Ensemble to warrant
not yanking the NBM`s PoPs for Friday in our far southern CWA
just yet, but did end up lowering them once again. Regardless
of exactly how Helene interacts with the upper low, tightening
pressure fields in the vicinity of these two interacting systems
will yield a period of breezy northeasterly winds Friday into
Saturday that will induce high wave action on Lake Michigan,
leading to dangerous conditions along the lakeshore.

Saturday onwards remains a fairly big question mark with
respect to precipitation owing to dependencies pertaining to the
aforementioned synoptic-scale interaction. In one possible
scenario, we could see rain showers on Saturday as Helene`s
remnants get whipped from east to west, then see daily periodic
rain showers heading into next week as the upper low continues
to meander around nearby.

Alternatively, if Helene and the upper low meet up farther to
the east, or if the upper-level ridge ends up being stronger
than expected and continues to keep everything suppressed to the
south, then the upper low could remain bottled up well to our
south or southeast through early next week, keeping its
associated precipitation away from our forecast area.

There are several other possible "middle ground" outcomes as
well, but with how difficult it is to model such a complex
atmospheric interaction, it will still probably take at least
another day or two for us to gain some more clarity as to
whether or when it will actually rain this weekend into early
next week. For now, have continued to leave the NBM`s broad-
brushed slight chance and chance PoPs for this time frame
largely untouched. A fairly strong cold front passage pegged for
Monday night into Tuesday may then bring a true taste of
autumnal temperatures into mid next week.

Carlaw/Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 110 PM CDT Tue Sep 24 2024

The aviation weather concerns for the 18Z TAF period include:

* Continued showers into this afternoon with associated cig and
  vsby reductions

* MVFR cigs expected into this evening and possibly through
  tonight

* Fog possible overnight into early Wednesday, especially
  outside of Chicago

* Spotty lake effect showers tonight into the day on Wednesday,
  primarily over northwest IN

Showers, at times heavy, will continue to fall on Chicagoland
through mid-late afternoon before the rain pulls away to the
east. Through then, IFR cigs and vsbys will remain possible with
conditions likely to bounce around between categories like they
have been for a few hours now. Confidence is rather low on cig
and vsby trends this evening into tomorrow, but it looks as
though MVFR cigs will hang around through the better part of
tonight, if not stretching into Wednesday morning. Areas of fog
are also expected to develop overnight. Fog is mainly a concern
outside of the city but could certainly make its way over the
Chicago sites as well.

Additionally, pockets of lake-effect showers are expected to
develop tonight and continue into tomorrow. Moving north-
south, these showers will primarily impact GYY, but a shower or
two could fall on/near the Chicago sites as well, especially
during the night.

Meanwhile, predominantly NNW winds should largely remain below
10 kt through the rest of today and will go light tonight.
Wednesday will begin with more NNWs below 10 kt with a shift
over to NE expected sometime around mid afternoon.

Doom

&&

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

&&

$$

Visit us at weather.gov/chicago