650
FXUS63 KLOT 240836
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
336 AM CDT Tue Sep 24 2024

.KEY MESSAGES...

- Showers, drizzle, and even a couple of thunderstorms today,
  with some occasional bursts of heavy rain likely

- Lake effect showers could continue tonight into early Wed over
  NW IN

- Chances for showers this weekend into early next week, but
  confidence in specifics remains low at this time

&&

.DISCUSSION...
Issued at 336 AM CDT Tue Sep 24 2024

Through Wednesday:

Bottom line up front:

Waves of showers, possibly heavy at times, and a couple of
thunderstorms are expected today, especially this morning
over our western CWA, continuing into the afternoon eastern CWA.
In between showers, drizzly conditions are expected today,
especially this morning. System rain is expected to move out of
the CWA later this afternoon or very early this evening,
however lake effect showers threat is expected to continue over
northwest Indiana tonight and possibly into Wednesday morning.

Details:

Surprisingly tricky forecast today with still substantial spread
in model guidance in the short term with respect to
precipitation trends over the next 18 hours. Currently, a 1008
mb surface low is located just north of STL with an inverted
trough/weak warm front extending north from this low to
northeast IL and north across eastern Wisconsin. A secondary
warm front extends east from the low across central IL into
central Indiana.

Currently, a northeast-southwest axis of showers, and even a
couple of thunderstorms, extends from northeast Missouri to the
Rockford area. This activity seems to be tied to the elevated
low level baroclinic zone where moisture convergence is
maximized. Objective analysis suggests fairly modest elevated
instability with just a couple hundred J/kg of MUCAPE. Model
guidance is not handling this activity well and recent satellite
and radar trends have shown a slow increase in coverage and
intensity of this activity over the past few hours. The elevated
baroclinic zone should continue to shift slowly eastward this
morning and while guidance generally isn`t producing much
precip associated with it, it will be moving into equally if not
more moist/unstable air mass. Given this, seems reasonable that
this axis of showers/isolated storms should hold its own if not
show a bit of an uptick this morning as it moves east across
northern IL out ahead of the upper trough.

Seasonably high PWATs, rather deep warm cloud, and weak low
level instability near the 925-850 mb boundary should result in
strongest ascent below -10C, which all favor mostly warm rain
processes and the potential for particularly efficient rainfall
rates from rather benign looking convection on radar.

Additional convection is forming near/ahead of the sfc low
closer to the surface warm front(s). Model soundings and
modified 00z KILX sounding suggests that this convection is
likely rooted in the boundary layer with a few hundred J/kg of
MLCAPE in objective analysis ahead of this low across central
IL. This modestly unstable air mass should continue northeast in
conjunction with the upper trough and sfc low overspreading the
southeast 1/3rd or so of our CWA early this morning. Stronger
synoptic low level wind fields will remain east of our CWA, but
given the very low LCLs and with LFCs just barely off the
surface, it isn`t out of the question that there could be a
brief weak tornado/landspout threat this morning in our SE CWA
with any more robust storms that are able to ingest the low
level vorticity maximized near the warm front and just ahead of
the sfc low.

Between the slowly eastward moving axis of showers/isolated
storms over our western CWA and the expected increase in
convection near/ahead of the sfc low, we should see a couple
swaths of widespread beneficial rainfall this morning. The
convection near/ahead of the surface low could expand in
coverage developing north-northeast along the inverted
trough/warm front early this morning.

Given the generally favorable set-up for efficient heavy rain
producing showers and storms, there is certainly a potential for
some isolated 1 to maybe 2 inch rainfall totals this morning
into early this afternoon. Outside of the heavily urbanized
Chicago metro corridor, the recent drought-like conditions
should limit the threat of flooding with most areas able to
handle a 1-2" rainfall over a couple-few hour period. If one of
the localized heavier rainfall areas sets up over Chicago metro
area, this could lead to a urban flooding threat this morning.

Final wild card is the potential for the lake to get involved.
Most short range guidance suggests that this inverted surface
trough will move slowly east this morning and back out over the
lake. Any potential lake enhancement to convection and rainfall
should be near and east of this surface trough, so close meso-
analysis this morning will be need to monitor the evolution of
this feature. If it remains near/over the metro area, it is
possible that could become a focus for quasi-stationary
moderate to heavy rainfall and increase the flood threat. If the
trough moves east over the lake early this morning, as a lot of
the short range guidance suggests, then that would lessen,
though not completely eliminate a Chicago area urban flooding
risk. By late morning/midday, the surface low should move east
of Chicago`s longitude pulling the trough east of Chicago with
it.

Thermodynamically, lake induced instability is increasing now
and should be sufficient for lake effect/lake enhancement to the
rain/convection prior to sunrise. The thermodynamics will remain
favorable for lake enhancement to the convection today, tonight
and possibly into early Wednesday. Behind the inverted trough,
the lake effect threat will focus over the lake and potentially
into northwest Indiana this afternoon, tonight, and possibly
into Wednesday morning as prevailing flow should push convection
into Indiana and keep it east of Chicago.

Normalized lake induced CAPE values tonight into Wednesday
morning are quite low, with extremely skinny, but potentially
fairly deep convective layer. Given the very moist environment
in place, cannot rule out a locally heavy rainfall threat into
northwest Indiana tonight into Wednesday morning if lake effect
convection is on the more robust side. Given the very low
normalized CAPE, the margin or error between enough instability
for robust LE convection and little if any LE convection is
razor thin. There is a very real possibility that lake induced
CAPE is just a tiny bit weaker than progged and that could
lessen the LE threat dramatically. The extremely large QPF
values from lake effect that the HRRR is producing over
northwest Indiana seems pretty unlikely, with the more likely
scenario being weak/minimal convection tonight and Wednesday.
Nevertheless, it will be something we will be watching closely
today.

While the lake effect/enhanced shower threat should be east of
Chicago this afternoon, upper trough taking on a more negative
tilt could result in showers lingering across northeastern IL
well into the afternoon.

Summary:

Rain should taper off/end over western CWA later this morning
and eastern CWA later this afternoon or early this evening. The
exception will be the potential for lake effect showers
continuing into northwest Indiana tonight into Wednesday
morning. Otherwise, Wednesday should feature decreasing
cloudiness and very mild temps.

- Izzi


Wednesday Night through Monday:

A complex evolution to the synoptic weather pattern continues
to limit forecast confidence for the long term forecast period.
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 00Z 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 entirely just yet,
but did end up lowering them a bit 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.

Ogorek

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 125 AM CDT Tue Sep 24 2024

Key Messages:

- Rain showers and drizzle at times through the TAF period.

- Ceilings as low as LIFR and visibilities as low as IFR
  expected at times through this afternoon.

A baggy low pressure system centered near KSTL at press time
will continue to lift northeastward throughout the day today and
bring periodic showers through the area today. During the
overnight hours, drizzle may be observed at times and produce
IFR/MVFR visibilities and LIFR/IFR ceilings. Towards and
especially after daybreak, precipitation should become more of a
true rain, with the potential for a few-hour long period of
steady rainfall to produce visibility and ceiling reductions
that will be comparable to what the overnight drizzle has been
producing. RFD may be far enough removed to the northwest to
avoid seeing the more impactful ceiling and visibility
reductions that are expected in the Chicago metro, but could
still nevertheless experience MVFR conditions at times.

A few lightning strikes could not be ruled out at/near the
Chicago metro terminals during the daytime today, but it
appears now that the better chances for thunderstorms will
remain southeast of the terminals. Most of the rain should come
to an end by this evening with visibilities and ceilings likely
to improve by this time as well, but at least isolated showers
and MVFR ceilings will likely linger around in the vicinity of
Lake Michigan Tuesday night into Wednesday morning.

The wind direction forecast for today is a bit tricky with the
low pressure center passing by and the expected rain, but
guidance has generally trended towards winds turning northerly
and northwesterly a little earlier compared to previous model
runs, so have adjusted the wind directions in the TAFs a bit to
account for these trends.

Ogorek

&&

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

&&

$$

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