961
FXUS63 KIWX 241040
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
640 AM EDT Tue Sep 24 2024

.KEY MESSAGES...

- Numerous showers and a chance of thunderstorms today.

- There is a marginal risk of severe thunderstorms for a small
  portion of the forecast area. A tornado is possible.

- Additional rain is expected late this week.

&&

.DISCUSSION...
Issued at 321 AM EDT Tue Sep 24 2024

As a surface low moves through Indiana, severe thunderstorms capable
of producing a tornado are possible this afternoon. The area of
greatest interest is from roughly Wabash, to Fort Wayne, to Ottawa;
South to Portland and west to Marion. Forecast soundings, including
ensembles, depict a weak cap this morning that quickly mixes out
prior to noon. Outstanding directional shear in the low-levels
exists in the vicinity of the low`s triple point, paired with low
LCLs near 500m. Bulk shear maxes out near 25-30 knots paired with
modest instability to start, but perhaps ballooning toward 1500
j/kg. The potential failure mode here could be one of two things: 1)
lapse rates today are overall rather weak at less than 7C/km (thus,
instability is marginal) and 2) the best directional wind shear
could occur prior to this afternoon`s destabilization, thus the
tornado risk would be notably less with subsequent thunderstorms.
This is somewhat depicted in marginal helicity tracks within the
CAMs guidance. In lieu of the tornado risk, a few forecast soundings
depict low-level lapse rates approaching 7.5 C/km as afternoon
temperatures warm into the mid-70s, which presents a non-zero risk
of damaging wind gusts and perhaps more likely, small hail. Overall
confidence in severe weather occurring is medium.

Elsewhere, a nice steady rain (drizzle) is falling over a large
portion of the forecast area this morning. This is ahead of a broad
40-knot 500-mb jet over the Tennessee and Ohio river valleys. As the
trough aloft deepens this morning, this upper-level wind field will
increase, enhancing lift and promoting additional shower
development. As destabilization occurs within this dynamic
environment the chance of thunderstorms increases through the
afternoon (see preceding paragraph).

While the synoptic rain shield departs early Wednesday night, lake
enhanced rain showers are anticipated tonight amid modest delta-Ts.
Low-level wind quickly veers prior to daybreak Thursday putting and
end to any lake effect. Patchy fog is anticipated overnight as well.

For the end of the week, all eyes will be on Tropical Cyclone 9 as
it impacts the Gulf Coast, drifting up over the southern
Appalachians. Guidance continues to advertise this cyclone wrapping
around an upper-level low over the Southern Mississippi River
valley. Closer to home, lingering high pressure just northeast of
the Great Lakes delays the overall shower chance here until the
weekend when a strong ridge over the Four Corners steers this low to
the Midwest. The in-house blend significantly increased POPs Thursday
and Friday. This was out of line with the latest model runs and
seems to ignore the aformentioned Great Lakes high pressure.
Therefore, I leaned closer to the existing (i.e.previous) forecast
for that time period.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 640 AM EDT Tue Sep 24 2024

Surface low pressure is approaching the area with abundant
moisture convergence yielding IFR/LIFR ceilings and light rain.
Ceilings will slowly improve through the morning as diurnal
mixing increases. Return to VFR likely at KFWA with surface low
passing just to the northwest allowing for better mixing.
Thunder is possible (mainly at KFWA during the late afternoon)
but confidence in timing and coverage is still too low to
include in the TAF. Return to IFR (or at least low-end MVFR)
likely again Tuesday night with minor CAA and plenty of residual
boundary layer moisture/cyclonic flow in the wake of departing
low.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...None.

&&

$$

DISCUSSION...Brown
AVIATION...AGD