638
FXUS63 KIND 250656
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
256 AM EDT Wed Sep 25 2024

.KEY MESSAGES...

- Cool and mostly cloudy with isolated sprinkles or showers possible
  today

- Seasonable temperatures for much of this week with highs in the
  70s and lows mostly in the mid 50s to mid 60s

- Potential widespread beneficial rain late this week and weekend
  from the combination of a cutoff upper low and the remnants of
  Helene

&&

.SHORT TERM (Today and tonight)...
Issued at 256 AM EDT Wed Sep 25 2024

A large cutoff upper low will persist over the mid Mississippi and
lower Ohio Valleys today and tonight, leading to a cool and somewhat
dreary early Autumn day, with plentiful cloud cover.

Forecast soundings depict up to 50-100 J/kg of shallow upright
instability may be present near peak heating this afternoon, though
with subsidence aloft as high pressure nudges into the region from
the northwest. Suspect that a few very shallow showers may form,
producing relatively light rain or sprinkles. Will include a blanket
slight chance PoP, maximized at and just after peak heating, but
cannot justify much more than this.

The vast majority of any shower activity will be convectively-
driven, and thus the overnight hours should be dry tonight, though a
few sprinkles or patchy drizzle wouldn`t be entirely out of the
question. Measurable precipitation is unlikely however, and do not
anticipate a mentionable PoP being required.

High temperatures today and diurnal range tonight will be limited by
persistent cloud cover underneath the upper low, and made some
adjustments to blend numbers to account for this. That said, some
dry advection and breaks in cloud cover tonight may allow for
slightly cooler readings, primarily north and west of Indy metro.
Today will be one of the cooler days in what looks to be a fairly
mild week overall.

&&

.LONG TERM (Thursday through Tuesday)...
Issued at 256 AM EDT Wed Sep 25 2024

The combination of a nearby cutoff upper low and the remnants of
Helene are expected to lead to beneficial rainfall for especially
south central parts of central Indiana late this week and weekend.

Tropical Storm Helene can clearly be seen strengthening off the
coast of the Yucatan Peninsula when examining the GOES-16 IR channel
and this is backed up strongly by NHC. Helene will initially move
northwest and into the very warm Gulf of Mexico. This should result
in Helene quickly strengthening into a Hurricane and eventually a
major Hurricane. Meanwhile, an upper low can be clearly seen in H20
vapor imagery over northeastern Missouri. Models in general
agreement this feature will move south into the Ozarks and
eventually gobble up the the remnants of Helene somewhere near
southwestern Indiana late Friday and Friday night. As this occurs a
baroclinic boundary will set up somewhere nearby, perhaps over
southern Indiana and combined with deep moisture per model
soundings, result in widespread rain with the best combo of banding
expected over this area. Currently, the NHC supports 1-2 inch and
higher total amounts over this area with the best chances Friday and
Friday night. This would be be a big help in mitigating current
drought conditions while also keeping the biggest risk of flash
flooding well to the south. We want to stress though, that potential
future changes in the forecast track will also result in changes to
the precipitation forecast, and Helene is still well away from even
the southeastern coast.

As is most often the case with the remnants of tropical
systems, weak lapse rates and lack of instability will mean very
little lightning.

In addition to the rain, tightening surface pressure gradient would
support gusty winds to possibly 30+ knots at least Friday and Friday
night. In addition, could see remnant brief spin ups, if the track
mirrors or is at least close to the current NHC forecast probability
track.

Later in the weekend, the deterministic models starting diverging
regarding the eventual evolution of the upper low. This is related
to an expansive upper ridge, that will extend from the desert
southwest to the Great Lakes. A stronger ridge, like in the case of
the 00z GFS, would weaken the upper low and send it quicker
southeastward than the 12z ECMWF, which would take the remnant
showers with it earlier. This generally favors adding more weight to
the ensembles later in the weekend and next week. With confidence
not great on the solution, will not make any big changes to the long
term and keep at least low PoPs into next week.

With such a moist column, extensive cloud cover should limit
afternoon temperatures to the 70s and also not promote large diurnal
temperature swings. So, look for overnight lows mostly in the 60s
through at least the weekend.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 1256 AM EDT Wed Sep 25 2024

Impacts:

- Fairly widespread MVFR conditions with spotty IFR possible through
  daybreak

Discussion:

Lingering low level moisture underneath a stagnant upper low will
allow for persistence or redevelopment of low clouds overnight, with
widespread MVFR expected and pockets of IFR possible. Ceilings are
the primary concern, though areas of fog will be possible,
particularly under any breaks in the cloud cover.

VFR conditions will return across the area as mixing ramps up
Wednesday morning, though at least a broken low end VFR cloud deck
should persist.

Winds will generally be light throughout the period, variable at
times, but generally shifting from west/northwesterly to more
northerly through the period.

&&

.IND WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Nield
LONG TERM...MK
AVIATION...Nield