577
FXUS63 KIND 260701
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
301 AM EDT Thu Sep 26 2024

.KEY MESSAGES...

- Strong wind gusts of 40-45 MPH likely with locally higher gusts
  possible Friday into Friday evening with the remnants of Helene

- Widespread beneficial rainfall expected tonight into the weekend,
  also associated with Helene and a large upper low in the region

&&

.SHORT TERM (Today and tonight)...
Issued at 301 AM EDT Thu Sep 26 2024

The earliest effects of Hurricane Helene will begin to be felt
across central Indiana later today into tonight, as the potent
tropical cyclone makes landfall along the Big Bend Coast of Florida
this evening and rapidly proceeds inland toward north Georgia and
the Tennessee Valley by daybreak Friday.

As the large closed upper low to our southwest begins a Fujiwhara
interaction with Helene during the course of the day into tonight,
it will retrograde, allowing the warm conveyor belt to our east,
well-visualized on GOES satellite imagery, to also retrograde back
to the west, bringing back with it deeper moisture and more
expansive cloud cover as the day moves into evening, though
scattered to broken convective stratocumulus is likely to develop
during the daytime hours ahead/west of this deeper expansion of
cloud.

Some weak isentropic upglide arrives over the far southern portions
of the area late this afternoon, and a few showers may be able to
develop and sneak into the area, though suspect that activity will
largely hold off until the late evening and overnight as much
stronger forcing and richer moisture push into the area from the
south/southeast ahead of Helene.

Cloud cover today and increasing deep moisture/precipitation tonight
will limit diurnal range of temperatures, and have made adjustments
to blend output to this effect. Expect highs today per expected
cloud cover and low level thicknesses slightly warmer than
persistence, from the mid to upper 70s in an east to west gradient,
and lows tonight in the low to mid 60s from northwest to southeast,
well-supported by model wetbulb progs with precipitation
overspreading the area.

A somewhat more pressing impact from the Helene remnants will be
increasing winds late tonight and beyond as the pressure gradient
strengthens rapidly. While the vast majority of this (potentially
significant) impact will hold off into the day Friday into Friday
evening, gusts approaching 20-25 MPH can be expected at times this
afternoon into this evening, with gusts in excess of 25 to perhaps
30 MPH likely late tonight.

&&

.LONG TERM (Friday through Wednesday)...
Issued at 301 AM EDT Thu Sep 26 2024

The main concern with the long term will continue to revolve around
the remnants of Helene and a cutoff upper low initially to the west
of the remnants track and resulting QPF and winds.

Models initially in good agreement that the remnants of Helene will
be near Chattanooga 12z Friday while the upper low will be over the
the Mid South. Meanwhile, a wavy front stalled front will extend
from the center of Helene to northeast across the Appalachians. The
upper low and remnants will mimic the Fujiwhara effect as the
remnant low center moves into western Kentucky and the upper low
circles around it and eventually consumes it somewhere around
western Kentucky Friday night. Meanwhile, a baroclinic zone will
setup near the Ohio River. Latest indications are with copious
amounts of tropical moisture, good synoptic lift and potential
banding due to the baroclinic zone, widespread rain will lift
northeast to likely somewhere around the I-70 corridor Friday
afternoon and night perhaps a bit earlier. WPC continues to forecast
1 to 2 inches of rain from I-70 and south and perhaps higher amounts
over our southern tier. The area near and to the south of I-70 is in
the DAY3 WPC Marginal Risk for Excessive Rainfall while our
southern tier or two is in the Slight Risk for Excessive Rainfall.
This matches up nicely with the DESI LREF grand ensemble 25th to
75th percentile spread QPF through 8 AM Saturday. Although the
following is very unlikely, the worst scenario in this case would be
4 to 7+ inches over our forecast area per the max LREF grand
ensemble. If that occurred, flash flooding, potential even life
threatening would be a huge issue over south central Indiana. The
more likely outcome would be for perhaps some localized minor
flooding over south central Indiana where the stronger bands occur.
For the most part, we expect this to be a beneficial rainfall event
in terms of partially mitigating the ongoing drought conditions.

If the path of the remnants is captured reasonable well by NHC, a
few brief spin up tornadoes can not be ruled out over south central
Indiana Friday afternoon and night. Otherwise, a tight pressure
gradient, strong 55+ knot low level jet and soundings suggest winds
could gust to borderline or briefly higher wind advisory criteria
40+ knots Friday afternoon and early evening. Weak low level lapse
rates would be one argument against the higher winds completely
mixing down. We would have one more run before having the confidence
to pull the trigger on a headline though. For now, will go with an
SPS and enhanced wording in the HWO and Weather Story. We did raise
both surface winds and gusts from the NBM to match up. Thanks for
the coordination adjacent offices!

The upper low will eventually exit northeastward into the
Appalachians late this weekend or early next week with its track
highly defendant on how strong the broad ridge from the Rockies to
the Great Lakes turns out to be. Deterministic solutions start to
differ a lot around this time, addition of ensembles is usually the
best way to go. This supports the NBM holding on to small PoPs
through early next week.

Diurnal temperatures will be on the low side of the spectrum based
on all the projected cloud cover. Good confidence in highs mostly in
the 70s and lows mostly in the upper 50s and 60s. The next big
change in temperatures will be toward the middle of next as a cold
front moves in in the wake of the upper low.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 105 AM EDT Thu Sep 26 2024

Impacts:

- Increasing winds Thursday afternoon and night as Helene begins to
  influence the area late in the period

Discussion:

Light northeasterly winds and primarily a small amount of mid and
high cloud can be expected throughout the night. As the day wears on
Thursday into Thursday night, winds and cloud cover will gradually
increase as landfalling Hurricane Helene rapidly pushes northward
toward the region and the pressure gradient tightens. Periodic gusts
Thursday afternoon and evening in the 17-20KT range can be expected,
potentially increasing further beyond the end of the TAF period.
Winds will veer slightly during the period to a more
east/northeasterly direction.

Precipitation and restricted ceilings associated with the Helene
remnants should hold off until very late or beyond the end of the
current TAF period.

&&

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

&&

$$

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