210
FXUS63 KIND 261745
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
145 PM 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

&&

.FORECAST UPDATE...
Issued at 1013 AM EDT Thu Sep 26 2024

The forecast is largely on track for the next 6 to 12 hours across
Central Indiana with rather benign conditions expected in the near
term and rapidly deteriorating conditions overnight tonight.

Surface high pressure is currently centered over the Great Lakes
with a stacked mid and upper level low centered near Paducah, KY. An
inverted low level trough will continue to develop and strengthen
across the SE US with its axis stretching northward toward the OH/IN
border. This inverted trough will continue to strengthen and slowly
pivot north and east as Hurricane Helene tracks northward over the
next day. Latest regional ACARs soundings indicate a fair amount of
moisture in the lower levels with dry air aloft above a subsidence
inversion around 3km agl. Frontogenetical forcing increasing
throughout the day as the inverted trough and associated energy
approaches from the southeast this afternoon. Potential is there for
scattered showers to develop for portions of Eastern and Southern
Indiana this afternoon and evening with rainfall increasing into the
overnight hours. Along and north of the I-70 corridor, and points
northwest, expect a dryer forecast as high pressure and drier air
will have more of an influence on surface conditions in those areas.
Overall, by this evening, expect increasing clouds from the
southeast in most areas.

Currently watching the potential for strong to high winds Friday
associated with the remnants of Hurricane Helene approaching the
region. Will be updating the forecast later today to reflect the
high wind potential as headlines may be needed as confidence
increasing.

&&

.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 (18Z TAF Issuance)...
Issued at 144 PM EDT Thu Sep 26 2024

Impacts:

- Increasing impacts from Helene beginning late tonight
- NE winds steadily increasing, ceilings deteriorating, and showers
overspreading the area from southeast to northwest 06-12z
- Period of LLWS mid afternoon tomorrow into tomorrow evening

Discussion:

VFR conditions are expected to persist through the rest of this
evening; however expect deteriorating conditions overnight tonight
and into tomorrow as the remnants of Hurricane Helene approach from
the southeast.

Expect NE winds to steadily increasing overnight tonight and through
the day tomorrow. Wind gusts of 30-40kts expected around and after
15z. Watching the potential for gusts of 45-50kts+ 20z-00z tomorrow
evening as a strong 50-75kt low level jet pushes in overhead from
the southeast.

VFR cigs will continue through the late evening hours; however
expect cigs to fall into the MVFR and then IFR categories by the mid
morning hours as rainfall spreads over the region. Rain will begin to
overspread the area from the southeast tonight, reaching BMG first,
then IND/HUF and then LAF last. Vis will be deteriorating as well,
possibly under 3sm at times Friday. Lowest cigs and vis more likely
at KBMG.

&&

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

$$

UPDATE...CM
SHORT TERM...Nield
LONG TERM...MK
AVIATION...CM