-- Discussion containing changed information from previous version are highlighted. --
614
FXUS64 KMRX 262325
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
725 PM EDT Thu Sep 26 2024

...New AVIATION...

.SHORT TERM...
-- Changed Discussion --
(This evening through Friday)
Issued at 333 PM EDT Thu Sep 26 2024

Key Messages:

1. Major, near record flooding possible on French Broad and Pigeon
rivers, with potential for life threatening flash flooding
elsewhere. Some uncertainty in the far south and southwest though.

2. Widespread high wind event likely to unfold tonight into Friday
morning. Expectations are that power outages, and downed trees will
be very prevalent across the area.

Discussion:

A fairly historic situation is about to unfold across the southern
Appalachian region over the next 24 hours as the remnants of
Hurricane Helene move through.

Synoptically speaking, Helene is rapidly gaining strength this
afternoon and is expected to make landfall late this evening
along the Florida coast. Afterwards, it will be quickly pulled
northward through a gap between an upper low over western TN and a
ridge over the western Atlantic. Ahead of the storm, tropical
moisture will continue to be funneled into the southern Appalachian
region, making for continued light to moderate rainfall across the
forecast area before the last slug of tropical downpours arrive with
the core of Helene late tonight into Friday morning.

Flooding along with high winds continue to be the two main concerns
with this event.

Winds:

The forward speed of this hurricane after landfall is expected to be
on the extreme upper end of the spectrum. As such, the wind field
will stay intact much further inland than normal. There remains
some slight uncertainty in the center track of this storm, but the
general consensus, along with the NHC track, brings the center
across the southern TN valley around and shortly after daybreak
tomorrow morning. Ahead of the system low level winds will turn
southeasterly across the mountains, with winds in the higher terrain
ramping up to warning levels by around 2 AM tonight. As the core of
the system approaches the southern TN valley, winds in the lower
elevations should ramp up considerably, especially down the TN
valley as pressure gradients rapidly tighten. Those areas should see
warning level winds arriving in the 5-7am time frame roughly.
Believe that there will be some widespread areas of 55-65 mph winds
in the TN valley, the plateau, and even areas up into our VA
counties. Most trees still have their leaves on them, and these
winds will occur with very saturated soils. As such, expect
widespread power outages to occur tonight and tomorrow. Went ahead
and upgraded the watch to a warning and now have our entire CWA is
now covered by a high wind warning. This excludes our NC counties
which are in a Tropical Storm Warning. I`m quite sure there will be
some locations that do not see winds close to these values. But I
think the likelihood of warning level gusts and resulting impacts
warrants the upgrade.

Rain & Flooding:

Increasing southeasterly winds across the Appalachians will result
in some rainshadowing effects and will cut down on additional
rainfall north of I-40 relative to the southern valley tonight into
Friday. However saturated soils mean that any additional rains will
go immediately to runoff. Couple this with the immense amount of
rains expected on the NC side of the border and some historic river
flooding is expected in area rivers. The Pigeon and French Broad
rivers in Newport will be very close to record crests with this
event and that`s not to speak of the flash flooding potential across
the area. The current forecast for the French Broad vier at
newport is 23`, which is one foot shy of the record crest set in
1867! So, this is certainly an impressive event to say the least.
Further south, additional rainfall amounts of 3-5" are expected in
the southern valley and southern plateau with the approach/passing
of the core of Helene. The uncertainty here is that those amounts
closely match the 3hr and 6hr flash flood guidance values. Will the
rainfall rates and amounts be enough on any time scale to overcome
that and cause runoff and flooding issues? I`m not entirely sure.
However, tropical events like this have a tendency to over perform,
so will keep those areas within the flood watch and continue to
advertise the possibility of flooding across the entirety of our
forecast area with the remainder of this event.
-- End Changed Discussion --

&&

.LONG TERM...
(Friday night through next Thursday)
Issued at 333 PM EDT Thu Sep 26 2024

Key Messages:

1. Unsettled weather will continue with seasonable temperatures
into early next week.

2. Dry weather appears to return late in the period on Wednesday and
Thursday.

Discussion:

Headed into Saturday, we will be notably on the downward trend from
impacts from Helene. Her center is forecast to be over the state
line of western Kentucky and Tennessee by that time. Southerly to
southeasterly flow will continue to bring moisture from her remnants
northward into our area. Although winds will be decreased across
lower elevation spots, gusty conditions can still be expected across
higher terrain of the southern Appalachians and Cumberland Plateau.
Wind gusts in the 40`s to near 50`s is still possible there. The
later we get into Saturday, the more the winds will diminish,
especially over the typical windier spots.

What`s left of Helene and a closed low aloft that will eventually
envelop her, will keep cloudy and rainy conditions around at times
through early next week. In the middle of the weekend, the closed
low that phased with Helene will absorb into the mean flow, become
elongated and then ripped off of the continent Monday into Tuesday.
Upper level ridging across the lower tier of the US will then expand
eastward, as well as surface high pressure from the High Plains that
will reach the western Ohio Valley by Wednesday, bringing dry
weather back to the area the last couple days of the long term
period. High temperatures will mostly remain steady Monday through
Thursday with upper 70s to low 80s for valley locale.

&&

.AVIATION...
-- Changed Discussion --
(00Z TAFS)
Issued at 722 PM EDT Thu Sep 26 2024

Flight conditions over the next 24 hours will be very poor across
the region with widespread rain, vcts, very strong winds, and LLWS
associated with the landfalling Hurricane Helene. IFR or LIFR
conditions will setup overnight and are forecast to persist as
the pressure gradient/winds increases and moderate to heavy rain
overspreads the area. LLWS will also be a risk across the region
with winds increasing at the surface by 12z Friday. Wind gusts on
Friday will likely be over 35kt at all sites with some isolated
gusts over 50 kt expected.
-- End Changed Discussion --

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             66  74  64  73 / 100 100  40  60
Knoxville McGhee Tyson Airport, TN  68  76  64  73 / 100 100  20  50
Oak Ridge, TN                       66  74  62  72 / 100 100  40  60
Tri Cities Airport, TN              67  78  60  76 / 100 100  10  30

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
NC...Flood Watch through Friday afternoon for Cherokee-Clay.

     Tropical Storm Warning for Cherokee-Clay.

TN...Flood Watch through Friday afternoon for Anderson-Bledsoe-Blount
     Smoky Mountains-Bradley-Campbell-Claiborne-Cocke Smoky
     Mountains-East Polk-Grainger-Hamblen-Hamilton-Hancock-
     Hawkins-Jefferson-Johnson-Knox-Loudon-Marion-McMinn-Meigs-
     Morgan-North Sevier-Northwest Blount-Northwest Carter-
     Northwest Cocke-Northwest Greene-Northwest Monroe-Rhea-
     Roane-Scott TN-Sequatchie-Sevier Smoky Mountains-Southeast
     Carter-Southeast Greene-Southeast Monroe-Sullivan-Unicoi-
     Union-Washington TN-West Polk.

     High Wind Warning until 8 PM EDT /7 PM CDT/ Friday for Anderson-
     Bledsoe-Blount Smoky Mountains-Bradley-Campbell-Claiborne-
     Cocke Smoky Mountains-East Polk-Grainger-Hamblen-Hamilton-
     Hancock-Hawkins-Jefferson-Johnson-Knox-Loudon-Marion-McMinn-
     Meigs-Morgan-North Sevier-Northwest Blount-Northwest Carter-
     Northwest Cocke-Northwest Greene-Northwest Monroe-Rhea-
     Roane-Scott TN-Sequatchie-Sevier Smoky Mountains-Southeast
     Carter-Southeast Greene-Southeast Monroe-Sullivan-Unicoi-
     Union-Washington TN-West Polk.

VA...Flood Watch through Friday afternoon for Lee-Russell-Scott VA-
     Washington VA-Wise.

     High Wind Warning until 8 PM EDT Friday for Lee-Russell-Scott VA-
     Washington VA-Wise.
-- End Changed Discussion --

&&

$$

SHORT TERM...CD
LONG TERM....KS
AVIATION...JB