328
FXUS63 KLMK 261054
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
654 AM EDT Thu Sep 26 2024

...Updated Aviation Discussion...

.KEY MESSAGES...

*  Widespread moderate to heavy rain and windy conditions on Friday
   as the remnants of Hurricane Helene move through the Lower Ohio
   Valley.

*  Potential for 2 to 4 inches of rain from late Thursday night to
   early Saturday morning. Locally higher amounts are possible.

*  Wind gusts of 35 to 45 mph are possible during the day Friday as
   the center of Helene works into KY and the Ohio Valley.

&&

.SHORT TERM /THROUGH THIS AFTERNOON/...
Issued at 340 AM EDT Thu Sep 26 2024

Synopsis...Cut-off upper low will slowly move south across the Ohio
Valley and meander over the western half of Tennessee Valley during
the next 24 hours while upper ridging consolidates virtually across
the rest of the CONUS. This stagnant pattern will allow Hurricane
Helen to quickly move northward to make landfall later today under
the increasing influence of the upper low to then become absorb by
it. The increasing tropical moisture influx along with typical post-
tropical transition dynamics will account for surging rainfall
chances and strengthening wind gusts with worst impacts in our area
expected on Friday and onwards (see Long Term section below).

Today...Current surface analysis depicts a weakening frontal zone
oriented north to south across eastern Kentucky. Radar and satellite
imagery show shower activity well ahead of the front leaving central
Kentucky and southern Indiana in the cool side of the boundary
characterized by dry conditions, calm surface winds, and building
low-level clouds/fog. As a result, expect reduced visibilities
during the early morning commute along I-75 and southern KY highways
with patches of dense fog close to river valleys and protected
areas. Any fog/visibility issues should burn off by midmorning as
daytime heating mixes the BL and surface winds slowly increase from
the northeast with intermittent gusts of 15 mph or more during the
afternoon. Rain chances will augment from east the second half of
the day as the leading edge of the moisture influx clashes with the
weakening front and starts to wrap around the upper low circulation.
Considering PW values and synoptic lifting mechanisms, nature of
today`s rainfall should be mostly light with a isolated chances of
moderate rainfall along axes of stronger mesoscale/localized
lifting.

Tonight...Expanding moisture advection associated with the
approaching 850-mb jet will take PW values over the 2-inch mark with
surface winds gusting around 20 mph or more. Therefore, heavier and
more persistent rain chances are reflected with perhaps a low chance
of elevated thunder with the strongest cores. However, no flooding
or severe concerns are anticipated at the moment.

&&

.LONG TERM /TONIGHT THROUGH WEDNESDAY/...
Issued at 340 AM EDT Thu Sep 26 2024

Friday - Sunday Night...

The center of Helene is expected to travel from northern GA to
western KY through Friday, with a transition to an extratropical
state as it does so. Steady weakening is also expected to occur
during this time, although current data supports a low pressure
still sub 990 mb as it crosses into KY. That is still an anomalously
strong low pressure system, and it will bring impacts to our region,
most notably on Friday.

Widespread rainfall is expected to be ongoing by sunrise Friday,
along with increasingly gusty winds. Precipitable water (PWAT) values
are expected to rise to the 2-2.25" range as the core of the system
moves into the region, and not surprisingly, forecast soundings show
a saturated and deep warm cloud layer. This will result in efficient
rainfall processes through the day. Will also note that an extremely
strong wind profile will also accompany this system, and low level
helicity values will be very high. Looks like we will be lacking in
the instability department, however at least have to be wary of
potential for a brief spin-up tornado. CSU machine learning does
show some low probabilities for tornadic activity 12z Friday to 12z
Saturday.

The more widespread concern with regard to the wind profile will be
with how much wind energy aloft will make it to the surface?
Wouldn`t have to mix very deep to get into a 50 knot layer of wind.
While clouds and rain should limit that potential to mix very deep,
do see some potential in some of the data. For instance, models are
fairly consistently showing a "QPF shadow" on the NW side of the
Appalachian spine through the morning hours of Friday that could be
the result of dry air being entrained into the weakening system,
and/or some downslope warming/drying of the low level airmass off
the Appalachians. The low level jet does look to be oriented
perfectly perpendicular at that time, and have seen this a time or
two in the past with this type of setup. Should that occur, we could
see some clearing across eastern/northeastern portions of our CWA
through midday or so. Something to watch for that could result in
higher gust potential in those area. Areas across our southern and
western CWA are expecting more rain/cloud cover so thing the gust
potential is a little lower there, although that area will be closer
to the low pressure center. Right now, like the idea of messaging 30
to 40 mph with some locally higher gusts, but let`s keep that
potential drying and any associated clearing potential in the back
of our minds through midday Friday. We`ll be pretty close on Wind
Advisory criteria, and the main focus in the long term will be
Helene and how it impacts our region for the end of the week and
into the weekend. Main impacts continue to be the potential rainfall
as well as strong gusty winds associated with the system as it moves
into the Ohio Valley.

As of the 11am EDT update, Helene had become a category 1 hurricane
with the National Hurricane Center forecast anticipating Helene to
rapidly intensify into a potential category 4 hurricane over the
next 36 to 48 hours before making landfall along the FL Panhandle
Thursday evening. The current NHC track for Helene appears to be a
bit further west and south compared to the 12z run of the
deterministic models, which brings the center of the storm northward
through GA then more into eastern KY before hooking it to the west
and meandering over the area into the weekend as it weakens further.

Our first impacts from Helene will be felt Thursday night into early
Friday morning as moderate to potentially heavy rain work in from
the south as the system quickly works through GA as it slowly
weakens. Rainfall amounts could range from a quarter of an inch near
and just south of the Parkways to nearly an inch along the KY/TN
border. Rain will continue during the day on Friday as Helene works
into the Ohio Valley then starts to hook and loop back to the west
Friday night into Saturday. An additional 1 to 2 inches will be
possible across our CWA from 12z Friday to 12z Saturday. Total
rainfall from Helene is still anticipated to range between 2 to 4
inches with locally higher amounts possible.

The second impact will be the potential for strong advisory criteria
wind speeds and gusts during the day on Friday. NBM has continued to
trend too low on its wind values for Friday and with the 12z HREF
Ensemble probability of wind gusts equal or greater than 40 mph by
12z Friday is between 80-100% with EURO Ensemble showing the same
during the day on Friday. While confidence is becoming high we will
likely need a wind headline in the form of a Wind Advisory on
Friday, locations needed in the advisory remain in doubt. Like the
rain mentioned above, the wind forecast will be determined on how
much Helene weakens as it works northward through GA and where the
center of the storm will be located as it reaches KY and the Ohio
Valley on Friday. Given this uncertainty, opted not to issue any
weather headlines at this time.

As Helene transitions into a post-tropical cyclone over the weekend,
showery weather will linger as it merges with the upper low over the
Ohio Valley. It will then open up and exit off to the east northeast
as we start next week.pect we will likely need some sort of
messaging at least for a portion, if not all of the CWA.

The rainfall portion of the forecast has overall remained pretty
consistent, and still thinking that a 2-4" range, with locally
higher amounts, looks pretty good. Since the bulk of those totals is
expected to fall over a 24 to 36 hour period, do think we`ll be able
to handle most of it okay. For example, 6 hour FFG values are
currently running from 2.5" to 4" across our area, and the latest
26/00z HREF data show probabilities of less than 20% for any 6 hour
totals over 2". That alone would suggest that widespread flooding
concerns are not likely at all. However, we also have to keep in
mind that as the event unfolds and amounts pile up, these thresholds
will drop, and flooding potential with any moderate or briefly heavy
showers will increase. As we get deeper into Friday and Friday
night, some flood concerns may begin to arise. WPC Slight Risk for
Excessive Rainfall Friday into Friday night seems pretty reasonable.
Possible we may need a Flood Watch, and will explore those
possibilities as we near the event. River ensemble forecast still
suggest that we will be able to handle this amount of rainfall, so
not expecting any real issues on the basins unless something
changes, or we get more rainfall than expected.

The remnant low continues to weaken on Saturday, with central
pressure likely rising above 1000mb, and a steady eastward wobble.
While, rainfall won`t be as widespread and consistent, we`ll still
see periods of showers through the day, with additional rainfall
ranging between .33" and 1" for most. Continued weakening occurs on
Sunday, with rain showers more spotty and weaker than Saturday.

Overall, looking for small diurnal ranges through the weekend as one
would expect with a saturated column/heavy cloud cover. Highs should
generally range in the low to mid 70s for most, while lows mostly
stay in the low to mid 60s for most.

Monday - Wednesday...

Lower confidence for the early week time frame as data starts to
diverge on how quickly the lingering closed upper low exits to the
east. Will keep some isolated to scattered shower chances
(especially in the east) in for Monday and Tuesday, with a
transition to fully dry by Wednesday. Highs look to remain in the
70s early week, possibly warming to around 80 by mid week.

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 653 AM EDT Thu Sep 26 2024

Impacts/Confidence:
- High confidence in lowering ceilings, increasing rain chances, and
enhancing wind gusts later this afternoon and especially tonight as
TC Helen moves inland.
- Low confidence in LLWS towards the end of the period.

Discussion...Current surface observations show slow improvement of
prior dense fog at BWG and RGA with total recovery to VFR after 12
or 13Z. Ceilings, on the other hand, might stay MVFR the rest of the
day at the easternmost terminals (LEX/RGA) before the cloud shield
moves to the west later tonight.  Otherwise, VFR conditions will
predominate during the afternoon hours. Additionally, northeast
surface winds will increase to the point that some gusts between 15-
20 knot gusts could be reported tonight. The last portion of the
period features worsening weather conditions as TC Helen rapidly
approaches the Florida coastline and moisture/cloudiness spreads
over the Ohio Valley. Finally, there might be a chance for LLWS as
the LLJ overspreads central Kentucky.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...None.
IN...None.

&&

$$

SHORT TERM...ALL
LONG TERM...BJS
AVIATION...ALL