729
FXUS63 KJKL 081644 CCA
AFDJKL

AREA FORECAST DISCUSSION...CORRECTED
National Weather Service Jackson KY
1244 PM EDT Wed May 8 2024

...CORRECTED FOR LONG TERM DISCUSSION...

.KEY MESSAGES...

- Our main thunderstorm potential lasts into early Thursday, with
  some showers then possible at times through early next week.

- Severe thunderstorms are possible until late tonight or early
  Thursday morning. Large hail and damaging winds are the primary
  threats, but there is also a risk of tornadoes. Locally heavy
  rainfall could also lead to flash flooding.

- Above normal temperatures are forecast through Thursday. A cold
  front passing on Thursday will bring cooler temperatures from
  Friday through early next week.

&&

.UPDATE...
Issued at 1121 AM EDT WED MAY 8 2024

Line of storms mainly across the border in northeast TN, but a
shield of ligh rain continues into southeast KY, with a few
lightning strikes embedded. Have updated the gridded forecast for
today/tonight to include updates from hi-res models that are
handling the current situation well. Expect the effective boundary
for storms to lift northward this afternoon, bringing some
potential for strong to severe storms back...but still with the
best chances for severe storms mainly south and west of the Hal
Rogers Pkwy.

UPDATE Issued at 720 AM EDT WED MAY 8 2024

Hourly grids were freshened mainly based on recent observation
trends to account for slight differences from the previous
forecast due to wetbulbing from shower activity and the where
there had been the passage of an outflow boundary.

&&

.SHORT TERM...(Today through Thursday)
Issued at 545 AM EDT WED MAY 8 2024

Early this morning, a meandering upper level low was centered near
the MT/WY/Dakotas border area with an associated trough axis south
across the Four Corners region to the northern Mexico. Downstream
of the trough southwest flow aloft extended from the Southern
Plains to the OH Valley/Appalachians to the mid Atlantic states.
An initial shortwave was passing well north of the Great Lakes
region though multiple weak disturbances were moving with in the
southwest flow. At the sfc, a parent low pressure was centered in
the Great Lakes with a triple point over southern Ontario. The
trailing cold front extended south and southwest to the mid MS
Valley to low pressure centered over KS with the frontal zone
continuing into the Southern Plains. This sfc low was developing
downstream of a shortwave trough moving from the Rockies into the
Plains. As of 5 AM, convection has spread south of the OH River
and into portions of eastern to south central and southwestern KY.
A southward moving outflow boundary was dropping south with this
largely outpacing the convection to the south. The intersection
of this southward moving outflow and convection in central KY was
moving toward the Rockcastle and Jackson county border area.
Mesoanalysis has MLCAPE of 2000 J/kg or higher in the southwestern
portions of the area and 1500 or higher south of the outflow
boundary. Low level lapse rates are meager though mid level lapse
rates are analyzed between 6 and 7C/km. Effective shear was still
around 45 to 50KT and these parameters are enough to sustain this
convection with an occasional stronger storm or two near the
intersection of the southward moving outflow and the convection or
ouflow its associated outflow.

Today, the upper level low is expected to meander to the NE and SD
border area with the shortwave trough to the south moving from the
Plains to the MS Valley region. The preceding sfc low should
reach the Mid MS Valley region this evening with the boundary to
the east stalling and eventually lifting north as a warm front in
the OH Valley region. The shortwave trough is expected to move
from the MS Valley into the OH Valley and toward the Appalachians
tonight to early on Thursday as the upper level low begins to
fill with a positively titled 500 mb trough extending from the
Maritimes to the Great Lakes to the Four Corners region on
Thursday morning. The shortwave trough working gradually through
the OH Valley should move east of eastern KY on Thursday with the
region remaining in southwest to west southwest flow to the east
of the 500 mb trough axis. The sfc low meanwhile should track
norht of the OH River and reach the Indianapolis vicinity by early
Thursday morning, continuing to the Mid Atlantic states by sunset
on Thursday. A trailing cold front would cross the region during
that point. However, where the outflow boundary from convection
moving across eastern and central KY attm settles near the TN and
KY border may play a role in the location and northward extent not
only of the focus for stronger convection with severe hazards
later today and tonight as well as risk for training convection
and a flash flooding risk.

0Z HREF has the highest probabilities for all severe hazards from
south of St Louis MO across southwest and south central KY and TN
with lesser probabilties further to the north and northeast.
Updraft helicity probabilties in the 2-5km layer are also highest
generally along this corridor and recent convective allowing
guidance also suggest more concentrated activity from near Lake
Cumberland to the KY/VA/TN tri state area from this afternoon into
this evening after some recovery following activity this morning.
Chances for at least a wind threat would likely increase later
Wednesday evening into Wednesday night in advance of the cold
front with prefrontal convection moving across portions of the
Commonwealth from the mid MS Valley. These more southwestern
locations across the JKL CWA should have better mid level lapse
rates reaching 6 to 7C/km though lapse rates during peak heating
may reach near 7C/km or higher areawide. Late afternoon to early
evening MUCAPE is forecast by the RAP in peak in the 1500 to 3000
J/kg range with bulk shear 30KT in the north and near 50KT in the
south. More substantial MLCAPE of 2000 J/kg or greater is also
expected in the southwest at that time. PW should climb to 1.5 or
higher areawide ahead of the front later this evening and tonight
though residence time of the higher PW of 1.5 to 1.6 inches in
addition to the best severe parameter vales should also be across
the Cumberland Valley to TN border area. There is some
uncertainty as to how far the boundary settles and some CAMS keep
the more substantial convection and training just south of the
border. Others bring this into the southern or southwestern CWA so
there is considerable uncertainty as to how this will unfold at
this point.

Not only is the PW expected to be above the 90th percentile for
this time of year, but the 0Z HREF is suggesting reasonable worst
case 3 hour and 6 hour amounts of about 3 inches and near 5 inches
respectively in the region. A flood watch was issued overnight
and is in effect through Thursday morning. WPC has a moderate
Excessive Rainfall Outlook in the southwest or areas generally
near or west of I-75 with a slight risk elsewhere.

Chances for convection will wave through Thursday behind the
front, with colder and drier air starting to advect into the
region at the end of the period.

.LONG TERM...(Thursday night through Tuesday)
Issued at 530 AM EDT WED MAY 8 2024

Cooler air will continue to flow into the region behind a
cold front departing to the southeast on Thursday night.
Significant precip will be gone by Thursday night, with only a
slight chance of showers lingering. Below normal temperatures are
expected to persist through the weekend. A mid-upper level trough
supporting the cool weather will be anchored over eastern Canada
and the northeast CONUS. Multiple waves will rotate around the
trough, with one passing over our area on Friday, and another on
Saturday. Each of these will bring a potential for an uptick in
showers. Have held back on the mention of thunder, with forecast
soundings suggesting convection won`t be deep enough.

The trough should be lifting out to the northeast on Sunday, with
rising geopotential heights locally. Along with warm air advection
and at least partial sunshine, warming temperatures will result.

After this point, model differences become more significant. An
upper low lingering above the western CONUS over the next several
days will begin to open up and get picked up in flow aloft. The
GFS is faster than the ECMWF to bring it eastward, and has light
precip arriving here from the west on Monday and carrying into
Tuesday. The slower ECMWF does not generate light precip until
Tuesday. A model blend would suggest nothing more than a slight
chance of precip Monday, followed by a chance on Tuesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
ISSUED AT 725 AM EDT WED MAY 8 2024

Convection is gradually dropping south across the area as is an
outflow boundary. Within the convection some lower CIGs and or
visibility down to MVFR to IFR is not briefly lower have been
reported. Another relative lull in activity should spread from
north to south between 12Z to 18Z. Another round or rounds of
convection are anticipated thereafter and perhaps mainly after 00Z
from the TAF sites north. This would also lead to some MVFR and
IFR reductions to ceilings and vis at that activity passes with
some areas briefly to airport mins. The winds will remain out of
the south and southwest at 10 KT or less sustained through the
period, but some brief higher gusts in excess of 30KT are possible
with any thunderstorm activity, especially after 18Z.

&&

.JKL WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Thursday morning for KYZ044-050>052-058>060-
068-069-079-080-083>088-104-106>120.

&&

$$

UPDATE...SHARP/JP
SHORT TERM...JP
LONG TERM...HAL
AVIATION...JP