294
FXUS63 KLMK 171447
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1047 AM EDT Sat Aug 17 2024

.KEY MESSAGES...

*   Strong to severe storms will be possible this afternoon and
    evening if morning convection doesn`t limit diurnal warming. The
    main threats are damaging winds and hail.

*   Scattered showers and storms will be possible Sunday afternoon,
    but severe weather is not expected. Lingering isolated showers
    will be possible on Monday, mainly east of US 127.

*   Dry weather is expected Tuesday through Friday.

&&

.UPDATE...
Issued at 1046 AM EDT Sat Aug 17 2024

So far this morning, isolated convection has moved across southern
Indiana and into north central Kentucky, with latest radar returns
showing this activity moving east of the I-75 corridor at this time.
Earlier in the morning, outflow from these storms produced gravity
waves which appeared to be trapped within the low-level stable
layer, and areas of low clouds developed along these waves. By this
time, a lot of that energy has mixed out as we have begun to
destabilize; however, a lingering area of additional clouds is noted
along the Ohio River, in addition to an unrelated area of low clouds
across east central KY. Otherwise, mostly sunny skies are present
across much of the area at this hour, allowing temperatures to
quickly warm into the upper 70s and lower 80s. We should continue to
warm into the upper 80s and lower 90s by this afternoon, with
current satellite showing relatively little cloud cover that could
stop the ascent in temperatures.

As far as the shower and storm potential for this afternoon is
concerned, preliminary 12Z guidance has trended a few hours later
for convective initiation. While cameras from across the area do
show nascent cu developing, SDF ACARS sounding in addition to 12Z
BNA sounding shows elevated LFCs just above 850 mb, so it should
take a while for things to destabilize enough to get additional
rounds of convection. Additionally, faster mid-level winds will
swing around the base of the trough later this afternoon,
contributing additional shear and possibly a bit of vort advection
which could add broad lifting. Finally, there is the sfc cold front
which should begin to drop through the region later this evening.
Any of the aforementioned features will assist in convective
initiation, and it is probable that there will be multiple waves of
showers and storms later today. Given latest guidance, would key on
18-20Z for afternoon convective initiation, with the main window for
showers and storms expected between 20-03Z (4-11pm EDT). Have added
PoPs across southern IN, though this would mainly be associated with
the cold front later this afternoon. With poor mid-level lapse rates
expected, and latest WV imagery showing quite a bit of mid-level dry
air (higher DCAPE), it still looks like damaging winds would be the
main severe threat, with hail being a secondary threat.

Updated products with tweaks to afternoon and evening PoPs were sent
a few moments ago.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 335 AM EDT Sat Aug 17 2024

The radar is quiet now, but another chance of thunderstorms is
expected to return today. To our north, a stacked low pressure
system sits over the Great Lakes, and as we head through the short-
term this system will only slide from the western side of the Great
Lakes to the eastern side. This will place the strongest winds on
the southern side of the trough directly over southern Indiana and
central Kentucky. Closer to the surface, the system`s cold front
which is really weak and slow moving, making it hard to pinpoint its
location at times, is approaching the CWA from its current location
near the top of Illinois. The speed of this front and its passage is
slower than what was expected a few days ago. It is now on track to
get to southern Indiana this afternoon.

Currently, ahead of the front, precipitable water values are near an
inch, but with the approaching system increasing the pressure
gradient over the Lower Ohio Valley, 10-20 mph southwest winds will
lift precipitable water values to around 1.7" ahead of the front
this morning. Dew points will be in the low 70s, so we have the
moisture in place to develop some convection.

Looking to our west, over southeast Missouri and central Illinois,
there is some ongoing convection which is producing cloud debris,
and some CAM model guidance has some more cells developing over
southern Illinois over the next few hours. If these cells do develop
they will produce more cloud cover and likely work east into the
CWA. This convection could produce lightning and some gusts, but
wouldn`t be severe. It could however produce enough cloud cover to
limit the strength of any afternoon convection.
.
Most model guidance places the bulk of afternoon convection in the
Bluegrass region, but this could also extend to near Louisville and
south through the Lake Cumberland area. Hodographs show
unidirectional marginal deep layer shear up to around 30 knots. The
driving force for any convection today will be the instability, and
this is where early convection and cloud debris could determine the
chances for severe weather later today. If cloud cover helps to
limit surface warming, MLCAPE values will likely be limited to
around 1,500 J/kg, but if cloud cover is more limited, MLCAPE values
could increase to around 2,000 J/kg, and where DCAPE values could
approach 1,200 J/kg, wind gusts could be more of a concern. Some
hail is also a possibility.

As the front slowly works south through the CWA, winds will veer
towards the west and west-northwest as they drop below 10 mph. Early
in the night, lines of convection will likely move to the southeast
as drier conditions follow from the northwest. Behind the front, an
isolated shower can`t be ruled out, but most will remain dry.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 335 AM EDT Sat Aug 17 2024

Renewed convection is expected on Sunday near the left exit region
of an upper jet streak advancing from the Midwest to the lower Ohio
Valley. Chances will be highest east of Interstate 65 closer to the
axis of a 5H trough over the Appalachians. Mid-level lapse rates and
instability will be weak, and deep layer shear will be low with a
fairly uniform NW flow throughout the column. Nevertheless, seasonal
atmospheric moisture and convective temperatures reached in the 80
to 85 degree range will lead to development of scattered popcorn
showers and thunderstorms. No severe weather is expected. High
temperatures will be in the lower and middle 80s.

On Monday the upper trough over the Appalachians will shift only
slightly eastward, with topographically assisted diurnal convective
development expected over eastern Kentucky. A few showers and
possibly a small low-topped thunderstorm or two could percolate as
far west as the Interstate 75 corridor. Afternoon highs will only be
in the upper 70s to around 80.

The Tuesday through Friday period still looks dry as a large dome of
high pressure slides from Ontario to the East Coast. This will
initially provide southern Indiana and central Kentucky will cooler
than normal weather, with highs Tuesday around 80 and lows Tuesday
night in the 50s. Each subsequent day after Tuesday temperatures
will moderate as the high moves by, and by Friday afternoon
temperatures will be back in the middle and upper 80s with dew
points in the 60s.

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 722 AM EDT Sat Aug 17 2024

As mentioned earlier convection is beginning to develop over
southern Illinois and southwest Indiana. This activity is expected
to move east towards HNB and SDF in the coming hours. Effects will
be limited, but cloud cover and cooling effects from the
showers/storms could help limit or delay afternoon thunderstorms at
area TAF sites. Still believe LEX and RGA are the sites most likely
to see thunderstorm activity during the afternoon hours, but these
early showers could determine how much activity is seen later.

&&

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

&&

$$

UPDATE...CSG
SHORT TERM...KDW
LONG TERM...13
AVIATION...KDW