Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/14/21

Area Forecast Discussion

National Weather Service Louisville KY

 830 PM EDT Fri Aug 13 2021

.Mesoscale Update...
Issued at 830 PM EDT Fri Aug 13 2021

Cloud tops have warmed and convection has decreased significantly in
coverage and intensity. The storms are well behind their surface
outflow boundaries. However, heavy rain will remain a threat for
another 30-60 minutes, especially where showers and storms are
training east of Louisville and north of Lexington. Precipitable
water values coincident with that area of storms have increased over
the last few hours to around two inches. Moderate instability and
low CIN are still in place, but further gradual weakening is
expected as we get into the post-sunset hours.

The cold front is still well to our north across central Illinois
and central Indiana. Showers and storms developed this afternoon
ahead of the boundary, but those too have weakened considerably.
Still, a few showers and even a rumble of thunder could pop up late
tonight and into early Saturday morning as the front moves into the
region. An upper disturbance over Missouri this evening will head
east and may help to encourage convection, but it will be weakening
as it approaches. Will keep late night rain chances low.

Issued at 556 PM EDT Fri Aug 13 2021

Thunderstorms have increased in coverage over the past hour in a
corridor of maximum moisture convergence stretching from middle
Tennessee to central Ohio as well as a local MLCAPE max around 2500
J/kg. The storms also are developing ahead and south of the
aforementioned MCV, which is now quickly approaching Cincinnati from
the west. DCAPEs are marginal (800-900 J/kg) and so far measured
wind gusts have mostly been in the 30-40 mph range. However, as of
this writing there is a particularly strong updraft in a
thunderstorm over Marion County that may result in stronger winds at
the surface as the cell collapses, and the Fayette County Mesonet
clocked a 44 mph gust with another cell. We are not out of the woods
yet with regard to gusty wind potential.

Having said that, most of the convection so far this afternoon has
struggled. Mid-level lapse rates are weak and shear is very weak,
especially over southern Kentucky but a little stronger over the
Bluegrass. The last couple of aircraft soundings out of SDF have
shown a warm layer at 900-850mb.

Given wet bulb zero heights around 12k` and lack of shear, severe
sized hail is unlikely. Smaller hail will still be possible in the
more robust storms, and has already been reported in Scott County KY.

Slow movement and training of storms has presented a local flood
potential as well.

Storms will continue to fire, with the greatest development and
coverage likely taking place over the next couple of hours or so.
The activity should slowly begin to wane this evening as we lose
insolation and the Cincinnati MCV moves off.

Additional showers and thunderstorm chances will continue tonight,
though, as the cold front to our north approaches.

Issued at 245 PM EDT Fri Aug 13 2021

...Strong to Severe Storms Remain Possible, Mainly East of I-65...

Very moist and moderately unstable air still sits over central
Kentucky, with a few lingering low-level boundaries from last
night`s convection and a mid-level MCV pushing across southern
Indiana. The focus for storms thus far has been east of I-65, with
the current effective boundary roughly from Frankfort to
Campbellsville to Tompkinsville and moving slowly eastward.

Most of our storm development should continue to focus along and
east of this boundary, including through the Bluegrass region and
Lake Cumberland. The storms across the Bluegrass have the best
potential to become severe if mid-level winds increase enough ahead
of the MCV. Without this enhancement of deep-layer shear, a warm
layer near 400mb has thus far been able to keep storms from getting
tall enough to produce severe winds. Harrison County KY Mesonet has
reported wind gusts to 36 mph, so gusty winds are still in play and
it wouldn`t take much improvement to open the door to severe winds.
So we will continue to highlight at least a localized damaging wind
threat.

For the I-65 corridor and west, mid-level clouds associated with the
MCV will put the brakes on development for at least a couple hours.
Still have opportunity for storms to fire later this afternoon into
this evening. Also worth watching is the cluster of storms in the
Missouri bootheel, as that could develop into another MCV that would
interact with our juicy air mass. However, it`s not a wide window of
opportunity, and the best focus for storms seems to be over western
Tennessee. It`s a lesser SVR threat west of I-65, but non-zero
especially in southern Kentucky. As you move north of the Western
Kentucky Parkway and into southern Indiana, the risk diminishes
greatly.

&&

.Short Term...(This evening through Saturday)
Issued at 245 PM EDT Fri Aug 13 2021

Once convection winds down around midnight tonight, what had
previously looked like a simple forecast of unusually pleasant mid-
August weather now seems more uncertain.

Cold front will take its sweet time pushing southward, leaving the
warm and muggy air mass in place overnight and at least into
Saturday morning. Will hang on to low POPs through the night, but
also keep QPF low as well.

For Saturday we will have to watch the southward progress of the
front, especially to see if any upstream MCVs will be able to
interact with it. POPs have been bumped up slightly, with 30-40%
chance south of the WK/BG Parkways and a 20% chance north of the
Parkways to the Ohio River. Afternoon showers and storms across
southern KY could be intense enough to drop some locally heavy
rainfall. Even with the muggy start, we should see dewpoints drop
through the day, especially across southern Indiana, so max temps in
the mid/upper 80s will actually feel like mid/upper 80s. South-
central Kentucky will not be as lucky, with dewpoints staying up
around 70 or a bit higher.

.Long Term...(Saturday night through Friday)
Issued at 255 PM EDT Fri Aug 13 2021

At the start of the period, expect to find general troughiness
moving into the MS Valley with areas of high pressure aloft centered
over the CA/NV border and the western Atlantic. The mid-level trough
is then expected to remain situated over the MS Valley through mid-
week before gradually lifting northeast towards the Great Lakes
region through the end of the week. At the surface, the cold front
initially located near the KY/TN border will continue to sink
southward, eventually stalling out over the TN Valley/Deep South,
before lifting back north as a warm front and washing out over the
area by midweek.

A quick return of southerly flow into the region as an inverted
surface trough sets up will help keep plenty of moisture over the
area as we move into next week, with PWATs remaining in the 1.7-2.0+
inch range. As pockets of energy ripple through the base of the
trough and over the area, there will be potential for precipitation
each day of the extended, keeping us in an unsettled pattern through
the end of the week. Will see a diurnal enhancement of precip
activity, with best chances of showers and storms occurring during
the afternoon and evening hours.

Temperatures throughout the period look to be near or just below
climatological normals. Highs will generally be in the lower to mid
80s, likely warming back into the upper 80s by next Friday.
Overnight lows will be in the mid to upper 60s with urban areas
possibly remaining in the lower 70s.

&&

.Aviation...(00Z TAF Issuance)
Issued at 654 PM EDT Fri Aug 13 2021

Thunderstorms will continue to affect LEX for the first hour or two
of the TAF period before waning as the sun sets. Can`t rule out a
thunderstorm near SDF and BWG...may have to make a last-minute
decision at press time over whether or not to include mention at
those locations. At this time HNB appears safe from thunder.

After the early evening storms fade, we`ll have several quiet hours.
Then late tonight a cold front from the north will approach and may
be enough to pop scattered showers and a thunderstorm or two.

The front will sink into central Kentucky tomorrow, with BWG
remaining near the front long enough that some afternoon convection
may affect that airport.


&&

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

$$

Mesoscale...RAS/13
Short Term...RAS
Long Term...JML
Aviation...13