Special Weather Statement, NWS Louisville, KY

Area Forecast Discussion
National Weather Service Louisville KY
853 PM EDT Thu Aug 12 2021

.Forecast Update...
Issued at 853 PM Thu Aug 12 2021

Earlier storms that cruised across southern Indiana and put out a
healthy gust front have weakened to scattered showers. The gust
front is still spreading southeastward but will weaken as it
approaches and crosses the Ohio River.

The next thing to watch is the severe MCS near St Louis, with an MCV
moving from west to east just north of the city at this time. Wind
gusts over 60 mph have been reported with this activity. Radar
animations show a gust front from the IL/IN storms surging well
south into southern Illinois along with another gust front pushing
southward along the Mississippi River ahead of the Missouri
activity. This should encourage new thunderstorm development to
concentrate more over southern Illinois and into western Kentucky
over the next few hours. As we proceed later into the evening the
storms will encounter a significant lack of shear as well as
increasing CIN post-sunset. So, some of the showers/storms may make
it into southern Indiana and central Kentucky, but they should be
weaker than they are currently.

Issued at 608 PM EDT Thu Aug 12 2021

All eyes are on the MCS sliding southeast across Illinois and
Indiana, with the big question being how far south it will make it,
and at what level of intensity.

So far the storms have maintained their strength as they move through
a very unstable atmosphere (3000-4000 J/kg MLCAPE, LI as low as -10C
just ahead of the line). They have occasionally produced outflow
boundaries, but new convection has developed along those boundaries.
It appears that a cold pool has begun to develop after the storms
initially struggled to establish one.

There are a few factors working against the storms. As they proceed
to the southeast they will be moving away from the better upper
support and more into the upper ridge sprawled across the southern
half of the country. Recent ACARS sounding from SDF shows a warm
layer between 700 and 750mb, though it's somewhat weak. Insolation
is decreasing, though we're still two and a half hours away from
sunset. Possibly most significantly, deep layer shear is extremely
weak ahead of the storms, especially along and south of the Ohio
River. This should hinder their ability to stay organized.

So, bottom line, with the storms still quite robust and only a few
counties away from the northern edge of NWS Louisville's area of
responsibility, will need to consider adding PoPs to at least our
southern Indiana counties over the next few hours. The feeling right
now is that they should begin a weakening trend soon given the lack
of shear and the possibility of the storms becoming outflow
dominant. In the forecast we will taper PoPs off significantly as
they approach the Ohio River, but will continue to monitor.

Thanks to SPC for coord.

&&

.Short Term...(This evening through Friday)
Issued at 305 PM EDT Thu Aug 12 2021

Another day of sweltering heat across the region as afternoon
temperatures have risen into the upper 80s to lower 90s so far along
with dewpoints in the mid to upper 70s and heat index values
exceeding 100 degrees for most locations. Overall have seen mostly
dry weather today as model soundings indicate drier air in the mid-
levels along with a slight warm nose around 700 mb. Upstream,
convection that has been poorly resolved by CAM guidance continues
to push across central IL and into IN with expansive cloud shield
remaining to our north. Latest model runs indicate this convective
complex may gradually approach the area later this evening into
tonight, progged to fall apart as it reaches our southern IN
counties. Confidence remains low however as models have struggled
all day to capture the evolution of these storms. Will keep a close
eye on this over the next few hours to see if mention of PoPs will
be needed tonight across northern parts of the area.

As we move into the overnight - Friday time frame, focus will shift
to the potential for stronger thunderstorms to develop across the
area ahead of an approaching system. A broad upper trough/closed low
will be centered over the Hudson Bay while an elongated area of high
pressure centered over the western Atlantic will extend across much
of the Southeast. For the lower OH Valley, this will maintain the
pattern of relatively weak flow aloft while southwesterly flow in
the lower levels continues to pump in warm and moist air. Moving
through the short-term period, expect a couple of mid-level
shortwaves to ripple through the base of the trough across the Upper
Midwest and into Upper Great Lakes, sending an associated surface
cold front southward towards the area. This will then result in
increasing precip chances throughout Friday ahead of the approaching
front.

Overall confidence remains lower for the evolution of storms
overnight and into tomorrow, but general thought is that another
round of convection will fire up ahead of the cold front across
northern IL/IN later tonight and gradually sink southward towards
the area, again generally weakening as it approaches. Expect precip
chances would then begin moving into southern IN and north-central
KY towards 13/1200Z. However, development of this round of
convection remains highly dependent on evolution of this afternoon's
convective complex and there is concern that models might be
overdoing this second round as they struggle to capture the current
environment.

In light of all of this, there remains potential then for some
redevelopment in the afternoon and evening hours Friday as
instability ramps up in the warm and moist airmass ahead of the
approaching front. Hi-Res guidance indicates PWATs will likely rise
back into the 1.7 - 2.0 inch range during the day tomorrow with
temperatures possibly warming into the upper 80s to lower 90s. In
this environment, moderate to strong instability would be possible
during the afternoon hours. However, this will all remain largely
dependent on how far south overnight convection/clouds are able to
progress and how well convection is able to stay together. In
general though, expect the warmest temps and best instability on
Friday to be across south-central KY. Additionally, expect Effective
Shear values to remain on the weak side, generally under 20 kts,
with disorganized multicells being the main convective mode. In this
setup, conditional on the evolution of overnight convection, the
greatest threat within any stronger storms able to develop Friday
would be locally gusty winds, brief torrential rainfall and frequent
lightning.

.Long Term...(Friday night through Thursday)
Issued at 245 PM EDT Thu Aug 12 2021

Weakness in upper ridging across the Ohio Valley will allow the heat
and humidity to break this weekend, even as the westerlies remain
solidly to our north over the Great Lakes. Cold front will be
pushing south across the area Friday night, with coverage and
intensity of convection being conditional on what happens during the
heat of the afternoon. Any strong to marginally severe storms will
weaken after sunset, and any localized damaging wind threat will
give way to a brief heavy rain threat.

Saturday morning will be a transitional period as sfc high pressure
builds from the north, with initially muggy conditions abating as
boundary layer growth mixes down drier air. Sat afternoon max temps
will run solidly below climo, with dewpoints mixing down into the
lower 60s with the exception of south central Kentucky, where
humidity levels will stay up and we could see scattered showers and
storms closer to the periphery of the high. Expect a similar trend
on Sunday, only with a more pleasant start and lower POPs as any
convection is suppressed farther to the south.

By Monday we return to more typical summer humidity as the sfc high
retreats into New England and gives way to an inverted trof just
west of the Appalachians. Upper pattern also looks more unsettled as
the East Coast ridging amplifies. Precip chances will also ramp back
up, with mainly diurnally driven convection each day next week.
Heavy rainfall is also on the table toward midweek as tropical
moisture from Fred is added to the mix. Greatest potential for heavy
rain will be in eastern Kentucky, perhaps as far west as Lake
Cumberland and the Bluegrass region. However, at this time we don't
expect much organization to it, but confidence remains limited this
far out.

&&

.Aviation...(00Z TAF Issuance)
Issued at 738 PM EDT Thu Aug 12 2021

A line of storms is working it's way towards the southeast through
southern Indiana. This line will impact HNB very soon and SDF over
the next couple of hours. Winds as high as 45 knots were observed in
eastern Illinois, so as this line moves through, expect winds to
shift to the north and become gusty. A couple of hours after the
line passes winds are expected to return to the southwest.

Models haven't handled today's weather well, so with the advancing
cold front that will approach the region tomorrow, precipitation
chances will increase across southern Indiana and central Kentucky.
However, confidence in timing remains low with tomorrow's showers
and thunderstorms.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...Heat Advisory until 9 PM EDT /8 PM CDT/ this evening for
     INZ076>079-083-084-089>092.

KY...Heat Advisory until 9 PM EDT /8 PM CDT/ this evening for
     KYZ023>035-038-039-045-046-053-054-061>065-070>076.

&&

Update...13
Short Term...JML
Long Term...RAS
Aviation...KDW

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Special Weather Statement
National Weather Service Louisville KY
903 PM EDT Thu Aug 12 2021

KYZ033-130130-
Henry KY-
903 PM EDT Thu Aug 12 2021

...A strong thunderstorm will impact portions of central Henry County
through 920 PM EDT...

At 903 PM EDT, Doppler radar was tracking a strong thunderstorm near
New Castle, moving east at 30 mph.

HAZARD...Winds in excess of 40 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
New Castle, Port Royal, Drennon Springs, Lacie, Delville, Slabtown,
Fallis, Lockport, and Franklinton.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, seek shelter inside a building.

&&

LAT...LON 3856 8506 3857 8504 3855 8500 3851 8503
      3852 8498 3849 8499 3848 8497 3846 8495
      3846 8492 3845 8491 3844 8493 3843 8492
      3843 8494 3845 8494 3843 8496 3843 8517
      3851 8521 3858 8508
TIME...MOT...LOC 0103Z 251DEG 24KT 3850 8510

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...40 MPH

13