Special Weather Statement, NWS Louisville, KY

Area Forecast Discussion
National Weather Service Louisville KY
206 AM EDT Tue May 4 2021

...Updated Aviation Discussion...

.Forecast Update...
Issued at 941 PM EDT Mon May 3 2021

...SEVERE STORMS POSSIBLE OVERNIGHT...

Currently, skies were generally clear to partly cloudy across the
region with temperatures in the upper 60s to the lower 70s.  23Z
surface analysis revealed a cold front out to our west from central
IL down into MO.  A strong dewpoint gradient was noted along the
KY/TN border area.  Dewpoints across the state ranged from the lower
60s over the north to around 70-71 down across the Pennyrile region.

For the overnight period, the forecast remains complex and contains
a fair amount of uncertainty.  Currently we have a band of storms
extending from roughly St. Louis northeastward to near Terre Haute.
These storms are within a CAPE instability axis and will pose a
threat of damaging winds, hail, and an isolated tornado or two as
they sink southeast.  The last several runs of the HRRR continue to
suggest that this line will diminish in intensity over the next
several hours.  The remnants of these storms could affect our
northern row of southern Indiana counties in the 11P-12A time frame.

Otherwise across the region, an elevated mixed layer (EML) with 700-
500 lapse rates in the 7-8C/km range continues to be present across
the region.  However, we have a strong cap in place across the area
that kept the evening cu field flat and convection out across
central MO that moved toward the MS river has continued to struggle,
if not just dissipate as it attempted to head toward Farmington, MO
and points east.  ACARS soundings out of KSDF showed the elevated
mixed layer with a fair amount of capping in place.  The latest CAMs
continue to suggest that this cap will likely hold for the next
several hours, perhaps weakening a bit later in the night.

Moving into the overnight hours, the CAMs have been migrating toward
a solution that suggests that convection may develop in an east-west
band near the KY/TN border region near the aforementioned dewpoint
gradient.  The last few runs have continued to shift this axis of
convection a little more south with each successive run.  Additional
convection looks to fire later in the night, probably after 300 AM
or so.  However, the main show for us will probably be an advancing
MCS that is starting to develop over eastern OK.  Shortwave trough
axis over TX will move northeast tonight and aid in the development
of a forward propagating MCS.  This feature should move northeast
across AR/southern MO/Western TN overnight and into western KY late
in the overnight period.  While the various CAMs have it arriving
around 12Z, they are probably a bit too slow with it, and we believe
it will arrive prior to sunrise.

In terms of hazards overnight, it really depends on how much forcing
we get and if it can break through the cap.  The EML aloft and the
steep lapse rates are pretty loaded, so any parcel that can get
lifted to the LFC and bust through the cap would take off at a
pretty good clip.  Any discrete cell could produce large hail and
damaging winds along with torrential rainfall.  With the arriving
MCS, a damaging wind threat would be most likely, though I could not
rule out an isolated spin up or two with any bowing line segments.

As far as the Tuesday severe weather threats, much of tomorrow will
hinge on how the early morning MCS affects the region.  It appears
that we may have some airmass recovery in the afternoon as a moist
boundary layer will still be in place.  Bulk wind shear values in
the 35-45 knot range along with MLCAPE values possibly rising into
the 1500-2500 J/Kg range would support organized severe weather.
Stronger forcing along an advancing cold front may result in another
round of strong/severe storms during the afternoon with all modes of
severe weather still possible.

&&

.Short Term...(This evening through Tuesday Night)
Issued at 222 PM EDT Mon May 3 2021

...Multiple Rounds of Severe Weather Possible Late Tonight Through
Tuesday Night...

A very challenging and complex forecast is expected in the short
term. Confidence is very high that there will be multiple waves of
strong to severe thunderstorms, some capable of significant wind and
hail damage along with an isolated tornado threat, across the Ohio
and Tennessee Valleys starting tonight and continuing into tomorrow.
Where confidence drops considerably is when and where these waves
end up. 00z and 12z HREF output shows convective timing and
placement all over the place, with little agreement amongst
convective allow models (CAMs) in the temporal and spatial fields.
We will likely see breaks in between waves of showers and storms,
but again, because confidence is very low in the details, its hard
to say when those will occur. The 'lull' time in between breaks in
the convection will be important, as if we are able to destabilize
in the wake of a wave, the subsequent wave of storms would have fuel
to keep it strong/severe.

At this point in time, we think the first wave could arrive as early
as 02-03z tonight in our western zones, with a second wave
potentially arriving near sunrise tomorrow morning. We may see a
break in the action during the mid morning to early afternoon
period, but additional showers and storms (final wave) could fire up
along an advancing cold front and persist up through midnight
tomorrow. It is entirely feasible that we miss out on some of these
waves, or that they weaken considerably by the time they get here
and we end up with little to no severe weather... conversely, if we
get hit by one (or more) of these waves at full strength, there
would likely be widespread wind and occasional hail reports
associated with it, some of which could be significant. If bowing
segments form, spin-up tornadoes would be possible.

Another item worth mentioning is flash flood potential. While most
models depict fast moving waves with little residence time over any
one given location, a couple of high-res models show convection
developing along an east-west oriented boundary across the region
where training of storms would occur. Would like to see more
agreement in the guidance before issuing a potential flash flood
watch, as mean model guidance output is generally in the 1-2" range
with isolated higher streaks, which most areas should handle fairly
well. This will be something that will be watched though.

The best advice we can give at this time is to be prepared for
multiple rounds of severe storms starting tonight and have a way to
get warnings during all times of the day and night. The severe
threat should diminish significantly behind the cold front late
tomorrow night.

.Long Term...(Wednesday through Monday)
Issued at 238 PM EDT Mon May 3 2021

Wednesday through Friday

The Ohio Valley will be situated at the base of a broad upper level
trough through the end of the week. This will result in below normal
temperatures for the region. Sfc high pressure will build on for
Wednesday behind the departing cold front. Clouds will clear from
the WNW to the ESE across the CWA (County Warning Area)through the
day, eventually becoming partly to mostly sunny. Temperatures will
be in the low 60s across southern IN/northern KY with mid 60s across
southern KY.

Thursday will be the coldest day of the week as clouds increase back
in over the region thanks to an embedded shortwave that will drop
southeast towards the OH Valley. This will also bring a slight
chance of showers to the area. Highs will be near or in the low 60s
with lows in the low 40s. Fair and unseasonably cool weather
continues for the end of the week as weak high pressure build back
in behind the passing shortwave. Mostly sunny skies and highs in the
low/mid 60s.

Saturday and Sunday Temperatures will start to slowly moderate over
the weekend and into early next week as southerly flow once again
takes over. This will warm temperatures into the up 60s/low 70s
Saturday and then the mid/upper 70s for Sunday and Monday. Moisture
will also increase across the region along with higher chances of
showers/storms. There remains differences between the long range
models on location of the surface boundary and placement of the next
storm system.

&&

.Aviation...(06Z TAF Issuance)
Issued at 205 AM EDT Tue May 4 2021

IMPACTS: - Strong to severe storms possible early this morning
- VFR conditions outside of storms, but IFR or lower within storms
- S-SW winds overnight shifting to the west late or toward sunrise

DISCUSSION:  Convection has blossomed over western KY early this
morning with locally heavy rainfall and large hail being the main
threats. Initially, BWG is the only TAF site threatened by these
storms, however SDF/LEX could come into play in the pre-dawn hours.
Will keep VFR conditions outside of storms, but drops into the IFR
range are possible with reduced vis for any storms. Later this
morning, a weakening convective complex is expected to move into our
area from the WSW, impacting the TAF sites through mid to late
morning. Thereafter, look for a return to VFR. Cold front then drops
NW to SE through our area this evening, bringing MVFR ceilings
toward the end of this cycle.

CONFIDENCE: Low confidence in T-storms and timing. Medium to high
confidence in ceilings.

&&

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

Update...MJ
Short Term...DM
Long Term....BTN
Aviation...BJS

Hazardous Weather Outlook
National Weather Service Louisville KY
947 PM EDT Mon May 3 2021

INZ076>079-083-084-089>092-KYZ023>043-045>049-053>057-061>067-
070>078-081-082-050200-
Orange-Washington-Scott-Jefferson-Dubois-Crawford-Perry-Harrison-
Floyd-Clark-Hancock-Breckinridge-Meade-Ohio-Grayson-Hardin-Bullitt-
Oldham-Trimble-Henry-Shelby-Franklin-Spencer-Anderson-Woodford-
Fayette-Bourbon-Nicholas-Nelson-Mercer-Jessamine-Larue-Marion-Boyle-
Garrard-Madison-Butler-Edmonson-Hart-Green-Taylor-Casey-Lincoln-
Logan-Warren-Simpson-Allen-Barren-Monroe-Metcalfe-Adair-Russell-
Cumberland-Clinton-
Including the cities of Paoli, Salem, Scottsburg, Madison, Jasper,
English, Tell City, Corydon, New Albany, Jeffersonville, Lewisport,
Hawesville, Hardinsburg, Brandenburg, Hartford, Leitchfield,
Elizabethtown, Shepherdsville, Louisville, La Grange, Bedford,
Milton, New Castle, Shelbyville, Frankfort, Georgetown, Cynthiana,
Taylorsville, Lawrenceburg, Versailles, Lexington, Paris, Carlisle,
Bardstown, Springfield, Harrodsburg, Nicholasville, Winchester,
Hodgenville, Lebanon, Danville, Lancaster, Richmond, Morgantown,
Brownsville, Horse Cave, Greensburg, Campbellsville, Liberty,
Stanford, Russellville, Bowling Green, Franklin, Providence,
Scottsville, Glasgow, Tompkinsville, Edmonton, Columbia, Jamestown,
Burkesville, and Albany
947 PM EDT Mon May 3 2021 /847 PM CDT Mon May 3 2021/

This hazardous weather outlook is for portions of south central
Indiana and Central Kentucky.

.DAY ONE...Overnight.

Strong to severe thunderstorms will be possible overnight and into
early Tuesday morning. The main threats with storms will be damaging
winds and hail, but an isolated tornado and flash flood threat will
be possible as well.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Multiple rounds of thunderstorms, some of which could be severe,
could impact the region most of the day Tuesday and Tuesday night.
The main threats with storms will be damaging winds and hail, but an
isolated tornado and flash flood threat will be possible as well.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed later tonight through Tuesday night.

DM/BTN/MJ

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Special Weather Statement
National Weather Service Louisville KY
157 AM CDT Tue May 4 2021

KYZ064-074-076-040730-
Metcalfe-Barren-Green-
157 AM CDT Tue May 4 2021

...STRONG STORMS NEAR GLASGOW...

At 156 AM CDT, radar indicated strong thunderstorms from 8 miles
northwest of Edmonton to just north of Glasgow. Movement was east at
30 mph.

Winds in excess of 30 mph and nickel size hail are possible with
these storms.

Locations impacted include...
Glasgow, Edmonton, Park City, Oil City, Echo, Cork, Coral Hill,
Lecta, and Knob Lick.

If outdoors, seek shelter inside a building.

LAT...LON 3700 8605 3710 8604 3713 8548 3711 8553
      3707 8552 3699 8558
TIME...MOT...LOC 0656Z 255DEG 28KT 3705 8574 3705 8597

13

Special Weather Statement
National Weather Service Louisville KY
215 AM CDT Tue May 4 2021

KYZ075-081-040745-
Monroe-Cumberland-
215 AM CDT Tue May 4 2021

...STRONG STORM NEAR TOMPKINSVILLE...

At 214 AM CDT, a strong thunderstorm was located near Tompkinsville,
moving east at 35 mph.

Winds in excess of 30 mph and penny size hail are possible with this
storm.

Locations impacted include...
Tompkinsville, Leslie, Waterview, Meshack, Persimmon, Forkton, Gum
Tree, Blythe, Freetown, and Ellington.

If outdoors, seek shelter inside a building.

LAT...LON 3667 8584 3677 8585 3682 8563 3682 8558
      3683 8558 3686 8544 3666 8540
TIME...MOT...LOC 0714Z 259DEG 29KT 3672 8575

13