Special Weather Statement, NWS Louisville, KY

Area Forecast Discussion
National Weather Service Louisville KY
235 AM EST Sat Dec 11 2021

.Forecast Update...
Issued at 235 AM EST Sat Dec 11 2021

A new Tornado Watch has been issued along and ahead of line of
storms currently moving across the area. These storms have had a
history of tornadoes and wind damage. The watch is in effect until
9am EST/8am CST but counties will likely be gradually shaved off of
the western edge as the threat progresses eastward.

Issued at 1250 AM EST Sat Dec 11 2021

The aforementioned corridor of 1.6" PWATs and surface dew points in
the mid 60s is translating eastward into central Kentucky and
central southern Indiana. This corridor is supporting SBCAPE of 1000-
1500 J/kg with decreasing CIN west of I-65 along with strong theta-e
ridging overhead. LCL heights are very low, around 500m and very
strong shear is still in place. Severe winds and tornadoes are still
a possibility as we move through the night (HNB recorded a 44mph
wind gust shortly after midnight).

Issued at 1130 PM EST Fri Dec 10 2021

Supercells have erupted to our west this evening in corridor of
strong shear and unusually strong instability (1000-2000 J/kg
MUCAPE) for this time of year. 0-500m SRH of 200-300 m2/s2 more than
sufficient for tornadoes, and unfortunately damaging tornadoes
appear to have occurred over the mid-Mississippi Valley this
evening. Dew points have surged well into the 60s over the western
half of Kentucky with precipitable water values around 1.6". Low
level jet of 60-70kt impinging on the region ahead of a 90kt 5H
speed max ejecting ahead of an upper trough over the central Plains.

The storms over western Kentucky have been moving east into central
Kentucky this evening, however the tornadoes, as far as we know thus
far, appear to be lifting before reaching LMK's CWA. There is a
sharp north-south instability gradient roughly from Salem to
Brandenburg to Bowling Green, and the storms have struggled as they
approach and cross over this gradient. Aircraft soundings out of SDF
continue to show a 850-950mb inversion, keeping low levels stable.
Earlier storm over Breckinridge County showed good rotation aloft
but the rotation was not able to work down to the surface.

As the larger scale storm system to our west approaches tonight,
surface based instability will spread into southern Indiana and
central Kentucky, though not quite as strong as what we saw out west
earlier this evening. The low level inversion is expected to weaken
overnight, though to what degree is still a question mark.

Supercells will continue to be possible over the next couple of
hours, and then a line (possibly more than one) of storms is still
expected to cross the region late tonight into early Saturday
morning. Low level SRH and STP will remain high through the night,
so a tornado threat will continue with any storms, whether discrete
or in a line.

Issued at 750 PM EST Fri Dec 10 2021

Tornado Watch #554 has been issued for the western two-thirds of our
CWA until 3 AM EST. Scattered pre-frontal convection has already
developed across the lower Ohio/Mississippi River confluence and
will quickly race northeastward into our western CWA within the next
hour. The VAD wind profiler data from VWX/HOP radars have shown
increasing low level wind shear resulting in very strong helicity in
the lowest 1 km (resulting in >400 m2/s2). Model soundings along our
far western CWA do show a bit of an inversion and farther east
toward the I-65 corridor we are seeing that inversion show up on
AMDAR soundings. RAP soundings over the next few hours show dew
points slowly increasing west of I-65, which may be enough to allow
those storms to become surface based when they move into the CWA.

Do want to convey that we expect another Watch will be needed
farther east in our CWA for the pre-dawn and around sunrise time
frame, but that will likely come at a later time tonight.

&&

.Short Term...(This evening through Saturday)
Issued at 344 PM EST Fri Dec 10 2021

...SIGNIFICANT EPISODE OF SEVERE WEATHER EXPECTED TONIGHT AND EARLY
SATURDAY MORNING...

====================================
Synoptic Discussion
====================================

Deepening mid-level trough axis over the Plains will translate
eastward this evening and overnight.  The trough axis is forecast to
remain positive to neutrally tilted as it moves toward our region.
Initial mid-level vorticity lobe moving NE across KS/OK this
afternoon will lead to initial convective development across
MO/AR/WKY/WIN/SCIL as early as late this afternoon/evening.  Deep
tropospheric wind fields are expected to overspread the Ohio Valley
along with a strengthening low-level flow that will transport
deep/rich Gulf moisture northward into the region.  The net result
of this combination of increasing instability and shear will lead to
a severe weather episode over the Ohio Valley tonight and early
Saturday morning.  Damaging winds in excess 60 MPH and isolated to
scattered reports of tornadoes are expected.  In addition, copious
amounts of moisture will lead to heavy rainfall which may result in
some minor flooding issues across the area.

====================================
Meteorological Technical Discussion
====================================

Surface warm front is rather diffuse this afternoon, but boundary
looks to be crossing through the region at this time, based on band
of elevated convection oriented from W/C IN southeast through
eastern KY.  This activity will continue to move to the northeast
this afternoon.   Once this activity clears out, we will be in the
open warm sector with moisture advection continuing through the
evening hours.  Dewpoints are currently in the mid 50s across the
region but are approaching 60 in our southwest areas.  A stronger
dewpoint surge is currently in progress out along the MS river and
dewpoints have already risen into the upper 60s over far southwest
Kentucky.

In the near term, a mid-level short wave evident in the GOES
moisture channel was moving NE across OK/KS. This feature will
translate northeastward into the Midwest this evening.  This feature
will likely lead to initial convection firing over AR/MO/WKY/WTN and
SCIL later this afternoon/evening.  Further east over our area,
continued moistening and theta-e advection will continue across our
region.  The front edge of a strong low-level jet axis will work
northeastward into very lower part of the Ohio Valley.  As this
occurs, additional cooling aloft is expected to overspread portions
of SWIN/WKY which should allow higher instability to develop as mid-
level lapse rates steepen.  These two items will also help erode the
elevated mixed layer of warm air aloft and result in a possible
uptick in surface based convection across our western CWA this
evening.  Given the parameter space, initial cells that develop will
like be discrete/scattered and given the shear profiles a mix of
multicell and supercells are expected.  These storms will likely
race off to the northeast at 55-60 MPH.  Threats with this activity
will be damaging winds, perhaps some marginally severe hail, and
isolated tornadoes.

Many members of the various short range ensemble models generally
support this idea, i.e., more members that not.  However, the
eastward extent of pre-frontal convective development this evening
remains uncertain due to unknowns involving how much of the EML will
erode and whether surface based convective inhibition will remain.
In general, the best chances of convective development this evening
will be in areas along and west of the I-65 corridor.

Later in the overnight, it appears that additional convection will
develop given the instability and deep forcing resulting in more of
a linear QLCS.  Given the strong shear and substantial front-
parallel components of the deep layer flow, downward momentum
transfer should yield instances of wind damage within the convective
line. Additionally QLCS mesovortices within the squall line where
bows and surges develop may produce several instances of embedded
tornadoes.  This line will work through region during the late night
hours and continue into our eastern sections after sunrise Saturday.
The expected progressive nature of this line should result in
generally short residence time of heavy rainfall over any one
particular area.  Additional QPF information may be found below in
the Hydrology section of the discussion.

We will continue to monitor observational data closely this
afternoon and evening especially upstream.  As cells develop and the
convective evolution becomes more certain, we will be able to update
our timing and speak a bit more confidently with regards to the
possible magnitude of this severe weather event.

====================================
Important Messaging
====================================

As we have mentioned over the past 24 hours, we want to stress the
danger in the overnight severe weather potential.  Historically,
tornadic and widespread severe weather events in the month of
December are rare, although we've had a couple of events in the last
10 years (12/2013 and 12/2015).  We would like to emphasize the
following topics:

1. Have multiple ways to get warnings overnight.  Leave your cell
phone on and have WEA alerts turned on.  Check the status of your
weather radio and make sure it is functioning.  Charge/replace
batteries in your alert devices.

2. Review your severe weather plans for your household.  Review
where your lowest level interior rooms in your home are and have
space ready in there to shelter. Put as many walls as you can
between you and the outside of your home.  If you have a basement,
make sure a shelter space in an interior room or under substantial
framing is available and ready in the event of a warning.  Stay away
from exterior windows and doors.

3. If you live in a mobile home, consider finding a more substantial
shelter this evening with friends and/or family.  Even in non-
tornadic winds, gusts of 60+ MPH can easily flip mobile homes
whether they are tied or bolted down.

4. These storms will be moving quickly with speeds of 55 to 60 MPH.
Warning lead time on these storms could be a challenge especially
involving mesoscale vortices within the anticipated squall line.

====================================
Forecast Confidence
====================================

Tornado Threat:
West of I-65: Medium-High
East of I-65: Low-Medium

Wind Threat:

High confidence across our region

Flooding Threat:

Medium confidence across our region

.Long Term...(Saturday night through Friday)
Issued at 320 PM EST Fri Dec 10 2021

After an stormy start of the weekend, the latter part of the
forecast will certainly be more relaxed. Ejecting upper wave will
give way to anomalous ridging aloft and surface high pressure for
the easternmost two-thirds of the CONUS. Meanwhile, upstream flow
will feature a series of digging troughs which will slowly advance
towards the central US by midweek, eroding the high-pressure dome.

Sunday-Monday... Morning temperatures both days will be below
freezing thanks to a combination of CAA and radiational cooling.
Afternoon highs will gently move from the upper 40s on Sunday to the
lower 50s at the start of the work week. A dry air mass in place
secures clear skies and plenty of sunshine as the surface high
pressure moves to the Mid Atlantic states and an West Coast trough
moves to Central Plains.

Tuesday-Wednesday...The aforementioned large scale configuration
imposes a southerly wind and accompanying increased theta-e
advection. As a result, temperature readings will progressively
climb and the forecast reflects maximum temps around the upper 60s
south of the Parkways by midweek. Expect winds to start picking up
on Wed with gusts in the 10-15 mph range while cloud coverage spread
our area.

Thursday-Friday...As the Central Plains trough progresses to the
Midwest/Great Lakes region, it pushes an attendant cold front to the
Ohio Valley. However, strong ridge aloft anchored over the SE CONUS
will inhibit further southward momentum of the baroclinic zone,
therefore, it could stall south of our area or right above us. There
is still some timing uncertainty with this evolution as noted in the
WPC mid-level EOF cluster patterns. That being said, there is
increasing agreement for warmer temps on Thursday which could result
in record high low for SDF. The latest NAEFS ESA guidance has 850-mb
temps around the 90th climo percentile. Winds gusts will be blowing
from the southwest around 15-20 mph on Thursday as the front sweeps
by central KY and diminishing by Friday. Finally, precipitation is
boosted for the end of next week and probably next weekend with the
frontal passage and subsequent stalling nearby. This scenario is
also reflected by the slight risk of heavy precipitation delineated
by CPC during the Dec 17-20 timeframe as return flow from the GoM
leads to high rain chances.

&&

.Aviation...(06Z TAF Issuance)
Issued at 1226 AM EST Sat Dec 11 2021

IMPACTS:

-Thunderstorms with torrential rain and gusty winds in the first
several hours of the TAF, especially before dawn
-Gusty winds outside of storms
-Low ceilings

DISCUSSION:

A powerful spring-like storm system will bring gusty winds and
strong thunderstorms to the region during the early morning hours.
Cold FROPA will occur from west to east during the daylight morning
hours, bringing an end to the storms. However, winds will remain
gusty as cold air rushes in behind the front. Winds will come in
from the south ahead of the front, switching to west behind the
front, then becoming light from the northwest tonight as high
pressure builds in.

CONFIDENCE:

Medium-high on all elements.

&&

.Hydrology...
Issued at 320 PM EDT Fri Dec 10 2021

Deep, moist SW flow characterized by PW~1.5" is being advected along
the warm sector ahead of an amplifying upper-level trough. Attendant
cold front will swing through our area Friday night/Saturday morning
bringing a strong-to-severe line of thunderstorms. Heavy rain from
this event might provoke localized flooding and river flow rises,
especially streams that experienced flow peaks during the last event.

Current forecast package is considering QPF in the 1-1.5" range
associated with the main frontal passage. Meanwhile 3-hr FFG sits
between 2-3" area-wide and the 0-40 cm soil moisture is still
average per the latest NASA SPORT analysis. Therefore, rainfall
should be largely managable giving minimal runoff. Additionally,
HREF 1-hr probabilities for QPF greater than 1" is less than 20%,
which aligns well with the progressive nature of the system. Only
problem for stagnant water is in poor drainage areas and low-lying
locations. WPC still manintains a Marginal Risk (1 out of 4) of
excessive rainfall due to the strong linear forcing, theta-e
advection, and any localized areas of training convection given the
alignment and strenght of the LLJ and the cloud-layer mean wind.

Another factor to consider is the antecedent hydrological conditions
of rivers that experienced flooding/rises this past Monday and
respond quicly to short periods of heavy precipitation, such as:
Rochester Ferry, Paris, Peaks Mill,and Boston. Other rivers worth
monitoring during the next 24 hours per the lastest AHPS/HEFS
forecast are the Green and White Rivers. It is important to mention
that the OHRFC does not expect any widespread risk of significant
river flooding.

&&

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

Update...13/DM
Short Term...MJ
Long Term....ALL
Aviation...13
Hydrology...ALL

Hazardous Weather Outlook...UPDATED
National Weather Service Louisville KY
741 PM EST Fri Dec 10 2021

KYZ036-037-040>043-047>049-055>057-066-067-077-078-081-082-120045-
Scott-Harrison-Woodford-Fayette-Bourbon-Nicholas-Mercer-Jessamine-
Clark-Boyle-Garrard-Madison-Casey-Lincoln-Adair-Russell-Cumberland-
Clinton-
Including the cities of Georgetown, Cynthiana, Versailles,
Lexington, Paris, Carlisle, Harrodsburg, Nicholasville, Winchester,
Danville, Lancaster, Richmond, Liberty, Stanford, Columbia,
Jamestown, Burkesville, and Albany
741 PM EST Fri Dec 10 2021 /641 PM CST Fri Dec 10 2021/

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

.DAY ONE...Tonight.

Evening and overnight storms may produce damaging winds and isolated
tornadoes as a cold front approaches and moves through the region
late tonight into early Saturday morning. Intense rainfall rates may
lead to localized flooding concerns.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Behind the front Saturday morning non-thunderstorm winds may gust to
40 mph.

.SPOTTER INFORMATION STATEMENT...

Spotters will be needed tonight into Saturday morning.

INZ076>079-083-084-089>092-KYZ023>035-038-039-045-046-053-054-
061>065-070>076-120045-
Orange-Washington-Scott-Jefferson-Dubois-Crawford-Perry-Harrison-
Floyd-Clark-Hancock-Breckinridge-Meade-Ohio-Grayson-Hardin-Bullitt-
Oldham-Trimble-Henry-Shelby-Franklin-Spencer-Anderson-Nelson-Larue-
Marion-Butler-Edmonson-Hart-Green-Taylor-Logan-Warren-Simpson-Allen-
Barren-Monroe-Metcalfe-
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, Taylorsville,
Lawrenceburg, Bardstown, Springfield, Hodgenville, Lebanon,
Morgantown, Brownsville, Horse Cave, Greensburg, Campbellsville,
Russellville, Bowling Green, Franklin, Providence, Scottsville,
Glasgow, Tompkinsville, and Edmonton
741 PM EST Fri Dec 10 2021 /641 PM CST Fri Dec 10 2021/

...TORNADO WATCH 554 IN EFFECT UNTIL 3 AM EST /2 AM CST/ SATURDAY...

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

.DAY ONE...Tonight.

Evening and overnight storms may produce damaging winds and isolated
tornadoes as a cold front approaches and moves through the region
late tonight into early Saturday morning. Intense rainfall rates may
lead to localized flooding concerns.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Behind the front Saturday morning non-thunderstorm winds may gust to
40 mph.

.SPOTTER INFORMATION STATEMENT...

Spotters will be needed tonight into Saturday morning.

BJS

Special Weather Statement
National Weather Service Louisville KY
130 AM EST Sat Dec 11 2021

KYZ028-029-045-110645-
Bullitt KY-Nelson KY-Hardin KY-
130 AM EST Sat Dec 11 2021

...A strong thunderstorm will impact portions of north central
Nelson, northeastern Hardin and Bullitt Counties through 145 AM
EST...

At 130 AM EST, Doppler radar was tracking a strong thunderstorm 8
miles southwest of Shepherdsville, moving northeast at 60 mph.

HAZARD...Wind gusts up to 50 mph and half inch hail.

SOURCE...Radar indicated.

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

This strong thunderstorm will be near...
  Shepherdsville around 140 AM EST.
  Pioneer Village around 145 AM EST.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Tornado Watch remains in effect until 300 AM EST for central
Kentucky.

&&

LAT...LON 3803 8550 3801 8551 3797 8546 3796 8544
      3790 8538 3783 8590 3794 8596 3808 8561
      3807 8558 3808 8554
TIME...MOT...LOC 0630Z 246DEG 55KT 3790 8583

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...50 MPH

DM

Special Weather Statement
National Weather Service Louisville KY
133 AM EST Sat Dec 11 2021

INZ078-079-092-KYZ029>034-038-110700-
Jefferson IN-Clark IN-Scott IN-Jefferson KY-Oldham KY-Shelby KY-
Bullitt KY-Henry KY-Spencer KY-Trimble KY-
133 AM EST Sat Dec 11 2021

...A strong thunderstorm will impact portions of Jefferson, eastern
Clark, southeastern Scott, northwestern Spencer, Oldham, Shelby,
Trimble, Henry, Jefferson and northern Bullitt Counties through 200
AM EST...

At 133 AM EST, Doppler radar was tracking a strong thunderstorm near
Madison, moving northeast at 85 mph.

HAZARD...Wind gusts of 50 to 55 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.

This storm will remain over mainly rural areas of Jefferson, eastern
Clark and southeastern Scott Counties in south central Indiana,
Jefferson, Oldham, Shelby, northern Bullitt, Henry, northwestern
Spencer and Trimble Counties in north central Kentucky, including the
following locations... Yoder, North Madison, Campbellsburg, Manville,
Buckner, Ballardsville, Smithfield, Crestwood, Clay Village and
Vesta.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Tornado Watch remains in effect until 300 AM EST for south central
Indiana...and central Kentucky.

&&

LAT...LON 3869 8520 3874 8527 3874 8533 3863 8523
      3858 8521 3860 8508 3856 8506 3857 8504
      3855 8500 3851 8503 3852 8498 3849 8499
      3848 8497 3810 8510 3796 8589 3891 8546
      3892 8520
TIME...MOT...LOC 0633Z 235DEG 75KT 3880 8534

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...55 MPH

KDW

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE LOUISVILLE KY
150 AM EST SAT DEC 11 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1232 AM     TSTM WND DMG     CLOVERPORT              37.83N 86.63W
12/11/2021                   BRECKINRIDGE       KY   911 CALL CENTER

            REPORT OF LARGE TREE DOWN IN CLOVERPORT ON
            105.

&&

BTN

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE LOUISVILLE KY
155 AM EST SAT DEC 11 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1245 AM     TSTM WND DMG     1 W HARDINSBURG         37.78N 86.47W
12/11/2021                   BRECKINRIDGE       KY   911 CALL CENTER

            REPORTS OF TREE DOWN ON 992.

&&

BTN

Special Weather Statement
National Weather Service Louisville KY
201 AM EST Sat Dec 11 2021

INZ076-084-089-110715-
Crawford IN-Orange IN-Perry IN-
201 AM EST Sat Dec 11 2021

...A strong thunderstorm will impact portions of northern Crawford,
north central Perry and southeastern Orange Counties through 215 AM
EST/115 AM CST/...

At 201 AM EST/101 AM CST/, Doppler radar was tracking a strong
thunderstorm 8 miles southwest of English, moving northeast at 60
mph.

HAZARD...Wind gusts of 50 to 55 mph and half inch hail.

SOURCE...Radar indicated.

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

This strong thunderstorm will be near...
  English around 210 AM EST.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Tornado Watch remains in effect until 300 AM EST/200 AM CST/ for
south central Indiana.

&&

LAT...LON 3825 8668 3829 8668 3843 8644 3831 8633
      3823 8667 3825 8669
TIME...MOT...LOC 0701Z 238DEG 52KT 3828 8660

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...55 MPH

KDW

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