Areal Flood Warning 25, NWS Louisville, KY

Area Forecast Discussion
National Weather Service Louisville KY
802 PM EDT Wed Oct 6 2021

.Mesoscale Update...
Issued at 802 PM EDT Wed Oct 6 2021

A deep southerly flow continues across our area ahead of the closed
low over MO/AR. So far, almost all of the activity has been focused
across our SE CWA and up the I-75 corridor where a few rotating
storms, and persistent light to moderate rain has occurred. The
Lincoln county supercell has weakened but has now merged into a
cluster over Garrard county that could become a wind producer. Will
continue to monitor that threat. Our Blue Grass region will still
have to be monitored for the next few hours as there is a modest
surface based CAPE gradient (1000-1500 J/KG), around 100-150 M2/S2
of 0-3 KM SRH, and continued low LCLs around ~750M. It appears the
SRH values are really being enhanced by the E and NE surface winds,
as the rest of the deep layer flow is fairly unidirectional. There
are some notable enhancements in some of the tornadic parameters
evident on the SPC Mesoanalysis, but overall the environment is
still fairly marginal and it will take pretty impressive signatures
(like earlier this evening) to prompt warnings.

Upstream, we are also watching a persistent tornado warned storm
coming out of MRX/OHX's area that could flirt with our mutual border
with JKL. There have been a few damage reports with this storm.
However, this area has been worked over so expect the storm to
weaken as it approaches.

Finally, we still have to keep an eye on our SW CWA where the best
instability remains thanks to afternoon clearing. Don't think the
tornado threat would be quite as high in this area as more NNW
movement to the left of the mean flow would take storms farther away
from good streamwise vorticity ingestion. Plus, we should be setting
up a near surface stable layer soon with the loss of heating so any
rotating storm should be somewhat elevated. DCAPE values are higher
across our SW CWA, so perhaps a few instances of gusty winds would
be possible.

&&

.Short Term...(This evening through Thursday)
Issued at 305 PM EDT Wed Oct 6 2021

GOES-16 Water Vapor shows stunning imagery of the upper low now
located over MO/AR with convection firing up along a N-S line from
central AL into central TN. Latest surface analysis places a low
center almost directly below the upper low over the MO/AR border
with a warm front extending NE across southern IL/IN/OH. With our
position on the eastern side of the low, deep southerly flow will
continue to pump moisture rich Gulf air over the region resulting in
above normal PWATs around 1.60 inches and dew points in the upper
60s. With that said, expect a chance of showers and thunderstorms to
continue through Thursday night.

Regarding thunderstorms, mesoanalysis shows an instability axis
across western KY with SBCAPE values around 1500 J/kg. More
concerning is the axis of higher shear values extending northward
into central KY with effective bulk shear values 25-30kt. Soundings
show unidirectional wind profiles with modest mid level flow. Can't
be ruled out that this could potentially split any stronger
updrafts, with slight backing H5 winds suggesting some left moving
storms. But with freezing levels around 12kft and saturated columns,
not expecting severe hail. There is some near severe wind gust
potential, especially in our SW counties, where higher DCAPE values
exist. There is also a low end threat for a brief spin up tornado in
any stronger storms. As a result, SPC does have a Marginal Risk over
our CWA west of I-75.

Showers and cloudy skies will make for another mild morning with
lows in the low to mid 60s. Thursday morning will see a mixed bag of
weather including showers, drizzle, patchy fog, and possible
thunderstorms. Showers and storm chances will continue throughout
the day Thursday as the upper low slowly makes its way into IL. With
these continuing showers and storms, there are minor flooding
concerns, but current flash flood guidance and QPF lean more toward
nuisance flooding of typically prone areas so no Flood Watch is
being considered at this time.

.Long Term...(Thursday night through Wednesday)
Issued at 320 PM EDT Wed Oct 6 2021

Thursday Night - Friday...

The closed upper low will move from the mid Mississippi River Valley
up into the Great Lakes to end the work week, transitioning to an
open wave as it does so. We'll hang on to some lingering showers
Thursday night into Friday as the system departs our area, but
overall things will trend drier from the previous 24 hours. Look for
lows in the upper 50s and low 60s Thursday night, and highs just
above normal in the mid to upper 70s on Friday.

Friday Night - Monday...

A strengthening upper ridge axis will build into our area over the
weekend, stretching from Mexico northeastward into the lower Great
Lakes. This will bring a return to dry conditions and temperatures
warming back into the upper 70s and low 80s on Saturday, and into
the low to mid 80s by Sunday. This would be about 10 degrees above
normal for this time of year.

The warm and dry conditions will likely persist into Monday as the
upper ridge tries to hold on ahead of a digging negative tilt
shortwave into the mid Mississippi River Valley. We'll continue to
advertise dry conditions with temps mostly in the low 80s.

Monday Night - Wednesday...

It is entirely possible that the early to mid week time frame also
stays dry or mostly dry and warmer than normal as a good amount of
the data keeps the upper ridge established enough over our area. The
forecast will mostly reflect that and will only have some smaller
chances across our western CWA to account for the shortwaves passing
to our NW.

&&

.Aviation...(00Z TAF Issuance)
Issued at 750 PM EDT Wed Oct 6 2021

IMPACTS: Periods of IFR/MVFR CIG/VIS in showers and thunderstorms.
Potential IFR CIGs Thursday morning. Continued periodic MVFR
conditions in showers and thunderstorms into Thursday afternoon.

DISCUSSION: Current radar imagery shows area of thunderstorms
approaching KLEX, with frequent lightning and brief torrential
rainfall expected. Scattered showers and storms will continue to be
possible across the area into Thursday, with brief reductions in
CIGs and VIS into IFR range possible in heavier showers/storms.
Additionally, expect CIGs to lower into low-end MVFR range and
potentially IFR towards sunrise Thursday morning then gradually
lifting through the afternoon. Outside of storms, winds will
generally be out of the E/SE below 10 kts overnight then shifting
more southerly Thursday afternoon.

CONFIDENCE: Medium on timing and conditions.

&&

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

Mesoscale...BJS
Short Term...CG
Long Term...BJS
Aviation...JML

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