Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 06/19/21

Area Forecast Discussion

National Weather Service Louisville KY

 937 PM EDT Fri Jun 18 2021

.Mesoscale Update...
Issued at 937 PM EDT Fri Jun 18 2021

An axis of 2000-3000 J/KG of CAPE still resides along and north of
the I-64 corridor this evening, with explosive convection ongoing
across south central IN into SW Ohio. Initial supercellular
structures are transitioning to bowing segments as storms evolve. Of
immediate concern is a cold pool dominated bowing segment coming out
of the Cincinnati area and heading for our NE Bluegrass counties of
Scott and Harrison. May have to warn on those if the damaging wind
threat persists into our area.

Of secondary concern is an isolated supercell currently over Brown,
Bartholomew, and Jackson counties. This storm is heading southeast
an an environment characterized by ~200 M2/S2 of effective SRH.
LCL`s are a little high for a more concerning tornado threat, but
something that will have to be monitored as it approaches Jefferson
county Indiana over the next 1 to 2 hours.

SPC went with a Severe Thunderstorm Watch until 07z for our
northeast CWA counties. Most confident about the severe threat in
our far north and east counties in the watch, with less confidence
on southward extent into the overnight. Nevertheless, the right
moving supercells and/or SE moving bowing segments could carry the
severe threat a bit farther south than what models had shown, hence
the extra buffer area. For what it is worth, a recent AMDAR sounding
showed a strong cap at 800 mb over Louisville, so don`t think the
threat will extend that far south, at least for the evening and
first part of the overnight.

Updated products already out. Will be monitoring.

Previous Update...
Issued at 650 PM EDT Fri Jun 18 2021

Latest mesoanalysis still shows a strongly unstable environment
characterized by 3000+ J/KG of surface based CAPE along and north of
I-64. However, latest AMDAR soundings all show a notable subsidence
inversion around H8. This feature is expected to hold pretty steady
through the evening hours, to the point that convection isn`t
anticipated across our CWA. It would take a pretty notable trigger
to overcome the capping, and given the lack of an outflow boundary
or other mesoscale feature agree with the the hi res models that
we`ll likely see a dry evening. Will continue to monitor just in
case.

After Midnight, there is enough data that suggests our NE CWA could
catch the tail end of the convective complex that fires over central
IN and translates southeastward. The placement of the Flash Flood
Watch from day shift still looks good, but overall don`t have a lot
of confidence in how much rain will actually fall out of this
feature. If it stays progressive doubt we`ll see enough rainfall to
get flash flooding, however if the southern end anchors against the
deep layer flow and continues training occurs then numbers closer to
the current WPC QPF may be realized. Will leave things as is at this
time to see how things play out.

Still think our best chance of any stronger or severe storms will
arrive around or just after sunrise in our northern CWA when a
convective complex drops in from the NE. Hi res models suggest that
this will be a cold pool driven gusty line of storms by this point,
with the main threats being widespread gusty winds/isolated damaging
winds along with heavy rainfall and lightning.

&&

.Short Term...(This evening through Saturday)
Issued at 313 PM EDT Fri Jun 18 2021

Rich low-level moisture, including surface dew points in the low and
mid 70s, is feeding northeastward ahead of a cold front stretching
from Michigan to Kansas. This moisture is beneath an EML reaching
from the central Plains to the Ohio Valley with local soundings
showing large CAPE above a low level cap. Forcing ahead of the cold
front and a 5H speed max over the upper Mississippi Valley have led
to elevated thunderstorm development along a mesoscale boundary from
northeast Illinois into northwest Indiana this afternoon. This
activity is not the main show, but it is indicative of the primed
atmosphere in this region for thunderstorm development.

Tonight convection should increase in coverage and intensity,
initially to our north, as the night goes on. Low-mid level flow will
become more unidirectional with time, becoming general west-east
after midnight. A potent low level jet will develop from northeast
Missouri to northeast Indiana. This west-east jet should line up
with the convection with thunderstorms heading east but backbuilding
westward into the jet and diffluent 1000-500 thicknesses upstream.

The corridor of maximum surface theta-e tonight will be from central
Illinois to central Ohio, with a 850mb theta-e ridge axis over the
northern half of the Ohio Valley in pool of high 850mb RH, further
enhancing the idea that storms will, at least initially, move from
west-to east from central Illinois through central Indiana to Ohio.
850-300mb MCS steering flow does start the evening in a WNW-ESE
orientation, but overnight becomes more west-east.

0-6km bulk shear values increase to 50kt after midnight from central
Illinois into central Indiana. This, plus the above, suggest a large
bow echo MCS may develop and sweep from west to east (SPC SREF
derecho composite is progged to be around 2). Shear and instability
decrease to the south, however a strong cold pool and sharp outflow
boundaries can cause this type of system to develop farther
southward than the data might initially suggest, so storms are still
in the forecast tonight for southern Indiana and north central
Kentucky, with the best chance of the strongest storms likely
between midnight and dawn. Freezing heights suggest small hail will
be possible (marginally severe in the strongest updrafts). Sounding
progs show that the storms may be elevated, but only slightly, so we
still can`t rule out some stronger gusts making it down to the
surface.

Another issue to consider is heavy rain. The main corridor of heavy
rain will, again, be to our north...especially if backbuilding does
develop. Surface dew points will be in the 70-75 degree range and
precipitable water values will be over two inches. So, after
coordinating with IND and ILN, will issue a Flash Flood Watch for
areas from Scottsburg to New Castle.

Tomorrow the synoptic surface boundary will stay well to our north,
while the tropical system moving northward through Mississippi will
stay well to our south. The models don`t print out much convection
here between these two systems. But, with a warm and slightly
unstable atmosphere in place and possible outflow or differential
heating boundaries from early morning convection and clouds, will
continue with a low PoP. It will be another hot and muggy day with
highs in the upper 80s to lower 90s and dew points of 65-70.

.Long Term...(Saturday night through Friday)
Issued at 257 PM EDT Fri Jun 18 2021

By Saturday night, rain showers are expected to have come to an end.
Most model guidance has the quasi-stationary front to our north,
over northern Indiana, keeping precipitation farther north, while
the low pressure system coming out of the Gulf of Mexico keeps its
associated moisture to our south. The trend is drying the CWA out
through Monday. High temperatures on Sunday are expected around 90,
a few degrees cooler on the eastern side of the CWA.

On Monday, a deep surface low, moving east-northeast through the
Great Lakes, will pull a trailing cold front through southern
Indiana and central Kentucky. With the front stretching southwest to
northeast as it passes through the CWA, The northwestern side of the
CWA will experience precipitation earlier in the day, probably
during the late morning hours. Showers will expand south and
eastward in the following hours. If this timing holds, expecting a
line of stronger storms and heavier precipitation to form in the
late afternoon to evening hours. This line would continue to the
east until out of the CWA by early Monday night.

Looking at the severe weather potential, model soundings show very
strong instability (over 3,000 J/kg) in the southern half of the
CWA, but deep layer shear (0-6 km) is around 30 knots. DCAPE values
reach around 850 J/kg, and there is some strong low level storm
relative helicity at times. These values don`t guarantee severe
weather, but dew points in the 70s and precipitable water values
over 2" in unstable environments with some forcing do cause heavy
rainfall. Expect gusty winds, lightning, and heavy rainfall with the
line. As the event nears, timing and details will come into better
focus.

This is a strong June cold front. Monday`s high temperatures around
90 degrees will be reduced to the 70s on Tuesday as northerly winds
keep temperatures suppressed, but on Tuesday night, high pressure
quickly moves past the region. Return flow will slowly lift
temperatures into the mid 80s by Friday.

By Thursday or Friday, another shortwave pushing along upper flow
will bring the next round of precipitation to the area. Guidance
still hasn`t found agreement, so kept the blend in the forecast.

&&

.Aviation...(00Z TAF Issuance)
Issued at 737 PM EDT Fri Jun 18 2021

IMPACTS: Showers and thunderstorms possible Saturday morning.

DISCUSSION: A W-E oriented boundary across central IL/IN will be the
focus of convection this evening with a line a showers and storms
expected to move south through central KY Saturday morning.
Potential for heavy rain and strong convective winds exists with
this convection that could reduce VIS to MVFR and possibly to IFR in
the heavier storms. Latest guidance suggests the best time frame for
storms will be from roughly 19/12Z through 19/15Z affecting HNB,
SDF, and LEX. Another round of showers and thunderstorms is possible
after 20/00Z.

CONFIDENCE: Medium confidence in timing of thunderstorms.


&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...Flash Flood Watch through Saturday morning for INZ078-079.

KY...Flash Flood Watch through Saturday morning for KYZ032-033.

&&

$$

Mesoscale...BJS
Short Term...13
Long Term...KDW
Aviation...CG