Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 271658
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1258 PM EDT Sat May 27 2017

...Updated Aviation Discussion...

...Forecast Update...
Issued at 1132 AM EDT Sat May 27 2017

A few showers and storms have already begun to develop across south
central KY this morning with hail reported in Morgantown. Storm
development still should be somewhat limited over the next couple of
hours due to the cap still in place. However, the latest AMDAR
soundings have shown this inversion eroding this morning with 700 mb
temps cooling to 9-9.5 C from 12+ C earlier this morning. Scattered
storms are still expected to develop this afternoon. Currently, the
greatest wind threat looks to be across southern IN and north
central KY where DCAPE is maximized at 1000-1200 J/kg. Any stronger
storm across the forecast area could produce marginally severe hail
as well. The forecast was updated based on current radar trends and
to decrease cloud cover through the early afternoon hours.

.Short Term (Now through Sunday)...
Issued at 335 AM EDT SaT May 27 2017

...Multiple Severe and Flooding Threats Expected Through Sunday...

An active and complex 2 days of weather is expected to unfold across
the Ohio River Valley. During this time, multiple periods of severe
weather are possible along with a localized hydro threat for areas
that see repeated thunderstorms.

This Morning Into Early Evening...

Recent AMDAR soundings from early morning arrivals into KSDF reveal
a very stout 700 mb inversion around 12+ C. This has kept activity
quiet across our area, despite CAM and operational models trying to
develop isolated to scattered showers and storms. Forecast soundings
have had a good handle on this inversion, and it should hold through
sunrise, before gradually weakening as we move toward midday and
afternoon. So, early morning model depiction of shower and storm
chances appear to be overdone, but do buy into scattered development
by this afternoon as the atmosphere becomes strongly unstable.
Warm/moist low levels characterized by surface T/Td around 80/70
beneath very steep mid level lapse rates should yield MLCAPE Values
around 2500-3500 J/KG. Some higher instability values may be
realized where low 70s Tds are able to pool. Overall deep layer
shear profile for this afternoon isn`t that impressive with marginal
20-30 knot values likely supporting a mixture of pulse and briefly
organized clusters of storms. Given the amount of instability and
dry air in the mid levels, DCAPE values around 1000 J/KG should
support a localized damaging wind threat. In addition, a large hail
threat will exist with the strongest updrafts. At this point, feel
the strongest threat for an afternoon severe storm will be across
south central KY where capping looks a little weaker than in the
northern CWA.

Will also have to watch for a localized Flash Flood threat if any
training or repeated t-storm activity takes place. In particular,
worried about a late morning/midday complex surviving from the
Ozarks into our SW CWA, and about an afternoon E-W oriented boundary
somewhere near I-64 that could be the focus for training storms. In
coordination with JKL, part of the WPC Excessive Rainfall Slight
Risk, and the potential for multiple rounds of convection with high
PWATS, will hoist a Flash Flood Watch along and south of I-64 this
afternoon through Sunday.

Overnight...

Scattered convection should continue into the early evening before a
relative lull occurs ahead of the next severe weather threat. Hi-res
models have come into relative agreement of an organized MCS that is
currently over NE/KS re-intensifying this evening in an extremely
unstable environment over MO and surrounding states. It appears this
complex will take an ESE direction with a compromise between almost
due east 700-300 mb flow and the stronger instability gradient to
it`s south. This would result in the strongest forward propagation
occurring across the Mo Bootheel into western KY and down into
western TN. The strong cold pool associated with the complex would
still pack a punch into our CWA, especially along and west of I-65,
and across southern KY. Moderate instability should still exist in
the 1500-2500 J/KG range in these areas. A reasonable estimate would
be for fairly widespread 45-55 mph winds, with localized pockets of
60+ mph winds sometime between Midnight and 6 AM EDT. The best
chance for the strongest winds would be in our SW CWA where some
more significant wind gusts can`t be ruled out. Also don`t want to
leave out that lightning could be quite prolific at times this
afternoon and overnight.

Looking at other severe threats, can`t rule out an isolated spin up
tornado, however low level shear profiles are not ideal. Best threat
for this occurrence would also be in our SW CWA, or along any remnant
boundary from previous convection. The large hail threat should be
relatively low, but not zero, as the complex will be very outflow
dominant by this point. Finally, localized flooding will be a
concern, especially for areas that see afternoon storms. The
progressive nature of the forward propagating MCS is a limiting
factor to a more widespread flooding concern.

Sunday...

We`ll spend much of the day recovering from the stabilized airmass
that the massive cold pool leaves behind. However by late afternoon
and early evening, forecast soundings show destabilization again
ahead of an approaching cold front. Won`t spend too much time on
Sunday, but the idea is that we will have another severe threat if
we are able to destabilize. Main threat would be localized damaging
winds, hail, and an isolated flooding concern.

.Long Term (Sunday night through Friday)...
Issued at 335 AM EDT Sat May 27 2017

Sunday Night...

By Sunday night, a roughly 1002mb surface low will move through the
eastern Great Lakes ahead of shortwave rotating through the parent
trough. As the system pushes east, it will drag a cold front through
the region by dawn on Monday. Ahead of the front, expect scattered
showers and thunderstorms to continue. Instability should be waning
by the evening hours, however lingering modest instability may be
enough to support a lingering gusty wind threat. Also some localized
flooding may be a concern depending on how much rain is received
earlier in the weekend. Lows should drop into the low 60s by dawn on
Monday.

Memorial Day - Night...

Expect to salvage a nice day Memorial Day as mid level dry slot
works into our region south of the occluded and expansive upper low
over the western Great Lakes. Meanwhile, cold front should be far
enough southeast to keep mentionable pops away from the Lake
Cumberland region. Expect plenty of sunshine with highs topping out
in the upper 70s and touching 80.

On Monday night, a few light showers could work into southern
Indiana after Midnight as another shortwave rotates through the
parent trough. Otherwise, expect dry with lows bottoming out in the
upper 50s to near 60.

Tuesday - Friday...

The eastern CONUS trough stays established through the remainder of
the work week, with individual disturbances rotating through the
base of the trough. Meanwhile, a weak frontal boundary may set up
around or just south of our CWA by mid to late week. Will carry low
chances of mainly diurnally driven shower and storm chances during
this time.

Temps during this time should mostly be in the upper 70s to near 80.
Meanwhile, lows should mostly be in the upper 50s and low 60s.

&&

.Aviation (18Z TAF Update)...
Issued at 1250 PM EDT Sat May 27 2017

Forecast confidence is fairly low as the hi-res models are starting
to lose their handle on what will happen. Main challenges are
convective potential this afternoon, and then the impact of the MCS
likely to affect at least some part of Kentucky late tonight.

MVFR stratus deck that has taken SDF and LEX in and out of fuel-
alternate this morning is finally dissipating. Initialization looks
like a last-minute call but may be able to start off VFR, but any
cloud deck will be between 2000-3000 feet.

For this afternoon, the air mass is quite moist and unstable, with
quite a few boundaries left behind by convection that passed well to
our north last night. Most model runs show scattered storms popping
up after 19-20Z, and that seems reasonable but we will limit the
VCTS mention to SDF and LEX, where the strong mid-level cap is
likely to erode first. Look for any storms that develop to dissipate
around sunset, as wind fields appear too weak to provide much
organization.

The evening looks mainly dry, but the much-anticipated, outflow-
dominated MCS will roll quickly across the area after midnight.
Given the expected structure of this system, will hit winds hard,
with 30-40 kt gusts at times. Still some question as to the track,
and will only carry a TEMPO at BWG, which seems most likely to be
impacted. With the lower confidence at SDF and LEX, will only carry
a PROB30, and not hit the winds quite as hard.

Low ceilings will linger into the morning behind the convective
complex, but the best chance of continued precip and lower ceilings
will be at BWG as the heavier convection could train across
Tennessee on Sunday morning. Will carry fuel-alternate ceilings and
mention VCSH. Afternoon convection along the synoptic cold front
should be late enough to be beyond the scope of BWG and LEX TAFs,
but will mention VCTS/CB at SDF late in the day.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...Flash Flood Watch through Sunday evening for KYZ027>030-034>036-
     038>042-045>049-053>057-061>067-070>078-081-082.

IN...NONE.
&&

$$

Update.........EER
Short Term.....BJS
Long Term......BJS
Aviation.......RAS


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