Area Forecast Discussion
Issued by NWS Louisville, KY

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494
FXUS63 KLMK 210546
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
146 AM EDT Sun May 21 2017

...Updated Aviation Discussion...

.Forecast Update...
Issued at 954 PM EDT Sat May 20 2017

Convection has been weakening over the last couple of hours as the
storms have pushed into east central KY. A stronger storm or two
with gusty winds will will still be possible over the next couple of
hours but the severe threat has diminished, so the Severe
Thunderstorm Watch will be allowed to expire. There has been a break
in precip along and west of the I-64 corridor. This should last at
least another couple of hours, but mesoscale models do indicate more
showers and storms will develop overnight.

&&

.Short Term...(This evening through Sunday night)
Issued at 245 PM EDT Sat May 20 2017

...Severe Storms & Flash Flooding Possible This Afternoon and
Evening...

Convection continues to fill in along a remnant MCS outflow boundary
from southern Indiana down through western Tennessee. Cumulus field
is expanding quickly in the unstable environment across central KY,
mainly west of I-65 just out ahead of the main convective band. The
second boundary of note is the surface warm front, which has been
making steady northward progress through southern Indiana. It
appears this boundary currently drapes back to the ESE through the
northern Bluegrass Region. Surface winds are SW in Harrison County,
KY, but easterly flow was noted at Covington. A few cells have
developed in the vicinity of this boundary, to the north of
Lexington.

South of the warm front, rain/storm chances will increase from west
to east through the afternoon and evening hours. With the upper low
spinning over the Central Plains, convection will move northeast
into central KY and southern Indiana. Moderate buoyancy has
developed, with 18z SPC meso page indicating 1000-1500 J/kg MLCAPE.
Effective bulk shear is marginal, with 30-35 kts along convective
boundary in western KY/TN. Stronger mid-level winds will remain off
to the northwest, closer to the upper low. Deep layer shear vectors
oriented roughly parallel to the boundary will support primarily
a linear mode, with damaging winds as the main threat. DCAPE
values exceeded 1000 J/kg across the region also support a
damaging wind threat. Isolated hail is possible, but freezing
levels are much higher today than yesterday, so it will take a
really strong updraft. The bigger risk is likely flash flooding,
particularly in Washington County, Indiana and Allen County,
Kentucky. Any additional heavy rainfall may quickly pose a flash
flood threat, so have a way to receive warnings this evening and
tonight.

Scattered storms early this evening will become more numerous late
this evening and overnight as axis of deeper moisture ahead of
surface cold front advects in. The surface cold front lingers way
back across central Missouri this afternoon and isn`t forecast to
push through the area until Sunday afternoon. With convective
activity overnight likely lingering into Sunday morning, and with a
good deal more cloud cover, less instability is expected just ahead
of the cold frontal passage. Favorable vertical shear will lag the
surface cold front as well, so severe weather looks unlikely at this
time. But an uptick in convection looks likely east of I-65 during
the afternoon. Lows tonight will be quite warm, in the mid 60s to
around 70 degrees. Highs on Sunday will make it back into the mid
and upper 70s prior to frontal passage. Showers and storms will exit
to the east Sunday evening, with lows falling into the low to mid
50s by Monday morning.

.Long Term...(Monday through Saturday)
Issued at 310 PM EDT Sat May 20 2017

The upcoming work week looks distinctly cooler than the last. Monday
looks very pleasant, with lower humidity, plenty of sun, and highs
in the low to mid 70s. Much drier air advects in as surface high
pressure builds through the region.

Clouds return Monday night, however, with rain chances returning on
Tuesday. The better chance for rain will be late Tuesday into
Tuesday night as another cold front approaches. This is another
surface low that develops in response to a deep upper trough
dropping southeast. This upper low will control the weather for the
midweek period, with cooler temperatures and a good deal of cloud
cover. Highs Wednesday and Thursday will likely be limited to the
mid and upper 60s, with nighttime lows in the upper 40s and 50s.

Weak upper ridging along with surface high pressure should result in
dry weather Thursday night through Friday night. Blended guidance
suggests highs back in the mid 70s to low 80s for Friday and
Saturday.

&&

.Aviation...(06Z TAF Issuance)
Issued at 140 AM EDT Sat May 20 2017

On and off light rain showers will continue overnight through this
afternoon ahead of a slowly approaching cold front.  There is a
chance for some t-storms this afternoon in convection but feel those
chances are too slim to mention attm.  Thunder may be added in
coming TAFs though.  Rain showers area expected to end this
afternoon after the cold front finally clears the area.

The TAFs are largely VFR except for a period around sunrise where
low level moisture may form light fog in a relative lull in precip.
Also MVFR cigs below 2 kft are expected to form around or before
sunrise and continue through the morning hours before lifting this
afternoon.  Lower flight conditions will be possible in any
convection today.

Winds will veer from SE to SW by mid morning and then W this
afternoon behind the cold front.  Wind speeds will generally be
between 6-12 kts throughout the TAF period with higher gusts
possible in convection.

&&

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

$$

Update...EER
Short Term....EBW
Long Term.....EBW
Aviation...AMS



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