Severe Thunderstorm Warning 59, NWS Louisville, KY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
331 PM EDT TUE MAY 21 2013

.Short Term (Tonight through Wednesday night)...
Issued at 315 PM EDT Tue May 21 2013

A difficult and complicated forecast as a storm system approaches
from the Plains...

A parade of MCSs has taken shape in the WSW flow from the southern
Plains into the Ohio and Tennessee Valleys. One MCS dissipated as it
moved across Kentucky this morning, and the next MCS in line has a
leading convective line extending from just SE of Elizabethtown,
southward into middle Tennessee. Near-severe wind gusts have been
observed already, and the potential remains for SVR winds in an
unstable and mostly unidirectionally-sheared environment. Hail is a
lesser threat, and tornado threat will be very minimal.

Expect showers and storms to continue for the next few hours, and
will try to delineate between the more numerous storms south of the
Bluegrass Parkway and the hit-and-miss activity that remains
possible farther north, but has not developed yet.

The evening looks to be dry as the heating of the day will be lost,
and we will be in a lull between systems. POPs will ramp up again
toward morning, especially in the west as low-level jetting
increases and fuels yet another MCS.

Tomorrow morning will look fairly similar to this morning, only with
the MCS closer to central Kentucky and probably lasting longer as
the low-level jet is a bit more persistent just ahead of the front.
MCS should still weaken or dissipate by midday, but will again leave
behind plenty of boundaries in an juicy environment. This bodes well
for re-development in the heat of the afternoon, which supports
likely POPs again.

Precip will diminish from west to east, mainly Wednesday evening as
the front pushes in and deeper moisture is shunted to our south and
east.

Not much confidence in the temp forecast as it will be modulated by
clouds and precip. Expect mins in the lower/mid 60s, with daytime
temps hitting 80. Max temps have the higher bust potential, as too
much rain will keep us in the mid 70s, while substantial breaks of
sunshine will give us mid 80s.

.Long Term (Thursday - Tuesday)...
Issued at 300 PM EDT Tue May 21 2013

At the start of the long-term period, the slow moving cold front
that is currently across the Plains and Middle Mississippi Valley
will be exiting our forecast area to the east. The convection
associated with the front will be pushed east and we should start
Thursday dry. However, the upper trough axis will be swinging across
the forecast area during the afternoon hours. Believe some
low-topped showers will develop in afternoon, which could produce
locally gusty winds. There could be some thunder with these showers,
but it will be isolated as cloud tops will be around or just
slightly above the freezing level. Breezy southwest winds will
become northwesterly through the day Thursday, with high
temperatures ranging from the upper 60s to middle 70s.

Distinctly cooler air with low dewpoints will continue through the
weekend, as high pressure of Canadian origin builds southward
through the Mississippi Valley Friday and into the Ohio Valley for
the holiday weekend. Despite clearing skies, Friday will stay quite
cool, with highs ranging from the middle 60s to right around 70
degrees. Temperatures will slowly moderate to low 70s in our north
to upper 70s in our south by Sunday, as ridging aloft edges back
over the Ohio Valley. Overnight lows during the weekend will be
rather cool with the low humidity. Expect lows around 50 degrees
Saturday morning, rising into the middle 50s Sunday morning.

By late Sunday, ridging will strengthen across the southern Plains,
with northwest flow aloft developing across the Ohio Valley. A warm
front will move northward across the forecast area, with moisture
beginning to pool late Sunday along and north of this boundary. The
warm front is expected to stretch along a line from Iowa southeast
through northern Kentucky Monday and shift north Tuesday. However,
there is a lot of model spread associated with the placement of this
boundary. The placement is significant because isolated to scattered
convection is expected to periodically develop along this boundary
and propagate in waves southeastward late Sunday through Monday and
perhaps even into Tuesday. This will spread showers and storms
across the region, but timing and locations to be impacted are much
in question. So, will continue with the running forecast of a slight
chance.

As for temperatures Monday through Tuesday, we should see a warm up
if heights aloft increase and the warm front lifts north. Will go
with high temperatures ranging from the middle 70s to lower 80s, but
this has a high bust potential if waves of storms track across the
region and cloud cover inundates the forecast area.

&&

.Aviation (18Z TAF Issuance)...
Issued at 115 PM EDT Tue May 21 2013

Still a tricky TAF set, but we seem to be coming into line with
previously advertised conditions as we get enough sunshine to
support heating.  Will initialize BWG with VCTS as a cluster of
storms makes its way east along the KY/TN border. Any direct impact
on the airport will be too brief to carry in the TAF. Convection has
yet to get going near SDF or LEX, but as temperatures warm we do
expect scattered storms to develop. As we are still a few degrees
shy of convective temps, will keep with the previous idea of
introducing VCTS beginning around 20-21Z.

Still looking for this activity to diminish after sunset with the
loss of heating. Will re-introduce precip as prevailing TSRA around
or just after midnight as the nose of a 40-50 kt low-level jet finds
its way into Kentucky. Will most likely see brief restrictions of
MVFR or even IFR later in the night, but look for prevailing VFR for
the vast majority of the time.

The latter few hours of the TAFs on Wednesday morning will be a lull
between complexes, with dry weather and VFR conditions. Only real
concern in this period will be SSW winds gusting into the 20-25 kt
range.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
KY...NONE.

Short Term.......RAS
Long Term........MJP
Aviation.........RAS

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