Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 220056
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
856 PM EDT TUE MAY 21 2013
Issued at 855 PM EDT Tue May 21 2013
The next line of storms on radar will be moving into western
portions of the forecast area within the next hour. The strongest
portion of this line is at the northern end and this area should
track along and north of the Ohio River. How long this system holds
together is somewhat in question. Most mesoscale models show it
weakening as it continues east tonight and becoming more scattered
in nature. The models don`t have a great handle on the ongoing
strength. However, the general trend seems reasonable. The next line
of storms will them move in towards morning as a weak LLJ develops
and a shortwave approaches. Rather than try and time this out
specifically, have kept pops in the likely category for most of the
night across the western portion of the forecast area. Further east
the precipitation chances will be more scattered in nature overnight.
Issued at 630 PM EDT Tue May 21 2013
The storms have moved out of the area this evening and the Severe
Thunderstorm Watch has been cancelled. The next batch of storms is
just starting to move into far western Kentucky this evening. These
storms are expected to move into the forecast area late this evening
and into the overnight hours. Have adjusted pops down over the next
few hours before ramping them up again overnight. Updates are
.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
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
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 (00Z TAF Issuance)...
Issued at 722 PM EDT Tue May 21 2013
The TAF forecast remains tricky in regards to convection as there are
several waves of thunderstorms which may affect the sites over the
next 24-36 hours. The most immediate concern is the line of
thunderstorms currently in western KY. Many of the models have this
line weakening as it moves east. However, still think it will affect
SDF and BWG overnight. Timing the line at its current speed puts it
in to these sites between 04-05Z. Have therefore adjusted the TSRA
in the TAFs to reflect this. Beyond this time frame, additional
thunderstorms do look like they may develop towards the morning
hours. However, with uncertainty of development will carry VCTS
after 07Z through the mid morning hours. There should then be a lull
before redevelopment may occur Wednesday afternoon. Will keep the
latter portion of the TAF period dry for now, but thunderstorms may
need to be added as confidence in the forecast increases. Storms at
LEX will be more scattered in nature, especially during the first
half of the TAF period so will keep prevailing TSRA out of the TAFs
there for now.
The other concern for tomorrow will be the development of gusty
winds. Winds will strengthen out of the south through the morning
hours, with gusts in the 20-25 knot range expected.