Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 201732
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
131 PM EDT MON MAY 20 2013
...Updated Aviation Discussion...
Issued at 1100 AM EDT Mon May 20 2013
Low-level debris clouds from morning convection across the Middle
Mississippi Valley have overspread the Lower Ohio Valley. These
should break up a bit this afternoon as winds become gusty out of
the southwest and mix the boundary layer. Even with the cloud cover,
temperatures have increased into the low and mid 70s as of 10 AM
EDT. With clouds expected to break up later, will not make many
adjustments to the running forecast at this time.
Otherwise, ridging aloft remains in control. According to the latest
guidance and aircraft soundings, there is a cap aloft. Some models
break that cap this afternoon, while others hold on to at least some
weak inversion. Believe some models are advecting dewpoints that are
too high into the region, creating more available surface-based
CAPE. With this, will side with slightly lower dewpoints and at
least a weak cap. If bombarded with outflow boundaries today, then I
would say there is the potential for pulse storms to develop which
could go big with 2-3K J/kg available. However, outflow from morning
convection washed out before reaching our area and do not see any
convection close by that would help trigger development. So, plan to
continue with a dry forecast this afternoon, but will monitor trends.
.Short Term (Now - Tuesday)...
Issued at 309 AM EDT May 20 2013
The biggest concern for the early morning hours continues to be
patchy fog formation, however steady southerly winds will likely
keep this potential confined to sheltered valleys and spots that
received rainfall yesterday. Will only keep patchy fog wording in
the zones for now. Otherwise expect a dry, mostly clear, and mild
overnight in the mid and upper 60s.
An upper level ridge axis is in place over the Ohio Valley and will
remain that way today. Meanwhile, central CONUS trough and its
associated features will continue to produce severe weather across
the eastern Plains into the Great Lakes. Will keep the forecast dry
over our region as there will be no real trigger for convection. A
couple of concerns worth noting, however. First of all, forecast
soundings do not show as much of a subsidence inversion as had been
forecast in past days. Secondly, the generally uncapped environment
will be quite unstable as temperatures in the upper 80s and dew
points in the upper 60s combine with steep low and mid level lapse
rates. There is potential that differential heating (from upstream
convective debris) or a remnant outflow boundary (also from upstream
convection) could set off a few thunderstorms. The higher resolution
20/00Z SPC WRF model shows this, but has not done well over our area
recently. Anything that is able to go in our environment will likely
go big due to the instability. After collaboration with surrounding
offices, have decided to keep the forecast dry as there is a low
probability of this occurring and coverage is expected to be less
than 20 percent. Will have to watch for areas of moisture
convergence and cu development this afternoon. If any storm does go,
large hail, cloud to ground lightning, and heavy rainfall would be
the main threats.
Temperatures are expected to be even warmer today under partly
cloudy skies and steady southerly flow. The forecast temperature for
Louisville (90) is expected to fall just 1 degree short of the old
record (91 set in 1934). A few spots will likely be right around the
90 degree mark, although the majority of locations will settle for
the upper 80s. Much will depend on cloud cover, especially if
upstream convective debris makes it this far east.
As we move into tonight and Tuesday, the upper level ridge over
our area will break down as the trough axis moves into the eastern
Plains and upper Midwest. With stronger westerlies aloft and a
strongly unstable airmass in place, upstream convection is expected
to survive into the Ohio Valley. Tonight`s chances will only be
across southern Indiana, however by Tuesday much of the CWA will
have solid storm chances (40-50%). This will begin an active period
of weather as the associated cold front hangs back to our west for a
couple of days. SPC has placed areas along and west of a Bowling
Green to Frankfort line in a slight risk. This makes sense given the
increase in 0-6 KM bulk shear around 25-30 knots and the strongly
unstable airmass. Multicell clusters with heavy rainfall and cloud
to ground lightning, along with isolated instances of large hail and
damaging winds will be the main threats.
Tonight`s temperatures will again be mild in the upper 60s and low
70s. Tuesday will see a warmup to the upper 80s in most spots.
.Long Term (Tuesday night - Sunday)...
Issued at 306 AM EDT Mon May 20 2013
Thunderstorms will erupt from Illinois to Texas around the beginning
of the long term and will push east into our region overnight
Tuesday night. While severe weather looks likely out to our west,
the storms will become increasingly elevated as they head eastward
as shown on sounding progs advertising a strong nocturnal inversion
setting up around 925hPa. So, while the low level jet does crank up
from Memphis to Indianapolis overnight, most of those winds should
stay off the surface. Wet bulb zeroes look to be a little too high
for anything more than isolated marginally severe hail. The best
chance for any strong storms will be along and west of Interstate 65.
Wednesday remains complicated. Surface low pressure will move from
Illinois to Michigan and will drag a cold front into the Mississippi
Valley. We`ll be under the entrance region of the upper jet and
additional convection is likely. However, how strong the storms
will be is still a question mark. Surface based instability may be
hampered depending on how much cloud and shower activity there is
from the previous night`s rain. Total totals in the upper 40s and
SWEAT index around 300 are marginal for any severe storms (and are
actually lower on Wednesday than Tuesday night). Wet bulb zeroes do
lower some on Wednesday, though, supporting a little more of a hail
Scattered storms will remain in the forecast Wednesday night and
Thursday until the aforementioned low pulls off into New England and
drags a secondary cold front through here Thursday evening.
Canadian high pressure will then move in from the north and keep us
dry through at least Saturday.
The forecast for Saturday night-Sunday is almost nil confidence,
with the GFS keeping us dry and the ECMWF bringing in widespread
shower/tstorm activity. Will lean toward the generally more
reliable ECMWF this far out in the forecast, and even the drier GFS
has a warm front in the Ohio Valley during this time period, further
supporting the idea of a chance of rain. Will keep PoPs very low.
Will go for high temperatures on Wednesday around 80, though the
prevalence of clouds/precip will have a big effect on that. Highs
should then be in the 70s for the rest of the forecast period.
.Aviation (18Z TAF Issuance)...
Issued at 130 PM EDT Mon May 20 2013
Morning low-level debris clouds should give way to VFR conditions
this afternoon and evening as south-southwest winds help mix the
lower atmosphere and an upper-level ridge of high pressure holds
overhead. Expect a few Cu around 2-4 K feet and some upper level
clouds. Southerly winds will increase to between 10 and 15 mph by
late morning with a few gusts up around 20 mph. Scattered showers
and storms may move into southern Indiana late tonight, in a
weakening state. These appear to approach SDF in the pre-dawn hours.
Not sure what form they will possess, given the weakening. With low
confidence and differences between the latest suite of guidance,
will not mention in the SDF TAF at this time. Another concern is
marginal LLWS at SDF and BWG around 09-13Z. There is a decent
inversion at low levels, but if outflow moves in during this time,
LLWS will be void. Given the uncertainty and marginal status, will
not mention in the TAFs.