Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 160149
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
949 PM EDT Wed Apr 15 2015
Issued at 949 PM EDT Wed Apr 15 2015
A cluster of showers and thunderstorms has developed over west
Tennessee and is moving to the north and northeast. Though the best
chances for precip will be to our south and southwest tonight,
there`s enough elevated instability to suggest that anything over
Tennessee could survive into Kentucky, and produce some lightning.
Though precipitable water amounts will remain around 1.25", low
level jet winds overhead are weak and disorganized and the best
theta-e ridging stays to our south, so nothing too strong is
expected. So, increased PoPs just a bit into high chance category,
and included some scattered thunder.
.SHORT TERM (Now through Thursday Night)...
Issued at 325 PM EDT Wed Apr 15 2015
Mild but unsettled weather will continue in the short term.
Low-confidence forecast as most of the models, including hi-res
output, are struggling to get a handle on current conditions.
Surface high pressure ridging SW from the Great Lakes is mostly
winning the battle thus far, with frisky ENE winds just off the deck
pulling dry air into the Ohio Valley. Precip trying to spread north
out of middle Tennessee has mostly dissipated as a result of the dry
air, but some light returns now showing up over central KY, most
likely in response to a WSW-ENE oriented boundary.
Expect at least a slight increase in coverage tonight as an upper
impulse lifting out of the Lower Mississippi Valley interacts with
this surface boundary. However, there remains the potential that
much of central Kentucky could be gapped if this impulse runs up the
spine of the Appalachians, and the stronger impulse lifting out of
the Ozarks stays far enough west. Therefore will limit POPs to a
high-end chance through the short-term period. Forecast soundings
hint at some elevated instability tonight, so we can`t rule out a
rumble of thunder, but think chances are too slim to mention.
Perhaps a better chance for thunder on Thursday as there could be
marginal surface-based instability. Will carry a slight chance then.
QPF could be quite variable, especially on Thursday, but expect most
locations to be around a half inch before ridging takes over and
rain tapers from west to east Thursday night.
.LONG TERM (Friday through Wednesday)...
Issued at 300 PM EDT Wed Apr 15 2015
...Unsettled Weather Continuing Into Early Next Week...
Friday - Saturday...
A closed upper low will be centered over Colorado on Friday morning.
Meanwhile, upper ridging will be in place over the Ohio Valley, with
a weak surface front washing out over our CWA through the day. Will
continue with mention of an isolated shower or storm on Friday as
soundings try to show enough heating/instability to overcome the mid
level inversion beneath the upper ridge. Highs should find their way
to the mid 70s.
Other than a lingering slight chance early in the evening, will go
dry Friday night with lows mostly in the mid and upper 50s.
The upper ridge axis slides east of our area on Saturday with SW
flow aloft ahead of the opening upper low as it moves into the high
Plains. As this occurs, precipitation chances associated with deeper
moisture and an isentropic lift component will only very slowly
increase from SW to NE through the afternoon hours. Therefore, will
only mention a slight chance late Saturday afternoon north and east
of a line from Jasper, IN to Stanford, KY. South and west of this
line, scattered to numerous light rain showers can be expected by
late in the afternoon. Saturday highs are again expected to be in
the mid 70s.
The wettest period of this forecast looks to be Saturday night as a
surface low develops across the mid Mississippi River Valley ahead
of the aforementioned upper system. The associated low level jetting
and moisture transport will lead to more widespread rain showers
with some rumbles of thunder also possible. Latest data suggests
that the heaviest rain will hold off for the Louisville metro until
just after Midnight. A few light showers will be possible between
sunset and Midnight. At this point, most locations should expect
around a half an inch with locally higher amounts. Look for mild
overnight lows in the upper 50s and low 60s.
Sunday - Monday...
Off and on showers are then expected as we end the weekend and start
the new work week. This will occur as our weekend upper system gets
absorbed into a much larger eastern 2/3 CONUS trough and individual
waves slide through the mean flow. These shower and iso T-storm
events will be more scattered than the Saturday night precipitation.
However, the prolonged rain chances and a shot at periods of heavier
rain with any storm could continue to agitate local hydro problems.
As mentioned previously, 1 to 2 inches of rain seem reasonable Friday
Temperatures will be slightly cooler on Sunday, more in the low and
mid 70s, with Monday falling into the mid and upper 60s.
Monday Night - Wednesday...
Lower confidence forecast for the middle of the week as we find
ourselves in relatively benign zonal flow south of the big closed
low centered north of the Great Lakes. Have elected to go dry for
this period as moisture will be harder to come by in this pattern
and individual waves are not apparent. What can be said with more
confidence is that this will be a cooler period with highs in the
60s and lows in the 40s.
.AVIATION (00Z TAF Update)...
Updated at 715 PM EDT Wed Apr 15 2015
A low confidence forecast with a mix of challenges including
ceilings and precipitation trends. A warm front lies near the KY/TN
border this evening and will gradually lift northward overnight into
the day Thursday. Increasing low level moisture and an upper level
weather system will bring scattered rain showers to BWG/SDF/LEX this
evening and again tomorrow. It`s difficult to pinpoint prevailing
periods of showers or dry times as the air mass will be moist with
plenty of overrunning precipitation.
Currently ceilings are high-end MVFR and the expectation is that
they will lower below fuel alternate levels overnight. It`s possible
that high-end IFR conditions could develop for a period tomorrow
morning (12-16z) but confidence is not high with that scenario. Once
the front passes in the afternoon and with precipitation around,
expecting ceilings to lift, reaching high-end MVFR or VFR toward the
end of the period.