Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 241920
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
320 PM EDT Thu Apr 24 2014
.SHORT TERM (Now through Friday Night)...
Issued at 310 PM EDT Thu Apr 24 2014
In the near term, afternoon visible satellite imagery shows high
level cirrus clouds progressing northeast across central KY and
southern IN, in advance of a shortwave to our west, with surface
temperatures warming steadily to near 70 east and lower and mid 70s
west. Temperatures will continue to rise a couple/few more degrees
through late afternoon before falling back this evening.
Pronounced shortwave over KS/OK at this time will move steadily east
tonight accompanied by an increase in low-level moisture and
southerly winds, which will effectively limit/end our evening fall
in surface temperatures overnight. Surface winds should become gusty
overnight. Currently, this system is causing widespread convective
development from western MO to eastern OK and AR. Models are in good
agreement bringing these storms in a band into central KY and
southern IN late this evening and overnight. The 15z HRRR and 12z
in-house mesoscale models bring the leading edge of the convection
to our western forecast area border around 04 UTC.
Looking at model soundings, boundary layer cools this evening but
steep lapse rates and elevated instability remain if lifting a
parcel from about 900-950 mb. This will support continuation of
convection as axis of precipitation moves across our area,
especially western part of forecast area. With increasing winds at
925 and 850 mb, there is a chance that greater momentum air could
mix down to surface and create a few strong wind gusts with the
strongest cells (again, more likely in western sections), but
presence of weak-to-moderate nocturnal boundary layer inversion
should keep strongest winds aloft, unable to penetrate to surface.
After this band passes, there could be a brief lull in precip, but
models bring another area of moisture through from early to mid
morning (from west to east) associated with the actual mid-level
trough axis and surface front. As a result, additional scattered
showers and thunderstorms are expected during that time, with models
again indicating presence of some instability.
Front passes through the area first half of Friday. Winds behind the
boundary will become quite gusty from the west-northwest around 30
mph. Morning clouds and lingering showers will move off to the east
followed quickly by a clearing trend. Highs Friday afternoon should
be in the upper 60s and lower 70s, depending on how rapdily skies
clear. Friday night will be mainly clear with lows again down in the
40s in most locations.
.LONG TERM (Saturday through Thursday)...
Issued at 318 PM EDT Thu Apr 24 2014
Saturday - Sunday...
Heights will rise through the weekend as a progressive upper ridge
axis approaches and then slides through. Meanwhile, a cold front
will sag southward across southern Ohio and into central IN,
stalling by Saturday evening as it becomes parallel to the upper
level flow. Technically, it would be a back door cold front if it
were to make progress through the area, but it looks to stall just
to our north. Either way, the boundary will be moisture starved and
we should stay on the warm side of it. This will lead to a dry and
warm weekend. Expect highs on Saturday in the upper 70s north to low
80s south. Lows Saturday night should dip into the lower 50s. Should
start to see an gradual increase in mid and upper level clouds on
Sunday as the ridge axis starts to slide east, however 1000-850
thicknesses and 850 mb temps will support a 2-3 degree increase from
Saturday. Therefore, have gone with mostly lower 80s on Sunday. A
few Bluegrass or southern IN counties may stay confined to upper 70s.
Will continue to leave the slightest of chances for a shower or
storm in our far western tier or two of counties late Sunday
afternoon, however bulk of precip will not arrive until Sunday night
Sunday Night - Tuesday...
By Sunday night, blocking upper ridge axis will be off to our east
with an impressive closed upper low over the central Plains.
Southwesterly flow through a deep layer will overspread the Ohio
Valley, including a 40 knot low level jet nosing in. The quasi-
stationary frontal boundary will be just off to our northeast with
expansive mid level deformation aloft. By dawn on Monday, expect
scattered to numerous showers and a few t-storms mainly along and
west of I-65. These likely shower and t-storm chances will then
encompass the entire CWA on Monday. Tough to tell how well we
destabilize in this environment, but it appears that a stronger to
possibly severe storm threat will be conditional upon
destabilization, especially along and west of I-65.
Another threat of stronger to severe storms may move in later Monday
night into early Tuesday as the exit region of another impulse
rotating around the upper low ejects out of the southern Plains.
Again, strength of storms would be conditional upon destabilization.
Scattered to numerous showers and storms could then plague the area
again later Tuesday afternoon/evening.
Will continue to monitor threat for stronger storms to start the new
work week, but will also keep an eye on the threat for periods of
heavy rain and flooding. With repeated rounds of t-storms possible
and PWATs jumping up around 2 standard deviations above normal
for this time of year, flooding could become a concern. Tough to
judge temperatures during this time, solid reading in the 70s seem
reasonable each day on the warm side of the boundary and warm
advective flow. Obviously, convection will be the deciding factor.
Lows will be mild during this time, at least in the low 60s in
Tuesday Night - Thursday...
We begin to transition to a cooler period by the middle and end of
the week as the wobbling upper low and its associated surface
reflection slide into the Midwest. Precipitation chances look to
become more scattered in nature during this time with thunder
chances becoming less likely. These days will likely not be
wash-outs, but will be mostly cloudy and considerably cooler. Look
for highs Wednesday and Thursday in the 50s and lows in the low and
mid 40s by Wednesday night.
.AVIATION (18Z TAF Update)...
Issued at 130 PM EDT Thu Apr 24 2014
In the near term through this evening, VFR conditions will prevail
at all 3 TAF sites, with a southeast surface wind becoming south at
5 to 15 mph. Tonight, a strong shortwave trough will progress
eastward across the Ohio Valley, accompanied by a band or 2 of
showers and thunderstorms. Low-level wind fields will increase from
the south with surface gusts over 20 kts at times late tonight.
Winds at 850 mb may increase to around 50 kts, but winds will
increase steadily from the surface to 850 mb with no abrupt change
in wind direction, i.e., strong low-level/sudden wind shear should
not be a significant concern.
A band of convection should reach SDF and BWG in the 04-06z time
frame or so, and then just thereafter at LEX. Convective cells would
be more likely at BWG and SDF, and then more scattered within a band
of showers at LEX as the line moves east. This should take vsbys
and/or ceilings into MVFR category as predominant. After this axis
of rain moves through, there may be a break of precipitation with a
return mainly to VFR. But then scattered thunderstorms are again
possible a couple/few hours either side of 12z as the surface front
and mid-level trough pass through the TAF sites. Have accounted for
this with VCTS in the grids.
After frontal passage Friday morning, winds will become west to
northwest and gust from 20-30 kts at times. Low clouds will also
clear during the day.