Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 260532
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
132 AM EDT Sat Jul 26 2014
...Updated Aviation Discussion...
.SHORT TERM (Now through Saturday Night)...
Issued at 338 PM EDT Fri Jul 25 2014
In the near term, partly to mostly sunny skies were noted across the
region this afternoon. Decaying MCS over MO/IL continues to drop
southeastward and will bring a thin veil of high cirrus clouds to
the region later this afternoon and evening. Extensive area of fog
and low stratus mixed out early this afternoon and has left a
cumulus field down across portions of east-central and south-central
KY. Combination of ASOS and Kentucky Mesonet data show afternoon
temperatures in the upper 70s to the lower 80s. The warmest
temperatures were down along the Natcher Parkway area of
west-central KY. We anticipate quiet weather this evening and
overnight with just some high cirrus passing overhead. Temperatures
this evening will fall back into the 70s. Overnight minimums look
to cool into the lower-middle 60s. We stuck very close to the
bias-corrected AllBlend guidance which has verified quite well on
minimum temperatures in the last 3-5 days.
For Saturday...the day looks to start off dry and under partly to
mostly sunny skies we`ll see temperatures warm up quite a bit more
courtesy of a southerly flow. Temperatures should be some 7-9
degrees warmer than today with highs in the upper 80s to the lower
90s. This is very close to the SuperBlend guidance which has been
verifying quite well on daytime maximum temperatures.
By Saturday night, a cold front will begin to approach the region
from the northwest. Many of the numerical simulations develop a
cluster of thunderstorms across IL/IN by late afternoon/early
evening. This is likely to organize into a linear MCS which should
plow east-southeastward across IN/OH/Northern KY and into WV. While
the data suggests that our northeastern CWA stands the best chance
of seeing precipitation and active weather, there continues to be
some uncertainty on the westward extent of precipitation. For now,
have kept the highest chances of PoP NE of a line from roughly
Jasper Indiana down to near Somerset KY. We expect some sort of
strong gradient of PoP in areas southwest of that line.
Strong to severe storms are likely with wind damage and large hail
being the primary severe weather threats. Highest chances of severe
appear to be across our northeastern CWA including the Lexington
metro area. While there remains uncertainty with the convective
evolution, future refinements to PoP and weather will likely need to
be made in subsequent forecasts. Temperatures Saturday night will
be mild with lows generally in the lower 70s.
.LONG TERM (Sunday through Friday)...
Issued at 300 PM EDT Fri Jul 25 2014
...A Conditional Severe Threat Sunday...
The synoptic pattern at the start of the long term will feature an
amplified regime, with a large ridge across the western CONUS and a
digging trough across the Great Lakes. This trough will amplify
over the Ohio Valley through the long term period, bringing
unseasonably cool temperatures by early next week.
Before the cooldown arrives, we must first visit severe potential on
Sunday. A strong PV anomaly will be diving southeast across the
Upper Midwest early Sunday, pushing into the Ohio Valley by Sunday
night. In response to this feature, isentropic ascent on the nose
of a 30-40 knot LLJ will likely result in MCS development overnight
Saturday (see short term section above), which will be ongoing
Sunday morning. The placement/coverage of this convection will be
critical in any severe risk Sunday, as morning clouds/precip may
linger into the afternoon which will significantly cut down on
available instability. The 25/12Z NAM supports the lingering MCS
idea and less of a severe threat (except some redevelopment possible
across southern KY), while the 25/12Z GFS is a bit more progressive
with the MCS and supports atmospheric recovery and the potential for
better afternoon redevelopment.
Will lean a bit more toward the GFS solution as it has some support
from the GEM. Therefore, think there will be ongoing convection
Sunday morning, mainly across areas along and north of I-64. This
convection should weaken with the diurnal weakening of the LLJ,
which should allow for some breaks in the clouds by the late
morning/early afternoon, especially in areas along and south of I-64
where morning convection looks less likely. Instability should then
build by the afternoon hours to 1500-2500 J/kg MLCAPE. Convection
should then fire along the synoptic cold front or any lingering
outflow boundaries and push through the region Sunday afternoon into
early Sunday evening. Initially, storms could become supercellular
along the cold front as 0-6km shear vectors (40 to 50 knots) will be
orthogonal to the frontal orientation and the region will lie within
a favorable right exit region of an 80-knot jet streak. These
storms may then congeal a bit into bowing segments as cold pools
interact. Given the steep mid-level lapse rates and drier mid-level
air, large hail will be possible along with damaging wind gusts.
Again, uncertainty in how this unfolds remains high and the
mesoscale details will need to be fine-tuned as confidence in the
overnight MCS evolution increases. This system certainly has the
dynamics to produce severe weather over the Ohio Valley, but as
we`ve seen time after time this spring/summer, morning convection
tends to throw a wrench in otherwise favorable synoptic setups for
severe storms. Stay tuned to coming forecasts.
Otherwise, drier and cooler air will work into the region overnight
Sunday into Monday as the strong front pushes southeast. High
temperatures on Monday will struggle to get out of the 70s,
especially across north-central KY and southern IN. There may be a
few light showers that develop under the cold core low during the
heat of the day across the Northern Bluegrass, but coverage appears
as if it will be rather isolated.
The remainder of the long term period will be dry, with cool
conditions continuing as the upper trough remains fixed over the
Ohio Valley. Highs through the upcoming workweek will be in the
upper 70s to lower 80s, with overnight lows in the 50s and lower
60s. We may even flirt with a few record lows early Wednesday
morning. Certainly very pleasant weather to send off
.AVIATION (06Z TAF Update)...
Issued at 130 AM EDT Sat Jul 26 2014
VFR conditions are expected to prevail now through Saturday evening
but then becomes more questionable as an MCS is expected to develop
and directly impact southern IN and northern KY. Have introduced
VCTS in this update for SDF given growing confidence in thunderstorm
activity in the overnight period in and around the terminal.
However, less confidence exists for the exact timing as these
systems have a tendency to accelerate beyond what the models are
able to capture, especially this far out. Therefore, have
compromised based on the latest guidance with a time frame of
04Z-08Z for SDF. This would put LEX in the 06Z-10Z time frame but is
beyond this TAF period. BWG should escape direct impact from this
MCS but could still see some concerns to aviation interests as it
brushes past to the north.
Winds for the remainder of tonight will be light to calm and either
southerly or variable as high pressure exits the region. By midday
Saturday, the pressure gradient will tighten over the Ohio Valley as
a surface low vies to move right on in, approaching the southern
Great Lakes from the Plains by the end of this TAF period. Winds for
much of Saturday are expected to be greater than 10 knots and from
the southwest, gusty at times. Winds could briefly die down as the
sun sets at SDF but are anticipated to pick back up with the