Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 172227
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
627 PM EDT Sun Aug 17 2014

...Forecast Update...
Issued at 610 PM EDT Sun Aug 17 2014

Elongated sfc low extends from NE Arkansas into western KY, with a
mid-level MCV somewhere over southern Indiana, but a bit more
indeterminate. Deep convection has had a difficult time firing over
the Ohio Valley as the environment remains worked over from earlier
convective system. Heavy rain and better SVR potential is mostly
suppressed into Tennessee, but still a decent cluster of showers and
embedded T-storms coving out of the Pennyrile region of west-central
Kentucky, and just entering Ohio County at this hour. A more steady
rain continues over southern Indiana, but has produced fairly
impressive precip rates with a few spots picking up over an inch in
an hour.

Expect the highest rain chances to gradually shift south and east as
we progress through the evening hours. Hourly POPs are fairly low
confidence due to any pop-up showers and differing storm motions.
Still believe most locations will pick up measurable precip at some
point. Text forecasts seem well on track at this point, and updates
to gridded forecasts are limited due to lack of confidence.

.SHORT TERM (Now through Monday Night)...
Issued at 315 PM EDT Sun Aug 17 2014

...Localized Heavy Rain Possible Across South Central KY This
Evening...

The initial two waves of precipitation have weakened as they pushed
east, however still dropping light to moderate rain. Will continue
to push this precipitation eastward over the next couple of hours,
with focus shifting more to our southern CWA this evening for heavy
rain and stronger storm potential.

Currently, a deep area of low pressure sits over the lower Ohio and
Wabash River Valleys. This feature will slowly push east this
evening and overnight, sliding a very moist and moderately unstable
airmass across south central Kentucky. The moisture will continually
be reinforced by an H85 jet around 30-35 knots, along with low level
convergence ahead of a cool front. Thunderstorms have already begun
to develop across western KY and TN in a partial clearing slot.
Expect these to grow upscale over the next few hours as they push
east, with locally very heavy rainfall possible at times.
Additionally, these storms are forming on the nose of a relative mid
level dry punch, which could aid in the gusty wind (~40 mph)
potential. Basically, think that the 12z Nam solution is pretty
reasonable if you displace it about 75 miles southward, putting the
axis of heaviest rain across central and northern TN. Considered a
Flash Flood Watch for the southern tier or two of counties, but
decided to hold off due to the more localized threat of a flooding
problem, rather than widespread.

Elsewhere, southern Indiana and north central Kentucky will continue
to be in a moist airmass with the deep area of low pressure kicking
off scattered to numerous showers and a few thunderstorms. Locations
should expect another quarter to half an inch with additional
rainfall, with totals going up as you approach central and southern
KY.

Heaviest rainfall threat will switch to the Lake Cumberland region
in the pre-dawn hours toward dawn, with isolated to scattered
coverage elsewhere. Look for a mild overnight with lows around 70
and some light fog or low clouds lingering.

The upper low will quickly push off to the east by Monday, with the
surface low and associated moisture lagging a bit behind. Will leave
isolated (west) to scattered (east) coverage in the forecast for
this time, however the heavy rain threat should be over as moisture
profile won`t be as deep. Look for another day with highs below
normal, ranging in the 80-85 range. Areas to the west should see
some partial clearing late in the day allowing for temps to jump a
bit.

.LONG TERM (Tuesday through Sunday)...
Issued at 319 PM EDT Sun Aug 17 2014

=============================
Long Term Synoptic Overview
=============================

A negative height anomaly the Aleutians is forecast to remain in
place through the extended forecast period.  This will lead a bit of
ridging continue across the NE Pacific. At the same time, a Rex
Block looks to remain in place across Quebec and the upper St.
Lawrence River Valley.  These features combined will lead to a
rather progressive flow across the CONUS throughout the long term
period.  A slight northwest component to the flow will keep a steady
flow mid-level disturbances passing through the region as our region
remains on the periphery of a developing mid-level ridge axis to our
south/west. While the timing of these mid-level disturbances will be
hard to time, the multi-model consensus suggest the strongest one
will likely still come through late Wednesday and Thursday.  The
impact of these disturbances will depend on the strength of ridging
that develops.  The long term multi-model consensus is still gung ho
on developing a major ridge axis across the southern Plains by the
end of next week.  While this still looks rather plausible, the
strength and location of the ridge axis is still a bit in question
as the operational models continue to variability in their run to
run solutions.

=============================
Model Preference / Confidence
=============================

As mentioned above, the operational runs of the GFS, Euro, and GEM
continue to variability in their model runs.  The general consensus
of the models has stayed the same in forecasting the progressive
pattern with the Ohio Valley remaining within the baroclinic zone.
However, the strength of the developing ridge toward the end of the
period is a bit less clear.  Generally, have trended this forecast
closer to the 17/00Z Euro ensembles with some detail from the 17/12Z
operational Euro. Some GEFS data was utilized as well to form the
forecast.  Forecast confidence remains average throughout much of
the forecast period but slowly falls as we head toward the end of
the week.

=============================
Sensible Weather Impacts
=============================

It will be a warm and muggy week across the Ohio Valley next week.
Within then northwesterly flow, multiple weak disturbances will move
through the region.  With no real focused areas of forcing aloft,
convection will likely be diurnally driven in nature with the most
coverage occurring during the afternoon and early evening hours.
Warm, muggy nights with patchy fog will be quite likely.  Afternoon
PoP coverage will probably average around 30-40% in most locations.
A stronger signal is still seen arriving by late Wednesday and
Thursday.  This forcing may keep the best chances of convection
across the NE third of the forecast area.  Thus, plan on keeping a
gradient of PoP across the region with the highest in the Bluegrass
region and lesser chances of precipitation down toward the Bowling
Green area.  Depending on the development of the upper ridge, it is
possible that drier and warmer conditions may become more widespread
by Friday and Saturday.

As for temperatures, we`re not buying into the operational raw and
MOS temperatures for next week.  Looking at a variety of
verification data from the last few weeks reveals a relative warm
bias in temperatures in the extended.  Therefore, a more
conservative approach to temperatures has been incorporated into the
upcoming forecast.  We expect highs Tuesday to warm into the
low-middle 80s across the NE with middle to upper 80s across the
central (I-65 corridor) and points south/west.  A continued
moderation in temperatures will continue into Wednesday with highs
warming into the middle 80s in the NE and upper 80s to near 90 in
the central and SW sections.  Overnight lows will be in the upper
60s to around 70.  For Thursday and Friday we will continue to go a
bit more conservative on temperatures with readings warming into the
middle to upper 80s in the NE with upper 80s to the lower 90s in the
central and southwest.  The operational models really turn up the
heat by the weekend with raw model surface temps warming well into
the 90s.  Feel that this unrealistic given the relative green
vegetation and expected wet soil conditions from diurnally driven
convection.  Thus, will be keeping daytime highs in the middle to
upper 80s in the NE with upper 80s to the lower 90s in the central
and SW.  If lesser amounts of precipitation do occur and the
mid-level ridge develops more prominently as the 17/12Z Euro
suggests, then a run at lower-middle 90s seem very plausible across
western and southern sections of KY (SW of a line from Owensboro to
Lake Cumberland).

&&

.AVIATION (18Z TAF Update)...
Issued at 1256 PM EDT Sun Aug 17 2014

A deep area of low pressure is currently sitting over the lower Ohio
and Wabash River Valleys, with a lot of moisture streaming into the
area ahead of this system. The TAF sites have experienced a couple
of rounds of showers and storms this morning, and should experience
a relative lull this afternoon before things get going again late
afternoon and evening. Will continue to keep the best chance for
thunder and heaviest rainfall reducing visibilities significantly in
the BWG area. Best timing for a heavy round of showers and storms
looks to be between 7 and 10 PM CDT. With the focus shifting to
south central KY this evening, SDF/LEX will likely see moderate rain
showers with a chance for a few rumbles of thunder. Have more
optimistic conditions for these sites in the evening from a
visibility standpoint.

After the heavy rain passes, all TAF sites look to be subject to
lowering ceilings at least into the MVFR range below fuel alternate.
Sites could possibly go into the IFR range for an extended period of
time after Midnight through the dawn hours.

Surface winds will generally be out of the south the rest of the
afternoon and evening, gradually veering to southwesterly and then
westerly as a surface front passes along the with the low pressure.
Some improvement is expected to occur in ceilings by late morning or
mid day on Monday.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...NONE.
IN...NONE.
&&

$$

Update.........RAS
Short Term.....BJS
Long Term......MJ
Aviation.......BJS




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