Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 270502
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
102 AM EDT Sun Jul 27 2014

...Mesoscale Forecast Update...
Issued at 100 AM EDT Sun Jul 27 2014

Cancelled the watch as severe convection has moved out of the area.
Rain with scattered storms will continue over the next few hours
before possible redevelopment of stronger storms during the pre-dawn
hours.  Stay tuned for more details.

Issued at 1215 AM EDT Sun Jul 27 2014

Ongoing MCS continues to track across central KY with the severe
portion of the line weakening as it loses support. However, pockets
of instability still exist in untapped areas, particularly in
southern KY. Have trimmed back some KY counties from the watch and
will likely clear all counties with the next update. The MCS is
moving away from the better speed shear for now, although the winds
aloft will strengthen by the pre-dawn hours on Sunday as both the
low-level and upper-level jets overtake the forecast area. DCAPE
continues to be stout for this time of night, with SPC`s meso
analysis indicating over 1100 J/kg over the eastern counties. So
far, flooding has not been an issue but will continue to monitor as
this system has had a history of producing copious amounts of rain.
For reference, 3-hour FFG exceeds 2.75 inches for much of central
KY, especially in the south and counties where less rain has been
seen over the last several weeks.

Issued at 1010 PM EDT Sat Jul 26 2014

Strong to severe storms roughly along the Ohio River at this hour.
SPC has issued a second Severe Thunderstorm Watch covering all of
central Kentucky ahead of the line. Storms were sub-severe for quite
a while, but NW-SE oriented axis of downdraft CAPE over central
Kentucky, and ingestion of isolated pop-ups ahead of the line, have
combined to push storms to SVR limits.

Watch is valid until 08Z, but the action will most likely be over
before then. Instability will diminish at least somewhat with the
loss of heating, and the NE portion of the line will outrun the
rest. At that point, will start to see training along an east-west
oriented line, which could result in short-fuse flooding problems
overnight.

Issued at 555 PM EDT Sat Jul 26 2014

Cluster of strong T-storms is ongoing over central Illinois, but has
yet to go severe. The air mass over the Ohio Valley remains quite
unstable and favorable to sustain convection for at least the next
few hours as this complex heads ESE toward the area. The faster end
of our current forecast timing still appears realistic, with
thunderstorms reaching Dubois County around 8 PM EDT, Louisville
Metro shortly before 10 PM, and the Lexington area around 11 PM.
This is definitely leaning faster than most of the short-term model
guidance, as they tend to underestimate the acceleration that
happens when a cold pool becomes established.

SPC has lowered the SVR risk to slight but we remain in play, with
damaging winds as the main threat.

.SHORT TERM (Now through Sunday Night)...
Issued at 309 PM EDT Sat Jul 26 2014

...Increasing confidence for severe threat tonight through Sunday...

The synoptic pattern this afternoon features a strong ridge across
the southern CONUS, placing the Ohio Valley within northwest flow
aloft.  A strong PV anomaly will dive through this northwest upper
flow, amplifying a trough over the region by the end of the short
term period.

Conditions remain quiet across the region early this afternoon, as
warm mid-level air has worked in, capping any convection.  Partly
cloudy conditions will continue through the remainder of the
afternoon which should allow temperatures to climb into the lower
90s in most spots.

An MCV/weak surface low visible on satellite imagery continues to
push east across northern MO.  Mainly elevated convection has
developed over the past few hours near this feature, as a firm cap
has limited surface-based development.  However, most short-term
hi-res model guidance including the 26/15Z HRRR, 20/12Z WRF-ARW and
WRF-NMM depict surface-based convection developing late this
afternoon and early this evening across MO and western IL, then
racing east across southern IL, southern IN, and into northern
Kentucky tonight.  Given the agreement in the hi-res guidance along
with support from the 26/12Z GFS, confidence in this convection
developing has increased from previous forecasts.  As this
convection congeals, it appears likely to develop a strong cold pool
given DCAPE values of 1200+ J/kg and 0-3 km shear vectors of 30-40
knots.  This will allow for a swath of potentially damaging winds to
push through IL/IN and into KY.  The timing of this convection puts
it into our southern IN counties around 2-4Z, and into the
Louisville metro area between 3-5Z.  Given the timing, the surface
will be stabilizing somewhat as we lose heating.  Therefore, think
the best damaging wind potential (some potentially significant wind
gusts) will be just northwest of the LMK CWA, but still expect a
swath of potentially damaging winds to push into southern IN and
northern KY tonight, with the system likely weakening as it pushes
further into KY.  This damaging wind threat will go all the way into
the Lexington metro, but will likely be more localized in nature at
that point.

A westerly low-level jet of 35-40 knots will ramp up tonight in
response to the approaching PV anomaly.  Therefore, any lingering
outflow boundaries will likely remain active (especially if they can
become oriented more north-south so convergence will be maximized)
overnight.  Given the expected path of the MCS, the southwestern
flank of the outflow will be the most likely spot for renewed
activity overnight, which will likely be across southern IN and
north-central KY.  While an isolated severe risk will persist with
this later convection, think the main threat will then transition to
heavy rain.  Given PWATs near 2 inches and the potential of training
storms, some localized flash flooding does appear possible, despite
the ground being so dry in recent weeks.

Confidence begins to decrease on Sunday as it remains unclear just
how much cloud cover and lingering precipitation will be around.
However, guidance continues to suggest the atmosphere will recover
ahead of the approaching upper trough and strong surface front,
setting the stage for additional severe thunderstorm development.
0-6 km shear vectors of 40-50 knots oriented mostly orthogonal to
the surface front, along with a favorable orientation to the
right-exit region of an 80+ knot upper-level jet streak will support
initial supercell development.  The best severe threat will be on
the Kentucky side of the river, where the front will be crossing
during peak heating.  Given the drier mid-levels and steep lapse
rates, large hail will be possible in addition to damaging winds
with these storms.  An isolated tornado is not out of the question
given the potential for semi-discrete storms, but it will likely
take interaction with any lingering outflow boundaries to make up
for a lack of ambient environmental low-level SRH to produce an
isolated tornado.

Otherwise, conditions will quickly clear out Sunday evening as the
front sweeps across the Ohio Valley.  Drier and cooler air will
filter in, as lows Sunday night drop into the mid 60s.

.LONG TERM (Monday through Saturday)...
Issued at 308 PM EDT Sat Jul 26 2014

A highly amplified upper level pattern will be in place for much of
the extended forecast period.  A very well advertised upper level
trough axis will remain parked across the eastern third of the US
through the period.  This upper trough is unseasonably deep and will
bring much below normal temperatures to our region next week.  The
below normal temperatures may tie or break record minimum
temperatures at some of our observation locations early next week.

A compact vorticity lobe may rotate around the base of the upper
trough on Monday which could bring some scattered rain showers and
clouds to mainly the eastern half of the forecast area.  After this
feature rotates out, the upper level trough remain in place across
eastern North America as the pattern generally remains blocked
aloft.  This will result in a cool northwesterly flow which looks to
keep the bulk of the week dry but with below normal temperatures.
Highs Monday and Tuesday will only top out in the middle to upper
70s with overnight lows in the lower to middle 50s. The upper trough
looks to open up slightly by Wednesday, but conditions will remain
rather dry with slightly warmer temperatures.  Highs by Wednesday
look to warm into the upper 70s and lower 80s with overnight lows in
the upper 50s to around 60.

The forecast becomes increasingly more complex by later in the week
as the upper level troughing becomes a bit more established once
again over the eastern US.  Both long term deterministic runs of
the Euro and GFS both eventually develop a cut off low that holds
sway over the Ohio Valley by late in the forecast period.  The
re-amplification of the pattern will likely result in increasingly
wetter conditions as we move from Friday and into next weekend.

In general, we have preserved much of the previous forecast which
started capturing the evolving trends.  This forecast simply
increases cloud cover and slightly increases PoPs from Friday and
into Saturday.  With the expected increase in cloud cover, we also
expect that afternoon maximum temperatures will likely be a bit
cooler as well.  Highs Thursday will likely be the warmest of the
week with readings in the lower-middle 80s.  Temperatures are likely
to drop back into the upper 70s by Friday and Saturday.  Overnight
low temperatures look to remain cool with readings generally in the
lower 60s.

&&

.AVIATION (00Z TAF Update)...
Issued at 705 PM EDT Sat Jul 26 2014

Main challenge is timing and impact of the convective system that
will push across SDF and LEX this evening. Even the high-resolution
models struggle with cold pool processes, and often end up too slow
in these situations. Knowing that, will stick with the previous
theme of T-storms impacting SDF 02-05Z and LEX 03-06Z. Believe that
the lowest conditions worthy of a TEMPO would be high-end MVFR
ceilings and borderline IFR visibility, but in the most intense part
of the line IFR ceilings and LIFR vis in heavy rain cannot be ruled
out. Wind gusts well in excess of 30 kt and other thunderstorm
hazards will also likely affect ground operations.

Models are hinting that overnight re-development is possible,
especially along the SW flank once the initial line of storms moves
through. Will keep VCTS in through roughly mid-morning.

Sunday will see a tight southwesterly pressure gradient as the front
makes its way south and east across Kentucky. Sustained winds will
be around 15 kt for most of the afternoon, with frequent 20-25 kt
gusts. Otherwise expect VFR conditions. Certainly can`t rule out
thunderstorms, but that will be so dependent on how tonight`s storms
evolve. Given that low confidence, will not include any TS on Sunday
afternoon.

&&

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

$$

Mesoscale......LG/AMS
Short Term.....KJD
Long Term......MJ
Aviation.......RAS




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