Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 262200
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
600 PM EDT Sat Jul 26 2014

...Mesoscale Forecast Update...
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.

While SPC has lowered the SVR risk from moderate to slight, we
still 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 (18Z TAF Update)...
Issued at 119 PM EDT Sat Jul 26 2014

VFR conditions will prevail through the rest of this afternoon,
before deteriorating conditions move in this evening and overnight
as a line of storms approaches.  For the rest of this afternoon,
expect some scattered cu and mid-level cloudiness, with southwest
winds around 10 to 15 knots, with a few higher gusts.

A line of strong to severe convection will develop to the west this
evening, pushing into the region late this evening through the
overnight hours.  Have attempted to time this convection into KSDF
and KLEX, but this timing will likely need to be refined as the
storms begin to develop this afternoon/evening.  These storms will
carry a damaging wind threat with them (especially at KSDF), so
aviation interests should continue to monitor latest forecasts.
Vsbys/cigs will also be reduced substantially in the stronger
storms.  After the initial line of storms late this evening,
additional convection is possible overnight thus will continue with
VCTS wording at KLEX and KSDF through the nighttime hours.

A cold front will bring one last round of convection through the
terminals on Sunday afternoon.  Sunday will be quite windy ahead of
the front, with WSW wind gusts pushing 25 knots at times.  Outside
of any thunderstorm activity, conditions should remain VFR.

&&

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

$$

Mesoscale......RAS
Short Term.....KJD
Long Term......MJ
Aviation.......KJD






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