Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 300502
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
102 AM EDT Sat Aug 30 2014

...Updated Aviation Discussion...

...Forecast Update...
Issued at 915 PM EDT Fri Aug 29 2014

Updated the forecast to remove POPs for the rest of this evening.
Diurnally driven convection has diminished over our area and we now
sit in a cull before the arrival of convection ahead of an
approaching trough.  Largely the overnight period should remain dry
with scattered showers/storms re-entering southwest IN/west central
KY around sunrise.  Only made minor tweaks to temps/dewpts.  Low
temps still look to bottom out in the upper 60s/lower 70s.

.SHORT TERM (Now through Saturday Night)...
Issued at 300 PM EDT Fri Aug 29 2014

Warm and unsettled pattern will gain a bit more focus over the next
36 hrs. For now, a summertime regime continues with a weak surface
boundary lifting north across the Ohio Valley. Moisture pooling
along the boundary is responsible for a CAPE axis running from near
SDF to just south of LEX, and just ahead of it there is an enhanced
cu field over the Bluegrass region. Slightly drier air has worked
into south-central Kentucky, as surface dewpoints have dropped to
around 60F.

Any convection this afternoon will be focused along the
aforementioned boundary, so will initialize with isolated T-storms
roughly along and north of I-64. Wind fields have strengthened just
enough that any thunderstorms should be more progressive than the
last couple of days, and the forcing should be out of the area
before sundown.

For Saturday the S/SW flow will deepen and heights will fall as the
sharp upper trof over the Plains opens up and lifts toward the Great
Lakes. Precip chances will increase from west to east through the
day, with likely POPs creeping into south-central KY from the
Pennyrile region late in the day. Otherwise will stay in the chance
range until evening.

Saturated column and modest SW low-level jetting Saturday night will
support likely POPs across the board. Most likely QPF will be around
half an inch in some areas, but models do point to a swath of 1-2
inch rainfall where the best forcing sets up. What little CAPE there
is will become quite tall and skinny, so will limit thunder to
chance. Confidence in the placement of any heavier rainfall is not
high enough to issue a Flood (or Flash Flood) Watch this early, but
later shifts may need to reassess as confidence in the placement of
this heavier rain swath increases.

Above normal temps will continue, but there is real bust potential
on Saturday highs if the precip is quicker moving in than
advertised.


Weak frontal boundary is roughly located along our southern border
this hour. This boundary is forecast to lift northward today, with a
wind shift from easterly ahead of it to southerly behind it. Lift
along this front as well as instability from late afternoon heating
should produce isolated to scattered storms, with the best chance
over southern Indiana and the Bluegrass region of Kentucky. The
stronger storms will produce brief gusty winds and heavy rain.

.LONG TERM (Sunday through Friday)...
Issued at 300 PM EDT Fri Aug 29 2014

...A cloudy and potentially wet Sunday with warmer and more
scattered precipitation for Labor Day...

Upper air pattern early Sunday will feature zonal flow along the
northern tier of the CONUS, with sub-tropical ridging firmly
established near the Georgia coast. Two shortwaves of note: the
first will feature low pressure moving over Michigan early Sunday,
while a stronger shortwave will just be ejecting eastwards of the
northern Rockies.

The Commonwealth and southern Indiana will light right in the middle
of a tropical airmass early Sunday, as an arc of very humid air with
PWATs of greater than 2 inches is forecast to extend from a
disturbance in the western Gulf through the eastern Great Lakes.
Showers, with some embedded thunder will likely continue from
Saturday night right into a good portion of Sunday. Various models
agree with the assessment from the Weather Prediction Center of
substantial rains during this Saturday night-Sunday period with a
swath of widespread 1 to 2 inch rains with possibly locally higher
amounts. Exactly where this falls is still to be determined, with
the NAM showing the heaviest accumulations along the Ohio River with
the latest GFS showing this a bit farther south. Localized runoff
problems may develop where any showers repeatedly move over a
specific location. Highs Sunday may not exceed the lower 80s due to
cloudiness.

By late on Labor Day a surface cyclone associated with this second
wave ejecting from the Rockies will deepen north of Lake Superior.
The Lower Ohio Valley will lie in a humid airmass left over from
Sunday`s disturbance. Temperatures will be warmer, reaching the
upper 80s, with isolated to possibly scattered afternoon to evening
thunderstorms.

By Tuesday, this second cyclone will bring a frontal boundary that
will likely lay along an east/west line somewhere across Indiana and
Illinois. This front will likely weaken and dissipate late Tuesday
or early Wednesday along or north of the Ohio River. Widespread
convection is likely Tuesday north of us. However some organized
storms may reach as far south as southern Indiana during the
afternoon or evening. Only isolated unorganized convection is likely
farther south towards Tennessee Tuesday afternoon and evening. Hot
humid weather will continue Tuesday with highs around 90.

.Wednesday through Friday...

A fast zonal flow will set up Wednesday through Friday with a
relatively robust jet aligned almost exactly over our northern
border with Canada. Strong ridging will become established across
practically the entire southern tier of the CONUS. This pattern will
mimic ones that we occasionally see during the "dog days" of August.
Expect generally light winds with hot humid weather and warm muggy
nights this whole period with highs near 90 and lows at 70 or above.
Isolated thunderstorms may develop each afternoon. However no
organized precipitation is expected.

&&

.AVIATION (06Z TAF Update)...
Issued at 102 AM EDT Sat Aug 30 2014

The early morning hours should be pretty quiet across the TAF sites
as we await the next plume of gulf moisture ahead of an approaching
wave. In the meantime, expect scattered to broken mid level clouds
streaming overhead, with light SSE winds.

Will notice an increase in surface winds by mid to late morning, and
a shift to more SSW. Low and mid level clouds will increase during
this time, however should stay above the VFR threshold. Will start
prevailing rain shower and VCTS mention by early afternoon at
SDF/BWG, and by mid to late afternoon at LEX. Conditions will likely
drop to at least low MVFR in any storm, however will nail down
better timing with later issuances.

Look for prevailing rain showers to end around sundown at SDF/BWG,
and more toward midnight at LEX. Some light showers may linger into
the overnight, with potential for some MVFR ceilings.

&&

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

$$

Update.........AMS
Short Term.....RAS
Long Term......JSD
Aviation.......BJS





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