Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 290520
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
120 AM EDT Fri Aug 29 2014

...Updated Aviation Discussion...

...Forecast Update...
Issued at 955 PM EDT Thu Aug 28 2014

Convection has faded with the loss of daytime heating. Forecasts
have been updated to remove essentially all POPs for the rest of the
night. The lone exception is from roughly Jasper to Brandenburg,
where another boundary is still trying to generate convection along
the edge of the higher-PWAT air mass. Believe that this will either
dissipate or move slowly west into southwest Indiana by midnight.

Issued at 640 PM EDT Thu Aug 28 2014

Surface boundary near the KY/TN border has interacted with a
moisture-rich air mass over western Kentucky. Slow-moving storms
have produced torrential rainfall and a few marginal severe and
sub-severe gusts,mainly near the Ford and Cumberland Parkways.
Forecast tweaks the last couple hrs have been to tighten the
gradient in POPs, with high-end scattered to low-end likely around
BWG and points to the south and west.

Similar to last night, expect this convection to dissipate fairly
quickly after sunset.

.SHORT TERM (Now through Friday Night)...
Issued at 321 PM EDT Thu Aug 28 2014

Currently through this evening:

Storms have already begun to erupt across portions of southern and
western KY and are expected to increase in coverage and intensity
through around sunset this evening. Moisture tongue draped across
western KY and digging into central KY is visible on the CIMSS
NearCast model and depicted in dewpoint surface obs. Latest SPC
MesoAnalysis shows the more significant DCAPE values to the south of
the TN/KY border but certainly cannot rule out
downbursts/microbursts still occurring with some of the taller
storms. OHX sounding at 12Z this morning had just over 2000 J/kg of
CAPE; 18Z GFSBufr sounding has over 3000 J/kg of CAPE in southern
KY, which lines up with the MesoAnalysis. In other words, plenty of
instability for these storms to work with, in conjunction with the
aforementioned moisture. Wind fields and shear are on the weak side,
and thus far, storms have very little motion to them, which combined
with the heavy to torrential rainfall threat, could lead to some
localized flooding concerns. Any storms that develop this
afternoon/this evening will dissipate after sunset.

Overnight through early Friday morning:

Skies will clear out for the most part overnight, winds will go
light to calm, and lingering low-level moisture (especially in areas
where rain had fallen) will equate to fog potential, particularly in
and around prone locations. Fog should remain on the lighter and
patchier side. Morning lows will range from the mid 60s in the
Bluegrass region to around the 70 degree mark in the Louisville
Metro and southwestern KY. No mentionable precip is in the forecast.

Friday through Friday night:

Weak frontal boundary currently laid out across southern KY will
lift back northward tomorrow as a warm front. This boundary will
serve as a primary focus for convection but as we`ve seen today,
leftover boundaries abounded and will likely see a similar setup
tomorrow. While there is moderate confidence in convection tomorrow
(again isolated to scattered in nature), model soundings are showing
much drier mid-level air filtering in and a cap that could limit
convection. With the warm front expected to lift north and southerly
flow kicking in, this mid-level dry air could be overdone.
Instability is also depicted as being a bit less as compared to
today but will have steep low-level lapse rates again. One thing to
keep in mind is that for the most part, deterministic models have
had a difficult time capturing the details and evolution of these
smaller scale pulse events and in some cases, underdone what has
actually happened over the last couple of days. Storms could once
again pose the threat of strong to possibly damaging winds. Any
development will once again wane after sunset as they lose diurnal
heating.

Highs tomorrow will be similar to today, ranging from the upper 80s
to the lower 90s. Dewpoints will again mimic today, generally in the
60s to 70s, although guidance is suggesting the higher values will
be in the vicinity of the frontal boundary. Lows overnight Friday
will be near or slightly above normal for this time of year, ranging
from the mid 60s in the Bluegrass to around 70 degrees elsewhere.

.LONG TERM (Saturday through Thursday)...
Issued at 320 PM EDT Thu Aug 28 2014

...Multiple Rounds of Precipitation Possible this Holiday Weekend...

An upper level trough and sfc low pressure will move east across the
Great Lakes region Sat/Sun with a slow moving sfc trough to approach
the Ohio Valley this weekend.  Precipitation will begin ahead of
this trough on Sat gradually spreading eastward throughout the day.
Soundings depict enough wind shear for perhaps some organized
multicell clusters of storms on Sat.  However, instability may be a
limiting factor depending on cloud cover early in the day Saturday.
The Bluegrass would be the best area for instability to build during
the day Sat and perhaps the best region for any strong storms to
develop.

The main rounds of rain will arrive for Sat night/Sun as multiple
disturbances move NE through the Ohio Valley ahead of the trough in
a warm, moisture rich environment.  With PWATs near 2 inches and a
deep warm cloud layer Sat night/Sun, multiple rounds of
showers/storms may cause minor flooding issues.

For the beginning of next week, convection chances will continue
although models vary quite a bit on exact timing.  The tail end of
this trough could linger long enough to cause on and off
precipitation Sun night/Monday according to most long range models.
However, convection timing does vary a bit among 12z models and
from run to run.  The next feature to cause yet more rounds of
convection will be a frontal boundary progged to lay out some where
over the Midwest for midweek next week.

Temperatures in the long term will stay near normal with highs in
the upper 80s/lower 90s and lows in the upper 60s/lower 70s most
days.  The one day that could be significantly cooler is Sunday as
multiple rounds of rain throughout the day may limit the diurnal
rise to the lower 80s.

&&

.AVIATION (06Z TAF Update)...
Issued at 115 AM EDT Fri Aug 29 2014

First concern is fog formation down at KBWG early this morning.
Already have MVFR conditions now, and vsby trend forecasts give a
good shot at IFR if not LIFR by daybreak. Bust potential could come
from a deck of clouds along the KY/TN border this hour, so will have
to see if that maintains itself on the way to the terminal.

KLEX did not get rain today, and consequently, the vsby is still at
10 miles this hour. With a solid east wind, have backed off on fog
potential there and gone with a tempo group for a few hours around
daybreak.

Next up will be dealing with scattered storms forming as a warm
front lifts north across the region today. The best chance for
development will be where this front coincides with peak heating.
Think this will happen over the Bluegrass later this afternoon. Have
enough confidence to go Prob30 for now, even though statistical
guidance is low. Cannot rule out development near KSDF, but think
chances are just a bit lower there so kept it out for now.

&&

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

$$

Update.........RAS
Short Term.....lg
Long Term......AMS
Aviation.......RJS




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