Severe Thunderstorm Warning 133, NWS Louisville, KY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
329 PM EDT Wed Aug 21 2013

.SHORT TERM (Now through Thursday Night)...
Issued at 310 PM EDT Wed Aug 21 2013

Water vapor imagery shows that the upper low now over eastern
Indiana has wrapped up into a cinnamon roll presentation, with the
axis of one last vort lobe trailing roughly along I-65. Isolated to
scattered thunderstorms have popped east of this axis, with heavy
rain and frequent lightning as the main hazards. The forcing for
this activity should push far enough east by sunset that any evening
coverage will be isolated at best, but it still warrants a pre-1st
period in the text products.

Expect a muggy night, as there is not enough of a surface reflection
to bring about any change of air mass. Low temps in the upper
60s/around 70 will cross over afternoon dewpoints, so patchy fog
could very well be a concern, mainly in the areas that receive the
heaviest rain from this afternoon's convection. Will leave any fog
mention out, and allow the evening shift to iron out those details
based on precip coverage.

A cold front will drop southward into the Ohio Valley on Thursday,
and take its sweet time pushing through Kentucky Thursday night.
Precip coverage will increase through the day, especially across
southern Indiana and northern Kentucky. Will go with slight chance
POPs for south central Kentucky, ranging upward to high-end chance
over southern Indiana. Would go for a likely, if not definite POP,
if the time frame was not straddling 00Z. Severe weather potential
is limited due to upper dynamics staying well to our north, but the
moist and unstable environment will support similar storms to today
with heavy rainfall and gusty wind potential. Should be enough
breaks to allow for temps close to climo.

Even with the front through, any small changes in the air mass will
lag behind. Therefore will keep Thursday night on the warm and muggy
side.

.LONG TERM (Friday through Wednesday)...
Issued at 320 PM EDT Wed Aug 21 2013

The start of the long term period will feature northwest flow aloft
over the Ohio Valley as a ridge builds across the west-central
CONUS.  This pattern will be short-lived, however, as the ridge
strengthens and expands its grip eastward into the Ohio Valley.
After potential storms on Friday, the building influence of the
upper ridge will make for generally dry and hot conditions for the
bulk of the long term period.

The start of the period will feature a very weak cold front/wind
shift that will slowly drift southeast into the Ohio Valley in
association with the weak northwest flow aloft.  Synoptic scale
models are struggling with the exact placement/strength of this
feature as it weakens and likely stalls across the Ohio Valley.
What is rather certain is that despite the frontal passage and light
northerly winds behind it, a rather unstable airmass will remain in
place Friday afternoon, with MLCAPEs 1500-2000 J/kg thanks to upper
60s dewpoints.  Think the front will make it into central/southern
KY by peak heating, thus have focused the highest pops here. But in
a generally uncapped atmosphere and northwest flow aloft, think
isolated storms could be possible as far north as the Ohio river on
Friday.  Temperatures will be tricky and highly dependent on
convection, thus will go with a general blend which puts
temperatures in the mid to upper 80s.

The front will make some southward progress by Friday night into
Saturday, allowing some slightly drier air to work into the region.
The real focus will then turn toward the west, as a rather
impressive upper ridge strengthens across the central United
States.  In fact, by Sunday afternoon, guidance projects the 500mb
high to be around 596 dam.  The real intense heat will likely stay
just west of the region, but the Ohio Valley will certainly feel an
increase in the heat and humidity throughout next week.  Have bumped
up highs a degree or two (now into the lower 90s next week)
throughout the long term period as this is quite a favorable pattern
for warm conditions (southwesterly surface flow, 850 temps around
18C, and generally dry conditions over the past few week or so).
This is far from a strong summertime "heatwave," but given the
generally cool conditions as of late, heat indices in the mid 90s
will feel quite uncomfortable.

Some of the guidance members try to spit out some precipitation on
Sunday and Monday afternoon.  However, with building heights aloft
and no real focus, just have a hard time buying into any
precipitation chances.  Therefore, will continue with a dry forecast
during this timeframe and into the middle of next week.

&&

.AVIATION (18Z TAF Update)...
Issued at 105 PM EDT Wed Aug 21 2013

Main challenge in this TAF set is afternoon convection. Best chance
will be near LEX, where it will be a press time decision whether to
include VCTS/CB. Can't rule out a stray storm at BWG or SDF, but
probabilities are too low to include in the TAFs. Either way,
chances will decrease late in the afternoon as the upper low over
Indiana pulls east, giving way to ridging.

Clearing skies and light winds overnight. However, confidence in any
fog forecast is fairly low. NAM forecast soundings seem to have the
best handle on the moisture profile, but winds just off the deck are
progged to shift to westerly and increase to 15-20 kt. In spite of
temps crossing over current dewpoints, will only take BWG down to
MVFR, with VFR conditions at SDF and LEX. Heavy rain directly
impacting any one of the airports will warrant re-visiting this part
of the forecast by the 00Z issuance.

Cold front drops in on Thursday, with somewhat deeper mixing
resulting in a higher diurnally-driven Cu deck. Convection should
hold off until after 18Z, but chances remain too low to even include
in the SDF TAF.

&&

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

Short Term........RAS
Long Term.........KJD
Aviation..........RAS

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