Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 210222
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1022 PM EDT Wed Aug 20 2014
Issued at 1015 PM EDT Wed Aug 20 2014
Warm and moist air mass remains in place over the Ohio Valley, and
has had some opportunity to recover after the earlier convection.
Cluster of T-storms near STL is pushing east into Illinois, and if
it holds together it could reach Dubois County by 06-07Z. Current
forecast seems fairly well on track with increasing POPs from
southern Indiana across to the Bluegrass as we head toward daybreak
Issued at 700 PM EDT Wed Aug 20 2014
Strong to severe storms have pushed across southern Indiana and
central Kentucky, with just a few cells now lingering along the
KY/TN border. The watch box has been cleared except for just a few
counties along the Tennessee line, and that should be cleared in
another hour or so as the storms exit.
Subsident regime in the wake of these storms will keep
us dry through the next few hours. Next wave upstream in the NW flow
is still poised to trigger another round of isolated/scattered
T-storms after midnight.
Issued at 430 PM EDT Wed Aug 20 2014
Complex over southern Indiana along with another over central TN
both are taking advantage of an unstable atmosphere across the
region to force scattered thunderstorms to develop. Given the
instability from SPC mesoanalysis, 4000 J/kg, as well as potential
for wet microbursts across parts of our region, went with Severe
Thunderstorm Watch box south of I-64 and roughly along and east of
.SHORT TERM (Now through Thursday Night)...
Issued at 300 PM EDT Wed Aug 20 2014
Temperatures in the mid 80s to around 90 with dewpoints in the
low-mid 70s is making for an uncomfortable afternoon, with a few
storms starting to pop up. Deck of high clouds streaming in from the
northwest has helped to hold off significant development across most
of our region to this point. Have a thunderstorm complex developing
in the unstable airmass over north central TN, and this complex may
force some new storms to develop along an outflow boundary in
southern KY. A few storms also are forming along the corridor from
Hancock county to Casey county in KY, with even more storms east of
the complex over south central IN.
Best instability and moisture still resides over southern KY, so
will keep most focus for severe weather down there the next few
hours, with a secondary focus over our southeastern counties. For
this evening, should continue to see scattered development away from
these focus areas, with the northeast forecast area having the best
chance to continue with storms after midnight. This region will
continue to be the focus for development the next few periods, as a
ridge aloft should help dry out our southwest forecast area Thursday
and Thursday night.
That ridge will provide hotter temperatures in our southwest as
well, with highs Thursday likely reaching the mid 90s. Heat indices
could top out around 100. Given potential for some cloud cover have
not gone into the next category, 100-105, for heat indices there.
Think we can hold off on our more prolonged period of near heat
advisory criteria until Friday, read on in the discussion below.
.LONG TERM (Friday through Wednesday)...
Issued at 306 PM EDT Wed Aug 20 2014
Friday - Monday...
...Heat, Humidity, and Generally Unsettled Weather This Weekend...
An upper ridge will be in place through this weekend and into next
week, keeping hot and muggy conditions in place. Upper ridge looks
"dirty" with very little in the way of a subsidence inversion
normally seen with this type of environment. Therefore, can`t do
away with storm chances each afternoon and evening. Models are
generally all over the place with expected precipitation fields each
day, despite the fairly good agreement in the overall pattern.
Therefore will just keep isolated to scattered thunderstorms in the
forecast pretty much every day and evening.
As far as temperatures go, highs look to top out in the low and mid
90s each day Friday through Monday as a core of H85 temps between
20 to 23C settle over the area. Some spots could touch the upper 90s
in our western Kentucky counties. Heat indices will be a bit of a
concern during this time period as the hot temperatures will combine
with dew points in the low and even mid 70s. Highest heat indices
each afternoon could top out in the 100-105 range, so forecast needs
to be monitored for a potential Heat Advisory due to a prolonged
period of days where heat indices approach criteria (105F). This
advisory would come with a later forecast issuance. One factor that
could limit temperatures and associated heat indices could be the
Tuesday - Wednesday...
Confidence lowers for the Tuesday/Wednesday time frame as some
degree of troughing over the western CONUS tries push the upper
ridge axis east. Should this scenario play out, it would bring
better chances for storms to our western CWA, along with
temperatures not as hot. As a result, will go with highs around 90.
.AVIATION (00Z TAF Update)...
Issued at 745 PM EDT Wed Aug 20 2014
Late afternoon convection has worked over the environment and left a
subsident regime in its wake, so expect VFR conditions and light
winds through much of the night. Main challenge will be
visibilities, especially in BWG where nearly 2 inches of rain fell
this afternoon. Models have not had a chance to bake this into their
initialization, so will just take the best shot at it. Will carry
prevailing MVFR with a TEMPO for IFR visibilities around daybreak.
Will also go with MVFR visibilities at LEX, but can`t rule out brief
IFR conditions given the earlier rain there.
Convective forecast confidence remains limited, as the models still
don`t have a great handle on ongoing activity. We are seeing some
developing storms over east-central Missouri, as well as in western
Kentucky, and either of those clusters could round the top of the
weak upper ridge, and affect central Kentucky well after midnight.
However, with low POPs at the TAF sites, will leave it out at this