Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 202034
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
434 PM EDT Wed Aug 20 2014
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 (18Z TAF Update)...
Issued at 125 PM EDT Wed Aug 20 2014
Confidence in convective forecast this period is low, as cirrus
shield from storms to our northwest quickly moves into the region.
Have some cumulus developing over the central forecast area this
hour, and SPC mesoanalysis indicates plenty of instability across
the region. However, the cirrus canopy should provide some
subsidence to inhibit convection. There is still a possibility that
the storms to our northwest can remain organized enough to bring
those storms to the terminals later this afternoon. Thus have gone
with vicinity storms over a larger period of time and will update as
the afternoon progresses. May even get into another round of precip
later tonight, as additional disturbances move in from the
northwest, but a lot will depend on what happens this afternoon. Fog
forecast for Thursday morning will depend on that as well. Given
very high dewpoints across BWG this hour, have enough confidence to
go with a period of MVFR conditions, as in current guidance.