Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 231038
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
638 AM EDT Sat Aug 23 2014
...Updated Aviation Discussion...
.SHORT TERM (Now through Saturday Night)...
Issued at 300 AM EDT Sat Aug 23 2014
...A Heat Advisory continues through Monday....
In the very short term, an area of diminishing thunderstorms will
sag south and generally dissipate west of Bowling Green during the
pre-dawn hours. With a lack of surface heating and no disturbances
moving south, only a very isolated shower at best is likely during
the morning hours. Some haze will greet the dawn across some areas,
especially the Bluegrass.
For today and Sunday, a persistent 500mb ridge centered near
Arkansas will build northwards as the whole upper air pattern across
the CONUS amplifies due to digging trough off the Atlantic Coast and
the northern Rockies. By late Sunday, the axis of the upper ridge
will extend from the lower Mississippi Valley through Michigan,
basically just to the west of central Kentucky.
A tropical air mass is trapped within this ridge and will continue
to bring warm and very humid weather to the Lower Ohio Valley
through the weekend. Earlier Friday, dewpoints reached 75 at
Louisville. PWATs of 2 inches or more will continue along a
north-south axis right over the Commonwealth through Sunday.
Scattered thunderstorms will likely develop both this afternoon and
also Sunday. Any storms that develop this afternoon will move slowly
to the south or southeast and produce tropical downpours with
localized heavy to possibly excessive rainfall. Scattered storms will likely
persist well into the evening, only slowly diminishing towards
midnight Sunday morning. Isolated to scattered afternoon and evening
thunderstorms are also anticipated Sunday.
Temperatures this afternoon will range from the upper 80s to near 90
across our eastern counties, and towards the lower 90s west of
Interstate 65. After overnight lows in the lower to mid 70s, highs
Sunday will reach the lower 90s as well.
.LONG TERM (Sunday through Friday)...
Issued at 325 AM EDT Sat Aug 23 2014
Long Term Synoptic Overview
Highly amplified upper level pattern is expected to be in progress
at the beginning of the forecast period. Large positive height
anomalies off the western US coast combined with a developing ridge
in the eastern US will lead to a trough west/ridge east pattern.
The eastern US ridge axis should will gradually expand and
strengthen through the midweek time frame. It appears that by
Thursday or so, ridging will push in from the eastern Pacific into
the western US. This will set up a more progressive pattern aloft
resulting the western US trough getting kicked off to the east. The
timing of this trough through the Ohio Valley remains a challenge as
there is considerable model spread at that time frame. There could
also be some effects of tropical development over the western
Caribbean and into the western Atlantic by mid to late week which
may complicate the pattern aloft. With the persistent ridging
expected in place, diurnally driven convection is likely to continue
across the region posing a risk of strong convection along with
tropical rains given the high moisture content in the atmosphere.
Model Preferences / Confidence
Multi-model agreement is fairly good in the front end of the
extended period. Spread becomes readily apparent by midweek as the
operational runs of the GFS are off to the races with pushing the
western US trough across the country. The 23/00Z is no different
than the previous 22/18Z and 22/12Z runs of the OP GFS. The 22/12Z
and 23/00Z OP Euro solutions are vastly slower with this
trough...with the 23/00Z perhaps slightly slower than the 22/12Z
run. The 23/00Z GFS ensemble is slower with the trough passing
through this week...and generally agrees with the 23/00Z Euro and
22/12Z Euro ensembles. We have generally dismissed the GFS OP runs
for this forecast and will trend the forecast to a 50/50 blend of
the 22/12Z Euro & GFS ensembles and the 23/00Z OP Euro. Forecast
confidence is above average early in the week with a drop to
slightly below average from midweek through the end of the week.
Temperature forecast still show a warm bias as the models are not
picking up on antecedent soil moisture conditions and lush
vegetation due to recent rainfall. Thus, prefer a more conservative
forecast on high temperatures throughout the week with the highest
readings in the west and lower readings in the east/northeast where
recent rains have fallen.
Sensible Weather Impacts
Upper ridge axis will start off to the west of the region early in
the forecast period (Sunday Night). The multi-model consensus
suggests this ridge will gradually expand through midweek. This is
not a typical clean, summer time ridge...but more of a dirty ridge
where diurnally driven convection can develop even near the actual
ridge axis. We believe that we`ll see diurnally driven convection
throughout the early part of the week...with perhaps a little less
coverage by midweek as the overall ridge aloft expands outward.
Highest PoP coverage in the period would likely be Monday and
perhaps into Tuesday before sufficient ridging allows a decent
mid-level cap to develop across the area. For now, plan on keeping
scattered PoPs in the forecast for Monday/Tuesday with more isolated
wording for Wednesday through Friday.
The ridge looks to break down by late week with an upper trough axis
coming through the region. As outlined in the model preference
section, the arrival of this trough axis looks to be slower with
each model run. So we would not be surprised to see the front and
return of cooler weather be displaced until the weekend...or even in
to the later part of the upcoming Labor Day weekend.
As for temperatures, we`re likely to see a continued gradient of
temperature across the forecast area with the cooler readings in our
NE/E and the warmest in our W/SW. Highs Monday/Tuesday will likely
be in the middle-upper 80s in the east with upper 80s to lower 90s
out in the west. Heat indices may reach 100 in some parts of the
region on Monday. Temperatures will largely be dependent on cloud
cover and precipitation coverage for Wednesday through Friday.
While the models have been insistent on developing hot conditions in
recent forecasts, these have not worked out well due to moist soil
conditions and lush vegetative cover...especially in the eastern
sections. The drier western sections have seen a bit better
verification numbers. Nonetheless, have opted to go a little more
on the conservative side for highs from midweek onward. Generally,
high will average out in the middle to upper 80s with overnight lows
in the upper 60s to the lower 70s. Much cooler and less humid
conditions will arrive later in the weekend when the aforementioned
frontal boundary finally makes it through the region.
.AVIATION (12Z TAF Update)...
Issued at 635 AM EDT Sat Aug 23 2014
Fog and occasional MVFR ceilings have developed at LEX with
visibilities lowering to as low as 1sm. Fog will dissipate and
ceilings will lift by 14z this morning. Elsewhere, only some light
haze has developed at SDF.
Calm winds this morning will stay light and variable this afternoon,
prior to becoming calm again tonight.
Scattered thunderstorms, with perhaps 30 to 40% coverage are
possible this afternoon. Although these storms will be unorganized
and difficult to time, some of them may produce localized heavy
rain. For now, will not insert VCTS into this afternoon`s TAFs.
KY...HEAT ADVISORY until 7 PM EDT /6 PM CDT/ Monday FOR KYZ023>031-
IN...HEAT ADVISORY until 7 PM EDT /6 PM CDT/ Monday FOR INZ076>078-