Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 201049
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
649 AM EDT Sun Jul 20 2014
...Updated Aviation Discussion...
.SHORT TERM (Now through Monday)...
Issued at 300 AM EDT Sun Jul 20 2014
The synoptic pattern early this morning features an upper trough
across the Great Lakes region extending into the Ohio Valley, with
an upstream ridge located off the West Coast. This trough will
begin to finally lose its grip over the Ohio Valley through the
short term period, leading to a warming trend for the start of the
The latest surface analysis shows a weak inverted surface trough
located from southwest to northeast, bisecting the state of
Kentucky. An approaching shortwave trough visible on WV imagery has
led to the development of a weak surface low across southern KY
along the aforementioned trough. To the northwest of this surface
low, showers have recently increased in coverage along a weak 850mb
These light showers will continue through the remainder of the
morning hours into the afternoon, before tapering off from west to
east throughout the day as the shortwave trough passes through and
the surface low weakens and meanders east. It appears the best
coverage of these showers will be to the north and east of the weak
low, which puts the Lake Cumberland region and the Northern
Bluegrass at the greatest risk (30% coverage). Forecast soundings
in these areas continue to support a very isolated risk of a
thunderstorm early this afternoon, thus will continue thunder
mention in the forecast. Highs today will be quite tricky, as
NAM/GFS guidance differs quite a bit on the depth of the low-level
moisture. Given the general pattern we`ve been stuck in and no real
entrainment of drier low-level air, have leaned toward the cloudier
NAM solution. This keeps clouds around through much of the day
across the Northern Bluegrass, which should once again help hold
highs down in the upper 70s to perhaps a few lower 80s. Further
west towards the I-65 corridor, a few peaks of sunshine this
afternoon should help temperatures rebound into the low to mid 80s.
The upper-level trough will finally begin to lose its grip on the
Ohio Valley tonight, which should allow for a drier forecast.
Cannot rule out a stray shower over near the I-75 corridor along the
stubborn inverted surface trough, but the potential looks too low to
warrant mention in the forecast at this time. Overnight lows will
be in the mid 60s.
Things will begin to warm up on Monday, as 850mb temperatures warm
to around 17-18C. In addition, sunshine looks to finally break out
in earnest Monday afternoon. However, it appears as if winds will
remain quite light and easterly, which will temper the warm up a
bit, thus highs in the mid to upper 80s seem reasonable, with a few
of the typical warm spots touching 90. Will continue to leave a
very slight chance of a thunderstorm near the surface trough across
the far eastern CWA, but this may be able to removed in the next
forecast package as it appears the threat is trending further
.LONG TERM (Monday Night through Saturday)...
Issued at 300 AM EDT Sun Jul 20 2014
Main synoptic influence Tuesday into Wednesday will be the hot ridge
building over the Rockies. Better model agreement with each
successive run that the ridge axis will extend eastward somewhere
near 38N, with strong capping over our northern and western
counties, especially on Tuesday. Will keep trending the forecast to
reflect this, pushing highs solidly into the lower 90s, with the
heat island of Metro Louisville creeping into the mid 90s. Heat
indices will be close to 100. Will also keep suppressing the POPs
farther south and east, with chance POPs limited to Lake Cumberland
and even holding the slight chance south of the Western Kentucky and
Pattern amplifies on Wednesday with heights falling as a NW flow
impulse pushes the cold front toward the Ohio Valley. Should be just
about as hot as Tuesday, but with better chances of precip,
especially late in the day over southern Indiana.
Best shot at showers and storms will be Wednesday night with the
cold front. Will carry high-end chance POPs with confidence in
timing being the main limiting factor. There will be strong
instability ahead of the front and decent mid-level winds, but
nocturnal timing is not as favorable for severe storms. Dynamics are
just enough to support some organization, so can`t completely rule
out a few strong storms. Will continue to mention in HWO.
Latter part of the week will be marked by east coast trofiness and
NW flow over the Ohio Valley. Temps will run a few degrees below
normal Thu-Sat, especially by day. Surface high over the Great Lakes
will provide at least a day of dry wx and comfortable humidity
levels on Friday. Rain chances return on Saturday as the next system
takes shape in a flattening NW flow.
.AVIATION (12Z TAF Update)...
Issued at 648 AM EDT Sun Jul 20 2014
LIFR cigs have pushed into KLEX and KBWG early this morning as
an unseasonably cool airmass interacts with copious low-level
moisture. KSDF has even gone high-end IFR as some light rain
overnight has helped cigs to slowly lower. Given only light winds
expected today and no real entrainment of drier air, think these low
clouds will be slow to mix out, with KBWG and KLEX likely not going
VFR until this afternoon. However, it appears that by this
afternoon all sites should return to VFR. A stray shower is
possible at any site this morning into early this afternoon, with
the best coverage likely occurring near KLEX.
Conditions may once again deteriorate tonight, with light fog and/or
low cigs possible once again, especially towards KLEX. Given the
uncertainty in the amount of available low-level moisture, will
leave restrictions out of KBWG and KSDF for now, but they may be
needed in coming forecast packages.