Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 202309
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
709 PM EDT Sat Sep 20 2014
...Updated Aviation Discussion...
.SHORT TERM (Now through Sunday Night)...
Issued at 250 PM EDT Sat Sep 20 2014
SW return flow is well established over the Ohio Valley as high
pressure retreats off the Delmarva coast, and a cold front dives SE
across the Land of Lincoln. Temps have punched into the lower/mid
80s with dewpoints solidly in the 60s, in a definitive last hurrah
for astronomical summer. Modest enhancement in the cu field over
southern Indiana has generated a few isolated showers, but even the
traditional 20 POP is a bit generous to describe their coverage.
The heat of the afternoon will keep the isolated showers in play
until around sunset, and there is enough enhancement in the cu that
north-central Kentucky would be fair game as well. However, there
remains a strong enough mid-level cap that any showers could not
grow very tall, so no mention of thunder.
Low-level jetting overnight will keep temps unseasonably mild. Given
the typical model bias in pre-frontal SW flow, will keep forecast
temps at least a couple degrees warmer than the warmest of the model
guidance. Main bust potential would be anywhere that it rains before
daybreak, as wet-bulb effects would knock temps down closer to model
guidance. Confidence in precip is again quite limited, as model QPF
fields are backing off in response to the track of the upper impulse
remaining to our north. Will go above guidance POPs with 30-40 POP
north and just a slight chance over south central Kentucky, but QPF
looks quite meager.
Best rain chances on Sunday will be across southern Indiana early in
the morning. The cold front could dry up as it pushes southeast, as
the dynamics are not as favorable farther south. Fropa will be
around the time of minimum instability, but with decent mid-level
lapse rates and a 30-40 kt low-level jet at 925mb, will at least
mention isolated thunder. POPs never go above slight chance south of
the Western Kentucky Parkway and west of I-65.
Decent cold air advection will set up behind the front. However with
deep boundary-layer mixing and decreasing cloudiness, we do expect
at least some diurnal temp recovery. Sunday night mins will be above
guidance once again, as the N-NW winds stay up around 5-7 kt.
.LONG TERM (Monday through Saturday)...
Issued at 300 PM EDT Sat Sep 20 2014
Upper air pattern will set up Monday through the entire work week to
bring a remarkable stretch of fine late September weather that, all
told, will probably last a full week or more. In fact, 500mb heights
over the northern Ohio Valley from Wednesday onwards will rise to
values more typical of summer than early fall.
Basically, this fundamental shift in the pattern aloft will begin
Tuesday as a deep upper trough develops off of or near the Pacific
Northwest coast. Due to the configuration of the northern
hemispheric flow, this trough will move very little, possibly coming
ashore the Pacific Northwest coast late Friday. Strong 500mb ridging
will develop over Indiana or Ohio Tuesday and will remain nearly
stationary through probably at least Saturday or Sunday of next
Now for the details...
Expect a rain free week with the first real chance of precipitation not
really apparent until a week from tomorrow.
High pressure building across the upper midwest early Monday will
bring northerly winds and a cool day Monday, despite ample sunshine.
Temperatures Monday will struggle to reach 70, especially across
southern Indiana and the Bluegrass Region. After a clear night with
light northeast winds and good radiational cooling, Tuesday morning
will be chilly, with the coolest early morning lows of the season so
far. Some typically cool sheltered rural locations may approach 40
for a low early Tuesday. Tuesday will warm into the lower 70s under
ample sun and light east winds.
Wednesday through Friday will be mostly clear with a slow warming
trend. Stationary high pressure over Ohio throughout this period
will bring light winds and several days of relatively wide diurnal
temperature swings. Expect warm afternoons with coolish nights.
Highs will warm from the mid 70s to the lower 80s, with lows in the 50s.
Clear to partly cloudy skies, slightly higher humidities and warm
temperatures are anticipated next weekend. Deep layer moisture will
return to the Tennessee Valley with some chance of afternoon or
evening thunderstorms. However, for now, will continue with a dry
forecast as drier air is still expected to hold over the
Commonwealth. Temperatures this weekend will not stay warm, with
highs near or in the lower 80s.
.AVIATION (00Z TAF Update)...
Issued at 709 PM EDT Sat Sep 20 2014
Cluster of convection east of KBWG will continue to slide off to the
east and weaken over the next hour. VFR conditions are expected at
the terminals this evening and into the overnight hours with a
breezy southwesterly surface flow continuing.
While KBWG did pick up some precipitation this afternoon, current
thinking is that low-level jet cranking up overnight will prevent
fog formation down that way. However, will continue to monitor for
a possible amendment later if fog developers.
Surface cold front will approach from the northwest tonight. Model
time height sections continue to show a light southwesterly flow at
the surface with a low-level jet developing after midnight.
Therefore, will keep LLWS in the TAFs going for this update as
current forecast looks good. Did add some VCSH at the terminals for
late tonight and toward daybreak Sunday as the cold front swings
through the region. Surface winds will then shift to the west and
northwest Sunday morning as the cold front slide southeastward
across the region. Surface wind gusts of 15-20kts will be common at
the terminals on Sunday.