Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 280721
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
321 AM EDT Tue Jul 28 2015
.SHORT TERM (Now through Wednesday)...
Issued at 320 AM EDT Tue Jul 28 2015
Ohio Valley remains under weak NW flow on the periphery of the hot
upper ridge, currently centered over the southern Plains. Low-level
air mass is quite juicy, with moisture pooling near a weak
quasi-stationary front draped across southern Indiana. However,
coverage of showers and thunderstorms has remained limited by a
respectable mid-level cap.
Expect today`s convective behavior to be fairly similar to Monday,
with isolated storms popping any time after noon. Instability is
decent but not impressive, and shear is almost nonexistent, but
heavy rain will be the main threat under the stronger updrafts.
While storms today will be diurnally driven, coverage will be
greater on Wednesday as a disturbance moving into the Great Lakes
flattens out the upper ridge, and a stronger surface front heads
toward the Ohio Valley. With enough destabilization, a few of the
stronger storms could produce marginally severe winds, especially
later in the day.
Very warm and muggy conditions will continue through Wednesday, as
the surface front will not wave far enough in any direction to
change the air mass at all. Temps in the lower 90s and dewpoints in
the lower/mid 70s both day will yield afternoon heat indices just
above 100 along and west of I-65, so will cover that in a SPS.
.LONG TERM (Wednesday Night through Monday)...
Issued at 245 AM EDT Tue Jul 28 2015
The synoptic pattern at the start of the long term period will
feature a ridge across the south-central CONUS, with a trough
building into the north-central CONUS. This trough will build into
the Ohio Valley through the period, shunting the oppressive
heat/moisture to the south making for a generally dry long term
The long term period will start out with a cold front right on the
doorstep of the LMK CWA. Mass convergence along the front should be
enough to lift a moderate to strongly unstable environment,
characterized by MLCAPE values of 2000-3000 J/kg, resulting in
scattered thunderstorms. Troposheric flow is rather meager with
0-6km shear on the order of 10-20 knots, so storms will be have
trouble organizing. That being said, precipitation loading coupled
with relatively steep low-level lapse rates will support an isolated
damaging wind threat in the stronger downdrafts. Given this threat,
agree with SPC`s Marginal outlook for Thursday evening into early
The front and associated precipitation will push east of the region
Thursday morning, allowing cooler and drier air to filter into the
Ohio Valley through at least Friday night. Highs Thu/Fri will be in
the mid 80s with dewpoints in the lower 60s.
On Saturday, a rather weak and moisture-starved front will attempt
to push towards the Ohio River. While an isolated shower/storm is
possible with this feature, chances appear to low to warrant any
mention at this time. This front will wash out before it makes full
progress through the Ohio Valley, so temperatures should begin to
warm Sunday into Monday as readings creep back into the upper
80s and lower 90s.
.AVIATION (06Z TAF Update)...
Updated 105 AM EDT Tue Jul 28 2015
Sultry summertime air mass remains over the Ohio Valley, so expect
the next 24 hrs to be fairly similar to the last 24 hrs. Main
question overnight is fog formation, especially in BWG where there
was a quick burst of rain Monday afternoon. Debris cloudiness from
convection to the west, along with a still-respectable 6 deg F
temp/dewpoint spread, will work against fog formation. However, with
both the GFS and NAM guidance showing restricted vis, and the
climatology of that site, we can`t downplay the potential too much,
and will carry MVFR with a TEMPO IFR vis arouund daybreak. LEX may
have some reduced visibilities, but no worse than MVFR.
Isolated showers and T-storms will be in play again during the heat
of the afternoon, but given Monday`s low coverage and an atmosphere
that looks to remain at least as capped, probabilities seem too low
to include in the TAFs. Diurnal cu should remain scattered, with
bases just high enough that any ceiling will stay VFR.