Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 291911
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
311 PM EDT Fri Aug 29 2014
.SHORT TERM (Now through Sunday)...
Issued at 300 PM EDT Fri Aug 29 2014
Warm and unsettled pattern will gain a bit more focus over the next
36 hrs. For now, a summertime regime continues with a weak surface
boundary lifting north across the Ohio Valley. Moisture pooling
along the boundary is responsible for a CAPE axis running from near
SDF to just south of LEX, and just ahead of it there is an enhanced
cu field over the Bluegrass region. Slightly drier air has worked
into south-central Kentucky, as surface dewpoints have dropped to
Any convection this afternoon will be focused along the
aforementioned boundary, so will initialize with isolated T-storms
roughly along and north of I-64. Wind fields have strengthened just
enough that any thunderstorms should be more progressive than the
last couple of days, and the forcing should be out of the area
For Saturday the S/SW flow will deepen and heights will fall as the
sharp upper trof over the Plains opens up and lifts toward the Great
Lakes. Precip chances will increase from west to east through the
day, with likely POPs creeping into south-central KY from the
Pennyrile region late in the day. Otherwise will stay in the chance
range until evening.
Saturated column and modest SW low-level jetting Saturday night will
support likely POPs across the board. Most likely QPF will be around
half an inch in some areas, but models do point to a swath of 1-2
inch rainfall where the best forcing sets up. What little CAPE there
is will become quite tall and skinny, so will limit thunder to
chance. Confidence in the placement of any heavier rainfall is not
high enough to issue a Flood (or Flash Flood) Watch this early, but
later shifts may need to reassess as confidence in the placement of
this heavier rain swath increases.
Above normal temps will continue, but there is real bust potential
on Saturday highs if the precip is quicker moving in than
Weak frontal boundary is roughly located along our southern border
this hour. This boundary is forecast to lift northward today, with a
wind shift from easterly ahead of it to southerly behind it. Lift
along this front as well as instability from late afternoon heating
should produce isolated to scattered storms, with the best chance
over southern Indiana and the Bluegrass region of Kentucky. The
stronger storms will produce brief gusty winds and heavy rain.
.LONG TERM (Sunday night through Friday)...
Issued at 300 PM EDT Fri Aug 29 2014
...A cloudy and potentially wet Sunday with warmer and more
scattered precipitation for Labor Day...
Upper air pattern early Sunday will feature zonal flow along the
northern tier of the CONUS, with sub-tropical ridging firmly
established near the Georgia coast. Two shortwaves of note: the
first will feature low pressure moving over Michigan early Sunday,
while a stronger shortwave will just be ejecting eastwards of the
The Commonwealth and southern Indiana will light right in the middle
of a tropical airmass early Sunday, as an arc of very humid air with
PWATs of greater than 2 inches is forecast to extend from a
disturbance in the western Gulf through the eastern Great Lakes.
Showers, with some embedded thunder will likely continue from
Saturday night right into a good portion of Sunday. Various models
agree with the assessment from the Weather Prediction Center of
substantial rains during this Saturday night-Sunday period with a
swath of widespread 1 to 2 inch rains with possibly locally higher
amounts. Exactly where this falls is still to be determined, with
the NAM showing the heaviest accumulations along the Ohio River with
the latest GFS showing this a bit farther south. Localized runoff
problems may develop where any showers repeatedly move over a
specific location. Highs Sunday may not exceed the lower 80s due to
By late on Labor Day a surface cyclone associated with this second
wave ejecting from the Rockies will deepen north of Lake Superior.
The Lower Ohio Valley will lie in a humid airmass left over from
Sunday`s disturbance. Temperatures will be warmer, reaching the
upper 80s, with isolated to possibly scattered afternoon to evening
By Tuesday, this second cyclone will bring a frontal boundary that
will likely lay along an east/west line somewhere across Indiana and
Illinois. This front will likely weaken and dissipate late Tuesday
or early Wednesday along or north of the Ohio River. Widespread
convection is likely Tuesday north of us. However some organized
storms may reach as far south as southern Indiana during the
afternoon or evening. Only isolated unorganized convection is likely
farther south towards Tennessee Tuesday afternoon and evening. Hot
humid weather will continue Tuesday with highs around 90.
.Wednesday through Friday...
A fast zonal flow will set up Wednesday through Friday with a
relatively robust jet aligned almost exactly over our northern
border with Canada. Strong ridging will become established across
practically the entire southern tier of the CONUS. This pattern will
mimic ones that we occasionally see during the "dog days" of August.
Expect generally light winds with hot humid weather and warm muggy
nights this whole period with highs near 90 and lows at 70 or above.
Isolated thunderstorms may develop each afternoon. However no
organized precipitation is expected.
.AVIATION (18Z TAF Update)...
Issued at 1255 PM EDT Fri Aug 29 2014
Main challenge in this TAF issuance is afternoon convection and how
it will impact SDF and LEX. Based on what is still a fledgling cu
field, the effective boundary is roughly over SDF and still south of
LEX. Given recent progress, this boundary should be solidly north of
Louisville by the time it triggers any T-storms. Tougher call at
LEX, as there appears to be a narrow window for storms in the middle
of the afternoon. Once the southerly flow becomes better
established, there is actually less boundary-layer moisture as is
evident in the 12Z OHX sounding. Will start with a couple hours of
VCTS but otherwise just a scattered diurnal cu field.
Not too concerned with fog overnight as southerly flow will keep the
boundary layer somewhat mixy. Precip shield could start to creep
into BWG late morning, and SDF around midday Saturday. Not confident
enough in how solid this area of precip will be, so most likely will
just carry VCTS late in the TAF. LEX should stay VFR and rain-free
through 18Z Sat. Winds will pick up to 10-12 kt with mixing, but do
not expect enough heating to generate much gustiness.