Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 170535

Area Forecast Discussion
National Weather Service Louisville KY
135 AM EDT Sat Jun 17 2017

...Updated Aviation Discussion...

...Forecast Update...
Issued at 740 PM EDT Fri Jun 16 2017

Made some minor adjustments to fog grids this evening through
tomorrow morning based on latest model data. Latest trends seem to
indicate that the fog potential may not be as high as previously
thought, mainly due to dry air aloft and increasing southwesterly
winds around 1kft that could potentially mix down to the surface.
Best chances for patchy fog still look to generally south and east
of the Bluegrass Parkway.

Still planning on keeping the forecast dry for the evening, though
am closely monitoring some small potential for very isolated
showers/storms in northwestern portions of the CWA in southern
Indiana. Some isolated showers and thunderstorms have begun firing
off near the KVWX radar within the past hour or so and are slowly
drifting toward Dubois/Perry counties. Several outflow boundaries
are being picked up by radar/satellite as well. A quick mesoanalysis
of the environment reveals 1500-2000 J/Kg MUCAPE across portions of
southern Indiana, however, capping is quite strong and should keep a
lid on most convective activity. If the isolated activity holds
together, it should arrive into Dubois/Perry counties shortly after
02z. Will continue to monitor and update the forecast if it appears
activity will hold together into the late evening hours.

.Short Term (Now through Saturday Night)...
Issued at 230 PM EDT Fri Jun 16 2017

Latest preliminary GOES-16 derived precipitable water indicates a
narrow swatch of dry air over the region, stretching from central
Tennessee into Central Kentucky and then northeast to western New
York. This area is not moving much today, and AMDAR and current RAP
soundings show a subsident airmass in place across much of the
region. Thus have generally fair weather cumulus forming out there.
Temperatures are warm under the good insolation, with dewpoints in
the upper 60s to around 70, so the day is not overly perfect, but
typical for mid June. Only other concern then would be chances for
fog overnight.

A look at the GFS flow aloft indicates a weak disturbance moving
through the Midwest to our north tomorrow afternoon, and then
another one approaching the region late Saturday night. Precipitable
water is expected to increase between now and Saturday night as more
moist air pushes against our dry plume. Usually the edges of these
dry plumes are where convection gets fired during daytime heating,
so the current forecast for isolated to widely scattered pops across
southern Indiana seems reasonable, with just isolated chances along
the I-65 corridor. Warm temperatures should go for that mostly dry
forecast, with highs Saturday reaching the 90s for many locations.

Saturday night, the hi-res models, and to a lesser extent the
coarser ones, bring in a decaying line of storms ahead of a cold
front late. So here again will keep in the higher rain chances over
the northwest, southern Indiana, with a gradient into south central
Kentucky. Night time lows will depend on how quickly, or if,
showers/cold pool get into the region.

.Long Term (Sunday through Friday)...
Issued at 230 PM EDT Fri Jun 16 2017

The timing of precip Sunday will depend greatly on when/if rains
move through here Saturday night. Long-wave troughing aloft will
provide some energy for new development, with cold mid-level lapse
rates. Any boundary from the late Saturday night development could
serve as a focus as well for new development during the afternoon.
As for severe risk, the wind profile shows more shear potential than
with the last few days worth of convection, especially over the
northern half of the area. Temperatures for highs here again will
depend on location of these boundaries.

One other thing to note, precipitable waters from the models look
awfully high, especially given that the GFS/NAM/GEM all would argue
for record values for mid June.  Still expect at least above average
moisture into the region, so heavy rainfall will come with any
storms that fall.

By Monday, most of the precip should just be over or east of central
Kentucky, with the trough axis aloft swinging through here and
ridging trying to build in for midweek. Models have been struggling
with bringing in (or not) a frontal boundary and rains Tue night/Wed
and then beyond that we are back to a series of disturbances moving
through the flow making timing best rain chances difficult this far
out.  Will keep up with the slight chances for Wednesday and limit
peak rain chances the rest of the work week to that 20-30 percent

Monday should be the coolest day of the extended, and then we`ll
flip back to above normal by mid week, though again they will depend
on what should happen with that midweek system.


.Aviation (06Z TAF Update)...
Issued at 133 AM EDT Sat Jun 17 2017

VFR will prevail through this TAF cycle, with the exception of some
potential fog at BWG toward dawn. Afternoon crossover T`s would
suggest that BWG could see some MVFR fog, especially given that T/Td
depressions are already at 3 degrees. One limiting factor will be
upper level debris clouds from strong convective complex over MO.
Otherwise, SDF/LEX should stay VFR with either a very light SSE wind
or calm winds. A light SSW wind will take hold by late morning with
Few-Sct cumulus and upper clouds.




Short Term.....RJS
Long Term......RJS
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