Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 281652

Area Forecast Discussion
National Weather Service Louisville KY
1252 PM EDT Sun May 28 2017

...Updated Aviation Discussion...

...Forecast Update...
Issued at 1109 AM EDT Sun May 28 2017

A few showers have developed across portions of the Bluegrass region
this morning and will continue to push off to the north over the
next hour, so have added in low pops for these showers. For this
afternoon, much of the high-res guidance as well as the new NAM12
have showers and storms developing across south central KY this
afternoon as a weak upper level disturbance moves in. Given the
relative agreement in at least scattered storms developing in this
region this afternoon, pops have been increased. Soundings show some
instability developing, but generally keep it around or less than
1000 J/kg, thanks in part to lower dewpoints than we had yesterday.
Sill, a few stronger storms are not out of the question. The other
concern for this afternoon is that south central KY had the most
rain yesterday, including some major flash flooding. Think that
storms should be fairly progressive and scattered in nature this
afternoon, but if there is any training it could lead to some
localized flooding issues.

.Short Term (Now through Monday)...
Issued at 315 AM EDT Sun May 28 2017

...Strong Storm Chances Later Today Conditional Upon Destabilizing...

Trailing stratiform rain from MCS now over the northern Gulf Coast
states will slide across southern KY for the remainder of the
overnight, so will continue high Pops there with a small chance for
an embedded rumble of thunder. Overnight rainfall, and expected
rainfall on Sunday are not enough to warrant the Flash Flood Watch
continuing. So, will let that fall off in coordination with JKL.
More on the Sunday precipitation below...

Once southern KY precipitation ends, we should see a relative dry
period through the remainder of Sunday morning and early afternoon.
As we move toward late afternoon, a shortwave and it`s associated
surface low over the southern Great Lakes will drag a cold front
near the area. Despite what is expected to be fairly heavy cloud
cover throughout the day, models show modest to moderate
destabilization by late afternoon. Most models suggest ML CAPE
values around 1000 J/KG, but a couple of models suggest values
closer to 2000 J/KG. Given the amount of cloud cover and that SW
surface wind will just be advecting old cold pool air from overnight
convection, feel the lower instability values are a better bet.
Instability will play a big factor because vertical shear profiles
are more than sufficient for organized updrafts, and even some
splitting/rotating storm modes given the long-straight hodograph. If
we are able to destabilize, still can`t take the threat of a few
severe storms off the table. Most likely place for this would be
over southern IN and northern KY late afternoon into the evening.
SPC has us in a Marginal Risk that is ultimately conditional upon
destabilizing. Otherwise, expect mostly cloudy skies with highs
topping out in the upper 70s and touching 80.

Shortwave trough axis and cool front move through overnight, with
shower and storm chances ending from west to east. Overnight lows
should drop into the upper 50s and low 60s by dawn on Memorial Day.

Still expecting a dry and pleasant Memorial day with plenty of
sunshine and highs on either side of 80. The dry relief will come
thanks to a mid level dry slot wrapping into a deep occluded low
over Lake Superior, and post-frontal surface high pressure.

.Long Term (Monday Night through Saturday)...
Issued at 315 AM EDT Sun May 28 2017

Monday Night - Wednesday...

Deep and occluded cyclone centered over Lake Superior will slowly
meander into eastern Ontario through Tuesday night. Mid level dry
slot that had been over the area will push SE Monday night and a
weak front will approach from the NW. Models have been hinting at
some potential light showers ahead of this front on Tuesday, and
will only mention small chances mainly across southern IN in the
morning, and across east central KY in the afternoon. A second front
looks to swing through on Wednesday as another shortwave rotates
around the parent trough. So, will leave slight chances for a shower
overnight Tuesday, and a chance for a few showers and storms on
Wednesday as instability looks a little better. Look for highs
around 80 and lows in the upper 50s to near 60 during this time.

Wednesday Night - Thursday...

Have higher confidence that the front will be through the area
during this mid-week time frame, with dry NW flow aloft and surface
high pressure briefly in control. So, will continue a dry forecast
with lows in the mid to upper 50s and highs in the upper 70s to
around 80.

Thursday Night - Saturday...

Overall placement of upper and surface features to end the work week
becomes a bit muddy, but there is a general idea that a strong storm
system will drop out of Canada and into the Great Lakes, dragging a
cold front into the Ohio River Valley. Expect to see some unsettled
days (Fri-Sat) ahead of this feature with scattered thunderstorm
mention. Highs should continue mostly in the upper 70s to near 80,
with lows in the low 60s.


.Aviation (18Z TAF Update)...
Issued at 1245 PM EDT Sun May 28 2017

Stratus continues to hold on from this morning, and will make
initialization tricky. BWG could start off in fuel-alternate, but
figure best case is MVFR. Could still have MVFR in SDF and LEX, but
that too is an issuance time decision. Expect to improve to VFR by
mid/late afternoon as the boundary layer continues to deepen. Winds
are already around to WSW and gusting near 20 kt.

The slower destabilization and lackluster convergence ahead of the
approaching cold front will limit thunderstorm chances later in the
day. Still hard to rule it out especially with a decent upper
impulse still progged to move through early this evening. Will carry
a few hours of VCTS/CB from roughly 23Z til 03-04Z Mon, but no
restrictions to cig/vis.

SDF could have some lingering post-frontal stratus overnight but
will stay above fuel-alternate. Otherwise VFR conditions and light,
generally westerly winds expected through the TAF period.




Short Term.....BJS
Long Term......BJS
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