Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 210735
Area Forecast Discussion
National Weather Service Louisville KY
335 AM EDT THU JUL 21 2016
.SHORT TERM (Now through Friday)...
Issued at 329 AM EDT Thu Jul 21 2016
...First Significant Heat of the Summer Coming...
Strong 500 mb ridge is centered over the southern Plains, and will
stay in place through the short term. Meanwhile, the Ohio River
Valley will be situated on the eastern periphery of this upper
ridge. Today will mark the first in a series of hot days as a
noticeable rise in the 1000-850 mb thicknesses and temps occurs on
the eastern edge of the ridge. Have most highs in the 93-95 range
for today, and with dew points in the lower 70s west of I-65 this
will allow for heat indices to jump into the low 100s. Given the
prolonged nature of the upcoming heat, will leave the Heat Advisory
as is, even though we likely won`t meet actual criteria until
Friday. Did want to note that we should have significantly clearer
skies to the east of I-65, where dew points should mix out nicely
into the 60s this afternoon.
The overall pattern should keep us dry today given the lack of any
noticeable trigger, and the 650 mb subsidence inversion noted on
forecast soundings. Can`t completely rule out a brief shower or
storm, but any updraft that is able to overcome this inversion would
be very short lived given the stagnant deep layer flow.
Will continue with a virtually dry forecast for the overnight
period, but will have to at least introduce a slight chance of
storms across our NW. 4km ARW is suggesting that the MCS currently
over MN survives all day and dives SSE through the NW flow aloft
toward our region, arriving later this evening. 4 km NMM has a
similar idea, but is much slower with the arrival. While our
environment will have plenty built up instability, the overall deep
shear parameters are very poor this far south so do not expect a
complex to survive into the region. Nevertheless, it is worth noting
and keeping an eye on through the day. Obviously, if this system
were to survive into the region tonight it could have a huge impact
on Friday temperatures...
Upper ridge does lose a little strength on Friday, and resulting
subsidence is not expected to be as strong. Perhaps a little better
chance of an isolated afternoon or evening storm, but will cap any
chances below 25 percent across our north. Big story will again be
the heat. H85 temps and thicknesses jump even higher by Friday which
should allow for temps in the 93 to 97 range. Mid 70s dew points
will be better established across our west, and will allow for even
higher heat indices mostly in the 105 to 109 range. Further east,
100 to 105 is expected.
Do want to note that the idea of an Excessive Heat Warning due to at
least 4 days (Fri-Mon) of heat indices above 105 has been tossed
around for areas west of I-65. However, T-storms will become more
likely to impact temps the later we get into the weekend, and
overall confidence is lowered as a result. We`ll have time to nail
down convective chances over the next couple of periods and still
have time to pull the trigger for longevity if need be.
.LONG TERM (Friday Night through Wednesday)...
Issued at 320 AM EDT Thu Jul 21 2016
...Continued Hot and Humid Likely Through This Weekend...
Friday Night - Sunday Night...
The Ohio River Valley will remain on the eastern periphery of an
upper ridge through the weekend, however the ridge will begin to
lose a little strength during this time. As a result, the ridge
takes on a more "dirty" characterization where subsidence will not
be as pronounced, and mid level lapse rates will be slightly more
favorable (evident on forecast soundings). In addition, the weakened
ridge will allow for the zonal northern stream to drop a little
further south, and a weak trailing cold front to potentially drop
into the region from the north. All of these factors will result in
slightly better chances for afternoon/evening showers and storms.
We`ll still be hot and humid during this time, but some relief could
come in the form of a shower/storm. Speaking of humidity, highs
should top out solidly in the mid 90s on Saturday, with most peak
heat indices in the 100-107 range. Heat Advisory looks good. May see
a subtle drop off in highs for Sunday, but solid low to mid 90s is
still a good bet, along with heat indices in the 100 to 106 range.
Will mention that any shower or storm will temper the threat, but
with coverage only expected to be 30%, other areas will stay around
criteria that stay dry.
Lows each night will be mostly in the mid 70s. A few upper 70s will
be possible in the typical mild spots.
Monday - Monday Night...
Models generally agree on another weak frontal boundary dropping
over the region on Monday, which could help to provide enough focus
for scattered (40%) coverage of showers and storms. Highs should be
right around 90 for Monday, mainly dependent on timing of the front
and when storms fire.
Tuesday - Wednesday...
Models generally agree that the front stalls close to the KY/TN
border for the mid week time frame, with shortwave ridging aloft.
This should help to keep us mostly dry, except for some small
chances down along the border. In addition, temps will not be quite
as hot in the post-frontal airmass. Still, upper 80s to around 90
looks reasonable for Tue/Wed.
By Thurdsay, stalled frontal boundar may lift back north as a warm
front, where another round of scattered showers and storms would be
.AVIATION (06Z TAF Update)...
Issued at 1252 AM EDT Thu Jul 21 2016
Plan on generally VFR conditions through the TAF period outside of
light fog visibility restrictions possible at BWG early this morning.
Widely scattered storms from yesterday combined with the light winds
and abundant low level moisture overnight could promote fog
development at about anywhere in the region, the greatest likelihood
is across southern Indiana and in the favored south-central KY
locations including BWG.
After mid-morning, plan on light/variable winds with a scattered cu
deck developing by afternoon. Lack of surface convergence and upper
level forcing should preclude storm development today.
KY...Heat Advisory from noon EDT /11 AM CDT/ today to 8 PM EDT /7 PM
CDT/ Saturday for KYZ023>030-045-053-061>063-070>075.
IN...Heat Advisory from noon EDT /11 AM CDT/ today to 8 PM EDT /7 PM
CDT/ Saturday for INZ076>078-083-084-089>092.