Area Forecast Discussion
Issued by NWS Louisville, KY

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311
FXUS63 KLMK 211445
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1045 AM EDT THU JUL 21 2016

...Forecast Update...
Issued at 1045 AM EDT Thu Jul 21 2016

Adjusted temperatures for current observations. Also made an
adjustment to the dewpoint gradient over the region from west to
east. Forecast soundings show quite a bit of dry air in the east
that should be able to mix down under abundant sunshine.  We easily
could see lower 60s dewpoints in our eastern counties compared to
the low to mid 70s in our west. These changes made for a slight
downward trend in peak heat index values for today.  Now am just
getting to a little over 100 in the heat advisory area. Could
justify cancelling today`s heat advisory, but with the possible
prolonged nature of this heat event and with the focus in those
products being the higher heat index values Friday and Saturday,
will keep headlines in place as they are.

Another thing to focus on is the line of storms over Wisconsin and
Michigan this morning. Almost all of the models have no handle on
these storms, even ones just now running with 12Z initialization!
The 00Z WRF-ARW looks the best with its 15 hour forecast, and this
model takes the line and makes it take a hard right, dropping into
Indiana this afternoon and into our southern Indiana counties this
evening. It should be dropping into a lobe of the ridge over us
today, so not willing to go straight with those showers making it
all the way south. But, we will have to watch to see if it drops any
boundaries down for new storms to develop on Friday, possibly short
circuiting our heat. The ARW has the boundary making it to the
Appalachians and firing off storms there tomorrow, so all it would
do for us is add more humidity, and bring heat index values closer
to warning levels for some. Lots to look at as we digest the rest of
the 12Z model runs.

.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)...
NZUS01 KLMK DDHHMM
WRKLNG

Issued at 329 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 (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. Heat Advisory may need to be extended into
Monday if timing of front is slow or non-impactful from a
precipitation standpoint.

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
possible.

&&

.AVIATION (12Z TAF Update)...
Issued at 635 AM EDT Thu Jul 21 2016

Early morning MVFR visibility restrictions at BWG will continue
through about 13z before improving. Otherwise, light/variable winds
are expected at all TAF sites today under mostly clear to partly
cloudy skies. The lack of strong surface convergence or upper level
forcing will keep the forecast dry through tonight. Toward the end
of the TAF period and into Friday morning, showers and storms could
be moving into parts of the region.

&&

.LMK Watches/Warnings/Advisories...
KY...Heat Advisory until 8 PM EDT /7 PM CDT/ Saturday for KYZ023>030-
     045-053-061>063-070>075.

IN...Heat Advisory until 8 PM EDT /7 PM CDT/ Saturday for INZ076>078-
     083-084-089>092.

&&

$$

Update.........RJS
Short Term.....BJS
Long Term......BJS
Aviation.......ZT



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