Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 200700

Area Forecast Discussion
National Weather Service Louisville KY
300 AM EDT Thu Jul 20 2017

.Short Term (Now through Friday)...
Issued at 255 AM EDT Thu Jul 20 2017

Upper ridging centered over the southern Plains continues to build
into the Ohio Valley, and will match or exceed the hottest temps of
any of the last few summers. Main challenge is when and where Heat
Advisory thresholds may be breached. Not too concerned about
convection with 700mb temps at 11-12C to keep a lid on things, but
Thursday night convection to our north could push an outflow
boundary into the Ohio Valley to focus isolated storms along/north
of Interstate 64 on Fri.

Temps finally managed to overachieve on Wednesday, and both 850mb
temp and 1000-850mb thickness progs suggest Thursday could run 3-4
degrees warmer. That puts most of the area in the lower 90s, with
mid 90s possible across parts of south-central Kentucky and upper
90s in urban Louisville. Dewpoint forecast is toned down slightly
from most guidance given how the boundary layer mixed out Wednesday,
but is still enough to push max heat index values to 101-104 degrees
west of I-65.

Heat wave will peak on Friday as the upper ridging is the strongest.
Temps may be only a degree or two warmer, but dewpoints could rise
into the mid 70s, and that would push the heat index solidly into
advisory territory, especially west of I-65. However, in
coordination with surrounding offices, will hold off on any
headlines for at least one more forecast cycle as it remains a lower-
confidence dewpoint forecast. Will continue to highlight the heat
and humidity via Special Weather Statements and web/social media

.Long Term (Friday Night through Wednesday)...
Issued at 255 AM EDT Thu Jul 20 2017

Upper ridging starts to break down on Saturday, allowing for a bit
more cloud cover and isolated showers and storms, especially east of
Interstate 65. Temps look a couple degrees cooler than Friday, but
heat indices will still top 100 degrees across most of the forecast

By Sunday a respectable upper trof swings through the Great Lakes
and pushes a cold front slowly into the Ohio Valley. Expect
scattered showers and storms Sunday into Monday, but weakly sheared
environment doesn`t support much organization. Canadian surface high
settles into the Great Lakes for Tue-Wed, knocking our temps below
climo and dewpoints into the lower 60s, providing a respite from the
oppressive heat and humidity.


.Aviation (06Z TAF Update)...
Issued at 133 AM EDT Thu Jul 20 2017

High pressure will remain in control of the weather through this TAF
period, with prevailing VFR and light winds. The one exception will
likely be shallow, MVFR fog at BWG between 10-12z. BWG didn`t
mix/dry out as much earlier this afternoon as SDF and LEX. With
clear skies and calm winds, shallow fog looks likely at BWG. The fog
potential looks lower at LEX, and confidence in MVFR is low. Vsby at
LEX will likely drop to 6-7SM. After a calm morning, a southwest
wind around 5 kts will develop after 15z.




Short Term........RAS
Long Term.........RAS
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