Area Forecast Discussion
Issued by NWS Nashville, TN

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FXUS64 KOHX 190447

Area Forecast Discussion
National Weather Service Nashville TN
1147 PM CDT Sun Mar 18 2018




It`s relatively quiet at update time. A little overrunning light
rain/drizzle is on the radar and with a small shield of rain
crossing over the Mississippi, our western counties could see a
little more rain prior to midnight, likely less than 0.25 inches.
I revamped overnight PoPs a little to account for this potential.

Of course, the one thing on everybody`s mind right now isn`t these
piddly rain showers -- it`s tomorrow`s severe potential. The 00Z
models aren`t in just yet, but there`s a couple of things based on
models from earlier today I want to mention. First is the cap
being depicted on forecast soundings. This is always major player
in discrete storm development. Breaking through this cap is going
to depend on morning showers and whether or not the sun can come
out and play. The morning showers are looking more likely, based
on latest Hi-Res guidance, so we`re probably going to need the sun
come out to maximize surface instability. I mention the sfc
instability because the other thing is surface dew points. Right
now, models are struggling to get us into the 60s for dew points
during the afternoon. While not getting into the 60s for dew
points isn`t a negating factor, it could very well be a limiting
factor, especially for discrete development ahead of the front.
That said, shear won`t be an issue. There`s plenty of shear, both
deep layer and low-level throughout this event.

Even if we don`t see afternoon discrete development, there will
still be a concern with any storms that form later in the
afternoon/early evening closer to the cold front. The forcing
alone should overcome any lower dew points that may be around. So,
as we get into the evening hours and low-level winds veer, then
we`ll run into more of a QLCS situation where straight line winds
become the bigger issue, with the potential for embedded
supercells and rotation.

I also can`t fail to mention the upper levels of these forecast
soundings. Lapse rates are very good and there is dry air aloft,
like we haven`t seen yet this spring. This means the potential for
larger hail stones to develop as CAPE will be high in the hail
growth zone.

The bottom line here is everybody in Middle TN needs to be weather
aware tomorrow. We`ve been touting on the social medias for people
to have a plan in place ahead of time. This isn`t to ring the bell
any louder than normal, but hopefully to get people to pay closer
attention tomorrow.



Showers are showing up on radar this evening, and heading towards
middle TN. Have showers in all TAF sites during the overnight
hours, and ending before sunrise. Models have backed off on MVFR
ceilings until thunderstorms arrive to the area in the afternoon.
Again used short term models to try and time best chances for
thunderstorms at each terminal. Winds look to be easterly
overnight, then southeasterly for most of the day, peaking around
10 knots with gusts 15 to 20 knots in the early afternoon. A few
of the thunderstorms may be severe during the afternoon, and
could produce brief gusty winds at impacted terminals.
Thunderstorm chances will end from west to east from late
afternoon through the evening, and winds will become northwesterly
by 06Z.





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