Area Forecast Discussion
Issued by NWS Nashville, TN

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FXUS64 KOHX 181733

Area Forecast Discussion
National Weather Service Nashville TN
1233 PM CDT Wed Jul 18 2018




The waterworks have finally closed up shop and while we have a
really nice 24-36 hours in store for Middle TN, the dry air
hasn`t yet begun to filter into the state. Dew points are still
running in the low 70s across the mid-state which, as temperatures
come down and dew points depressions close in on 0 to 2 degrees,
fog is beginning to form. While mainly relegated to valleys and
low-lying areas, some of it is dense, especially for counties
closer to the Alabama state line. I don`t know that it will be
widespread, but be prepared to encounter some fog on your
Wednesday morning commute. Those low to mid 60s dew points that
are just north of the KY state line at forecast time will push
southward throughout the day and while temperatures will still
warm to around 90 degrees this afternoon, it should be a much more
comfortable airmass for everyone by late this afternoon.

For the most part, Middle TN should be rainfree both today and
tomorrow as high pressure builds into the region, however, models
are suggesting a few thunderstorms will be possible on the
Cumberland Plateau Thursday afternoon and evening.  Nothing too
widespread, but an inverted trough south of the Appalachians might
provide enough moisture for this to occur.

Besides the brief respite from the muggy air, the other story of
this forecast package revolves around the intense upper level low
that will develop in the Great Plains tonight and push into the
Great Lakes region Friday.  As it does, areas from the mid-
Mississippi Valley into the Tennessee Valley will be squarely in a
northwesterly flow aloft. Models continue to show a pre-frontal
MCS (though being depicted to arrive earlier in the day Friday now
in both the Euro and GFS) and a surface low developing to our
west Friday evening. While the MCS could be troublesome enough,
both the GFS and NAM are off the charts regarding the deep layer
shear being generated by this surface low. What was 40-50 kts of
0-6 km shear is now 60 kts and in the 00Z to 03Z Saturday time
frame (Friday evening), low level SRH values bounce up to about
300 m2/s2. Instability values are also sky-high due to the
southerly influx of surface moisture. The first thing that comes
to mind in the northwesterly flow aloft is a Friday evening
derecho. Obviously, a scenario like this carries with it damaging
straight line winds, heavy rains (PW values are running about 2
inches) and yes, the potential for July tornadoes. I am going to
add this Friday severe weather threat into the Hazardous Weather

As Saturday rolls around and the Great Lakes upper low amplifies and
pushes southeastward, a blocking high in the Atlantic could help to
keep shower and thunderstorm chances in the forecast for at least
the eastern half of Middle TN.  In fact, similar PoPs can be
expected into the first couples days of next week, as blocking high
in the Atlantic keeps the upper low from progressing off the
coast. Fortunately, by mid-week, both the GFS and Euro are showing
a drying trend as the high pressure the Atlantic gives way and
another high pressure system pushes into our neck of the woods.
This could actually be another break in the humidity, as well.
Temperatures still look to hold around normal throughout the
forecast package.



VFR conditions are expected at all sites throughout the forecast
period. Drier air has made its way into Middle Tennessee and will
keep skies mostly clear today. However, by Thursday morning cloud
cover will have increased and winds will shift out of the south
ahead of an approaching system that could bring some precip late





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