Area Forecast Discussion
Issued by NWS Nashville, TN

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000
FXUS64 KOHX 250806
AFDOHX

Area Forecast Discussion
National Weather Service Nashville TN
306 AM CDT Tue Apr 25 2017

.DISCUSSION...

Latest satellite imagery shows some low clouds lingering across
eastern zones while clear skies prevail elsewhere. Light winds and
plentiful low level moisture have allowed for patchy dense fog to
form across Middle Tennessee early this morning, with a few sites
such as Gallatin and Sparta near zero visibility. Expecting fog to
become more widespread as night goes on then rapidly dissipate
after sunrise. Dense fog advisory remains in effect for all zones
through 14Z.

Very active weather pattern will commence this forecast period
with several rounds of showers and potentially strong to severe
thunderstorms. For today and tomorrow, however, two beautiful and
warm spring days are anticipated as H5 ridge builds over the
Tennessee Valley, with highs in the upper 70s today and well into
the 80s on Wednesday. By Wednesday night, a sharp upper level
trough will eject out of the Plains into the Mississippi Valley
while taking on a negative tilt. This will force a QLCS to rapidly
evolve across Arkansas and race towards us late Wednesday night
into Thursday. 00Z models continue to show QLCS outrunning main
moisture/instability axis to our west and falling apart once it
enters Middle Tennessee, although some rejuvenation of the line
is possible by Thursday morning across eastern zones once daytime
heating commences. At this time appears any severe weather
potential with this system appears quite limited to maybe a few
damaging wind gusts due to the weak instability but impressive
shear. Temperatures will also be much cooler on Thursday due to
the QLCS passage with highs only in the 70s.

A more substantial severe and heavy rain threat appears possible
on Friday as a large warm sector develops across the southeast
CONUS ahead of a very large H5 trough develop across the country.
00Z guidance depicts a warm front lifting northward across the
state Friday morning with a very warm and moist airmass moving in
from the south, with dewpoints rising into the mid to possibly
upper 60s and PWATs increasing to near the daily max of 1.5-1.6
inches. Forecast soundings indicate MLCAPE potentially rising to
around 2000 J/Kg by Friday afternoon with deep layer shear
increasing to around 50 knots. Main uncertainty revolves around a
potential trigger for convection as GFS/ECMWF show no significant
vort max or shortwave trough aloft, main jet streak passing to our
north/northwest, and a weak cap in place. Nevertheless, both
models develop considerable convection across the area, especially
the Euro, with forecast parameters indicating potential strong to
severe storms along with heavy rainfall. For now have kept pops
low on Friday due to the aforementioned uncertainties.

Any showers and storms will mostly lift out of the area Friday
night with Saturday being the warmest day of the forecast period
as well as likely the most humid, with highs well into the 80s.
With airmass remaining fairly unstable, cannot rule out a pop up
storm Saturday afternoon so will have a low pop in place. The
large H5 trough in the Plains looks to finally eject out into our
neck of the woods on Sunday into Sunday night with 00Z GFS/ECMWF
in good agreement on timing. Forecast parameters and soundings
once again show potential for strong to severe storms and heavy
rainfall with this system, with weak to moderate MLCAPE, strong
deep layer shear around 60 knots, and very high PWATS up to 1.6
inches. However, once again several uncertainties exist this far
out in time, including potential for a Gulf Coast MCS disrupting
moisture return this far north as 00Z GFS suggests. Regardless,
showers and storms appear likely during this timeframe and will
show likely pops for late in the weekend. Once this system exits
by Monday, a return to near normal temperatures and dry weather is
anticipated for early next week.


&&

.AVIATION...
06Z TAF DISCUSSION.

Residual moisture from recent rainfall will lead to widespread
fog formation overnight. All sites are expected to fall to IFR or
LIFR late night. Sunny high pressure will lift all sites to VFR
by 15Z.


&&

.PRELIMINARY POINT TEMPS/POPS...
Nashville      79  58  86  63  75 /   0   0   0  50  60
Clarksville    78  58  83  61  71 /   0   0   0  60  30
Crossville     73  54  82  60  72 /   0   0   0  20  70
Columbia       78  57  85  61  74 /   0   0   0  60  50
Lawrenceburg   78  57  85  61  76 /   0   0   0  50  60
Waverly        78  60  83  61  71 /   0   0   0  60  30

&&

.OHX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CDT this morning FOR Bedford-
Cannon-Cheatham-Clay-Coffee-Cumberland-Davidson-De Kalb-Dickson-
Fentress-Giles-Grundy-Hickman-Houston-Humphreys-Jackson-Lawrence-
Lewis-Macon-Marshall-Maury-Montgomery-Overton-Perry-Pickett-
Putnam-Robertson-Rutherford-Smith-Stewart-Sumner-Trousdale-Van
Buren-Warren-Wayne-White-Williamson-Wilson.

&&

$$

DISCUSSION......Shamburger
AVIATION........13



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