Area Forecast Discussion
Issued by NWS Nashville, TN

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FXUS64 KOHX 260837

Area Forecast Discussion
National Weather Service Nashville TN
337 AM CDT Wed Apr 26 2017


Very active weather pattern in the cards for Middle Tennessee
over the next week with three rounds of potentially severe
storms to round out the month of April. For today, some patchy
fog will be possible this morning over mainly eastern zones, but
nothing widespread like yesterday morning is anticipated. Another
beautiful albeit warm spring day is on the way today as
temperatures soar into the 80s by afternoon under mostly clear
skies. LLJ strengthening up to 60 knots later today will bring
increasingly gusty south winds, possibly to near wind advisory
criteria this evening.

Latest models continue to show development of a QLCS to our west
that will race east across our forecast area tonight into Thursday
morning. 00Z guidance has sped up timing of the QLCS, with NAM
showing line of storms reaching the Tennessee River around
midnight give or take an hour. All models continue to weaken the
QLCS as it moves further into our area and outruns the narrow
instability axis, but line remains intense enough that a few
strong to severe storms may occur especially over west and
southwest zones. Although MLCAPE will be on the low side at around
500 J/Kg, very intense low level and deep layer shear may allow
for some damaging winds and possibly even a QLCS tornado with the
line before it diminishes significantly by 12Z. With models now
faster with the QLCS, all convection should clear the Plateau by
18Z on Thursday if not sooner so will remove pops from Thursday
afternoon. Still, cooler temperatures are expected on Thursday and
Thursday night in the wake of this system with highs in the 70s
and lows down into the 50s.

For Friday, models continue to show a warm front lifting
northward across the state during the day with dewpoints rising
well into the humid 60s by late afternoon, creating a very
unstable environment south of the boundary with MLCAPE rising as
high as 2500 J/Kg per GFS forecast soundings. Lack of a forcing
mechanism and weak cap now looks to delay convection until very
late afternoon or especially Friday evening as an approaching
shortwave trough from the southern Plains interacts with the warm
front, with majority of guidance developing numerous showers and
thunderstorms over northwest half or so of the area. High MLCAPE
combined with forecast strong low level and deep layer shear
suggests several strong to severe thunderstorms will be possible,
with large hail, damaging winds, and even a couple of tornadoes.
PWATS rising above 1.5 inches also suggests a threat for heavy
rainfall and localized flooding. SPC has now highlighted the
northwest half of our area in a slight risk for Friday/Friday
night, with an enhanced risk over far northwest zones.

Convection looks to diminish or lift northward out of Middle
Tennessee by 12Z Saturday morning. Very warm and humid conditions
still expected on Saturday with highs reaching the mid to upper
80s. Atmosphere will remain quite moist and unstable so cannot
rule out a storm or two Saturday afternoon. Main H5 trough also
still anticipated to eject out into the Tennessee Valley on
Sunday into Sunday night, although latest 00Z ECMWF is
considerably slower than previous runs and the 00Z GFS. Despite
timing differences, synoptic pattern indicates likely pops are
warranted during this timeframe, with numerous showers and storms
anticipated for all zones. Although MLCAPE will be weaker with
this system versus Friday, strong deep layer shear will still
support yet another risk for potentially severe thunderstorms
this forecast period. PWATS also rising to near the daily maximum
of 1.6 inches indicate heavy rainfall will also be possible on
Sunday into Sunday night. Cooler and drier conditions still look
to return for Monday and Tuesday, but GFS/ECMWF suggest another
system may impact the area by the middle of next week.



Surface pressure gradient will start to slowly tighten over
western Middle Tennessee late tonight, and tighten further over
all of the Mid-State by Wednesday afternoon. This will occur in
advance of a cold front, that is expected to be near the
Tennessee River by the end of the forecast period.

Decoupling of surface winds tonight will be most likely over
eastern areas, where a period of IFR conditions, due to fog, is
expected at CSV (between 09z and 13z). After 13z, south winds will
become gusty, with gusts increasing to between 20kt and 30kt
after 00z/27.

Aside from the period of early morning IFR conditions at CSV,
conditions should be VFR, with cigs at or above 10k feet.


Nashville      86  63  73  54  85 /   0  70  30  10  20
Clarksville    83  58  69  51  82 /   0  70  30  10  20
Crossville     80  61  72  52  80 /   0  40  60  10  20
Columbia       84  61  72  53  84 /   0  70  30  10  20
Lawrenceburg   84  62  73  54  85 /   0  70  30  10  20
Waverly        84  57  70  53  83 /   0  70  30   0  20





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