Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN

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FXUS64 KMRX 211922

Area Forecast Discussion
National Weather Service Morristown TN
322 PM EDT Fri Jul 21 2017

.SHORT TERM (Tonight and Saturday)...
The southern sections of our area should have the best chances of
showers and storms through the evening, with most activity being
diurnally driven, ending before midnight. However, as the
mid/upper level ridge to our west drifts slightly to the south
tonight, convection across IN may track in a more SE direction and
into WV/SW VA late tonight. The NAM may be overdone with
sustaining convection overnight, but the RAP supports it and to a
lesser extent, the HRRR as well. So a slight chance PoP will be
kept for SW VA overnight. On Saturday we will have very little
forcing and a weak cap in place, so most convection will likely be
terrain-based and diurnally-driven. Temperatures tomorrow look
pretty similar to today, but with slightly higher dewpoints, heat
index values near or a bit above 100 are expected in the central
and southern TN Valley.

.LONG TERM (Saturday night through Friday)...
A weak upper level high pressure ridge will move south through
the Mississippi Valley through the beginning of the work week. At
the same time...a weak low level boundary will move south through
the Ohio Valley and just to the east of the upper level high
pressure. Models have moved this boundary through the area a
little faster...bringing the rain chances beginning early Sunday
afternoon and lasting through monday afternoon and evening. Ample
instability is present Sunday with greater than 3000 J/kg
expected...along with PW values around the 90th percentile for
this time of year. With low level flow from the SW...mid level
flow from the west...and upper level flow from the NW...decent
shear is present. However winds will be far too weak to influence
storm structure or motion. For Sunday afternoon and evening...can
expect scattered strong to isolated severe showers and
thunderstorms for downburst winds and large hail...with the
greatest threat in areas north of I40. With the large PW values
for this time of year...and the mid level flow parallel to the
boundary...could see the possibilty of localized flooding in many
areas for storms training over the same locations.

Monday...models disagree on timing of the frontal boundary moving SE
of the southern Appalachians. Right now...have left rain chances in
through the day. Instabilities and winds will be much weaker on expecting scattered showers and thunderstorms...with a
chance for a few strong storms but nothing severe expected at this
time. There does remain a threat for localized flooding with storms
that move across the same area as PW values remain high. Coverage of
showers and storms is expected to weaken overnight...with only a few
showers remaining along the higher elevations.

Tuesday...scattered showers can be expected in the higher elevations
of E TN and areas south of I40 where forcing is still
present. By Wednesday and Thursday...models disagree on timing of a
frontal boundary swinging through the area. Have included slight
chance precip chances for both days for scattered showers and


Chattanooga Airport, TN             76  95  75  90 /  20  20  20  50
Knoxville McGhee Tyson Airport, TN  73  94  74  89 /  20  20  20  50
Oak Ridge, TN                       74  94  75  89 /  10  20  20  50
Tri Cities Airport, TN              69  92  71  88 /  20  30  20  60




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