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

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FXUS64 KMRX 100121
AFDMRX

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
921 PM EDT Sun Aug 9 2020

.UPDATE...
EVENING UPDATE.

&&

.DISCUSSION...

Previous forecast reasoning still looks good overall with the
expected convection overnight ahead of the short wave. Have just
made a few tweaks mainly to precip timing with this update. Other
changes include some adjustments to sky cover to better fit
latest trends, as well as hourly temperatures and dew points.

&&

.AVIATION...
00Z TAF DISCUSSION.
There will be some showers and thunderstorms around at times
starting tonight and into Monday, with TYS looking to be the most
likely to be affected and the lowest chances/latest timing at TRI.
Timing of convection is still quite difficult, but will try to
bracket better chance times with PROB30 groups. Outside of any
convection, mainly VFR conditions are expected with winds on the
light side.


&&

.PREV DISCUSSION...
/ISSUED 317 PM EDT Sun Aug 9 2020/
SHORT TERM...(This Afternoon through Monday)...

Diurnal convection has developed across the southern valley and
southwest North Carolina this afternoon with MLCAPEs of 1000-1400.
Isolated to widely scattered showers/storms will dissipate by sunset.

The main forecast challenge remains the short-wave moving across
southern IL/IN/W KY this afternoon. Models differ in how this system
evolves through the next 24 hours. The NAM is much more agressive in
developing a area of convection from the northern Plateau, central
valley into the central/southern Mountains. Will trend this
direction given the HREF is also following this solution by
confidence is moderate at best.

The latest NAM shows increasing isentropic lift over the Platea into
the central valley late tonight and Monday morning. This lift focus
on a quasi-stationary boundary to produce scattered to numerous
showers and storms. HREF shows mean PWs of 1.9-2.0 inches with NAM
showing high freezing levels even for summer-time producing deep
warm-rain processes.

In result, some locally heavy rainfall possible especially from the
northern Plateau, central/southern valley into the central/southern
Mountains. WFP QPF also suggests of this potential. There are
isolated flash flood concerns in this area.

For Monday, HREF shows CAPEs of 2000-2500 across the southern valley
with NAM DCAPEs near 1200 and Theta-E difference approaching the
middle 30s. Isolated damaging winds are possible in these areas
during the afternoon. Will continue to include this possibility in
the HWO.

LONG TERM...(Monday night through Sunday)...

The general theme of the extended is relatively unchanged. Showers
and storms will mainly be driven by diurnal activity as well as a
few weak impulses rotating through the mostly zonal flow. Also,
continue to undercut NBM guidance on POPS as it continues to push
out Likely and Categorical POPs across the extended which seems way
overdone.

A few showers and storms will be ongoing to start the period but
with coverage decreasing. This is short-lived though as showers and
storms fire back up early Tuesday morning as another impulsed dives
down from the northwest. The biggest question for Tuesday is will we
see any strong-to-possibly severe storms. The NAM continues to be
the most bullish model in terms of instability with soundings
showing roughly 2000 to 3000 J/kg MLCAPE values. Upper level
dynamics however are not favorable. There is a weak amount of flow
in the lower levels but not much: the NAM only shows 15 to 20 kts of
0-3km shear at best. Because of this, most of these storms should be
of the pulse variety and short-lived. If we do end getting the 15 to
20 kts of shear, perhaps a few weakly organized storms are possible.
Having said that, the showers, storms, and resultant clouds during
the morning hours will likely hamper some of the afternoon activity.
The afternoon activity that develops is in response to a cold front
that slowly drifts in from the west-northwest. Areas that can clear
out and re-destabilize will have the better chances to see the
potentially strong-to-severe storms as the front moves through
during the late afternoon hours. Again, overall confidence on storm
strength is low at this time due to the above mentioned factors.
Main threats at this time are locally heavy rains and isolated
strong and gusty winds.

Lower POPs on Wednesday with no real forcing mechanism in place,
other than the orographic terrain-based activity. POPs increase
again on Thursday as another weak impulse moves through the flow and
into our area from out of west TN. Models then show the weak,upper
low becoming closed off and lingering across our area on Friday.
Then, another weak trough moves down into our area for the weekend
from out of the northwest.

Temperature still expected to be near to slightly above normal
through the period.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             73  91  73  94  72 /  30  60  40  60  40
Knoxville McGhee Tyson Airport, TN  71  87  71  92  71 /  40  60  30  60  40
Oak Ridge, TN                       70  87  70  92  71 /  50  60  30  60  40
Tri Cities Airport, TN              63  86  66  88  67 /  10  40  30  60  30

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...NONE.
TN...NONE.
VA...NONE.
&&

$$



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