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

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FXUS64 KMRX 271932

National Weather Service Morristown TN
332 PM EDT Tue Oct 27 2020

.SHORT TERM (Today and Tonight)...

For the rest of today and tonight expect generally quiet weather as
a the upper ridge to our west remains the main weather player.
Mostly sunny weather across the eastern Tennessee Valley, but expect
cloud coverage to increase tonight as moisture builds ahead of the
incoming Tropical Storm Zeta. Models are indicating that the first
band of this system may move into Southeast Tennessee tonight just
after midnight, and rain chances will continue through the rest of
the overnight period. Some high resolution models are trying to show
fog development in the morning as the dew points increase with the
incoming moisture, but think most of the area will see enough clouds
from the tropical system that fog will stay confined to low lying
areas in Northeast Tennessee and Southwest Virginia.


.LONG TERM...(Wednesday through Tuesday)...

Key Messages:

1. Periods of rain from the remnants of Zeta Wednesday through
Thursday night.

2. The steadiest and heaviest rain will occur Wednesday night
through early Thursday afternoon.

3. Gusty winds up to 30 kts on the southern Plateau, southern
valley, and SW North Carolina with over 40 kts in the far E
Tennessee mountains late Wednesday night through mid day Thursday.

4. Drying Friday through early next week with much cooler temps.


Wednesday through Thursday Night...

Alot to talk about in this period as Zeta interacts with a mid/upper
low bringing widespread rain and gusty winds to the region. The
mid/upper low will move into the S Plains Wed with Zeta forecasted
by the NHC to make landfall near the LA/MS coasts Wed evening. The
mid/upper low will shift E Wed night reaching the TN Valley Thurs.
This will pick up Zeta and accelerate it to the NE across N AL and N
GA Wed night while it weakens to a depression. The remnants will
then track across the S Appalachians Thurs and off the Delaware or
New Jersey coasts Thurs evening. All guidance has come into much
better agreement on the timing and track of Zeta`s surface low, as
well as the timing of the eastward movement of the mid/upper low,
leading to greater confidence on rainfall amounts and wind impacts
compared to 24 hrs ago.


The mid/upper low moving into the S Plains Wed and strong
subtropical ridge still holding strong offshore of the SE CONUS will
strengthen deep layer southerly flow and moisture advection through
the day. Increasing moisture and isentropic lift will focus an
expanding band of showers across the S half of the region Wed
morning which will expand N during the afternoon. Forcing is weak,
so expect there to be breaks in the rain with QPF generally a few
tenths to 0.75 inches with isolated 1 inch amounts in the S Valley
and Plateau. As Zeta moves northeastward, much more significant
tropical moisture will surge into the area Wed night through Thurs
morning with upper jet forcing also slowly increasing as the strong
upper low moves into the TN Valley. This will be the period of
widespread moderate to heavy rainfall. PWATs of 1.9 to 2+ inches
will be between the 99.5th percentile and the climatological maximum
value for Oct 29 combined with IVT in the 99 to 99.5th percentile.
This will combine with a warm, tropical cloud layer depth to produce
efficient rainfall rates. Agree with WPC`s marginal risk of
excessive rainfall in day 2 and day 3 since the fast movement of
this system will confine the best rainfall to a relatively short
window Wed night and Thurs morning keeping flooding localized. As
Zeta`s surface low quickly moves NE of the area, a dry slot is
likely from W to E before the upper low moves in Thurs night, so the
heavy rain will transition to scattered showers Thurs afternoon. As
the upper low moves overhead Thurs night, forcing from a 130+ kt 300
mb jet punching into PA will produce one more band of showers that
will rotate across the area, but decreasing tropical moisture will
lead to lighter rainfall rates. Storm total rainfall will average 2
to 2.5 inches in most areas Tues night through Thurs night with
isolated 3 inch totals around the Plateau and far eastern mountains.


Zeta`s surface low crossing from N GA into the S Appalachians Thurs
morning will produce a brief southeasterly low-level jet around 50
kts in the 09 to 15Z timeframe. Soundings show a weak inversion, and
one that is also too high for standard mountain waves (around 800-
700 mb). This should limit wind gusts to 40-50 kts across the peaks
and foothills of the E TN mountains during this time window, but
some downsloping and stronger winds mixing down near the core of
Zeta`s circulation will also lead to gusts to near 30 kts in the S
Valley and SW NC. Elsewhere, gusts will be 10-20 kts. Winds will
quickly decrease and veer to the WSW as the circulation departs
Thurs afternoon.

Friday through Tuesday...

The region will quickly dry out Fri as a strong cold front pushes E
behind the departing upper level low/trough. Cold surface high
pressure building in Fri afternoon through Sat will bring mostly
sunny skies but much cooler temps in the upper 50`s/low 60`s. Frost
is likely in northern areas Fri night. Highs will rebound a bit into
the low/mid 60`s Sun ahead of another approaching trough and
associated cold front. This front lacks deep moisture, so kept the
NBM dry forecast for Sun. Deep mid/upper troughing over the eastern
CONUS and another strong, cold surface high Sun night through Tues
will bring back below normal temps, including potentially a
widespread freeze Sun night and Mon night.




MVFR to VFR clouds are sticking around the area this afternoon,
and will likely remain through the evening. Some model guidance is
trying to indicate fog overnight, but think enough cloud coverage
will persist to inhibit it across most of the area. Tonight into
tomorrow expect rain from Tropical System Zeta to move into the
southern TN Valley. Rain from this will continue for most of the
end of this forecast leading to MVFR to IFR conditions at the end
of the TAFs.



Chattanooga Airport, TN             65  73  68  77  51 /  60  90  90  80  20
Knoxville McGhee Tyson Airport, TN  61  71  66  74  51 /  20  70  90  90  30
Oak Ridge, TN                       60  70  65  74  49 /  20  90  90  80  30
Tri Cities Airport, TN              56  71  62  73  48 /  10  70  90  90  50



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