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

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FXUS64 KMRX 211321
AFDMRX

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
921 AM EDT Wed Apr 21 2021

.UPDATE...
MORNING UPDATE.

&&

.DISCUSSION...

Morning update mainly to tweak PoP chances across northeast
Tennesee and southwest Virginia the rest of the morning.
Precipitation is moving through a tad faster than previously
forecasted, so have adjusted hourly PoP forecast to reflect this.
In addition some of the high resolution CAMs are indicating there
may be enough lift lingering in this area that we continue to see
a few sprinkles this morning and maybe into the early
afternoon... So have included mention of sprinkles into the
forecast, but accumulations are expected to minor (if any).
Otherwise the forecast remains largely unchanged and the
discussion from this morning is still relevant.

ABM

&&

.AVIATION...
12Z TAF DISCUSSION.

Winds are shifting to W and NW behind the front. We have seen
some gusts around 20 kts at TYS over the past hour, but do not
expect this to persist. Scattered SHRA will bring periods of MVFR
this morning, mainly at TYS and TRI. The showers will end by Noon,
but some MVFR cigs could linger into early afternoon before
returning to VFR. NW winds will gust to 15-20 kts at TRI and CHA
this afternoon before lighting this evening into tonight.

Garuckas


&&

.PREV DISCUSSION...
/ISSUED 313 AM EDT Wed Apr 21 2021/
SHORT TERM...(Today and Tonight)

Key Messages:

1. Falling temps early this morning with scattered showers changing
to a brief period of snow on the Plateau (trace to 0.1 inch).

2. Snow showers persist in the mountains of E TN/SW VA through early
afternoon before ending with up to 1 inch above 5000 feet.

3. Highs 20 degrees below normal today struggling to reach the low
50`s in many areas followed by widespread freeze/frost event tonight.

Discussion:

Early This Morning...

Latest surface analysis shows the cold front approaching the Plateau
as of 230 AM with a band of showers behind the front in a classic
ana front set up. This band of showers stretches from the lower OH
Valley into middle TN and will move into the Plateau after 09Z. This
is tied to the left exit of a 100-110 kt upper jet over AR and W TN
which is rounding the base of a strong mid/upper trough currently
digging into the OH and TN Valleys. Deep moisture is lacking, and
the best frontogenetic forcing/tightest isothermal profile is
actually up in the OH Valley where a second jet max is strengthening
a surface low this morning. This will keep the precip scattered and
light as it moves into our area between 09 and 12Z. Current temps as
of 2 AM are still in the low/upper 50`s in most areas, but these
will quickly drop from W to E behind the fropa, and expect to see
low 30`s on the Plateau and upper 30`s/low 40`s elsewhere by 13Z.
This leads to a brief changeover to snow on the Plateau with
accumulations of a trace to 0.1 inch before quickly ending around
13Z.

Today...

As the mid/upper trough axis continues to deepen over the OH and TN
Valleys, strong CAA will drop 850 mb temps to -5 to -7 C behind the
front this morning. Some lingering upper jet forcing at the base of
the trough will keep the band of light precip behind the front for
areas roughly E of I-75 through 14 or 15Z, but due to the limited
moisture and overall forcing, QPF will be very light with most areas
barely seeing 0.05 inch of rain. The strong CAA, NW upslope flow,
and lingering moisture below 800 mb will keep upslope rain and snow
showers going in the E TN and SW VA mountains through mid afternoon.
The latest HREF guidance has caught onto this and now shows bands of
precip in these typical NW flow areas through mid afternoon, so kept
high chance PoPs in the mountains through 18Z before drier air
finally wins out. Latest RAP forecast soundings show the available
moisture to about 800 mb, but the profile is not very well saturated
with some dry air closer to the surface. The exception is LNP which
is projected to have better saturation between the surface and 800
mb. Also, the boundary layer flow is only about 20 kts on average,
and typically 30 kts is more favorable. These factors combined with
the high late April sun angle and warmer ground temps will limit
accumulations to the highest peaks where up to 1 inch is possible
above 5000 ft. Will issue an SPS for the E TN and SW VA mountains to
discuss this light snow. For the rest of the area, temps today will
struggle to reach the low 50`s in many areas given the strong CAA
and anomalously deep trough.

Tonight...

A large surface high will be building in from the MS Valley tonight
allowing skies to clear, and with the continued deep NW flow aloft,
a cold night is in store. Some clouds could linger in NE TN, SW VA,
and along the mountains through this evening, but expect rapid
clearing after 04Z. Winds will also become light as the high builds
in, so temps should easily fall into the low 30`s in most areas with
some upper 20`s on the N Plateau, NE TN, and SW VA. Issued a Freeze
Warning for much of the area and a Frost Advisory for the S Valley.

Garuckas

LONG TERM...(Thursday through Tuesday)

-A cold start with possible frost (maybe even a freeze in some
colder spots) Thursday night.

-Widespread rain Friday night into Saturday.

-Dry and warmer late in the period.

Some weak mid/upper level short wave energy will brush our northern
areas as it moves through the eastern US trough Thursday but should
only bring some clouds. Surface high pressure will move east over
the area and will provide calm/light winds Thursday night. It will
be cold Thursday night, but exactly how cold will partially depend
on how much cloud cover there will be, as models are showing
increasing moisture/weak isentropic lift as warm advection begins
ahead of the next system Thursday night into Friday.  This may hold
temperatures up a bit and tend to lessen the frost/freeze potential.
Will include areas/patchy frost over much of the area but no freeze
watch for now as confidence not high enough for significant freezing
temperature coverage. Friday may see quite a bit of cloud cover
especially south/central. The NAM tries to bring some sprinkles/very
light showers into the area, but it has little support from other
models and given the dry lower levels will lean toward the drier NBM
PoPs for now.

A significant short wave will be moving out of the Southern Plains
Friday night and then across our region, with a surface low moving
into KY and then off to our northeast. This system will bring rain
to the area Friday night and Saturday before ending Saturday night.
Models indicate there may be some very limited convective energy for
southern parts of our area, so will carry slight chance thunder
there for Saturday. The heavier precip signal still looks to be
south of our area, but we may see precip total exceed an inch across
some of the south and central counties.

Surface high pressure will move in behind this system on Sunday,
with a strong mid/upper level ridge building over the region early
in the week.  Sunday through Tuesday look to be dry with a warming
trend. Temperatures will likely return to the 80s across much of the
valley by Tuesday.

LW

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             55  35  64  43  67 /  10   0   0  10  10
Knoxville McGhee Tyson Airport, TN  51  32  60  39  64 /  60   0   0  10  10
Oak Ridge, TN                       52  32  60  38  64 /  20   0   0  10  10
Tri Cities Airport, TN              49  28  54  33  63 /  60   0  10  10  10

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...Freeze Warning from midnight tonight to 9 AM EDT Thursday for
     Cherokee-Clay.

TN...Freeze Warning from midnight EDT /11 PM CDT/ tonight to 9 AM EDT
     /8 AM CDT/ Thursday for Anderson-Bledsoe-Blount Smoky
     Mountains-Campbell-Claiborne-Cocke Smoky Mountains-Grainger-
     Hamblen-Hancock-Hawkins-Jefferson-Johnson-Knox-Loudon-
     Morgan-North Sevier-Northwest Blount-Northwest Carter-
     Northwest Cocke-Northwest Greene-Roane-Scott-Sequatchie-
     Sevier Smoky Mountains-Southeast Carter-Southeast Greene-
     Southeast Monroe-Sullivan-Unicoi-Union-Washington.

     Frost Advisory from midnight EDT /11 PM CDT/ tonight to 9 AM EDT
     /8 AM CDT/ Thursday for Bradley-East Polk-Hamilton-Marion-
     McMinn-Meigs-Northwest Monroe-Rhea-West Polk.

VA...Freeze Warning from midnight tonight to 9 AM EDT Thursday for
     Lee-Russell-Scott-Washington-Wise.

&&

$$


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