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

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000
FXUS64 KMRX 070838
AFDMRX

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
338 AM EST Sun Mar 7 2021

.SHORT TERM...(Today and Tonight)...

The main concern during the short term will remain the
exceptionally dry air and resulting fire weather concerns. The
period will feature upper troughing over the eastern seaboard,
with high amplitude ridging over the rockies. Upper flow from the
western Great Lakes down through our forecast area will become
increasingly meridional in nature today in response to another
shortwave dropping south from Canada and the western ridge
building east. All guidance, to varying degrees, show this
shortwave ushering another batch of very dry air into the southern
Appalachians, categorized by PWAT values below 0.20". Given that
dewpoints were still in the single digits to low teens in portions
of southwest VA at 300am, decided to take the forecast for this
afternoon towards the driest side of the guidance envelope. Mostly
have a blend of the RAP and ARW guidance for dewpoints this
afternoon. PWAT data suggests the driest air will traverse
southwest VA and the far northeast TN zones, so have RH values
lowering to around 20 percent there. For most everywhere else,
have afternoon RH values of around 25 percent.

By this evening, surface ridge axis begins to settle into the
southern Appalachians. Light winds and clear skies will mean prime
radiational conditions again tonight. Expect many places will see
lows in the low 20s, especially in the Tri Cities area and
northward.

CD

.LONG TERM...(Monday through Saturday)...

Key Messages:

1. Low RH values will continue fire concerns Mon, but increasing
moisture will ease this threat Tues and especially Wed through Sat.

2. Mostly sunny with near normal temps Mon warming much above normal
mid week into the weekend.

3. Slight chances of showers Thurs night into Sat, especially over
the northern half of the region.

Discussion:

Monday and Monday Night...

The eastern CONUS mid/upper trough will begin to lift out Mon as a
mid/upper ridge amplifies over the Plains and Midwest leading to
height rises across the TN Valley and S Appalachians. The mid/upper
flow will still be NW on the eastern periphery of the ridge axis
which will continue to advect very dry air into the region, but the
large surface high shifting toward the Carolinas and Mid Atlantic
will begin to shift low-level winds more westerly and eventually
southwesterly as the day progresses setting up WAA. This combined
with the height rises and full sunshine will boost highs into the
upper 50`s/low 60`s across the N Plateau, N and central Valley, NE
TN, and SW VA with low/mid 60`s across the S Plateau, S Valley, and
SW NC. Despite the beautiful early March day, the aforementioned dry
air will continue to cause fire weather concerns. NAM forecast
soundings look to be well mixed to above 850 mb across most of the
Valley and to above 800 mb in SW NC and SW VA. This will mix
afternoon Td`s down into the mid teens/low 20`s in many areas
leading to minimum RH values of 15-25%. NBM Td`s continue to look
too high, so leaned toward the ARW for Monday`s Td`s. Increasing WAA
Mon night will lead to a milder night with lows in the low/upper
30`s.

Tuesday through Thursday...

The mid/upper ridge axis will shift over the TN and OH Valleys Tues
and into the eastern CONUS Wed and Thurs in response to deep
troughing digging into the western CONUS. The large surface high
will become established over the western Atlantic and SE CONUS
during this period allowing for deep southwesterly flow/strong WAA
across the TN and OH Valleys. This will set the stage for
spectacular Spring-like weather with highs Tues warming into the
low/mid 60`s areawide, upper 60`s/low 70`s Wed, and generally low
70`s Thurs (10-12 degrees above normal). Increasing moisture
advection from the Gulf will raise Td`s into the low/mid 30`s Tues,
upper 30`s/low 40`s Wed, and mid 40`s/low 50`s Thurs. This will lead
to increasing RH values allowing fire concerns to ease. Cloud cover
will increase Thurs as shortwave energy coming out of the western
CONUS trough begins to interact with a diving northern stream trough
pushing a cold front into the OH Valley. Overnight lows will
moderate each night.

Thursday Night through Saturday...

The aforementioned northern stream mid/upper trough will drop into
the Great Lakes and OH Valley Thurs night into Fri as the western
CONUS trough evolves into a cut off low. A cold front tied to the
northern stream trough will sag into the area Thurs night and Fri
but should stall in response to the cut off low starting to move E.
Kept slight chance PoPs late Thurs night through Fri with the best
chance over the north half of the region. QPF continues to look
light. Deterministic guidance continues to differ on how the cut off
low will eject eastward and possibly interact with the northern
stream trough late Fri into next weekend with the GFS continuing to
suggest more phasing and greater amplification compared to the ECMWF
and CMC which keep these features separate. Stayed with the NBM
which is more in line with the ensemble guidance in keep slight
chance PoPs Fri night and Sat as the frontal boundary remains in the
vicinity or starts to lift north. Temps will remain well above
normal.

Garuckas

&&

.AVIATION...
06Z TAF DISCUSSION.

VFR conditions will prevail through the period. Could see some
gusts to 16-18kt at KTRI from late morning through early evening,
but otherwise winds will be 10kt or less at all terminals. High
pressure builds into the region late, for calm or nearly calm
winds after 01z-02z timeframe.

CD


&&

.FIRE WEATHER...

Low afternoon RH values will once again be a concern today. While
mixing heights won`t be quite as high as yesterday, very dry air
aloft will be efficiently mixed to the surface. Expect to see RH
values drop to around 20 percent for much of southwest VA, and
some of far northeast TN as well, with values around 25 percent
fairly common elsewhere. It is worth noting that guidance has
been too high with afternoon humidity levels the last few days.
The forecast reflects the lower/drier side of guidance, but was
reluctant to go lower due to very slight improvements in overall
moisture levels.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             61  33  65  37  68 /   0   0   0   0   0
Knoxville McGhee Tyson Airport, TN  56  29  61  35  65 /   0   0   0   0   0
Oak Ridge, TN                       56  30  62  35  64 /   0   0   0   0   0
Tri Cities Airport, TN              51  22  58  32  62 /   0   0   0   0   0

&&

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

$$


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