Special Weather Statement, NWS Morristown, TN

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
325 PM EDT Thu Oct 21 2021

.SHORT TERM (Tonight and Tomorrow)...

Key Messages:

1. A line of showers and thunderstorms will move through the region
through the evening hours ahead of an approaching frontal boundary.

2. A few stronger storms are possible with isolated damaging wind
gusts as the main concern, mainly near and west of I-75. An
isolated, brief tornado also can't be ruled out in southern
portions of the region.

This Evening/Tonight

Currently, a mid/upper shortwave is centered over the Mississippi
River Valley. At the surface, a low is located in the eastern Great
Lakes Region with its associated cold front extending southwest
through western Kentucky and into west Tennessee. As these features
continue to move eastward, ongoing convection will do the same with
further development into the early evening hours. Notable clearing,
especially along and west of I-75, has allowed for and will continue
to allow for destabilization with mid-level lapse rates of ~6 C/km
also realized, following mid/upper height falls. The overall upper
support/divergence is fairly limited. But, low-level convergence
with the frontal boundary, MLCAPE of 500 to 1,000+ J/kg, and deep-
layer shear of 30 to 40+ kts area-wide will keep the ongoing concern
of isolated organized multicellular/linear to possibly even
supercellular convection through the evening hours. With the loss of
diurnal heating later in the evening, the potential becomes lessened
further to the east of I-75 with places near I-75 south of I-40
being the areas of primary focus. This is also currently seen in a
relatively tight theta-e gradient based on the latest surface metar
observations.

Overall, the main concern continues to be locally damaging wind
gusts for much of the area as surface-based instability will remain
limited outside of the aforementioned areas. This is also given
potential for downbursts as is seen with DCAPE values > 500 J/kg.
With wind profiles revealing notable veering in the low-levels,
combined with a 25-30 kt 850mb jet ahead of the frontal boundary, 0-
1 km bulk shear is still expected to reach into the 20 kt range.
This also comes with notably low LCL heights below 1,000 meters due
to the notable moisture in place from early in the day. As such, a
brief, weak tornado can't be ruled out, in addition to the damaging
wind threat. Regarding hail, CAPE in the hail growth region of >200
J/kg is also expected, but freezing levels well above 10,000 feet
will allow for significant melting before any hail reaches the
ground. This will largely add to the concern for damaging winds due
to momentum transfer. But, as with most organized convection in this
pattern, some subsevere hailstones could reach the ground. Following
the frontal passage by the early morning hours, northwesterly flow
will funnel in cooler air into the region. Lingering low-level
moisture and orographic considerations will keep potential for
lingering sprinkles/light showers overnight.

Tomorrow

During the day tomorrow, the passage of the aforementioned
frontal boundary will keep the region in a northwesterly flow
pattern, also as ~1,020mb high pressure settles in over the
Mississippi River Valley. With this surface pattern and mid/upper
troughing in place, cooler and drier weather will be ushered into
the region. With pooling of low-level moisture, lingering cloud
cover is likely to moderate temperatures as well.

BW

.LONG TERM (Friday Night through Thursday)...

Key Messages:

1. Dry through the weekend, with rain returning on Monday.

2. Lower confidence beyond Tuesday morning, but another system
possible either Tuesday night or Wednesday.

Discussion:

Period begins with deep, but low amplitude, upper troughing north of
the Great Lakes region and ridging over the Rockies. Embedded
shortwave trough will traverse the Ohio river valley area Friday
night which will keep some clouds lingering into Sat morning.
Shouldn't see lows drop too much into the 40s Fri night as a result,
but expect highs to be 2-3 degrees below normal on Sat due to
ongoing CAA regime.

By Sunday, the sfc high flips to the eastern seaboard while surface
low pressure develops over the central plains in response to upper
trough ejecting from the Rockies. Significant differences still
exist amongst deterministic models regarding how strong this
shortwave will be and where the surface low tracks. Main impacts in
that regard are going to be how quickly rain moves in on Monday and
how long it lasts into Tuesday. Stuck pretty close to the NBM due to
the uncertainty that remains, which features widespread showers with
some embedded thunderstorms, arriving mainly Mon afternoon. This
route also has some lingering 20-30 percent PoPs well into Tuesday
which may wind up being too high in areas outside of the mountains
as upper ridging does move into the region by late Tuesday
afternoon. Did taper back the PoPs in the valley to that extent, but
didn't go completely dry like some deterministic runs are currently
showing.

For Tue night onward, stuck with the NBM guidance entirely.
Uncertainty is very high for that time period. Upper trough ejects
east off the Rockies Tue evening, but strength and placement is not
well agreed upon in the models. GFS is weaker and well north of the
stronger/southern ECMWF solution. Once again the implications are in
the arrival time of rainfall into the TN valley, with the GFS
showing widespread rainfall by daybreak Wed morning and the ECMWF
holding off until late Wed afternoon or evening. NBM is a middle
road approach mainly, which is reasonable for the time being since
confidence in which direction this will go is pretty low.

CD

&&

.AVIATION...
18Z TAF DISCUSSION.

Notable aviation impacts are expected late this afternoon and into
the early evening at all 3 sites as a line of showers and a few
thunderstorms will move through the region. This will come after
increases to VFR at CHA and TYS with notable SW winds, possibly
gusty at times. Within any of the storms, temporary reductions
below MVFR are possible, along with locally strong and variable
winds. These impacts are more likely at CHA and TYS than at TRI as
the storms will likely weaken by the time they arrive. By the
later evening hours, the sites are likely to return to VFR with
limited confidence in potential for fog at TYS and TRI. There is
also the potential for some lower ceilings early in the day,
mainly at TYS and TRI, but the forecast is currently for VFR and
largely lighter winds to prevail.

BW

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             52  67  48  72  53 /  40   0   0   0   0
Knoxville McGhee Tyson Airport, TN  51  64  48  69  52 /  60   0   0   0   0
Oak Ridge, TN                       49  63  47  69  51 /  40   0   0   0   0
Tri Cities Airport, TN              50  62  46  66  48 /  60   0   0   0   0

&&

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

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Special Weather Statement
National Weather Service Morristown TN
509 PM EDT Thu Oct 21 2021

TNZ067-083-212200-
Rhea TN-Roane TN-
509 PM EDT Thu Oct 21 2021

...A strong thunderstorm will impact portions of northeastern Rhea
and southwestern Roane Counties through 600 PM EDT...

At 509 PM EDT, Doppler radar was tracking a strong thunderstorm 8
miles southwest of Rockwood, moving northeast at 35 mph.

HAZARD...Wind gusts up to 50 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Rockwood, Fairview, Midtown, Harriman, Grandview and Eagle Furnace.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3569 8481 3578 8488 3583 8480 3583 8477
      3586 8473 3590 8471 3593 8464 3577 8452
      3572 8470 3574 8470 3574 8472 3573 8471
      3572 8472
TIME...MOT...LOC 2109Z 240DEG 48KT 3578 8476

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH

McD

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