Special Weather Statement, NWS Morristown, TN

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
254 AM EDT Tue May 4 2021

.SHORT TERM (Today and Tonight)...

Key Messages:

1) Multiple rounds of showers and thunderstorms possible today with
the late morning line expected to bring the most widespread coverage
of strong to severe storms

2) Another round may occur in the afternoon/evening, but confidence
is still low on this second round actually materializing.

3) Flash Flooding still a big concern due to possible multiple
rounds of heavy thunderstorms today.

Discussion:

Currently we are already seeing isolated showers and thunderstorms
across the eastern Tennessee Valley early this morning. Expect
coverage to increase through the morning as ripples of energy are
coming out of the main complex currently west of Tennessee. They are
being aided by falling heights ahead of the front and a mid/upper
level jet helping to bring divergence aloft. The environment for
these isolated early morning storms will be favorable for large hail
and strong winds. Steep lapse rates, good CAPE up to the hail growth
zone, and moderately low freezing heights this time of night means
that hail over the size of quarters will be possible, and as these
storms collapse they could cause brief wind gusts over 50 mph at the
surface. An isolated tornado can't be ruled out with any of the
morning convection, but think that wind and hail are the more
widespread threats.

Expect these early morning storms to continue through sunrise, and
likely increase in coverage as the morning wears on. The most
widespread line of thunderstorms to impact the forecast area is
currently moving through Arkansas into western TN/KY/AL and will
eventually make it's way to the eastern Tennessee Valley. CAMs are
still having a difficult time with how far this round of storms will
make it eastward and how far the northern extent of the line will
be. Most models agree that the line will maintain into the
Cumberland Plateau. This morning line of storms is expected to move
onto the Cumberland Plateau between 12-15z and the front edge of the
storms should be east of the mountains by 17-20z. The main threat
with this line of storms will be strong straight-line winds in
excess of 60 mph, especially if any segments of the line try and bow
out. There is a much lower risk for large hail and tornadoes, but
neither of these can be completely ruled out, especially with any
storms that get out ahead of the line.

This is where the forecast gets a bit tricky, due to how morning
convection will impact convection later in the afternoon/evening.
Once this initial line blows through we should get a break in
precipitation, and will have to prepare for the possibility of
another round of thunderstorms as the actual front moves through in
the evening/overnight hours. Models aren't in great agreement on
if/when this may occur. HRRR and several other models are not
impressed with this secondary line, while the NAM and a few others
are very bullish on bringing in more widespread storms along the
front. Right now I am leaning towards the HRRR solution...Think that
this morning round of storms will do a fairly good job of
stabilizing the atmosphere, and as it moves to the east/southeast it
could really cut off additional moisture and warm/unstable air
transporting into the area. In similar set-ups like this in the past
a line of storms across the Carolinas into AL can really cut off
storms forming further to the north. If this scenario does pan out,
I would expect the atmosphere to have a hard time destabilizing and
the round of storms along the front will (hopefully) weaken as they
approach the forecast area.

The other aspect of storms today that can't be overlooked is the
flash flooding risk. PWAT values will reach up to around 1.5 inches
across the area which is around the 90th percentile for this time of
year. This very moist atmosphere will allow storms to produce heavy
rainfall very quickly. Storms that moved through earlier on Monday
were able to produce 1-2 inches of rain in a short period of time,
and I think that will be very possible with storms later today.
Luckily storms today should be moving at a fairly decent clip, BUT
if multiple storms roll over the same area today and dump heavy
rain, then flash flooding can quickly occur. Could see an average of
1-2 inches of rain across the area today, but think it could be a
feast or famine situation with some places missing out on rain and
only getting a few tenths up to a half inch, but just as likely some
places could see several rounds of rain and pick up 3+ inches of
rain during the day today. The threat for flooding increases
throughout the day as rain accumulates in flood prone areas.

Some showers may linger after the front pushes through by Wednesday
morning, but if they do occur they should be fairly weak and pretty
isolated. Fog will be possible tomorrow morning, especially in areas
that got heavy rainfall. A cooling trend will begin tomorrow after
the front has passed through.

ABM

.LONG TERM (Wednesday through Monday)...

The surface cold front will be moving across the area Wednesday
morning, with the mid/upper trough axis lagging a few hours behind.
Will keep some chance PoPs in northern and eastern areas until the
passage of the trough, which should occur in the late afternoon.
Cold advection through the latter half of the day will result in
cool temperatures mainly in the 60s, with some clearing allowing
highs near 70 in southern sections.

Another trough will be crossing the region on Thursday night,
dropping out of the northern Plains and amplifying as it moves
across the OH/TN Valley region. Moisture ahead of the trough will be
limited, and models differ on QPF amounts. Will have just a slight
chance PoP in the northern half on Thursday night, which is closer
to the drier NAM/ECMWF solutions than the wetter GFS.

The remainder of the forecast period will be characterized by a
nearly zonal flow with embedded disturbances bringing some chance
PoPs through the weekend and early next week. Confidence is low
regarding the details as these weak disturbances are not well
resolved by the models this far out. The general trend will be
increasing rain chances as we go from Saturday to Monday, with a
warming trend bringing highs back to the upper 70s to lower 80s by
Monday.

DGS

&&

.AVIATION...
06Z TAF DISCUSSION.

Unfavorable flying conditions and an uncertain forecast for the
next 24 hours. Already thunderstorms across northeast TN early
this morning. Chances are low that one of these isolated storms
moves over KTRI or KTYS, but will mention a TEMPO group in the
TAFs since it's the likely the most impactful weather in the first
6 hours. Could see some patchy fog this morning, but coverage is
expected to be limited under the cloudy skies. Round of showers
and thunderstorms will move through the area this morning, and
have fairly high confidence on the timing of this round of morning
storm's, but didn't get into the weeds in the TAFs since it's
more than 6 hours out at this time. Another round of storms could
form this afternoon/evening, but confidence is much lower on if
this secondary round of storms will even occur. Will include a
PROB30 during the most likely time period.

ABM

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             81  63  73  49  72 /  80  80  20   0   0
Knoxville McGhee Tyson Airport, TN  80  62  71  47  67 /  60  80  40   0  10
Oak Ridge, TN                       79  60  70  46  67 /  60  70  40   0  10
Tri Cities Airport, TN              80  59  70  43  65 /  60  90  60   0  10

&&

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

Hazardous Weather Outlook
National Weather Service Morristown TN
257 AM EDT Tue May 4 2021

NCZ060-061-TNZ012>018-035>047-067>074-081>087-098>102-VAZ001-002-
005-006-008-050700-
Cherokee-Clay-Scott-Campbell-Claiborne-Hancock-Hawkins-Sullivan-
Johnson-Morgan-Anderson-Union-Grainger-Hamblen-Northwest Cocke-
Cocke Smoky Mountains-Northwest Greene-Southeast Greene-
Washington-Unicoi-Northwest Carter-Southeast Carter-Roane-Loudon-
Knox-Jefferson-Northwest Blount-Blount Smoky Mountains-
North Sevier-Sevier Smoky Mountains-Sequatchie-Bledsoe-Rhea-Meigs-
McMinn-Northwest Monroe-Southeast Monroe-Marion-Hamilton-Bradley-
West Polk-East Polk-Lee-Wise-Russell-
257 AM EDT Tue May 4 2021 /157 AM CDT Tue May 4 2021/

This Hazardous Weather Outlook is for portions of southwest North
Carolina...east Tennessee and southwest Virginia.

.DAY ONE...Tonight

Strong to severe thunderstorms will be possible this morning and
again this evening. Damaging wind, large hail are the main
threats, but an isolated tornado cannot be ruled out. In
addition, multiple rounds of moderate to heavy rainfall will
further increase the potential for isolated flash flooding today.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to relay reports of severe weather and
flooding to our office today.

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