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

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FXUS64 KMRX 252012
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
312 PM EST Sun Feb 25 2018

.SHORT TERM...(Tonight and Monday)...

The cold front is making slow progress to the east across the
forecast area. The surface boundary currently bisects the
forecast area with southerly winds still being reported in East
TN. Current radar imagery indicates widespread light returns
across the Tennessee Valley. However, there is extremely dry air
in the 600- 850 mb layer preventing this precipitation from
reaching the ground. This dry layer was sampled very well by the
upper air balloon this morning in Nashville.

The main upper level trough is lifting out to the northeast and is
now in the Great Lakes into Southern Canada. Current water vapor
imagery and RAP upper analysis depicts another shortwave trough
across the Lower Mississippi River Valley moving to the northeast.
This shortwave will move into the Tennesse River Valley later this
evening into the overnight hours. The frontal boundary has not made
it as far to the south and east as previously thought and as this
shortwave moves in later tonight, widespread showers will develop
across the Southern Appalachians. The dry level in the 600-850 mb
level will saturate in the 02-06z timeframe. Due to this, have
increased PoPs for the overnight period and have stretched likely
PoPs further to the north. With widespread cloud cover and
precipitation overnight lows will range from mid 40s to low 50s.

PoPs will decrease from northwest to southeast during the day on
Monday as a more robust shortwave trough across the Mid-
Mississippi River Valley. This system should finally push the
boundary to the east with drier air building in behind the front
with some clearing forecast later in the day. The cold air
advection behind the boundary is not overly impressive and high
temperatures will be near to slightly above average highs for late
February.

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

Shortwave trough moves to the east with ridging building in on
Monday night and Tuesday. This should result in a clearing sky and
mostly sunny conditions on Tuesday. Low temperatures on Tuesday
morning are expected to be near normal with efficient radiational
cooling due to high pressure centered over the area, calm wind, and
a clear sky. During the day, low-level ridge axis moves to the east
with south-southeasterly flow developing. This may lead to some weak
downsloping with warm max temperatures about 10 to 15 degrees above
normal across the area on Tuesday afternoon.

Isentropic lift ahead of the next low pressure system will result in
increasing clouds late Tuesday and Tuesday night with rain showers
becoming likely across southern and central portions of the forecast
area by Wednesday morning. As surface low pressure strengthens and
moves into the Great Lakes region, a warm front will move north
through the forecast area late Wednesday bringing additional
precipitation chances. PW values on Wednesday night into Thursday
morning will be near 1.0 to 1.25 inches which places the region in
the 90th-97th percentile compared to climatology. This rich moisture
combined with deep forcing due to an approaching surface cold front,
upper-level shortwave trough, and boundary layer convergence along
the low-level jet axis will result in periods of moderate to heavy
rain late Wednesday night and Thursday morning which could lead to
some localized flooding. Instability is weak, but cannot rule out
the slight chance of a thunderstorm. Could have some gusty winds
across the higher elevations Wednesday night/Thursday with strong
850mb winds, but uncertainty still exists in the strength and
position of the low-level jet.

Behind the cold front, northwest flow develops across the region
Thursday night and Friday with a chance of light rain showers, and
possibly some light snow showers or flurries across the higher
elevations. Temperatures behind the cold front expected to be
slightly below normal on Friday. Models currently in good agreement
with ridging building in next weekend with drier weather and near
normal temperatures.


&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             53  61  41  68 /  80  30   0   0
Knoxville McGhee Tyson Airport, TN  50  57  36  66 /  80  30   0   0
Oak Ridge, TN                       49  58  37  65 /  80  30   0   0
Tri Cities Airport, TN              48  53  31  64 /  70  60   0   0

&&

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

$$

MA/JB



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