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

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FXUS64 KMRX 220909

Area Forecast Discussion
National Weather Service Morristown TN
409 AM EST Wed Feb 22 2017

.SHORT TERM...(Today and Tonight)...Upper low over Mississippi
will move slowly southeast into the northeast Gulf today.
Isentropic lift will continue this morning and afternoon and
trigger light showers across much of the forecast area. A weak
surface trough over Kentucky into middle Tennessee will also act
to enhance the shower activity. Rainfall amounts will be
light...heaviest near the Tennessee border and across southwest
Virginia. Skies will stay mostly cloudy. Mild temperatures this
morning in the mid 50s will rise to the mid and upper 60s this
afternoon. Tonight showers will end as upper low moves southeast.
Low level moisture will remain with mostly cloudy skies and mild
temperatures in the lower to mid 50s.

.LONG TERM...(Thursday through Tuesday)...
The main focus for the long-term forecast will be on the severe
weather expected to impact the area late Friday into early
Saturday morning.

On Thursday, an upper level trough will move out of the Rocky
Mountains with lee cyclogenesis. A warm front will extend eastward
from the surface low in Colorado to the Ohio River Valley. This will
put the forecast area in the warm sector. PW values will be around
an inch on Thursday. These values are around the 90th percentile for
late February. There is a weak mid-level trough that moves through
but not seeing much in the way of lift. Therefore, went ahead and
trimmed back PoPs on Thursday to mainly just the higher elevations
and only have around 20 percent there. Temperatures will continue to
be well above normal with highs in the upper 60s to mid 70s. Mid-
level ridging will build in Thursday night into Friday morning ahead
of the next approaching trough. The small PoPs will come to an end
and expect dry conditions during the overnight period. Southerly
flow will begin to pick up overnight as the pressure gradient
tightens as a cold front races across the Central and Southern
Plains. These southerly winds will keep overnight lows up in the 50s
across most of the forecast area.

On Friday, the focus will shift to the possibilty of severe
storms. Currently, it appears that most of the day Friday will be
dry with the ridge still providing some influence. In addition,
model soundings indicate a cap throughout much of the day with drier
air in the mid-levels. Temperatures will once again top out near
record values with highs in the 70s. Southerly winds will also be
strong throughout the day and approach advisory level criteria.
Expect the cap aloft and the linear forcing will keep most of the
convection along a line just ahead of the front. The NAM and GFS
indicate a surge in the low level jet around 03-06z Saturday up into
the 45-55 kt range. In addition, the strong warm air and moisture
advection will produce SBCAPE values in the 500-1000 J/Kg range
during the overnight hours. The strong upper level flow aloft will
lead to 0-6 km bulk shear values of 50-60 kts. Strong jet dynamics
will also setup at this time as a speed max rotates through the base
of the trough. The low level 0-1 km SRH will range from around 200-
300 m2/s2. Some model soundings indicate that the boundary layer
will decouple and produce lower instability values, however, this is
unrealistic given the strong southerly flow.  Therefore, have sided
more towards the NAM since it shows less decoupling. At this time it
appears the highest risk for severe weather would be from roughly 10
pm - 6am late Friday night into early Saturday with damaging winds
being the main threat.

Much drier and colder air will advect into the Southern Appalachians
on Saturday into Saturday night. So cold in fact, models indicate
some light snow possible across the higher elevations Saturday night
across the higher elevation. With the recent warm weather, flowers
and plants have been blooming and it looks like a widespread freeze
will occur Sunday morning. The cooler conditions will be short-lived
as a warm front lift across the area early next week. Rain showers
will be possible Monday night into Tuesday as the front lifts
northward with highs on Tuesday in the mid to upper 60s.


Chattanooga Airport, TN             70  54  73  55 /  60  10  10  10
Knoxville McGhee Tyson Airport, TN  67  52  72  53 /  60  10  10  10
Oak Ridge, TN                       67  52  72  52 /  60  10  10  10
Tri Cities Airport, TN              63  48  71  48 /  70  20  20  20




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