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fxus64 kmrx 271915 

Area forecast discussion
National Weather Service Morristown Tennessee
315 PM EDT Sat may 27 2017

Short term /this afternoon through Sunday/...
complex 12 to 24 hours across the eastern Tennessee Valley and
southern appalachian region due to the threat for scattered to
potentially widespread strong to severe thunderstorms.

Morning cloud cover continues to break up as heating is rapidly
breaking the capping inversion in place. Earlier convection
across northwest Tennessee dissipated as it tracked into the plateau, but has
laid a couple of outflow boundaries along the I-40 corridor and
across the northern plateau back into southern Kentucky. Additionally, a
weak front will be in the vicinity later on this evening across
SW Virginia and southern Kentucky. These boundaries will serve as a source
and focus for convection later on by the late afternoon and
evening as shown all of the mesoscale models. The peak timing of
this activity will be 4 PM to 10 PM EDT, with coverage being
scattered in nature at around 40-60 percent. In addition, an
upper-level perturbation/shortwave that will drop of the mid-
Mississippi Valley toward the southern Appalachians later this
afternoon and evening. This may provide enough forcing to overcome
the lingering inhibition currently in place by around 20-22z
later this afternoon.

As noted in the morning discussion though, a favorable
environment exist for any of this activity to quickly become
strong to severe. The main threats will be large hail and
downbursts. MLCAPE values are progged to be in the 2000-2500 j/kg
range, with MUCAPES over 3000 j/kg at times. Hail CAPES (1500 j/kg
and dcapes over 1000 j/kg) will create a very favorable
thermodynamic environment for strong updrafts, capable of
supporting the aforementioned threats. Bulk shear values are quite
high in the 40-50 kt range across the area, though unidirectional
in nature -- thus an east-southeast storm motion is expected. Straight
hodographs in this shear environment would favor organized,
multicell clusters and supercells (as well as numerous splitting
cells). A low-end tornado threat will exist thanks to the presence
of these boundaries -- though a less favorable 0-1 km shear
profile limits this. Still, areas to watch will be supercells
that can form along boundaries with a localized backed wind

The forecast for tonight into Sunday is less certain. Mesoscale
models do, however, continue to show a much faster progressing mesoscale convective system
pushing out of northwest and middle Tennessee into northern Alabama and the
central/southern valleys of east Tennessee late tonight. The main
question will be the exact timing and strength. Lower res models
are slower, but may be underestimating the higher forward
propagation that can occur in higher cape environments. Thus,
favored the mesoscale models (hrrr/NSSL-WRF/3-km nam) over the
GFS/NAM-12 solutions. Damaging straight-line winds would be the
main threat with this activity.

Storms will weaken as they push across the area Sunday morning and
may leave a lull in precipitation (thanks to a more stable air
mass) for most of the morning and afternoon. If some heating can
occur during the late morning thru mid afternoon, a third round of
severe storms is possible late Sunday afternoon, evening and
overnight -- along/near where the front stalls out. This part of
the forecast remains fairly uncertain at this time.

Long term /Sunday night through Saturday/...
will remain in an active weather pattern early in the extended as
a weakening cold front moves through the southern appalachian
region. A line of convection is expected to develop along/ahead of
this frontal boundary...effecting the mrx County Warning Area overnight Sunday
into Monday morning. With recent convection...the air mass in
place should be only weakly unstable and the chance of strong to
severe storms is limited. The frontal boundary is expected to
stall just south of the Tennessee/Georgia boarder Monday night and could see
scattered convection continue along our southern boarder Monday
night and Tuesday. Models begin to diverge Wednesday through the
end of the extended and confidence is low. Current trend is to
remain in a wet...unsettle pattern with the boundary remaining in
the vicinity Wednesday and Thursday and the next system moving in
around the Friday/Saturday time frame. All of this will keep 20 to
40 pops in place over our area through the last half of the


Preliminary point temps/pops...
Chattanooga Airport, Tennessee 68 85 69 82 / 60 60 60 40
Knoxville McGhee Tyson Airport, Tennessee 66 82 66 79 / 50 60 60 40
Oak Ridge, Tennessee 66 82 66 79 / 60 60 60 40
Tri Cities Airport, Tennessee 61 79 63 78 / 60 60 60 50


Mrx watches/warnings/advisories...

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