Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Morristown Tennessee
248 am EDT Wed Sep 18 2019
Short term...(today and tonight)...
the main feature of the short term will be a backdoor cold front
droping southward on the east side of the mountains through today.
Radar shows very little precip over the NC mountains, indicating
that the models are overdone with their quantitative precipitation forecast depictions and cloud
cover. The forecast for today will follow a drier and warmer
solution than what the models indicate, with only slight chance
pops in SW NC and the smokies through the morning. There will
also be a southeast low level flow that will produce a downslope wind
component in the valley, further enhancing stability and pushing
high temps above guidance. By the late afternoon the boundary
pushes across north Georgia and into the plateau in the evening. With some
surface convergence and diurnal instability in that area, an
isolated shower may be possible. The building high pressure ridge
extending SW along the mountains will bring lower dewpoints to the
area tonight, allowing low temps in the mid/upper 50s in SW Virginia
and NE Tennessee.
Long term (thursday through tuesday)...
after a slightly cooler and less humid pattern for the late week
period, heat will build again this weekend into early next week with
very limited opportunities for rain.
For Thursday and Friday, the southern Appalachians will be wedged
between a stregthening 500 mb ridge over the Ohio Valley and an 500 mb
trough offshore of the East Coast. Surface high pressure will be
centered overhead. This will allow dry Continental air to continue
to infiltrate the region leading to highs generally in the 80's and
td's in the 50's. Overnight lows will be quite cool in this drier
air with the high overhead, and expect 50's in protected valleys and
outside of urban areas.
The 500 mb ridge axis will shift over the southeast states this weekend
as a broad trough slowly moves into the central U.S. This will
result in deepening southwesterly flow and strengthening warm air advection in the
southern Appalachians, but td's will be slow to rise due to the
ongoing dry ground altering boundary layer moisture. With the
thermal cap overhead, lack of forcing, and limited moisture, will
maintain a dry forecast in all areas Saturday and Sunday. Used nbm
numbers for highs and lows this weekend.
As the trough to the northwest approaches Monday, the ridge will
begin to break down. Continued warm air advection will result in another hot day
with increasing humidity, but the latest European model (ecmwf) and GFS have both
trended slower and drier with the cold frontal passage. Will just
have slight chance pops in northern areas and higher terrain Monday
then slight chance Monday night. The weak front looks to stall near
the region Tuesday, so will continue to see a chance of
showers/storms, but the lack of deep moisture and the fact that
droughts are usually hard to break warrants keeping pops in the
slight chance range.
Preliminary point temps/pops...
Chattanooga Airport, Tennessee 92 66 85 61 / 20 10 0 0
Knoxville McGhee Tyson Airport, Tennessee 89 65 83 60 / 10 0 10 0
Oak Ridge, Tennessee 89 65 82 60 / 10 10 0 0
Tri Cities Airport, Tennessee 84 57 80 52 / 10 0 0 0