Scientific Forecaster Discussion
fxus64 kmrx 181913
Area forecast discussion
National Weather Service Morristown Tennessee
313 PM EDT Wed Sep 18 2019
Short term (tonight through friday)...
500 mb high pressure over the lower MS valley and Hurricane Humberto
in the Atlantic has provided northerly flow aloft, resulting in
the long awaited back door cool front moving through the
southeast. It will finally usher in some relief from the heat,
but the question is by how much. Model MOS continues to limit
high temps Thursday to the upper 70s throughout the Tennessee Valley
including cha, and so we will continue to go above guidance,
going with mid 80s in cha thurs and Fri. It is notable, however,
that temps today over northeast Georgia and the SC Piedmont have in
fact been limited to the mid to upper 70s under extensive cloud
cover. Can it happen for US tomorrow? Well, even the mid 80s
would be welcome relief from the lower 90s, and that's what we
will continue to go with, in general agreement with surrounding
There may also be isolated shower activity now through midnight,
mainly near the higher terrain both east and west.
As Hurricane Humberto moves out to sea, the southern U.S. 500 mb
ridge will reassert itself eastward over our area Friday and
squash the already limited rain chances and bring about a return
of the unseasonable heat in the days ahead.
Long term (friday night through wednesday)...
the extended forecast period kicks off on Friday evening amidst
and amplifying upper pattern featuring nearly full latitude
troughing out west, and ridging across the deep south. At the
surface, high pressure looks to control the pattern over the Tennessee
Valley and southern appalachian region, consequent of ample
subsidence aloft. Pattern evolution through the weekend will be
highlighted by persistence of the upper ridge, while the surface
anticyclone slides eastward allowing for improved return sly waa
into the area. Meanwhile to the west, a potent shortwave will
eject across the northern plains, driving a front across the mid
MS valley into the Great Lakes on Saturday.
Furthermore, guidance suggests Lee cyclogenesis over the Central
Plains beneath another potent 500 mb shortwave, with the surface cyclone
riding along the aforementioned front into the Ohio Valley by Monday.
This would favor a wetter pattern to our north, but as of now looks
to keep east Tennessee and southwest Virginia/NC dry, albeit with some flattening
of the upper ridge. By Monday morning the old front looks to be
somewhere in the vicinity, likely to the north. However, by this
point the operational guidance begins to highlight increasing
discontinuities with regard to phase of the upper flow, possibly
caused by the more amplified European model (ecmwf). Nevertheless, the next best
chance for precipitation looks to be on Monday night, increasing
slightly into Tuesday and the old front lays across the region.
However, the GFS seems much wetter than the ECMWF, thus will keep
pops low at this point.
Beyond that, uncertainty is high overall, but the pattern does
suggest at least a brief return to southeasterly ridging. As far as
any hazardous weather during the period, looks like the biggest
concerns will be extended above normal temperatures and dryness,
which will work to amplify fire danger.
Preliminary point temps/pops...
Chattanooga Airport, Tennessee 67 84 61 85 / 10 0 0 0
Knoxville McGhee Tyson Airport, Tennessee 65 82 60 84 / 10 0 0 0
Oak Ridge, Tennessee 65 82 61 85 / 10 0 0 0
Tri Cities Airport, Tennessee 57 79 52 83 / 10 0 0 0