Scientific Forecaster Discussion
fxus64 kmrx 200749
Area forecast discussion
National Weather Service Morristown Tennessee
249 am EST Wed Nov 20 2019
Short term...(today and tonight)...
a negatively tilted upper trough continues to traverse across the
northeast Continental U.S. This evening leaving gradually rising heights and
deep nwly flow across the Tennessee Valley and southern Appalachians. On
that note, 850 mb flow continues to veer nwly (albeit generally less
than 25kts) amidst sufficient moisture in h925-850 mb layer to
support a few orographically induced showers over the next 6 hours
or so. For elevations above 4kft a few of these showers could
yield a rain/snow mix with little to no accumulation expected.
Otherwise, the aforementioned llv moisture will remain trapped west
of the mountains beneath a modest inversion through much of the
morning yielding persistent llv stratus. This sky cover will be
most dense across the northern valley into southwest VA, gradually
thinning southward toward the Georgia/Alabama state lines. Into the late
morning hours guidance favors further nly veering of the llv flow,
which should allow this sky cover to slowly scatter out. All said,
these early/mid morning mostly cloudy skies will stunt daytime
heating somewhat for the more northern areas with highs topping out
a few degree below normal at tri, just below normal at tys, and
right at normal for cha. Overnight, upper heights will continue to
rise as a ridge approaches with only high cirrus streaming across
the region leading to a mostly clear to partly cloudy night,
possibly favoring some valley fog.
Long term...(thursday through Tuesday night)...
we begin the extended period dry on Thursday with surface high
pressure shifting to the east. This will bring a SW flow across the
forecast area with increasing moisture during the afternoon. With
low-level SW flow and ridging aloft, Thursday should be our warmest
day of the extended with Max temperatures 7 to 10 degrees above
normal. Models continue to show good agreement during the Thursday
through Saturday time period, so forecast confidence is high during
A shortwave trough and associated surface low across the Great Lakes
will move eastward Thursday night with a trailing cold front moving
southeast toward the Tennessee Valley. As it nears the forecast
area, it will slow and become stationary. The highest pops on
Thursday night and Friday will be near the surface front across north/northwest
portions of the forecast area. On Friday, strong upper divergence
from positive vorticity advection and a 110 kt jet streak will lead to cyclogenesis along the
front across the lower Mississippi Valley. This surface low will
move NE along the front on Friday night and Saturday spreading
precipitation across the entire area. With strong moisture
advection, precipitable water values will rise to 1.2 to 1.4 inches which will be
near or above the 90th percentile. For this reason, some heavy rain
will be possible at times on Saturday. With widespread cloud cover
and precipitation, temperatures should remain near normal.
Cold front moves east Saturday afternoon and evening with drier
conditions from west to east. A few showers, or even light snow
showers, may occur over the higher terrain of the mountains of east
Tennessee and SW Virginia Saturday night, but should quickly end as drier air
advects into the area. As troughing remains across the region, small
shortwaves could affect the area on Sunday with some light showers,
but confidence in timing, shortwave location, and available boundary
layer moisture is too low to include in the forecast at this time.
Temperatures will be slightly below normal on Sunday with troughing
across the area.
Dry weather is expected early in the week with surface high pressure
across the region. Temperatures remain near normal early next week.
High pressure will shift east midweek with another system expected
to bring precipitation to the region on Tuesday or Wednesday. The
GFS, ECMWF, and Canadian are all in good agreement with the general
pattern, but differ in the details with the system next
Tuesday/Wednesday. At this time, limited pops to chance but showed a
general increase in coverage on Tuesday night.
06z taf discussion.
VFR through the period at cha, while abundant llv moisture trapped
by the mountains among nwly flow will favor MVFR stratus at
tys/tri this morning, lifting into the late morning hours at tys
first. Afterward am expecting a few hours of low/mid VFR before
scting to high lvl clouds to round out the taf cycle. Otherwise,
light nly/vrb flow will dominate.
Preliminary point temps/pops...
Chattanooga Airport, Tennessee 61 40 65 48 63 / 0 0 10 30 70
Knoxville McGhee Tyson Airport, Tennessee 57 38 63 49 60 / 0 0 10 40 70
Oak Ridge, Tennessee 58 38 63 48 59 / 0 0 10 50 70
Tri Cities Airport, Tennessee 52 34 63 45 58 / 0 0 0 40 70