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

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FXUS64 KMRX 050822

Area Forecast Discussion
National Weather Service Morristown TN
322 AM EST Mon Dec 5 2016

.SHORT TERM (Today and tonight)...
Low level saturation under a strong inversion near 850 mb will keep
overcast clouds across the area this morning, but with no rain as
forecast soundings show sharp drying above the inversion. The models
are likely scouring out the clouds too quickly today given the
strong inversion and shallow CAD-like NE flow in the Valley,
especially the GFS, so will keep temps on the cool side of MOS and
maintain more cloud cover through the day.

Precip returns late this afternoon as the closed southern stream low
over northern Mexico tracks NE. The NAM and GFS continue to have
major differences in their depictions of POPs and QPF tonight, but
the NAM has support from the ECMWF and SREF, and the GFS is very
much an outlier in reagard to its late timing of rain onset. In
addition, WV imagery shows a tropical connection to the moisture
stream, so the GFS is likely underdone with its QPF. Isentropic lift
will quickly spread light rain into the area after 18z, with rain
intensifying in the evening as the LLJ picks up and the upper jet
becomes better positioned for upper divergence. Will also mention a
slight chance of thunderstorms tonight and Tuesday AM as there will
be some elevated instability along with the strong forcing. Precip
begins to taper off Tuesday afternoon with the passage of the 500 mb
trough and ensuing dry advection aloft. Will maintain low to slight
chance POPs up to 00z with the surface fropa.

With a LLJ expected to be near 50 kts tonight, winds in higher
elevations tonight into Tuesday morning will be near advisory
criteria. However, due to the S-SW orientation of the LLJ, this does
not appear to be a mountain wave event where the high winds impact
the foothills and favored Valley locations. Therefore the High Wind
Watch will be cut back to a Wind Advisory that will include only the
TN mountain zones.

.LONG TERM (Tuesday night through Sunday)...
We could see a hint of lingering precipitation in Northeast
Tennessee/Southwest Virginia into early Wednesday morning, but most
of the precip will have already exited the area to the northeast.
In place of the rain we will get dropping temperatures through the
rest of the work week and into the weekend. These cold temperatures
will be the main weather story for most people in the long term
portion of the forecast. Our coldest time frame will be Thursday
night through Friday night where temperatures will remain below
freezing for the vast majority of the time. Temperatures during
this time will be running about 15 - 20 degrees below normal for
this time of year.

The other story with the colder temperatures will the models
predicting much lower precipitation chances with the frontal passage
Thursday. GFS has been pretty consistent in showing little QPF with
the frontal passage while the ECMWF was much more bullish until the
latest run. Will be interesting to see what the new ECMWF comes in
showing, but at this time I will be hedging my bets towards the more
consistent GFS for this time frame.

We then get into a moderate warming trend behind the front over the
weekend with highs rebounding but likely remaining below
climatological normal.  Another system will move through Monday, at
the very tail end of the 7-day forecast, and long term models are
currently in decent agreement on timing/QPF with this system. It
will likely be the next best chance for precipitation after


Chattanooga Airport, TN             55  50  62  42 / 100 100  80  10
Knoxville McGhee Tyson Airport, TN  53  46  59  41 /  60 100  90  20
Oak Ridge, TN                       53  47  58  41 /  50 100  90  20
Tri Cities Airport, TN              52  45  56  39 /  20 100 100  30


TN...Wind Advisory from 10 PM this evening to 4 PM EST Tuesday for
     Blount Smoky Mountains-Cocke Smoky Mountains-Johnson-Sevier
     Smoky Mountains-Southeast Carter-Southeast Greene-Southeast



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