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

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FXUS64 KMRX 171933

Area Forecast Discussion
National Weather Service Morristown TN
233 PM EST Sun Dec 17 2017

.SHORT TERM...(Tonight and Monday)...

Isentropic ascent to the north of a warm front is leading to
widespread light rain across the area. Rainfall amounts have been
light and do not expect to see anything over a tenth of an inch.
Current water vapor imagery and RAP upper air analysis indicate a
mid-level shortwave trough moving to the east across the Ohio
River valley and Great Lakes with a ridge across the Florida
Peninsula. At the surface, a warm front stretches W-E across South
Alabama and Georgia. Current radar imagery indicates that the
back edge of the precipitation is moving into the Plateau. Due to
this, have decreased PoPs throughout the remainder of the
afternoon and into the evening hours. The isentropic lift is
forecast to move to the southeast during this timeframe. The lift
will exit the region tonight but the low level moisture will
remain with the boundary to the south. Due to this, expect fog and
drizzle to develop overnight. Not forecasting dense enough fog
for an advisory at this point but conditions will need to be
monitored throughout the evening and overnight hours. Have also
increased low temperatures overnight with widespread cloud cover
lows will generally be in the upper 30s to mid 40s.

On Monday, the upper level ridge will expand slightly to the north
with the front to the south nudging to the northeast. The clouds
will slowly dissipate late tomorrow morning. With the higher heights
aloft and partly sunny skies by the afternoon, highs will warm into
the mid to upper 50s. The boundary will not be too far to the south
but the lack of a lifting mechanism aloft and PW values in the 0.4-
0.5 inch range will keep the forecast dry.

.LONG TERM...(Monday night through Saturday)...

Fairly active pattern will bring three precip episodes into the
forecast area during the extended period, first on Tues night/Wed,
second on Friday night/ Sat, and the third potentially on Sunday.
Model solutions for the late portions of the forecast, around
Christmas Eve, continue to fluctuate wildly. The 12Z EMCWF just
arrived, showing a fairly warm Christmas Eve and Christmas Day
with the frontal boundary and rain field to our north over the
Ohio Valley, while the GFS is cooler and wetter with the frontal
boundary near us, but still not cold enough to produce wintry
precip. We will have to wait and see what comes of this epic
battle of model solutions. But as it stands presently, this week
looks to be warmer than normal, continuing into the holiday


Chattanooga Airport, TN             42  59  46  61 /  20  10  10  60
Knoxville McGhee Tyson Airport, TN  41  57  43  60 /  20  10   0  40
Oak Ridge, TN                       40  57  43  59 /  20  10  10  40
Tri Cities Airport, TN              38  55  39  59 /  30  20  10  30




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