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

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FXUS64 KMRX 200722

Area Forecast Discussion
National Weather Service Morristown TN
322 AM EDT Sun Aug 20 2017

.SHORT TERM (Today and Tonight)...Main concern early this morning
and early Monday morning will be patchy fog, some locally dense,
near area lakes and streams due to steam fog development.

Upper and surface ridging will be the rule for Today and Tonight.
Forecast soundings show a fairly strong mid-level inversion which
will essentially cap convection from developing today. A fairly
hot day is in store with a warming temperature profile.

We should start the week hot and mostly dry. On Monday (Eclipse
Day), a strong 500mb ridge will expand with heights expected to be
around 594-596 dm which is near the 97th-99th percentile compared
to climatology for this time of year. Subsidence aloft will act
to dry the air column and limit clouds and convection. Cannot rule
out about 10 percent PoPs around the higher terrain where
differential heating will create a valley-mountain circulation
with rising air across the higher elevations and sinking air
across the Tennessee Valley. Models is good agreement tonight with
only very limited convection anticipated across the higher
terrain features. Effective deep layer shear is very weak around
10 kts or less which means any ordinary convective development
would not be well maintained and would dissipate quickly.

The total solar eclipse will have some unique impacts on our diurnal
weather on Monday. Decreasing incoming solar radiation during the
mid-afternoon as the eclipse reaches its max for our area between
17z-20z will have several effects. Hourly temperatures are expected
to drop 2-8 degrees during maximum eclipse with the greatest drop in
temperatures occurring within the path of totality. High
temperatures expected to be slightly cooler by a degree or two as
well. With near surface temperature reductions, may actually start
to see a boundary layer profile similar to evenings near the path of
totality where a stable or neutral thermal layer forms near the
surface. This would likely further limit convection and cumulus
development during the hours around the eclipse, especially near the
path of totality. Even for local winds, may see some brief mesoscale
wind patterns due to the total solar eclipse where a weak divergent
flow occurs near the path of totality.

Overall, it is still looking like good viewing conditions for the
area with mostly clear skies across the valley and additional
cumulus/partly cloud skies across the higher terrain. At this point,
only few, to at most scattered, cirrus would be expected. After the
eclipse has ended on Monday afternoon, temperatures will quickly
rebound with highs in the upper 80s to lower 90s. Heat index values
will be in the 90-95 degree range on Monday afternoon. With the hot
and sunny weather, it`s important for anyone spending a lot of time
outdoors on Monday to stay hydrated, find shade, and use sunscreen
because of very high UV index values of 8-10 around midday before
the solar eclipse even begins.

The ridge breaks down on Tuesday as an upper-level trough
approaches. Model solutions in good agreement with a cold front
moving through the area on Wednesday with increasing PoPs late
Tuesday night and likely PoPs on Wednesday. May have a few lingering
showers across the mountains on Thursday before high pressure builds
in late week with dry weather expected. With troughing in place,
expect cooler, below normal temperatures late next week behind the
cold front.


Chattanooga Airport, TN             93  73  92  73 /   0   0  10  10
Knoxville McGhee Tyson Airport, TN  90  70  89  71 /   0   0  10  10
Oak Ridge, TN                       91  71  91  71 /   0   0  10   0
Tri Cities Airport, TN              89  65  89  66 /  10  10  10  10



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