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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS64 KMRX 170700
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
300 AM EDT Thu Aug 17 2017

.SHORT TERM (Today and Tonight)...
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An upper-level ridge centered across the Gulf Coast will remain in
place for much of the day Thursday resulting in a typical summer
pattern across the region. The combination of forcing mechanisms
due to terrain based differential heating and upper-level
divergence due to a 200-300mb jet streak will act to position
highest PoPs across northeast Tennessee, southwest Virginia, and
across the TN/NC mountains. Modest MLCAPE of 1000-2000 J/Kg and
effective bulk shear of 10-15 kts means that the risk of severe
weather is low with mainly ordinary convection expected. However,
the atmosphere remains very moist with PWs of 1.8-2.0 inches, so
some rain showers and thunderstorms may produce heavy rainfall.
High temperatures are once again expected to be several degrees
above normal.

The upper-level ridge will finally start to break down Thursday
night in response to an approaching trough and surface cold front
with chance PoPs across the area. Highest PoPs on Thursday night
expected to be across western portions of the CWA after midnight.
With increasing cloud cover and a very moist boundary layer, low
temperatures are forecast to be 5-8 degrees above normal on
Thursday night.

.LONG TERM (Friday through Wednesday)...
A cold front will be moving into the forecast area on Friday as a
shortwave trough passes to the north across the Great Lakes. PW
values ahead of the boundary will be in the 1.9-2.1 inch range.
These values are in excess of the 90th percentile for mid-August.
The timing of this front continues to be late morning into the
early afternoon. Due to the timing of the front, SBCAPE values
will only increase into the 750-1500 J/Kg range with 0-6 km Bulk
Shear values in the 10-15 range with the upper level trough
passing well to the north. Therefore, not expecting any strong to
severe storms with this frontal passage.

PW values will crash behind the boundary with much drier air
advecting in from the northwest. Chances for showers and storms will
come to an end from northwest to southeast during the afternoon and
evening on Friday. PW values by Saturday morning will be in the 0.7-
0.9 inch range, which are around the 10th percentile for mid-August.
Another mid-level shortwave trough swings across the Ohio River
Valley on Saturday, but with the lack of lift, only expect a slight
increase in cloud cover. Overall, the weekend is forecast to be dry
with highs in the mid 80s to low 90s both Saturday and Sunday with
sunny to partly sunny skies. An upper level ridge across the
Atlantic builds westward on Sunday. Temperatures will be 3-5 degrees
warmer on Sunday than on Saturday with the increasing heights aloft.

On Monday (Eclipse Day) start to see some weak southerly return flow
with surface high pressure across the Atlantic. In the upper levels,
the ridge continues to expand across the area with upper level
heights in the 594-596 dm range. This is a typical late summer time
pattern across the area. The main questions continue to be about
cloud cover and precipitation chances. Based on the latest model
trends with the further amplification of the ridge these chances
have gone down to around 10-20 percent. The lack of insolation
from the sun during the Eclipse will act to decrease these chances
as well and beginning to think based on the latest trends that
viewing will be good throughout the forecast area. Temperatures
will warm quickly into the early afternoon and beginning to
plateau as the eclipse begins and will decrease by 2-8 degrees
during the peak of totality. Without the eclipse expect highs
would be in the low 90s areawide with the strength of the ridge
but have forecast highs in the mid to upper 80s across most of the
area at this time.

Chances for showers and storms will increase on Tuesday and
Wednesday as the ridge begins to dampens with an upper level trough
passing to the north. Temperatures during this timeframe will be
near normal.
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&&

.PRELIMINARY POINT TEMPS/POPS...
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Chattanooga Airport, TN             92  75  90  69 /  30  40  40   0
Knoxville McGhee Tyson Airport, TN  89  73  87  67 /  30  30  50   0
Oak Ridge, TN                       90  73  87  67 /  30  40  40   0
Tri Cities Airport, TN              87  70  84  63 /  50  40  40  10
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&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...None.
TN...None.
VA...None.
&&

$$

JB/MA