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

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FXUS64 KMRX 180722
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
322 AM EDT Fri Aug 18 2017

.SHORT TERM...(Today and Tonight)...
A weak cold front will approach and cross the area today.
Scattered showers will push across the area this morning along
and ahead of the boundary. Just a few rumbles of thunder, if any,
will accompany these showers with very little instability to work
with. With PWAT values of 1.75 to 2 inches, could see some brief
heavy rain with this morning`s showers however their progressive
nature should preclude any flooding issues. Clearing skies are
expected this afternoon as drier air works into the area. Highs
will be a few degrees cooler today due to the northwest flow,
staying in the 80s areawide. For tonight, with clear skies and
drier air overhead temperatures will dip into the 60s in most
locations.

.LONG TERM...(Saturday through Thursday)...
We start the long term period with much drier air across the
forecast area in the wake of a cold front. A shortwave trough will
swing through the Ohio River Valley on Saturday, but with limited
lift and dry air in place, expect no precipitation and only a few
additional clouds. PW values will only be in the 0.8-1.0 range on
Saturday which is near the 10th percentile for this time of year.
An upper-level ridge begins to build in on Sunday increasing
temperatures by 1-3 degrees compared to Saturday. With the drier
low- level airmass and a building ridge, anticipate dry and
pleasant conditions with highs in the mid 80s to low 90s and low
humidity through the weekend.

The ridge continues to expand on Monday (Eclipse Day) with 500mb
heights around 594-596 dm which is near the 95th percentile for this
time of year. Subsidence aloft will act to dry the air column and
limit clouds and convection. Still cannot rule out about 10-20
percent PoPs, especially around the higher terrain where
differential heating could play a role as a forcing mechanism for
convection. However, effective deep layer shear is less than 10 kts
which means any convective development would have a difficult time
sustaining itself. It will also be interesting to see the effects of
decreasing incoming solar radiation during the mid-afternoon on
Monday. We may actually start to see a boundary layer profile
similar to evenings around the path of totality where a stable or
neutral thermal layer forms near the surface. This would help to
limit convection and cumulus development. Thoughts are that between
(1) the building ridge and associated subsidence, (2) weak shear,
and (3) decrease in boundary layer heating between 17z-20z on
Monday, it looks to be mostly dry with a mostly clear to partly
cloudy sky leading to good eclipse viewing conditions. Temperatures
will be very warm for this time of year, and without the eclipse,
highs would be several degree warmer. The eclipse will act to drop
temperatures between 2-8 degrees during max coverage with the
largest temperature drops occurring along the path of totality. With
dew points in the upper 60s to lower 70s, expect heat index values
to be in the 90-95 degree range on Monday afternoon.

Rain chances will increase through the middle of the week and peak
on Wednesday/Wednesday night as the ridge is replaced by troughing
and a cold front moves southward across the area. Long term models
are currently in good agreement with frontal passage occurring late
Wednesday and Wednesday night with likely PoPs along and ahead of
this front. Behind the cold front, much drier and cooler air settles
over the region late week.


&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             89  69  90  70 /  30   0   0   0
Knoxville McGhee Tyson Airport, TN  86  67  88  67 /  40   0   0   0
Oak Ridge, TN                       87  67  88  67 /  30   0   0   0
Tri Cities Airport, TN              82  64  87  61 /  30  10   0   0

&&

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

$$

EMH/JB



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