FXUS64 KMRX 191921

Area Forecast Discussion
National Weather Service Morristown TN
321 PM EDT Thu Jul 19 2018

.SHORT TERM...(Tonight and Friday)...

Moisture is increasing across the forecast area with light southerly
flow beginning. Dewpoints this afternoon are already in the mid 60s
to low 70s across most of the forecast area. In the upper levels
there is a bit of a weak jet to the north with a mid-level shortwave
trough currently across SE TN and SW NC. Due to this, there are a
few showers forming across SE TN and SW NC this afternoon and this
is where the highest PoPs are featured throughout the remainder of
the afternoon and evening hours. Further to the north, low level
winds just above the surface are from the southeast. These winds
have a downslope component and should limit any precipitation from
developing much further to the north. General troughing will
continue over the area aloft tonight but expect that most of the
precipitation should come to an end around sunset. Overnight lows
tonight will be few degrees warmer than last night and should range
from the mid 60s to low 70s.

On Friday, an elongated upper level jet will extend from the
Northern Plains southeastward into the Mid-Mississippi River
Valley. At 500mb, a low will cross the Upper Midwest into the
Great Lakes Region. A warm, moist, and unstable airmass will be to
the west of the forecast area where a MCS is expected to develop
late in the day. For now, models suggest that this MCS will not
move into the forecast area until after 00z. For Friday afternoon,
model soundings suggest a weak cap across a good portion of the
forecast area with the strongest capping inversion across the
southern Valley. Due to this, have the lowest PoPs for these areas
with the strongest cap. Model guidance has been trending drier for
Friday afternoon and have around 20-40 PoP for now. Afternoon
highs on Friday will range from mid to upper 80s for most

.LONG TERM...(Friday night through Wednesday)...

In short, the pattern will bring active and wet weather to the
forecast area. The latest Day 2 outlook from SPC shows the
enhanced risk over Middle Tennessee while areas generally along
and west of I-75 are under the slight risk area. Impressive shear
(for July!) along with abundant instability associated with the
upper low over the Great Lakes region will bring the best chances
of severe weather overnight Friday. The threats will be damaging
wind gusts, large hail, heavy rainfall, and isolated tornadoes.
The models have been fairly consistent with this feature however,
timing is more the problem as some models are much faster than
others. This is the first wave of precipitation that should end
Saturday morning, again - timing is the issue (could end before
0400 ET or after 1000).

As the upper low drops south, another shortwave and additional
showers and thunderstorms will occur on Saturday into Saturday
night. As the slow moving low transitions south/southwest of the
area through the middle of next week, unsettled and wet weather will
persist. There is a chance of drier weather to appear on Monday and
Tuesday when the upper low is progged to be around central AL/GA.
This would bring easterly winds across the mountains (downslope) and
limit rainfall totals. Still a long ways out...so confidence on
exact solution should get higher as this time approaches.

Another trough in the northern stream will begin to influence the
region bringing higher rain chances back into the forecast by
mid week through the end of the week.


Chattanooga Airport, TN 72 89 72 88 / 20 20 60 50
Knoxville McGhee Tyson Airport, TN 70 89 70 82 / 10 20 60 60
Oak Ridge, TN 70 88 70 83 / 10 20 60 70
Tri Cities Airport, TN 66 86 66 83 / 10 30 60 70




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