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

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FXUS64 KMRX 230815

Area Forecast Discussion
National Weather Service Morristown TN
415 AM EDT Mon Oct 23 2017

.SHORT TERM (Today and Tonight)...A deepening upper level low will
swing rapidly northeast from the Mid Mississippi and Ohio Valley
region today on its way through the Eastern Great Lakes by the end
of the short term period. A strong southerly flow ahead of the low
is pulling a broad swath of moist air out of the Gulf of Mexico
and this moisture is being lifted ahead of the upper trough and
surface cold front. Therefore, categorical pops were used for
showers today and there may be just enough instability for a
thunderstorm across the southern third of the forecast area.
Slightly drier air will flow into the region behind the cold front
from west to east through the day allowing the more persistent
rain to become a little lighter and more scattered in nature. The
chance for showers will continue tonight ahead of a reinforcing
cold front building southeast through the strong cyclonic flow
present within the Eastern U.S. mean trough. Much drier air behind
this front should bring an end to the light precipitation across
most of the forecast area by daybreak Tuesday. High temperatures
will be tricky today due to the fact that the lowest readings
around sunrise are elevated in the strong southerly flow regime
but cold air advection will be filling in behind the initial front
relatively quickly today. Tonight`s lows will drop into the middle
and upper 40s.

.LONG TERM (Tuesday through Sunday)...The cold front and lead
shortwave trough will be to the east of the area to start the
long-term forecast. A highly amplified pattern will being to take
shape on Tuesday as another trough dives out of the Mid-
Mississippi River Valley into the Tennessee River Valley. The
atmosphere will be dry behind the front with PW values ranging
from around 0.4-0.6 inches, which is around the 10th-25th
percentile for late October. Overall, there is sufficient lift for
precipitation but the lack of moisture will really limit any
showers to the higher elevations with dry conditions elsewhere.
Cold air advection will drop high temperatures into the upper 50s
to mid 60s across most valley locations on Tuesday. Cold air
advection Tuesday night into Wednesday morning will drop overnight
lows into the mid 30s to low 40s. Expect the pressure gradient
will remain elevated enough for some light winds overnight Tuesday
night and should prevent any widespread frost from forming.
However, there could be some patchy frost in the sheltered valley
areas Wednesday morning, especially for Northeast Tennessee and
Southwest Virginia.

The trough axis will shift to the east on Wednesday with
northwesterly flow aloft. Wednesday will be the coldest day of the
forecast period with highs only making it into the 50s across most
valley locations. By Wednesday night, surface high pressure builds
into the region. Ideal radiational cooling conditions will setup
Wednesday night into Thursday morning with lows in the 30s. Due to
this, expect a widespread frost on Thursday morning. For now, will
mention this in the HWO. A shortwave ridge moves in on Thursday with
dry conditions and moderating temperatures.

The GFS and ECMWF are both in good agreement with a highly amplified
pattern again over the weekend with a deep trough across the Eastern
CONUS. The models differ slightly on the timing of the system but
both suggest that the weekend will be cool and wet. For now, have 30-
50 PoPs throughout much of the weekend with below normal


Chattanooga Airport, TN             68  48  63  41 / 100  10   0   0
Knoxville McGhee Tyson Airport, TN  68  48  61  40 /  90  10  10  10
Oak Ridge, TN                       67  47  61  41 / 100  20  10   0
Tri Cities Airport, TN              68  46  61  36 / 100  60  20  10


TN...Wind Advisory until 3 PM EDT this afternoon for Johnson-
     Northwest Carter-Southeast Carter-Unicoi.

     High Wind Warning until 1 PM EDT this afternoon for Blount Smoky
     Mountains-Cocke Smoky Mountains-Sevier Smoky Mountains-
     Southeast Greene-Southeast Monroe.



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