FXUS64 KMRX 081724

Area Forecast Discussion
National Weather Service Morristown TN
1224 PM EST Fri Dec 8 2023

...New AVIATION...

Issued at 1016 AM EST Fri Dec 8 2023

No significant changes to the forecast this morning. Main
adjustment was editing hourly temps and dewpoints. Several obs
were running quite a bit cooler on temperature compared to the
forecast. The sites that were cooler still have calm winds. Once
mixing ensues, temps will start to climb more quickly. No changes
to today's max temp.


(Today and tonight)
Issued at 134 AM EST Fri Dec 8 2023

Key Messages:

1. Dry and warmer today.

2. Tonight southerly winds will increase in the higher elevations
of the East TN mountains and Cumberland Plateau ahead of an
approaching system. Some showers will move into the region, mainly
after midnight.


Ridging and high pressure remain over the Southeast today bringing
dry weather and a warming trend due to southerly flow. Highs will
be 7 or 8 degrees above normal.

Tonight, the ridge starts to break down as the trough over the
Midwest and Great Plains begins to move east. Some showers will
likely develop over the region, mainly after midnight as the
surface cold front approaches the Mississippi River. Southerly
winds will increase tonight in the higher elevations of the East
TN mountains and the Cumberland Plateau as the 850mb jet moves
into the region. The 850mb jet will be strongest over the Plateau
counties of Morgan, Scott and Campbell at 40 to 45 knots.
Elsewhere, the 850mb jet will be closer to 35 knots. The wind
direction may be favorable for downslope wind enhancement in the
East TN mountains and the NBM is picking up on that potential late
tonight. However, winds will likely stay below advisory criteria
during this time frame. Gusts will likely top out around 30 mph in
the aforementioned Plateau counties and may be only slightly
higher in the mountains and foothills of East TN since wind
direction may be more southwesterly and less favorable for
downslope wind enhancement.


(Saturday through Thursday)
Issued at 134 AM EST Fri Dec 8 2023

Key Messages:

1. A strong system is expected Saturday through Sunday night.
Threats include: High winds and mountain wave winds, widespread
rainfall with embedded thunderstorms, and northwest flow snowfall.

2. Dry conditions expected Monday through Friday with high pressure
across the region.


Shortwave trough across the Great Plains will move eastward on
Saturday with amplifying longwave troughing across the Mississippi
Valley. At the surface, a strengthening surface low will move into
the western Great Lakes with a trailing cold front from Michigan
southwestward across the Mississippi River. Across our forecast
area, we'll be within an area of deep southwest flow with
unseasonably warm temperatures and increasing clouds and chances for
light rain showers due to isentropic lift. Breezy conditions are
possible during the day, especially across the Cumberland Plateau,
as 850mb winds increase to around 30 kt.

The cold front shifts eastward and will be across our region by 12z
Sunday. Ahead of the front, 850mb winds will increase to around 50
kt with some mountain wave winds expected across the mountains and
foothills on Saturday night into Sunday morning. We will continue to
monitor the potential for a High Wind Watch of Wind Advisory across
the mountains and foothills with the forecast update later today. We
are also expecting to see quite a bit of rainfall across the region
with this system with at least 1 to 2 inches across most of the
forecast area. Lower totals will be possible near the western
foothills of the mountains where downslope flow will limit overall
rainfall totals. On the other hand, locally higher totals near 3
inches will be possible across the southern and western counties in
our forecast area near the Cumberland Plateau.

Behind the cold front, troughing amplifies and cold air will advect
into the region. We will see temperatures quickly drop Sunday
afternoon with a non-diurnal temperatures trend. This will change
lingering rain showers across the higher elevations over to snow
showers and rime ice as northwest flow of 15 to 20 kt provides ample
orographic lift for continued light precipitation across the higher
terrain. We won't be saturated through a great depth with this
system with drier air near the -10C level and above. However, it is
expected to be enough some some rime ice and snow showers across the
mountains on Sunday afternoon through Monday morning. Most model
guidance continues to suggest 1 to 2 inches across the higher
elevations above 2500 or 3000 feet with the potential for locally
higher amounts across the highest elevations such as Mt.LeConte.

High pressure builds in early next week with a clear sky and dry
weather. Temperatures will be below normal early in the week with
longwave troughing and a cold surface airmass across the region. A
moderating airmass and return of mid to upper level ridging will
result in warming conditions mid to late week with temperatures back
near or slightly above normal.


(18Z TAFS)
Issued at 1212 PM EST Fri Dec 8 2023

VFR at CHA through midnight then MVFR through the rest of the
period with a PROB30 for IFR tomorrow morning. TYS and TRI will be
VFR through the period but PROB30 for MVFR tomorrow morning.


Chattanooga Airport, TN             51  65  50  60 /  70  70 100  80
Knoxville McGhee Tyson Airport, TN  47  66  52  61 /  50  50 100  90
Oak Ridge, TN                       47  64  49  60 /  60  60 100  90
Tri Cities Airport, TN              39  66  52  61 /  30  30  90 100