210
FXUS64 KMRX 081745
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
1245 PM EST Wed Jan 8 2025

...New AVIATION...

.UPDATE...
Issued at 901 AM EST Wed Jan 8 2025

Morning update to address the snow across northeast Tennessee and
southwest Virginia. Pre-sunrise snowfall was able to accumulate on
grassy surfaces and some back roads. This could make some roadways
slick for the first half of the day. To account for this, have
issued a Winter Weather Advisory, with the possibilty of up to an
inch of snow in some locations as we`ve received reports of over
half an inch this morning. With the clouds in the vicinity the
chance to see flurries in the sky will likely remain throughout
much of the day today as is typical with cloudy northwest flow
events, especially in cold temperatures below 25 which helps
squeeze out that much more moisture in the PBL.

&&

.SHORT TERM...
(Today and tonight)
Issued at 350 AM EST Wed Jan 8 2025

Key Messages:

1. Northwest flow snow showers/flurries starting up again during
the early morning hours.

2. Mostly dry, outside of flurries again this morning expected.
Highs much below normal. Clouds decreasing from southwest to
northeast this afternoon. Highs below freezing northeast and mid
to upper 30s south.

3. A very cold, clearer and dry night over the southern Tennessee
Valley and Southern Appalachians tonight.

Discussion:

Some clearing occurred during the evening over northeast Tennessee
but lower clouds filled back in by around midnight and just in
the last hour or so light snow flurries and some snow showers have
developed across northeast TN and southwest VA and are moving
southeast out of eastern KY. Also some narrow bands are forming
near the Smoky Mountains in the east Tennessee Valley similar to
early Tuesday morning. Have added flurries for today and into the
evening across northeast sections where the low level moisture in
the weak NW flow is wringing out the flurries in the saturated
cloud layer. May have to update and expand the area if it
increases toward morning and add snow showers to the wording. As
skies very slowly clear or erode from the southwest the
precipitation should end by late today in central sections.
Timing this process is difficult but the NAM seems to be the best
at it now. Clouds with the model hang around until 00Z tonight for
the Knoxville area are partly to mostly cloudy all day around
Chattanooga and most or all of tonight in the northeast.
Temperatures will be held down to the upper 20s to mid 30s again
today with only Chattanooga likely to get a few degrees above
freezing. Winds will be lighter today as the high pressure to the
northwest continues to build in. There will be some gusts to 15 to
20 which will keep wind chills in the teens to lower 20s.

Tonight with the cloud cover lingering and a weakening disturbance
moving southeast flurries and light snow showers will still be
possible in southwest Virginia and northeast Tennessee. Lows
should be lower in the teens to lower 20s. Wind chills will be in
the teens to single digits.

&&

.LONG TERM...
(Thursday through Tuesday)
Issued at 350 AM EST Wed Jan 8 2025

Key Messages:

1. Confidence continues to rise that a significant winter storm will
impact the area Friday into Saturday with widespread accumulating
snowfall.

2. Given the increasing confidence, a Winter Storm Watch will be
issued for the entire area from late Friday morning to early
Saturday morning.

Discussion:

The main focus for the long term is the potential for widespread
heavy snow on Friday and Saturday, so the discussion will
concentrate on that as the remainder of the period is uneventful,
other than continued below normal temperatures through the first
half of next week.

The southern stream closed upper low over CA/AZ/NW Mexico this
morning will merge with a northern stream shortwave trough dropping
out of western Canada on Thursday, which will induce cyclogenesis
near the TX coast. This low will strengthen as it tracks along the
Gulf Coast region and off the Carolina coastline by Saturday
morning. For our area, cold air will be in place ahead of this
system as deep moisture begins to spread in from the SW Friday
morning. QG forcing peaks starting around 21Z Saturday as midlevel
frontogenetic forcing shifts north into the area, and pivots from an
W-E orientation to SW-NE by 06Z. This forcing exits through the
latter half of the night, followed by NW flow cold advection snow
showers in northern sections and the mountains through Saturday
morning.

Models have been showing better run-to-run consistency and agreement
on the general setup for this event and forecast snow amounts, so
confidence is increasing that we will have impactful snow amounts
across the entire area. As a result, a Winter Storm Watch will be
issued.

Timing:

Snow is expected to spread into the Chattanooga area Friday morning,
between 7-10 AM EST. Initially, low level dry air will need to be
overcome before snow reaches the ground, but top-down moistening
should occur fairly quickly in that time frame. The Knoxville area
should start to see snow starting in the 10 AM-1 PM window, and
Tri- Cities an hour or two later (11 AM-2 PM). Confidence on this
timing is moderate to high given the fairly good model agreement.
The period of heaviest snowfall rates will likely be from 4 PM EST
Friday to 10 PM EST Friday. Ending time is a bit more murky as we
will transition into a NW flow regime with scattered snow
showers, but precip will likely be tapering off from SW to NE from
midnight in the SW sections to around 7 AM Saturday in the NE
sections. Given this expected timing, and after collaboration with
neighboring offices, the Winter Storm Watch will be in effect
from 10 AM EST Friday to 7 AM EST Saturday.

Precip types:

Forecast soundings show a deep near (but just below) freezing
isothermal layer once the column moistens, and this persists through
Friday and Friday evening. All of the LREF members show a
temperature profile that is entirely below freezing through the
event, lending confidence to the forecast of an all-snow event.

Snowfall amounts:

The trends in ensemble guidance over the last 24 hours have
stabilized somewhat with regards to snow amounts and probabilities
of amounts greater than 4". The NBM probability of seeing 4" or more
by 12Z Saturday is between 40-70%, with a 6" threshold being 20-50%.
The spread of NBM members is largest across southern sections, with
a range of 2" to 9" in some spots, suggesting that the southern TN
Valley and SW NC have the highest variability in possible
outcomes. At this time, the most likely scenario is a widespread
3-6" snowfall across the entire forecast area. In my experience, I
have often seen these synoptically-driven events over perform the
model snow amounts, and given the expected thermodynamic profiles
and QG forcing, snowfall rates could be quite heavy at times.
Amounts above the forecast range would not be surprising, but we
are still over 48 hours from the start of the event, with plenty
of time to fine tune the amounts.

&&

.AVIATION...
(18Z TAFS)
Issued at 1205 PM EST Wed Jan 8 2025

MVFR ceilings and snow will be the primary weather for the first
6 hours of this TAF cycle. Overcast skies and cold temperatures
are allowing for flurries to be squeezed out of the atmosphere,
which will likely continue for much of the day today.
Accumulations are expected to be minor, but could reduce
visibilities to borderline LIFR conditions.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             19  39  26  34 /   0   0  10  90
Knoxville McGhee Tyson Airport, TN  16  34  22  33 /   0   0   0  90
Oak Ridge, TN                       17  34  22  33 /   0   0   0  90
Tri Cities Airport, TN              14  28  18  34 /  10   0   0  70

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...Winter Storm Watch from Friday morning through Saturday morning
     for Cherokee-Clay.

TN...Winter Storm Watch from Friday morning through Saturday morning
     for Anderson-Bledsoe-Blount Smoky Mountains-Bradley-
     Campbell-Claiborne-Cocke Smoky Mountains-East Polk-Grainger-
     Hamblen-Hamilton-Hancock-Hawkins-Jefferson-Johnson-Knox-
     Loudon-Marion-McMinn-Meigs-Morgan-North Sevier-Northwest
     Blount-Northwest Carter-Northwest Cocke-Northwest Greene-
     Northwest Monroe-Rhea-Roane-Scott TN-Sequatchie-Sevier
     Smoky Mountains-Southeast Carter-Southeast Greene-Southeast
     Monroe-Sullivan-Unicoi-Union-Washington TN-West Polk.

     Winter Weather Advisory until 1 PM EST this afternoon for
     Hancock-Hawkins-Johnson-Northwest Carter-Northwest Greene-
     Southeast Carter-Southeast Greene-Sullivan-Unicoi-
     Washington TN.

VA...Winter Storm Watch from Friday morning through Saturday morning
     for Lee-Russell-Scott VA-Washington VA-Wise.

     Winter Weather Advisory until 1 PM EST this afternoon for Lee-
     Russell-Scott VA-Washington VA-Wise.

&&

$$

SHORT TERM...
LONG TERM....DGS
AVIATION...