Great discussion by MRX.. LOVE the Trowal part!!
.SHORT TERM...(Tonight through Monday)...
Key Messages:
1. Difficult snowfall forecast for low elevations. Definitely a
battle of snowfall rates vs. air/ground temperatures. Impacts what
amounts people will wake up to Monday morning.
2. High confidence in accumulating snowfall occurring tonight, even
in southern parts of the TN valley.
3. High confidence in significant snowfall event for the higher
elevations of SW Virginia and the east TN mountains.
Discussion:
Synoptically speaking, a strong upper trough over the Arklatex
region this afternoon will transition to a cutoff low over the next
12-18 hour as it shifts east across Mississippi and Alabama. As this
occurs, this will place our forecast area beneath a strong
deformation band this evening and overnight, with the potential for
high snowfall rates and resulting significant snowfall event.
Precip associated with this system will spread north from Alabama
later this afternoon and early part of the evening. Surface
temperatures and thermal profiles universally support an onset ptype
of rainfall. This will quickly change however as a TROWAL develops
overhead around and shortly after midnight tonight. Strong lift
through the dendritic growth zone accompanied by negative EPV yields
the potential for convectively enhanced precip and subsequently
banded precip and high precip rates tonight. Model soundings show
rapid cooling through the column tonight, suggesting dynamic cooling
will contribute to snowfall reaching the ground with surface
temperatures that might not otherwise be supportive of much more
than a rain/snow mix. Meanwhile, more substantial CAA will be
ongoing after 06z to 09z as surface cyclogenesis is in full swing
east of the Appalachians and cold continental air begins to spill
into the valley, further aiding in potential for accumulating
snowfall.
Regarding confidence levels, confidence is very high in the presence
of snowfall tonight. That does not appear to be in question. The big
unknowns are 1) where does the deformation axis and associated
heavy, banded snowfall set up, 2) how high will the resulting precip
rates be, and finally 3) will these factors be able to overcome warm
ground temperatures and at least initially warm surface air
temperatures. The last two factors will have huge implications on
what people wake up to see in terms of snow depth. Heavy precip
rates, on the order of 2"/hour, are expected but will be short lived
generally only for 3-6 hours at most. This would be enough to cause
travel impacts and overcome the ground temperatures for a short
period. For those traveling overnight this could be an issue. The
big question is what snow depth will actually be seen by most
people. If a valley location gets 4 inches of snow overnight and it
ends at 3-4am, will the ground remain warm enough to melt most of
that by the time people wake up at 5 to 6am? That remains to be
seen. However, all of the available guidance I have at my disposal
points to a notable snowfall event for many places, especially at
higher elevations, and have adjusted the forecast in that direction.
Regarding snowfall amounts, accumulations along/east of the I-75/I-
81 corridor in the valley have been upped a bit and amounts in the
TN mountains have been increased significantly. It is quite possible
that portions of the Smokies could see a 12-13" snowfall out of this
storm. In the valley, expanded the 3-5" accumulations and resulting
winter storm warning to include the Knox metro area town to McMinn
county along I-75, as well as added eastern Polk, Cherokee and Clay
counties in NC, and Lee county in Virginia. Cherokee and Clay
counties will mostly be higher elevations though. One big
uncertainty I still have is the plateau counties. Those areas will
be the furthest remove from deformation precip, but will also see
the colder air quicker. Have left them all in the advisory for now
but would not be surprised to see some warning level accumulations
given how this system is shaping up.
On the back side, precip wraps up around or very shortly after
daybreak tomorrow. Perhaps some upslope showers continue in the
mountains through late morning but believe it will be mostly done by
then.
