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March 11th-13th Winter Weather Event. Winter's last gasp?


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3 minutes ago, Wurbus said:

MRX is going to say 1-3 for the valley with locally heavier amounts and issue a WWA and call it a day. I'd be surprised if they issue anything more than that today. They might even wait and let the night crew issue it.

My thoughts as well. The fact that it is a weekend probably makes it easier to downplay. Not the normal early morning rush to worry about. And it is easier to up totals later than downgrade. Most people I spoken too about it don't believe it will snow anyway. "It's been too warm to snow!" Is what I hear at work. 

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This could possibly be one of the best discos I’ve ever read…

Main concern for the long term is the looming winter weather event
for Friday night into Saturday. As such the discussion will focus
primarily on that event. Sunday and beyond is rather uneventful,
with mostly dry conditions and possibly some rain on Tue (mainly
south of I-40) as a closed upper low slides by us to the south. Now,
on to the winter weather event.

Friday evening into Friday night a northern stream trough/upper low
will shift east from the far northern plains and into the western
Great Lakes region, while a southern stream wave moves east from
Texas along the I-20 corridor across the deep south. Strong surface
high pressure will dive south out of Canada behind the northern
stream disturbance, pushing very cold air into the TN valley after
midnight Friday night. Meanwhile, the phasing of these two
disturbances coupled with the strong thermal gradient across the
Ohio and Tennessee river valley will lead to a very strong jet
streak developing over the eastern CONUS, stretching from the deep
south up through the mid-Atlantic seaboard after midnight Fri night.
Models forecast upwards of 160k jet streak to spread north from the
deep south through the southern Appalachians and mid-Atlantic
seaboard Friday night into Saturday. This will induce strong forcing
for ascent, with deep layer upward omega and saturated air extending
into and well beyond the dendritic growth zone for several hours. In
other words, this is setting the stage for widespread snowfall, with
precipitation rates that will likely overcome the warm ground and
lead to significant accumulating snow across much of eastern
Tennessee.

Precipitation begins to move into the region from the southwest
during the evening hours tomorrow, but really ramps up in intensity
and coverage between 06z and 12z Sat. Expect some areas in the
Cumberland plateau to begin seeing at least a rain/snow mix, if not
all snow, by 2-3 AM EST Sat morning, with most all of TN valley
seeing mostly snow by 5 AM EST or thereabouts. While some of the
potential snowfall during this period will be lost to melting or
mixed ptypes, I believe the strong CAA (and resulting increase in
snow ratios) and rapid cooling of the atmosphere due to dynamic
cooling will support a bulk of the QPF falling as snow. Further
lending confidence to higher snowfall amounts is that the strong
forcing will also be coincident with some negative EPV aloft, which
would favor some convectively enhanced snowfall rates. The strongest
signal in this regard will be mostly over the northern plateau and
further north into our western SW Virginia counties, which is where
the higher storm total snowfall amounts are. However, it also
extends across the central and northeastern TN valley as well which
is why snow amounts have been increased there.

It would be a lie to say there's not some uncertainty remaining.
Trends have been colder and more snowy over the last 24hrs, but
there's also less QPF to work with in most guidance as well. Will
that trend continue? Will the air temperatures drop quick enough,
and/or precip rates be high enough, to overcome warm ground
temperatures? These are questions that we won't know the answer to
until Saturday morning. But based on the guidance we have available,
it seems a significant late-season winter storm is in the offing.

Let's talk about storm total amounts real quick. In general,
accumulations of 2" or more should be mostly limited to areas from
Bledsoe over to Monroe county and points northward. Areas that get
3" or more snowfall should generally speaking be along or north of
the I-40 corridor, and amounts upwards of 4" should be limited to
the northern plateau and up into Lee and Wise county in Virginia,
and also in the Smokies. While the higher terrain of the southern
plateau (e.g. Bledsoe down to Marion county and also Signal Mountain
for instance) could see 2-3" as well, southern areas in general
should be more in the 1-2" range. For a more user friendly graphical
output, be sure to check our webpage and social media accounts.

Lastly, have gone ahead and hoisted up a Winter Storm Watch for the
entire CWA. A good argument could be made to have held off for the
far south. But given the official forecast, issuing a watch seems
like the best decision to make and leaving some southern counties
out is probably splitting hairs at this point. Will let the upcoming
night shift make a determination on what areas to upgrade to a
Warning and what areas to upgrade to an Advisory


.

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Getting ready to read the disco now, but my jaw dropped when I saw the quintessentially conservative MRX actually issued a WSW for even the valley. I thought it must be one of two things...either the models are amping up OR it is the end of this winter season and if they are wrong, no one will remember by next winter. ;)

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