Though it may not feel like it, we’ve managed to reach late
November and the commencement of another ski season. When the SCWB signed off
late last year, the worsening pandemic felt like a big black hole enveloping
us with no way out. As of November, the black hole still envelops us, but to
borrow (and slightly alter) one of Carl Sagan’s most famous quotes, “Us
Vermonters have a certain talent for adaptability” and we will apply that
adaptability to piece together a ski season while dealing with and overcoming
the many obstacles. As I see it, the weather is presenting a few obstacles as well, but its nothing we aren't used to. For those of you that will never get used to the wordy discussion, please skip ahead to the bottom and receive the less than wordy outlook !
The three months leading us into November were exceptionally normal in terms of temperature, landing . within a half a degree of normal. There was some concern early in the summer of a worsening
drought but a succession of very wet months (August/September/October) has alleviated
many of those concerns. The current US drought monitor still puts us in a D1 area - Drought Monitor, but
the overall water situation has improved since some very dry months in the
spring and early summer. Within the vicinity of the North American continent
the two most glaring aspects of weather was the persistent heat and resulting
forest fires in California and the remarkable hurricane season which
culminated in two historic landfalling storms (Eta, Iota) very late in the
tropical season. Many will refer to the 2020 tropical season as record breaking
because we blew through the alphabet and well into the greek letters. This is a
little deceiving since the technology is available to better indentify weather
systems that meet the various thresholds to earn an official tropical name. To see a
category 5 storm strike Central America in November is nothing short of
remarkable however and truly cements 2020 as a historic one for tropical
activity.
What about this La Nina
The tropical cyclone activity in 2020, particularly the strength and unusual late presence of that activity deserves a place in the
discussion about climate change (and it will certainly get one). That said, a
strong and active hurricane season is pretty typical given the presence of La
Nina and we’ve had a notable one since late in the summer. La Nina’s are best
illustrated by a line of anomalously cold water in the equatorial regions of
the Pacific and aside from the aforementioned impact on the hurricane season,
can, when strong enough, have a material impact on winter weather across
mid-latitude North America. So what are we looking at this year ? La Nina was
gradually strengthening, reaching a value, based on a temperature anomaly
of -1.7 C in late October though it has
recently subsided a touch since then. Anything under -1 is
significant and were it to stay there through the winter will most assuredly
have something to say about our prevailing weather pattern. So what is that weather pattern typically ? A
more concentrated storm track in the Pacific, generally north of California
(they could certainly use more rain), cold weather confined to the northern
tier of the US, warm, dry weather across the southeast U.S. and active weather
in the northeast. Active, because usually the storm track takes dead aim at New
England and can bring varying results.
(Above are some of the bigger La Nina's going back 70 or so years, some weaker but notable La Nina events not included)
I am not a fan of strong ENSO events of any kind (La Nina or
El Nino) as they bring with them many risks and are infamous for inconsistency. The 70 years of data does present a rosier picture however with several weaker La Nina's yielding huge snow years and a few stronger La Nina's delivering the same. Every inch of latitude-north is important and locations south of 40 N often perform quite poorly. The results really vary across Vermont as illustrated by both attached figures and results are highly dependent on what side of Vermont does the prevailing storm track establish itself. Is
this a forecaster speaking from both sides of his mouth ? Absolutely ! but the information
is revealing and it makes sense. A storm track aimed at Vermont as opposed to
well south of Vermont is actually a positive thing, it just doesn’t always
produce positive results. In colder La Nina years, and there are several like
1970-71, 1995-96 (weak La Nina), 2000-2001 (weak La Nina), 2007-2008 and 2010-11, the results are fantastic. This
is year 17 of the blog and I rank 2007-08 and 2010-11 as #1 and #2 respectively
for MRG ski years and both had significant La Nina’s. Unfortunately 1988-89,
2005-2006 (weak) and 2011-2012 (weak) also featured substantial La Nina’s and were awful snow
years. Either way, La Nina’s can produce up and down results not only from one
season to another but within that season. It is rare to go more than a few weeks without some classic Vermont-style rain/ice catastrophe.
(This illustration is pretty worthless to the naked eye except it shows the snowstake at Mt Mansfield which is never worthless and also shows the wide variation in results during La Nina winters)
PDO/Pacific SST
So how can one determine whether we get the good or bad version of La Nina this upcoming winter. It naturally leads us into
the Pacific Decadal Oscillation (PDO) part of the discussion which includes a
broader overview of sea surface temperature patterns in the Pacific Ocean. The
PDO phase is determined by sea surface temperature configurations in the mid
latitude Pacific. When cold SST anomalies hug the western North American
coastline it illustrates a negative phase with the reverse indicating a
positive phase. A positive PDO phase
does correlate well to a more favorable weather pattern for east coast cold and
snow but they are rare during significant La Nina winters. Officially however,
we remain on the positive side of neutral going into the upcoming winter. I
want to convince myself that this is a uniquely positive indicator but the data
going back 70 years is inconclusive at best. Several of the snowiest winters in
Vermont on this decorated list occurred with both significant La Nina’s and very negative PDO phases. What
is a forecaster to do aside from ramble on and on !
Autumn 2019
Autumn 2020
With each year of blogging and seasonal forecasting, I
continue to be a fan of PDO assessments. The winters of 2014-2015 in particular
was dominated by very large, sometimes record breaking positive PDO phase
underscoring the predictive value of this variable in a seasonal forecast. I’ve
also realized however that the nature of the PDO is also of great importance
and this speaks specifically to a deeper look at sea surface temperatures in
the mid-latitude Pacific Ocean. Last year, the persistent Pacific Ocean warmth
stretching from the Aleutian Islands through the Gulf of Alaska left us with an
invariably strong Pacific Jet and had us fending off mild air a lot more often
than we would prefer. Unfortunately, much of this SST configuration remains and
the blob of warmth appears especially formidable south of the Gulf of Alaska.
The only difference this year would be that the aforementioned warmth does not extend all the way north into the Gulf of Alaska. Though the map seems to indicate a more negative PDO index, it remains barely positive for now on the basis of some of that warm ocean water extending all the way to the North American west coast. The continued presence of the mid-latitude Pacific warm water is the most concerning variable right now and it underscores the importance of taking that deeper dive into the PDO/Pacific SST assessment. The weather pattern seems intent on dumping a lot of colder air into that region of the Pacific over the next 14 days and it will be interesting to see if that can weaken the area of anomalous SST warmth.
Snow Cover Expansion
We recorded another
strong snow build across the northern hemisphere in November. The 21.22 millions
of square kilometers observed in 2020 was the 9th consecutive October where the
land area of snow cover exceeded 20 millions of sq km. Prior to this 9-year run
of strong October snow expansion, the average extent of snow cover was about 18
millions of sq km with a standard deviation of roughly 2.5. Relative to that
average, all but two of the recent 9 years have been more than a standard deviation
above average. To borrow the phrase from former NBA star Rasheed Wallace, “Ball
Don’t Lie !“ and neither do these statistics. Either the prior almost 50 years of data are wrong or more
likely, the expansion of snow and ice in October is a byproduct of other trends
and changes occurring very quickly at arctic latitudes. The obvious player here
is the alarming loss of sea ice as a result of climate change which results in
more open water across the Arctic Ocean during autumn and an altered snow
climate across northern latitudes. I’ve had to consult with others to verify
this hypothesis and I am relatively convinced this is the cause. Without getting
too immersed in the weeds, this conclusion changes the way one would use this data
and would indicate that the old “normal” doesn’t apply. 21.22 millions of
square km is still a very strong number, especially since it is largely driven
by the 10.26 millions of square km recorded over the North American continent.
That said, this year’s data falls smack in the middle of what was recorded over
the last 9 years and is behind what was recorded last year.
Meanwhile arctic sea ice which has, what we think, is a
causal and inverse relationship with snow cover expansion been near or at record lows for
much of October into early November. We have seen ice expand into the Hudson
Bay in the past week or two which conforms to climatological expectations, but
as a whole, sea ice continues its longer term decline and that decline is more than alarming
when looking through the geological time scale lens.
Atmospheric Poker Tells
Has the atmosphere given
us any poker-style “tells” this year? Some things are obviously very different this
year verses last year like the emerging formidable La Nina and the many strong hurricanes. Some things are
the same such as the blob of sea surface temperature warmth in the mid latitude
Pacific. But how is the weather actually behaving ? We had those succession of “normal”
but relatively wet months this fall and what appears to be a relatively mild
November. In Vermont, the mild weather has been more garden variety but across the
U.S. as a whole, it will likely go down as one of the warmest November’s on
record (guessing top 5). I am particularly interested in the behavior of the
Arctic Oscillation (AO) since we would all like to avoid a recurrence of
2019-2020. When aggregated over the entirety of last winter, the AO was the 2nd
highest on record and though the season wasn’t completely free of good moments,
the persistence of the +AO was the elephant that thrashed the room and proved
too difficult to overcome. The AO did surge early this past November,
rekindling some of those bad memories but has since faded and is forecast to be
close to neutral through early December. You can’t say too much from what we’ve
seen so far except that we have yet to see good evidence that the weather
pattern is showing a propensity to get blocked. So far it has not !! This
combined with the mild turn this November and some more localized SST warmth off
the eastern seaboard is a little concerning.
Our Winter Outlook
The outlook is thus as follows. A mild winter can be
expected on the whole, but consistent with the traditions of La Nina, there
will be interludes of both very mild weather and cold and snow. What will hurt
this winter, in my expectation, is the likely inability for a cold, snowier
pattern to lock itself in for more than a span of 3 weeks; in fact, we may be
lucky to get 3 weeks. That said, I think the winter has some things going for
it. La Nina combined with a modestly positive EPO will keep the storm track
pointed in our direction and even if extended periods of favorable weather aren’t
expected, several broken up decent periods are likely. Additionally, we are in
a much better place for snow given this set up than southern New England, the Catskills,
Pennsylvania and points south. Latitude will be good and even 10 miles of Rt
100 might make a difference in certain events. I would expect the mild weather
will keep snowfall below average this year but we will have our days. Again, I
would be surprised to see 2015-2016 revisited. We could also surprise to the upside
as several La Nina winters have done. I am shying away from a snowy La Nina forecast
based on the SST configurations in the Pacific and recent behavioral pattern tendencies but it wouldn't be unprecedented or that surprising. In the end I am guessing MRG procures around 200 inches of snow.
Hope everyone has and will stay safe through the thanksgiving holiday. A special shout out to those essential folks working through the pandemic helping all of us get through this. It can't be easy and it hasn't gone unnoticed. See you all on the mountain !
