J.Spin

Here’s the Mt. Mansfield analysis I referred to in my post above – I just copied the text from the initial post below. The return rate on August snow seems to be 50+ years, but for September snow it’s about once every 6 years, and for October it’s almost a lock. “A comment in the main discussion thread had me pull up some of the first snowfall of the season data for Mansfield, but I’m putting it here for archival/discussion purposes as well. In a few more weeks and we’ll definitely be watching for those first dustings in the peaks around here in NNE. The earliest recorded accumulations on Mt. Mansfield are actually only a week from today (see inset in the graph below), and of course on Mt. Washington it can happen at almost any time. I ran the early snowfall numbers for Mt. Mansfield a few years back and found that first snows occurred in September at an average rate of about twice a decade, and only about once a decade do we not have that first snow by October. I recently assembled the actual numbers though, and I’ve plotted those below. The occurrence of first snow by October is actually a bit higher than 90% though, so the absence of snow by October is closer to once every 15 years vs. once a decade. First snows are potentially a bit more frequent than the numbers indicate too, since there are some seasons with chunks of autumn data missing, and with the ephemeral nature of early season snow and the variability in personnel making those observations, some occurrences of snow could have easily been missed.”

That’s really cool Tamarack; those observations certainly demonstrate the effect of that extra latitude up there.

I’d actually been thinking about putting together plots of earliest snowfall dates (First Frozen, First Accumulating Snow, First 1” Storm, First 2” Storm, etc.) because I have all those data, but figuring out a presentation format that I felt was effective has been tough. This inquiry got me working on it some more though. I wanted the dates to be on the horizontal axis in a plot because I like that format for chronological data. In addition, what I really wanted to do was have each season as a row in the plot, and stack all the first occurrence dates for each threshold of snow in the row as different colored dots/points for each category. Excel hasn’t quite been able to hit what I want, and I really haven’t felt like dealing with export to another graphing application just to achieve that result, but I think I’ve settled on a format I like. I’m putting just one category on a plot at a time, and it’s much less busy, so I can actually display the data points as the dates themselves. The date text markers aren’t placed perfectly with respect to the date axis (they’re on the left side of the desired position) because I’m sort of adapting a horizontal bar graph to get my result, but I still think effectively shows the distribution of dates by season. So to get to DS’s question about September snow, I’ve never seen it at our site, consistent with what backedge suspected. Below I’ve added the plot of earliest frozen precipitation at our site, which is the lowest threshold I record for early season occurrences of frozen precipitation. This first frozen precipitation observation doesn’t need to be specifically snow (although it often is), so it can be graupel or sleet, etc., as long as it’s frozen. It also doesn’t need to accumulate, just be observed as falling frozen precipitation. I record first accumulating snow as its own category, so I’ll do a separate plot for that. Anyway, as the plot below indicates, the earliest frozen precipitation in the records we have for out site is October 11th. That really doesn’t seem all that far from September, but it’s probably an eternity at that time of year. Even Mt. Mansfield struggles to get snow in September as PF noted, but I’ve got a plot of those data that I made for a post a couple of seasons back, so I’ll grab that for a separate post. So in terms of this past season, first frozen precipitation came on October 26th, which is a little on the late side (mean occurrence date is October 21st), but certainly right in that typical period where we see it. In this case, that was also the date for the higher threshold of first accumulating snow, which is very typical for that parameter with a mean occurrence date of October 27th.

I’ve never recorded snow in June at our site, so I consider May the final month of our snow season. The variability in the plot of snowfall by season speaks for itself, with accumulating May snowfall being quite inconsistent and only happening every few seasons or so. Similar to November and April, I consider October the counterpart to May on the front end of the season. Although there can be a lot of variability in both months with respect to whether or not we get any accumulating snow at all in a given season, I think a comparison of the plots for the two months is informative in the context of PF’s recent comments. Although both months have surprisingly similar mean snowfall values, the October snowfall mean comes from a much more consistent distribution of snowfall among the seasons. Those October snows often come as autumnal cold fronts push through and the mountains help wring out some moisture as the cold air arrives, which is certainly in line with PF’s idea about the types of setups that work well to produce snow here.

Great inferences PF; that makes a lot of sense. In addition, when I think about it, it fits the typical weather experiences for many of us who live along the spine of the Northern Greens. As with the November/April dyad, I’d say the trend is there even for the October/May dyad despite the fact that those months are the very fringe of the snow season and exhibit high snowfall variability. I’ll have the May snowfall plot next as the final in that series of analyses.

April is another one of those months with a lot of snowfall variability, and it came in a bit on the plus side this winter. Nothing has come close that the snowfall we had in April 2007, but it’s interesting to note that the past several years have shown much more consistent snowfall totals relative to the period before that really exhibited that April variability. I think of April as the spring equivalent of November when it comes to the frequency, amount, and overall tenor of snow. But as I look at the numbers and summary plots now, it’s obvious that November is much snowier; our site averages more than twice the amount of snow in November compared to April, and the number of storms is nearly double as well.

Moving into the last third of winter season, I have the March snowfall data for our site below. This past March was certainly below average in terms of snowfall, but not down there in the single digits as we’ve seen a couple of times. March shows much more variability than stalwart February, and on that note, it’s interesting to see that even March has exceeded a 55” monthly snow total in our data set, whereas February hasn’t. Despite the variability, there haven’t been any long runs of lean Marches, and on the higher end, that 2017-2019 period looks like it was a standout stretch of March snow totals.

Approved.

That winter was so outrageously anomalous in this area (snowfall at our site was 2.38 σ below the mean, statistical occurrence at 1 in 116 seasons). It’s hard to imagine how it even happened, especially around here in one of the most consistent snowfall areas in the eastern U.S. I did a quick look through my ski reports from the season, and it seems that for starters, it took forever to get going. My first day of backcountry skiing was on January 23rd, which is probably not all that late, but the snowpack at the Mt. Mansfield Stake didn’t even reach 30” until February 21st. In addition, the stake never reached even the 40” benchmark. I was wondering if it was some powerful combination of being both warm and dry. It was somewhat dry (October through May precipitation was about 5 to 6 inches below average at our site) so that likely contributed to part of it, but after reading through some of my reports, it sounds like “warm” was also a theme, and may have been the larger contributor to the overall result. Thank god we at least didn’t know beforehand that such a low snowfall winter was coming – I’m not sure how it would have affected the psyche of everyone involved in the various snow sports industries if we knew ahead of time what everyone would have to go through.

February is next in the monthly snowfall chronology, and it was a solid contributor this past season. At face value, I wouldn’t consider this past February’s snowfall anything more than roughly average, but as I’ve been seeing with these plots, actually putting together the visual is a nice way to get a feel for trends. The plot reveals that although 47.2” is nothing remarkable, it’s actually the fourth highest February in my records. Even before plotting the data, I knew that February was the king of consistency at our site among the various snowfall months, so that probably puts its reliability way up there for an area that already has a relatively high consistency in snowfall. The only February in the plot that really sticks out as much of a dud is the one from the highly anomalous ’15-‘16 season – all the other entries look quite decent. Consistency usually plays both ways of course, and I hadn’t noticed the lack of outstanding Februarys until making this plot. In 15 seasons worth of data now, there’s no February getting anywhere near the 70” range like instances I’ve seen for December and January. There hasn’t even been a February approaching 60”, and the month has yet to hit even a modest 55” total here in that entire period of record. I’d say that current February snowfall record for our site is certainly ripe for breaking at some point, but climatologically, it’s funny to think of how an incredibly snowy month like February has thus far been capped under 55” of snow here.

Indeed, snowfall consistency seems to be a big feature of our location, and I’d say in the Northern Greens in general. It feels like the climatological aspects that impart the snowfall consistency are likely playing a significant role in pushing the annual snowfall averages above the other mountain ranges in the region as well. If I had to rank the factors promoting consistency at our site relative to other regional locales, I’d say that orographics is probably at the top of the list. On the next tier for consistency contributors I’d put effects of LES by being relatively proximal/downwind of the lakes, and being one of the windward ranges in the region. Having Clippers (vs. other types of less consistent storms) as a major part of our winter snow climate is probably in there as well. The presence of Clippers themselves shouldn’t really be much of a consistency factor relative to the rest of NNE, since everyone at a certain latitude should get in on the storm, but the performance from those Clippers is enhanced by the other factors. I think where areas off to the south and east of NVT can capitalize more on snowfall is with coastal storms. It’s not that the Northern Greens miss out on coastals/nor’easters, and in fact we can often do quite well with the synoptic portions of those systems. However, if it’s one of those more compact systems, or something farther offshore, on average, it seems like areas off to the south and east are going to get more snow/liquid from the synoptic portions of those storms. The thing is, coastal storms are already a somewhat infrequent occurrence, and on top of that they lack consistency in positioning and behavior even when they do occur. So, potentially performing marginally worse in a phenomenon that is relatively infrequent with inconsistent performance, just isn’t going to be much of a hit to snowfall averages. It’s really not likely to affect snowfall consistency much, and if anything one might want to down weight our typical coastal systems in their snowfall climate repertoire if the goal was improved snowfall consistency.

Next up in the analysis of this past season’s snowfall is January, and those data are below. This past January was at least average with respect to snowfall, and it was even a bit better than the previous season. A trend that we’ve talked about in the forum, but certainly stands out when the data are plotted, is that notable stretch of Januarys with poor snowfall from 2012 to 2018. That period contains the five least snowy Januarys in our period of record, packed into quite a short span. Prior to that stretch, the January snowfall average in our data set was similar to the other midwinter months at ~40”, but it really started to fall off during that period. In many cases, the reduced snowfall seemed to be in part due to those January outbreaks of arctic air, where the storm spigot would shut off in NNE as the intrusion of dry, cold air pushed the storm track farther south. We’ll see how January behaves around here going forward, but these past three seasons seem to have been a return to the type of snowfall we’d had before that period.

It’s probably not coincidence that those are our two wettest months as well, although it is interesting that they’re flipped with June the wettest averaging ~7” of liquid and October next with ~5¼” of liquid. Perhaps they’ll even out somewhat as the data set continues to grow.

I see our site is on there at 15.53” – we were close to 5” behind average pace at the end of May. June is the wettest month in our data set, averaging almost 7” of liquid. We’ll have to see how the rest of the month goes, but since it makes such a big contribution to the average annual precipitation, it’s an easy month to put things farther behind average pace if it’s a slow rainfall month.

Looking at the December snowfall data for our site, we can clearly see that December 2020 was below average - it was actually the third lowest in the data set. With the low snowfall, I was thinking there might be an obvious segment of the month that was devoid of new snow to account for the deficiency (I know there was that somewhat warm period around Christmas day). The daily distribution of snowfall doesn’t show that though; the snow was actually distributed fairly evenly throughout the month. There was new snow on 19 of 31 days, and the longest period without new snow was three days. Even that period near Christmas day that came to mind only represented two days (the 24th & 25th) without new snow. I can put those daily snowfall data together in a different plot at some point. Similar to the November snowfall plot, the visualization of December snowfall really highlighted a trend that I hadn’t noticed. It was somewhat startling to see that we really haven’t had a strong December in our area in eight seasons. I actually think the long-term average for December snowfall at our site is around 40 inches, so even December 2017 was essentially average. That means one has to go all the way back to 2012 since we’ve really seen a solid December. After that notable stretch, we’re probably due for a rebound at some point.

Looking at the November monthly snowfall numbers from our data set, we can see where November ’20-‘21 stacks up. Indeed it was a fairly strong November overall, and with back-to-back storms bringing over a foot of new snow on the 1st and 2nd of the month, it got things off to a very early start that kept the cumulative snowfall ahead of average and helped buffer season snowfall through the relatively slow December. One thing that I hadn’t really noticed until putting the data into graphical form was what a solid run of Novembers we’ve had as of late. The past five Novembers have all been essentially at or above average, and that cluster definitely stands out on the plot.

My previous plot provided the average snowfall for each month to allow comparison to the ’20-‘21 totals, but we can also look at a month across the data set to see where ’20-‘21 sits among the distribution of data. October this season was about as average as it gets at our site for snowfall – it fits right in there among the high and low months, and it actually represents the median value for the data set.

That March value must be quite an outlier for a NNE location. March can certainly show a lot more variability in snowfall relative to DJF in my experience, and snowless/nearly snowless versions can happen, but this far north it’s only a certain combination of warmth and/or dryness that can get the month to come in that low. That’s a good topic though; I’ll start looking at the monthly snowfall comparisons in my data set next.

The next look at the ’20-‘21 season snowfall data is by month, with the plot below showing this past season’s monthly snow totals compared to the ’06-’21 averages (white diamonds) in my data set. There weren’t any really outstanding months this season, but November was solid. It was up there among the top five in my data set, although more than 10 inches behind the strong ’18-‘19 November we just had a couple seasons ago. The below average months are quickly evident, and they were December and March. Each was more than 10 inches below the mean snowfall value, and they obviously marked a couple of the major slow periods shown in the cumulative snowfall plot.

On the topic of snowfall for the 2020-2021 winter season, the next look is at cumulative snowfall, giving a sense of the points in the season at our site where it was ahead of, or behind, average pace. The first plot shows actual snowfall numbers (blue) relative to average (white), and the strong start in early November is very evident. That positive departure was gradually lost through the relatively slow December into the middle of January period before it picked back up from mid-January through mid-March. Then the pace of snowfall slowed down again, briefly getting behind average pace in mid-April before finally recovering at the end of the season. The second plot simply shows the deviation from average cumulative snowfall in inches, but more easily lets one view where the seasons was ahead of, or behind, average snowfall pace. That strong start in early November really jumps out here as well. The more dramatic “surprise” that I think this plot shows well is that despite a season with a relatively lackluster tenor, the vast majority of the season in this immediate area was ahead of average pace on snowfall. It’s really just that notable January dip where the pace fell of substantially, and even there it was still less than 10 inches behind average. The fact that two big chunks of the season (November through mid-January, and mid-February through mid-April) were periods where snowfall was simply losing pace vs. average (note the negative slopes in these parts of the plot) probably played into the potentially reduced perception of the season despite still being ahead of average snowfall pace.

Thanks for the data - I put together the graphics for the difference from mean snowfall at our sites by season, and with disparate averages, using percentage to normalize seemed like the best way to facilitate the comparison. As you noted, ’14-‘15 represented a somewhat substantial directional difference, with ’13-‘14 right next to it being a more muted version. And historically during this period of record, that’s really been it for directional differences aside from ‘06-‘07, but that season’s data are so tight to the means that the differences are really inconsequential. The rest of the differences have just been magnitude. The data certain speak to the general NNE trends in seasonal snowfall we often see, with that horrendous ’15-‘16 season being an extreme, but effective, example. The ’15-‘16 season was outrageously poor with respect to snowfall (an almost unbelievable season at more than 2 S.D. below the mean here at our site). I think NNE had some of the largest deviations from average in the region that season, and even within NNE, I think NVT showed some of the largest deviations. The whole season was quite poor region wide with respect to snowfall. ..and then we have the ’20-‘21 season. It’s quite an outlier on the plot that stacks both the directionality and magnitude of difference between our two sites like nothing else in the data set. It’s not at all like ’15-‘16 that had poor snowfall region wide. Weren’t some SNE sites even above average on snowfall for the season? So to see numbers at your site in the realm of ’15-‘16 (which it certainly looks like on the plot) would seem to make the season extremely anomalous. If someone’s got access to it, or knows how to generate it, I think it would be very interesting to see the New England map for snowfall deviation from average for the ’20-‘21 season. It seems that off to the east of here was affected the most, with NH affected somewhat, and then ME affected to an even greater extent? What was it like for northern and eastern ME?

June 1st is generally when I consider the snow season complete here at our site, so I can start finalizing the numbers. Total season snowfall is the main parameter I track, so I’ll start with that. There were certainly some dry spells with respect to total snowfall, but ultimately the season ended up with 160.6” here, which is about average for my period of record. Below I’ve got the plot of how this season’s snowfall compared to the others in my data set, with the mean value for the data shown by the black dashed line.

May Totals Days with new snow: 2 Accumulating Storms: 1 Snowfall: 0.2” Liquid Equivalent: 2.92” SDD: T I’ve put together my snow and liquid numbers for May, and snowfall was typical, but total liquid was two inches shy of the mean. For the calendar year here at our site we’re at 15.05” of total liquid, which is about five inches behind normal pace, so we’re running a bit on the drier side of average.