An area of overnight snow has spiralled across the northeast of Scotland overnight, and put a blanket of powdered snow down on all higher ground as this webcam of Cairngorm from Inverdruie shows (fig 1). Winds have now picked up on the summit and temperatures seem to have levelled off at around -6°C after the clearance of the occlusion like feature (fig 2).
After my blog about the possibility of a snowy Saturday, I was just looking for some extra detail in the GFS model for Saturday morning when I remembered the excellent wxcharts.eu website. The purple in the 06 UTC map indicates a large area of snow falling over northern England and Wales, and the meteogram for Eggleston in County Durham is forecasting 9 cm of snow.
Here are the 12 UTC temperature anomalies for 12 UTC on the 12th of November (fig 1) and it’s a cold day everywhere, especially the further east and north that you are. Temperature anomalies are generally in the range 2 to 4°C below the 1981-2010 long-term average for 12 UTC on the 12th of November. But the weather is bright enough away from the east coast and the far southwest, where showers have continued in the fresh or strong N’NW wind.
Meanwhile low Numa continues to track SE and deepen across the south of Germany (fig 2). There’s quite an area of snow developed now in the cold air to the north of the low, I should imagine the Alps are in for a pasting in the next 12 hours.
I can’t say for certain, but I would have thought that today was the first time that the high tops of the Cairngorms have seen a dusting of snow this Autumn, but I could be wrong (fig 1). It’s certainly been a very mild Autumn over the mountains of Scotland so far this Autumn (fig 2).
I didn’t expect to find a webcam at the top of the funicular railway at the Ptarmigan restaurant (fig 3) but here’s the evidence of that dusting of snow, although even that could be rime I suppose. I notice that they even have a smart phone app for this season so that you can check out the skiing conditions.
Forget all the waffle that I wrote about a warm start to November, after all that was last week, and as we know “a week is a long time” when it comes to NWP output. You can’t say the latest run of the GFS isn’t meridional at the start of November (fig 1), but in completely the opposite way that it seemed to be indicating last Wednesday. Although the start of November usually marks the start of the silly season for snow, the GFS model is well supported by the ECMWF model in this northerly outbreak, although the anticyclone is over northern Greenland in the GFS model, rather than the west of Ireland in the ECMWF (fig 2).
The cold air ushered in by storm Xavier has certainly dropped the temperatures on the Zugspitze in the last 24 hours (fig 2). From a balmy max of 2.9°C yesterday, temperatures have quickly fallen away to around -9.1°C at 15 UTC this afternoon, but then again it is 9,718 feet up in the Wetterstein mountains in southern Germany close to the Austrian border. The strong northwesterly wind has died down a bit and they have had at least 20 cm of fresh snow during today, but how they manage to find any snow surface that’s not severely drifted to accurately measure a snow depth beats me.
Interesting news item from the BBC about the lingering snow patches in various gullies across the mountains of Scotland. I wonder if the three patches in the Cairngorms will survive? I suppose it will depend on the kind of Autumn that we are in for, cold and dry, or mild and wet. Just across the way from Ben Nevis is Aonach Mor with an AWS on it (WMO #03041). The AWS is not quite at the top of Aonach Mor which is 1221 M high (4,006 feet), but a little way down at the top of the chairlift at 1130 M (3,707 feet), from where I guess there is some kind of extension lead buried under the ground that connects the two. So temperatures should be not far off what they would be on the north face of the Ben, but of course that face would be in perpetual shadow.
OGIMET wouldn’t let me download last Autumn’s data for the AWS on Aonach Mor, but here is the thermograph from the 4th of January through to April (fig 3), and as you can see although it was sub-zero for extensive periods, it wasn’t overly cold, and January was notably mild.
The rainfall anomalies for the period between November 2016 and March 2017 were generally near average, with November being a dry month (fig 4).
The temperatures through the extended winter period where all above average, except for the dry November which was colder than average, December was exceptionally mild (fig 5).
Iain Cameron is correct when he says that the demise of the snow in the gullies of Ben Nevis was down to the lack of snow last winter. Snow is obviously dependent on rain falling with temperatures near or below freezing, and the lack of it was down to two things, colder periods also tended to be drier, and the wetter periods were also usually mild, a combination of the two meant that not enough snow fell to fill the gullies to survive through till the following Autumn.
I suppose if you coupled up the temperature from Aonach Mor with daily rainfall totals from Tulloch Bridge you could with a bit of jiggery-pokery estimate how much snow fell and accumulated at a 1000 M. With a few other bits of climate data you could also try to calculate how much snow was lost due to sunshine, evaporation, sublimation, and warm rain. It sounds like the job for a sophisticated NWP climate modelling tool as far as I can see.
I reckon that there has been an almost 60% decline in annual snowfall since 1931. This won’t surprise a lot of people, because snow has become something of a scarce commodity in recent Winters, especially the further south that you are. Before I go any further the science behind this article is a bit thin, it’s based on a mix of daily Central England Temperatures [CET] and daily UKP rainfall (central region), but what the hell, you’ve got to start from somewhere, and I don’t think the Met office would have provided me with the required climate data to do this for free.
The biggest fudge factor, and don’t forget that even the most sophisticated and complicated NWP software employ some kind of fudge, is the algorithm that takes a daily maximum and minimum temperature, and decides if there’s any precipitation reported for that day, what likely probability is that it would fall as snow and accumulate. This is obviously easy if the maximum temperature for the day in question was below freezing, but not so easy say if the maximum is +5°C and the minimum is -1°C, so all you can do is give it your best guess. I suppose you could also look up the LWT for that day, and maybe get some kind of idea of what kind of air mass the country is under, but I didn’t go that far, and kept it as simple as possible.
Getting back to the chart (fig 1), you will see that my trusty algorithm has identified the snowiest winter in the last 86 years as being that of 1946-47, so that’s a good start. The second snowiest season it reckons was 1978-79, and having experienced of winters since the early 1960’s in various parts of the country I wouldn’t disagree with that. I guess that the accumulated snowfall for that season, ignoring melting of course, was close to a 100 cm, with 1946-47 producing an accumulation of just over 140 cm. I entitled the first article that I wrote about using daily CET and UKP to estimate a snow depth; ‘Central England Snowfall’; in fact it’s probably more appropriate to imagine the value as an index rather than a specific depth of snow. There have been some years with no snowfall when the algorithm couldn’t detect any snow, these were for the years 1988-89, 2013-14 and 2016-17, perhaps the code I calculate the daily probabilities requires a bit of tweaking, but then again, the last snow that I can remember settling down here in our part of Devon, was way back in 2010.
I did the original work for the application, which I call Central England Snowfall, about ten years ago now, but after deciding to give the code a bit of a spring clean today, I thought that I’d write an article around it. It’s a bit of fun because the science that I use could be regarded as suspect, but as far as I know, there are no long-term graphs available on the internet for annual snowfall totals for any climate station in the UK. If you do find one, or perhaps know of a graph than spans 60 years or more please let me know, because I would be very interested to see it, and would certainly include it in this article. I do have some evidence though, in the shape of daily climate records courtesy of Alistair McClean, Curator of Natural Science at the Weston Park Museum in Sheffield, for the period 1950-2010, which luckily includes daily values of fresh snow and snow depth. As you can see in this shorter period (fig 2), annual totals on the linear trend have also declined, by almost 40% in the last sixty years. The graphs are quite similar, even the estimate of snowfall that I make for the winter of 1978-79 are reasonably close (actual 116 cm estimated 98 cm). Having experienced that winter firsthand in the higher suburbs of that city though, I would say that the reported values maybe rather on the low side.
It’s a great shame that the Met Office stopped producing the Snow Survey of Great Britain in 1991, if that information could be digitised and collated it would make a wonderful climate resource for snowfall in the UK. Finally, I would just like to give a special mention to the website of Dr Richard Wild and is thesis ‘Spatial and Temporal Analysis of Heavy Snowfalls across Great Britain between the years 1861-1999‘, a fascinating read for the snow lovers amongst us.
The small low pressure in the Moray Firth at the moment (fig 1) doesn’t quite fit the bill as a classic polar low according to my old copy of the Meteorological Glossary. It describes a polar-air depression as:
A SECONDARY DEPRESSION of a non-frontal character which forms, more especially in winter, within an apparently homogeneous polar AIR MASS; near the British Isles the development is usually within a northerly or north-westerly airstream. The chief characteristics of this type of depression, which seldom becomes intense, are a movement in accordance with the direction of the general current in which the depression forms, and the development of a belt of precipitation near the depression centre and along a trough line which often forms on the side farther from the parent depression where also the pressure gradient (and surface winds) is increased.
Meteorological Glossary (6th edition 1991) Courtesy of the Met Office ©Crown Copyright
In this morning’s visible satellite image (fig 2) the trough, which the Met Office have as a wrap round occlusion in their midnight analysis, would usually occur on the western side of the low in this situation, or so says the glossary, but not with this feature looking at the imagery and the weather radar (fig 3).
The precipitation is of snow across the north of Scotland down to quite low levels this morning, and warnings have been issued by the Met Office (fig 3). I hadn’t notice that the Met Office had finally updated the dreadful mapping that they use in their warnings, a lot smarter, but about time.
Although this low might not be classed as a classic polar low, a proper one might be developing off the coast of eastern Iceland, because the latest pressure falls at Torshavn (fig 5), indicate either a trough or maybe even a proper polar low might be squeezing itself into the tight northerly flow and head southward later today (fig 6), so you never know.
The cold air has dug back in behind the cold front at Baltasound, turning the rain to snow, as the low pressure whizzes across the Northern Isles.
I wish someone would sort out that anemometer at Torshavn, I’m sure that the wind direction is the exact reciprocal of what it should be, it’s happened before, you just can’t trust those AWS.