WINTER 2006-7- part 3: The four-day winter - Feb 8-11!

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What is happening to our winters??

I'm old enough to remember cold snowy ones - I was born just before the 1962-63 winter - although obviously I don't remember that - and enjoyed weeks of freezing temperatures and snow in the winters of (in particular) the late 70s and mid 80s. Such conditions were "normal" enough that being sent home from school was a rarity - although I do recall one occasion when school didn't open because the snowdrifts were preventing the doors being opened! Nowadays, if I want to see snowdrifts like that it's necessary to get up into the mountains, it seems. And in addition, 2cm of snowfall brings everything grinding to a halt!

The last few winters have seen typically brief cold snaps with mild conditions dominating the rest of the time and 2006-7 has (so far) been no different. The synoptic pattern has remained mobile and zonal with the UK receiving air from well out to the SW most of the time. This is the main factor - climate change simply meaning that the temperatures have been a little higher than they would otherwise be. The synoptic pattern is what is crucial to giving the UK cold snowy conditions, as indeed it managed to on Feb 8-11.

Late January saw high pressure established over the UK and temperatures slowly fell due to overnight radiative cooling in the rather stagnant airmass. Then, in the first week of February, a ridge of high pressure over Greenland intensified: with low pressure over Scandanavia a cold northerly engulfed the UK bringing both daytime and nightime temperatures further down. However, by February 6th, the Greenland ridge had diminished somewhat allowing a developing Atlantic low to start heading for us. By the 8th, low pressure was centered just SW of Ireland, whilst the remnants of the cold air sat over the UK.

Such a set-up is known as a "battleground", the battle being between the resident cold air and the milder moisture-laden Atlantic air trying to get in. Where these meet, snow is commonly the result, and potentially an awful lot of it!

The snow arrived in Machynlleth in the morning of Thursday February 8th. It fell steadily all day, sometimes light, sometimes heavy, so that even at valley level around 10cm accumulated. This is the view from Penyrallt on that afternoon. Thursday's snow was well-forecast but in terms of normally being able to predict the weather 24 hours in advance with some accuracy, Friday's took everyone by surprise from professional meteorologists to amateurs like me. Complex developments in the Biscay area led to another series of fronts pushing up early on Friday morning so that new severe weather warnings were out by the morning and once again the snow fell all day, replacing the partially thawed fall of the previous day. I betted myself that no such thaw had occurred in the hills, though, and on Saturday 10th three of us set off up the Dylife mountain road, for a look....
The road was fine at 300m ASL...
...but around the 400m contour we found where the snowplough had given up! The cars belonged to sledgers in a nearby field. Having no sledge ourselves, or indeed the huskies to pull one along, we set off on foot....
...which proved exceedingly hard-going. The road, sunken between higher banks, was badly drifted-up, depths varying from ankle to thigh-depth. To make matters more complicated, advection-fog was forming at the snow-air interface due to the milder sou'westerly now blowing - its moisture was condensing at surface where the snowfields were cooling it, just as sea-fogs form in early summer when hot humid air is advected over a cooler sea. The end result was that the fog was thickest at this level, to the extent that it was impossible to tell where the snow was deepening or shallowing, making for very tough uphill progress! Here, we abandoned our march!
In a rare slight clearance I managed to catch one drift in more detail....
...before we ploughed our way back down and set off back to the valley. This was taken on the way back, looking up into the foggy snowfields.
Sunday saw overnight heavy rain clearing to a beautiful day, so alone this time I decided to give the pass another go. A substantial thaw was now underway and the road was passable to 4x4s with decent ground-clearance....
Inland, the scene was very wintry. Central and Southern Wales bore the brunt of this event...
Some major drifts over a metre in depth still remained, pock-marked where they had been melted from the surface downwards and with the sound of meltwater gurgling beneath them...
This one must have been huge originally! Now just 1.5m in depth.
I liked this pattern of furrows made into the remaining drifts on a nearby hillside, presumably by heavy rainfall running-off. So that, then, appears to have been that for winter 2006-2007. Disappointing for cold weather-fans, but when the snows did come, everyone made the most of them! I'm sure we'll see cold winters again, but for now my firewood-stash seems to grow year-on-year to an alarming extent!
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