I know I shouldn’t have tempted fate going on as I did about how warm it had been today. I wasn’t paying attention to the visible satellite image sequence this afternoon, but finally noticed the increasing cirrus and cumulus bubbling up over northwest Devon (fig 2). From the observations there’s certainly evidence of some kind of trough pushing into western areas of Wales and the southwest at 15 UTC, with a band of thick cirrus strung out along it. The increased humidity must have been enough to cause the convection, and if you look closer you’ll see a wind veer and increase in dewpoint, behind some small falls of pressure over East Wales (fig 1). It could all simply be down to a southward extension to the first of two occlusions that the Met Office are showing in their 06 UTC analysis (fig 3), but it’s certainly put paid to what had been a lovely afternoon here in south Devon till then.
The cold front that brought all the rain to parts of England earlier this week finally managed to penetrate the warm air over central Europe and bring thunderstorms and a sharp drop of temperature with it. The interesting thing about the 15 UTC visible satellite image (fig 1) is just how sharp an edge of the cloud the cold front had, it was either blues skies or frontal cloud, and in the image I can see at least three large embedded CB’s. The demarcation in cloud reminded me of what occurred earlier this week over southeast England. Here’s the Met Office analysis for yesterday at 12 UTC (fig 2), and as you can see there is not just one cold front but two.
Here’s the plotted chart for 15 UTC yesterday for the area (fig 3) and notice that temperatures at places ahead of the cold front in eastern Germany where as high as 30°C at that time.
Here’s the observation sequence from Regensburg, Bavaria for the last 24 hours, in the space of just a few hours the temperature fell from over 30°C to around 11°C (fig 4) during the late afternoon.
Judging by the 06-18 UTC rainfall totals (fig 5) the convective activity from the CB’s didn’t produce the heaviest rainfall, that occurred further west in the frontal cloud, even though there was a lot of SFERIC activity (fig 6).
I was just looking at last night’s ascent from Watnall to gauge the size of the inversion that’s trapping the SC sheet beneath it across the eastern regions of the UK. The inversion is quite sharp, at 2,615 feet the temperature is just above freezing 0.3°C, but by the time you ascend to 2,841 feet it’s shot up to +6.8°C and the humidity as fallen to just 4% (fig 1). If the Pennines where a little bit higher they would stand clear of this particular inversion, and it would have been a lovely scene last night as the radiosonde balloon pushed through the inversion, with a lovely moonlit sea of SC beneath it, some patches of the SC sheet bright from the lights shining up from the city lights beneath it. Anyway enough of waxing lyrical, here are the results when the sun came up from a little higher even than the radiosonde balloon can reach (fig 2).
The SC sheet can’t be particularly thick, maybe no more than a thousand feet or less, so it may fragment as it tries to push further southwestward (fig 3), helped by the rise in temperature in western areas (I notice that we are already up to 13°C in Devon), we shall see.
An interesting number of examples of just how shelter from mountains can make the difference between a cloudy day and one with sparkling spring sunshine in today’s visible satellite sequence. There are clear skies evident in the air coming south southwestward from over the mountains of Norway and into the northern North Sea. There’s also shelter as the air is warmed and dried on the southern slope of the Grampians over Perthshire and Fife. The mountain and fells of the northern Lake district are doing enough to stop the passage of the SC sheet extending southwards from the Scottish Borders, and the Pennines are just high enough, and the gradient is just backed enough to keep the same SC sheet from invading Lancashire, as it’s done east of the Pennines in Yorkshire.
Most years in spring you always seem to find a spell of anticyclonic easterlies over Scotland, and the Cuillins in Skye start calling to any intrepid Munro bagger out there. This year has been no exception with the weather since the 1st of May being absolutely glorious in the Northwest Highlands for the Munroist, as these visible satellite images reveal (fig 2). It’s been well over twenty years since I’ve been on Skye but one day I will return, if I’m not too old, and try to finish what I started, that’s if my wife will let me.
I could be wrong, but as moister air pushed up from the south over Northern England late this afternoon, a patch of orographic cirrus seems to explode out of the vicinity of Cross Fell and spread quickly away west-southwestward.
I say cirrus, because all I can see in the way of cloud is the 5 oktas at 22,000 feet reported in the 17 UTC by the LCBR at the Keswick AWS, whilst all of the other stations are reporting no cloud at all (fig 2).
There must be some Helm wind blowing down from the fells and into the Eden Valley as well at the moment, because there’s been a severe gale blowing all day on Great Dun Fell (fig 3).
An interesting vortex spins across a SC sheet lying over the East Midlands and Humberside in this morning’s visible satellite image sequence. I’m not certain what caused it, or if it’s an example of a single von Kármán vortex in the SC sheet, who knows. The animation would have been even better if we could have free access to 5 minute rapid scan satellite imagery, now that we can fly a spacecraft that’s over 900 million miles away remotely through the rings of Saturn, you wouldn’t have thought that 5 minute visible satellite images of the planet on which we all live would have been that difficult.
The earlier low that’s now scooted off down the east coast of Scotland may not have been a proper polar low, but as I suspected might happen, the one that’s just south of Sule Skerry at 1600 UTC is. Figure 1 is not from a kindergarten by the way, it’s my attempt to draw up the 1600 UTC chart using my Wacom tablet, so excuse the state of the isolines, it’s very difficult to get any kind of smoothness or get your hand inside a radius to draw a curved isobar, but it’s the best I can do under the circumstances!
The circulation around the feature is clearly seen in this 1545 UTC visible satellite image (fig 2) with curl of cloud marking the associated trough that lies to the west of the polar low and stretches north-northwest from Stornoway.
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 convective infill that has affected inland parts of southwest England, has spoilt what started off as rather a lovely day. Having said that it appears that a similar thing has happened but on a much larger scale over Ireland this afternoon, as the whole country seems to be under a sheet of CUSC.