A slightly premature press release from the Met Office regarding the latest cold spell and today’s snow (fig 1). Maybe it would have been better waiting for tomorrow to release it, in light of the fact that it’s still snowing across large parts of Devon and Cornwall (fig 2).
It would be fascinating to see a comparison between whatever mesoscale model MeteoGroup are currently using to produce the graphics with for their BBC contract, and the corresponding NWP output from the Met Office fine mesh model or whatever it’s called these days.
We live to the north of Exeter and it’s been snowing here since around 9 am this morning, it was moderate snow for two or three hours but now it’s generally slight. The top image is from last nights forecast from the BBC in Plymouth (fig 1), which has you can see has slightly mishandled the snow area that’s been affecting central and eastern Devon, and taken it westward much too quickly if you compare it with the weather radar (fig 2).
That forecast on the BBC was broadcast at 1910 UTC last night, and I’m guessing that they we are looking at the 12 UTC run of the ECMWF model (because as far as I know it’s only run at 00 and 12 UTC), and so they must have been using T+26 data (14 UTC) if there is such a time frame – if not then they must interpolate it in some way from the T+24 and the T+27 data.
I suppose it’s quite acceptable for the general public, and most will not have spotted that their forecast cleared away the snow far too quickly. Did it affect anyone? Well it may have, especially if you were a motorist trying to use the A380 near Exeter earlier this afternoon, because they had to close the road due to heavy snow, but the again I suppose that’s what amber warnings are for.
We’ve had around 5 cm of fresh snow this morning here in mid-Devon to add to the 5 cm or so that we got overnight, in a spell of moderate snow that’s now moving away W’NW across Devon. There’s an interesting convergence line that’s pluming a line of showers northwest across the Channel from the Cherbourg peninsula towards Devon. At the moment that looks like it might keep the snow falling in this part of the world for the rest of the afternoon, as the latest models suggest it will. I saw this type of convergence happen in an easterly from Cherbourg that affected Guernsey in the last cold spell, but this is a new direction for me.
Automatic weather stations now make up nearly all the network of observing sites in the UK, and they all have an inerrant problem when it comes to reporting precipitation totals in very cold weather, and that is they can’t!
Precipitation from AWS
Have a look at the 12 hourly totals [18-06] from 06 UTC this morning from across the country (fig 2) and you’ll see that almost all stations are reporting no more than a ‘trace’ of precipitation. These accumulations are obviously incorrect because most of the precipitation that’s fallen in the form of snow will still be stuck, frozen in the funnel of the gauge. Some AWS sites do have snow depth sensors, but these are not the answer, and can be easily fooled as we have seen this winter, by drifting when the wind is strong enough to lift any snow that has fallen.
Here’s are my estimates from the radar network of the precipitation that has fallen since 06 UTC on Saturday morning (fig 3). If this is anything to go by then, the deepest of the snow looks to have fallen across Norfolk and Suffolk in some kind of convergence zone, with precipitation totals as high as 16-24 mm. I estimate that there was around 6.9 mm from around High Wycombe, which I realise is not enough to account for the 27 cm of snow there at the moment. In my defence I offer the “spoking” that’s evident from the Chenies radar. Chenies lies not so many miles E’NE of High Wycombe, and I did speculate last year that the “spoking” that occurs there at times is caused by the radar being blocked by some of the large trees that surround the site.
I don’t know enough about why this is such a problem in the 21st century, you would have thought that it would be easy enough to detect when temperatures are close or below freezing and turn on heating elements inside the gauge to melt any snow that it catches. For all I know this might be already happening, because the SYNOP format is not how AWS report their readings these days, and much more detailed one minute or even more frequent observations from all the sensors are being taken and transmitted back to Exeter, which may include the water equivalent of any snow that has fallen, who knows…
Heavy snow at High Wycombe caused the Met Office to issue a very early (or should that be very late?) amber snow alert for London and the southeast of England at 0407 UTC this Sunday morning. The 27 cm of snow that fell there must have caused severe problems especially on Wycombe hill I imagine. It’s not the first time that High Wycombe has caught the Met Office out with snow this winter, it did it on the 11th of December 2017, and again on the 21st of January 2018.
The temperature at Gravesend yesterday was 13.6°C at 12 UTC, and today at the same time it was just 0.1°C, which makes it 13.5°C colder. To be honest there are a good number of stations which are 10°C or colder than they were 24 hours ago, and I’ve outlined them on the chart (fig 1). You can of course get large variations in temperature in the late winter and early spring, but today’s must take some beating, and looking at the latest NWP it looks like there may be more large swings in temperature in the coming week.
I’ve been monitoring the wind speeds from the Cairngorm SIESAWS this week where it’s been extremely windy. As you can see from the plotted observations for the last few days (fig 1), winds peaked at 15 UTC on the 15th of March, with a mean speed of 86 knots (99 mph) and gusts to 110 knots (126 mph), although mean speeds have never fallen below Beaufort force 10 in all that time. I’m beginning to think that they might have sensor problems up there at the moment, I know for the last few months reported wind speeds have been too low, but now they look like they could be too high.
There is a way to check if the wind speeds are too high though, because there is a second AWS on Cairngorm run by the Heriot-Watt Physics department (fig 2).
Even their sensors (the one’s that pop-up out of a can twice an hour) are also looking a bit suspect. The wind direction is definitely wrong, possibly because of a massive build up of rime in these condition. Wind speeds look too low at first, then went missing, and now look similar to the ones being reported by the Met Office SIESAWS.
Winds as severe as this are perfectly possible of course on Cairngorm, which makes it doubly difficult in deciding if they are right or wrong.