Yesterday afternoon the Met Office were promising the coldest night since February 2012 (fig 1) when they said on Twitter:-
“Temps in rural parts of Scotland could fall as low as -14 or -15 °C tonight, which would make it the UK’s coldest night since 11 February 2012, almost 6 years ago! High pressure will bring clear & calm conditions and is dragging cold arctic air from the north”
Unfortunately, the low cloud of yesterday evening didn’t finally clear till towards midnight, neither was it ever perfectly calm, and with only a patchy or thin cover of snow, the air temperature never got down as low as the -14°C that was promised.
In the end after trawling through all climate reports from sites which they like to keep to themselves, they did manage to find a -9.1°C at Dalwhinnie, just down the A9 from Aviemore (fig 2). The model does have problems, the -9°C instead of -15°C is just a symptom, it mishandled the overnight forecast of low cloud, surface wind, and probably the whole pressure field.
For the rest of us poor mortals who have to rely on the kindness of a man in Barcelona and his OGIMET website, here are the coldest spots from the SYNOP reports (fig 3). As you can see the other forecast temperatures outwith the Highlands of Scotland were much more accurate, except for southern areas and across East Anglia, where the strength of the wind prevented an air frost.
How did the BBC do?
The BBC who presumably use the same model data as the Met Office, almost got it almost right with a -10°C minimum (fig 4). I don’t know what they were thinking with a -4°C for the Western Isles and a -3°C for the Northern Isles though.
At one time the Met Office would have been just content in announcing the coldest night for six years after it had happened. But now because of the pressures of social media and to maintain their corporate image, they feel they must announce it before it does. This faux pas is certainly not on the same scale as the barbecue summer forecast was, and probably will go unnoticed by the vast majority of people anyway.
Here’s the forecast 04 UTC temperatures from the BBC (fig 1) in what constitutes the overnight minimum chart from yesterday evening’s news. As you can see the model colour contoured NWP temperatures have been totally ignored, and labels for much warmer temperatures of “towns and cities” overlaid on top. To me this is totally misleading to any viewer, but the BBC weather presenters continually to do this, ignoring the fact that Exeter airport is one of top UK cold spots, and last night it caught them out again (fig 2), with temperatures dipping to -3.5°C. Even sites closer to the sea such as Plymouth and Culdrose saw a slight touch of frost – their first of the Autumn or Winter.
Unusually the Met Office did respond with a very belated yellow warning of ice for the southwest at 0637 UTC (fig 3), no doubt when they heard about some road accident caused by the rain from yesterday’s showers freezing on roads across Devon and Cornwall.
There was really no excuse for the lateness of this warning, especially when the Met Office HQ lies just a mile or so west of the airport. They obviously have to follow a process to cover themselves from any possible repercussions from accidents and injuries caused by icy roads and pavements, the only problem is that it came at least 12 hours too late! Temperatures at the airport were already -2°C at 23 UTC yesterday evening so there really was little excuse (fig 4).
They didn’t do too well in the National forecast either, although Darren Bett did say that “cloud might be a bit more unreliable in central areas and that there might be a touch of frost” there (fig 5). I’m not sure if the -3.6° C at Pershore can be regarded as a “touch of frost” though.
What went wrong?
I think that the Met Office model, especially across the southwest, just kept producing too much in the way of cloud and showers for much of the night (fig 6) which just didn’t materialise, from looking at the weather radar. I would love to grab some forecast NWP evidence from the UKMO mesoscale model to back that up, but of course that’s on a need-to-know basis, and I don’t need to know!
I get the impression that things have been running just a little faster than forecast during the night. The cold front has fairly rattled through western parts (fig 1), followed behind in the fresh to strong westerly flow by a clutch of thundery showers (fig 2). Snow is falling further east from the Peak District northward much as expected in the yellow and amber alerts from the Met Office. No great snow depth reported from the sensors in the AWS network at 09 UTC, although it must be piling up quite nicely over higher ground of the Pennines. A small discrete low pressure centre has formed on the cold front just southwest of Birmingham in the large pressure falls just ahead of it, although this feature is not expected to persist.
A sharp frost in northern areas overnight. In parts of northern Scotland it was particularly severe with Loch Glascarnoch dipping to -12.3°C (fig 3). So last night failed to become the coldest of the year as some forecast it would.
I wonder what the verification score was for last night’s forecast minima temperatures across the UK by the Met Office? The forecast was for a moderate or severe frost (fig 1) which never quite made it. The forecast of a minimum of -10°C at Aviemore for example, ended up being -0.7°C (compare figs 1 & 2). A lot of places may have had a bit of a snow cover, but it was much cloudier in the north than forecast, and the wind never quite died down either to make it a more ideal radiation night (fig 3). Despite having no snow cover, Exeter airport were number two in the coldest spots with -3.7°C when the forecast minimum was -1°C. I wonder just how many times they have to get it wrong before they fine tune their forecast for Exeter Airport and get it right?
I would have liked to have added a graphic of what the BBC weather forecast for last night looked like, but the BBC has not made any news bulletin it broadcast yesterday available on iPlayer.
You might be thinking it’s been quite a snowy end to 2017 wherever you live in the country, but spare a thought for those of us down here in the southwest who so far have yet to see a snowflake. In Cornwall none of the SYNOP stations have reported an air frost so far this Autumn, and even here in mid-Devon although we have had four air frosts, and only one of these was for a temperature lower than -0.2°C.
I’d like to do a similar thing with snow that I do with frost, by checking for the occurrences of snow in the present weather from the hourly SYNOP files. Not only would that mean opening thousands of hourly SYNOP files, but many of the AWS reports either don’t report a present weather, or if they do they’re not that reliable when it comes to the reporting of freezing precipitation. The same thing applies to the reporting of thunderstorms, I have yet to see an AWS report a thunderstorm, even though I know the more sophisticated Vaisala systems are capable of sensing them.
Temperatures fell to -13.0°C overnight at Shawbury (fig 1). Not a real surprise, because it’s a well-known fact that Shawbury plus a clear night and light winds, plus a snow cover equals a very low temperature. It still holds the record for being the coldest place in England, when almost 36 years ago now, the temperature fell to -25.2°C on the 13th of December 1981, when there was a similar depth of snow on the ground. As far as I can see this table of top ten coldest places in the UK from the Met Office blog is still correct (fig 2).
Having said all that last night was not a perfect radiation night at Shawbury as you can see from the observations (fig 3).
The wind never really dropped out, although this didn’t prevent them going into fog at 23 UTC, when the temperature suddenly dropped from -3.0°C to -9.2°C. Fog or freezing fog, as it may well have been, will slow or even halt the fall in temperature depending on its thickness, and if these observations are correct the minimum may have occurred around 05 UTC, because the next hour it rapidly warmed from -12.4°C to -6.9°C, before falling again (fig 3).
Looking back almost 36 years, here’s the 00 UTC chart for the 13th of December 1981 (fig 4). As you can see the minimum at Shawbury that night probably occurred close to midnight. It’s interesting to speculate that if the ridge of high pressure had just hung on for another 6 hours instead of collapsing like it did, the minimum temperature at dawn may have matched the -27.2°C extreme minima recorded in Scotland.
It’s a pity that our nor’easter across the south this morning hasn’t brought more of the weather that our American cousins associate with their nor’easters. Across southern areas it just means a biting cold wind, leaden skies and outbreaks of rain or wet snow. Low Xanthos is now a shrunken shadow of its former self across northern Germany, overnight developments have meant that the new kid on the block is low Yves over Brittany, its centre is much deeper than Xanthos was yesterday, with a minimum central pressure that’s already less than 960 hPa (fig 1).
A hard frost further north across the borders were winds were lighter and skies remained clear (fig 2).
Rather surprisingly, the whole meridional theme of the last couple of days is simply washed away in the next 36 hours or so, as we switch back into zonal mode across the UK.