Very cold night across much of Europe with an anticyclone of over 1050 hPa across Norway. I notice that overnight temperatures fell to a minimum of -32.5°C (-26.5°F) at Drevsjo in central Norway (WMO 01393) (fig 2).
Unsurprisingly, it was also where the highest MSLP pressure was to be found (fig 3). It would be interesting to see the adjustment tables they use to calculate a QNH pressure from the station QFE for a station that’s already 674 M above sea level.
Meanwhile back in good old blighty, Dunkeswell in Devon did rather well to get down to -5.0°C overnight (fig 3), to make it the fourth coldest low-level station in WMO block #03 (fig 4).
Apologies for the weird title of this blog – it just came to me after listening to American Pie on Spotify!
You can tell just by looking at these sea surface temperature charts since 2015 (figs 1-3) that there’s been considerable warming going on in the last two years. The cold anomaly that was a feature for so long in the central Atlantic is now less intense and much smaller a feature than it once was. It’s shifted and has been squeezed further northeast towards the southeast coast of Greenland and the opening of the Labrador Sea. The other notable difference that’s appeared in recent months is the area of warmer SST anomalies ~1500 km to the west of Portugal (42N 17W), a warm blob if you like.
The warm blob and this coming winter
To be honest I don’t have a clue if this warm blob will have the slightest effect on the weather in the British Isles this coming Winter. It would be nice to think that the area of warmer SST might weaken the Azores high in some way, and that might increase cyclonic development in the southeast Atlantic which will run northeastward towards Biscay – who knows. Looking at the actual MSLP anomalies (fig 3) for so far this month, it’s true that the Azores high is displaced further to the northeast, but all this has done has been to increase the strength of the zonal flow across the Atlantic. In fact everything seems to be enhanced in some way, if you look at the underlying anomalies (red dashed line) both highs and lows have been more intense in the first three weeks of November 2017 than usual.
I’ve just downloaded the latest NCEP reanalysis data and run the stats for MSLP across the world, and you can see why the first half of October 2017 has been so mobile, and relatively mild over the British Isles, by studying the latest mean pressure and anomaly chart for the North Atlantic (fig 1). The MSLP anomalies across the North of Scotland are around 6 hPa below the 1948-2014 long-term average, whilst across southern England they are 1 hPa above.
In contrast a belt of higher than average pressure extends right from the western Atlantic, across America and into the central Pacific, and highlights the reasons behind the recent devastating forest fires across northern California (fig 2).
So why have temperatures in the first half of September 2017 been so cool across most of northwest Europe? One glance at the mean pressure chart for the first 16 days will give you the answer (fig 1). The Azores high has been +5 hPa stronger than average, and the Icelandic low has become elongated eastward, with pressure 10 hPa lower than average across the northern Isles. This has resulted in a strong west northwesterly flow across the central Atlantic across much of central Europe. I’m sure that the fast-moving ribbon of air which some people call the jet stream has something to do with it, I find that it usually does.
Here are the fine details of how the circulation has been behaving over the last couple of years with the help of some values from the objective LWT analysis (fig 2). I’ve highlighted September to show how strong and persistent the combined SW-W-NW theme has been this month.
It’s quite noticeable, that from the from the third week in July, maximum temperatures in the CET series have generally been rather flat and slightly below average (fig 3). If you look at the spells bar chart (the fifth chart down) there have been few if any prolonged warm or cold spells longer than 3 days or more with anomalies 2°C either above or below the long-term average, compared with previous summers.
Looking over a much larger area with the 12 UTC mean temperature anomalies for the first 16 days of September, you can see that the increased westerly flow has resulted in a large -3°C temperature anomalies across central Europe, and even higher +5°C warm anomalies over northeast Turkey (fig 4).
What about the coming Autumn and Winter?
What does all this portend for the coming Autumn and Winter? I have got absolutely no idea. But if this kind of strong anomalous west or northwesterly flow continue like it’s been doing, I would guess that it looks likely to be cold, windy and rather wintry at times.
I don’t know if the 937.6 hpa read at Stornoway at 0020 UTC on the 20th of December 1982, was indeed the lowest minimum pressure of the 20th century recorded in the UK, but it was certainly extremely low. This is the midnight chart that I have reassembled from the old SYNOP reports (fig 1).
Burt S.D, (1982) New UK 20th Century Low Pressure extreme; Weather 38(7) pp. 209-213
The pressure has fallen surreptitiously and steadily over the last few days here in the southwest, from around 1028 to 992 hPa without most of the population probably even noticing, unless of course they still have a hall barometer that they tap before going out to work, but that’s my very old-fashioned 1950’s suburban view of the world, that’s all but gone in these days of smartphones and tablets.
That fall of pressure and the recent dry spell, got me thinking about the obvious close correlation that exists between air pressure and precipitation, so I thought that I would overly the pressure and precipitation in a single graph, and hey presto a new graph, which I call a baro-hyetograph for want of a better word, is born. The top baro-hyetograph is for the period since the start of the year for Exeter airport, and shows the recent dry spells and anticyclonic periods very well (fig 1). The second baro-hyetograph is for Heathrow airport and paints a very similar picture (fig 2).
You may wonder why I can produce a fairly accurate barograph for the last seven months for some obscure farm in the heart of rural Shropshire. The reason why I can is that the six-hour reanalysis MSLP data that I download from the NOAA 20th century reanalysis project is based on a 2.5° x 2.5° grid, and Stockings farm is little more than a few hundred yards from latitude 52.5° north and longitude 2.5° west (fig 1).
The barograph (fig 2) shows the anticyclonic nature of last Autumn and Winter, the pink vertical bars are the five named storms that occurred during that period. I thought that I’d made a mistake when checking the results around Christmas, thinking that the anomalies were positive and much too high, but then I realised that the storm force winds from storm Barbara and Conor were caused as much by the reluctance of pressure to give way further south, as it was by the low pressure from the vigorous extratropical cyclones crossing the Norwegian sea further north. This did not happen with storm Angus, Doris or Ewan.
I’ve just been looking at the mean pressure chart for April 2017, and it was certainly an anticyclonic month with a large +12 hPa anomaly to the west of Ireland which produced a sizeable 1025 hPa anticyclone at 50°N 12°W, and a northwesterly flow across the British Isles. This made April 2017 a very dry and rather mild month across the British Isles, which although cloudy in the far west, was rather sunnier than average elsewhere. April 2017 does look to have a fairly strong resemblance to the April’s of 1967, 1980, 1982, 1995, 2015 and 1997.
Q: Just why has it been so dry and sunny and dry this April?
A: Because for the first 18 days of the month there has been a large (+11 hPa) positive MSLP anomaly sat just to the west of Ireland (fig 1). As I reported earlier this month (never thinking that the first half of April would turn out as anticyclonic as it has), 1938 was the most anticyclonic in records that started in 1871 (fig 2). The two April’s are indeed very similar, but the anomaly chart for 1938 was for the entire month, and not just the first 18 days, and were larger and even more pronounced. That’s not to say that the second half of April 2017 won’t continue to be just as anticyclonic as was the first.
I’ve just put quite a lot of programming effort into the program that I use to download, parse and visualise reanalysis MSLP data from NOAA, so hopefully I’ve got things right. The LTA that I have used to calculate the anomalies for years 2012 or earlier is for the whole of the 20th Century i.e 1901-2000. For the years after 2012 the LTA is for the 66 year period 1948-2013. This is because the older reanalysis data uses a 2 x 2° grid, whilst the data after 2012 is from the 20th Century reanalysis on a 2.5 x 2.5° grid.
There’s no doubt about it April 1938 was quite an extraordinary month across the British Isles. Not only was it the most anticyclonic April on record, it was also the most anticyclonic* of any month in the objective LWT series that started in 1871 (fig 4). Mean anomalies for the month were in excess of +16 hPa above the 1918-1947 long-term average across northwest Ireland (fig 2 & 3), and according to the MWR for the month :
Mean pressure markedly exceeded the average throughout the British Isles, the excess at 7h. ranging from 10.6 mb. at Lerwick to 16.7 mb. at Malin Head. The mean pressure over Scotland as a whole was the highest recorded in the month of April for at least 80 years. At Oxford the mean pressure was the highest for April since 1881 and at Southport the mean pressure was the highest in April since record were first taken in 1871.
I’m so pleased that the anomalies I generated for the month from the NCEP reanalysis data match the anomalies reported in the April 1938 MWR. NOAA doesn’t make things easy with their 6 hourly MSLP reanalysis data which is on a 2.5° x 2.5° grid back to 1948, but before then (from the 20th Century Reanalysis project) is on a finer 2° x 2° grid. This makes the file sizes much larger to download (~35 mb), and required changes to the code to handle both grid sizes, it also explains the strange LTA period of 1918-1947 that I’ve used in the anomaly chart (fig 3).
*I calculate a simple anticyclonicity index for the month by scoring the LWT for each day. Pure anticyclonic scores 1, while a hybrid anticyclonic type scores 0.5, add them and calculate a percentage of the maximum possible, and hey presto you have a simple anticyclonic index. You could of course have used the mean daily vorticity for the month that the objective LWT data series also produces.
I’m slightly concerned about the number of pure ‘A’ types in the objective LWT series from the CRU (fig 4). A few of the days look like they may have been better classified as more of a hybrid anticyclonic type rather than a pure anticyclonic type, take the 30th for example. Should that be an AE or ANE type perhaps rather than pure anticyclonic? Of course it’s impossible to be definitive about this though, because the objective LWT is derived from 12 UTC reanalysis data and the Wetterzentrale charts are generated using 00 UTC data. Going back to the original ‘subjective’ LWT data that Hubert Lamb developed, and who was the final arbiter on, and that scores an anticyclonicity index of 68.3. Who knows perhaps Lamb was being a bit hard on April 1938, especially in the first week. I will investigate this a little more.
It goes without saying that such an anticyclonic month was also very dry across the whole of the British Isles. Using the UK gridded rainfall data it was the driest April in the whole series that started in 1910 (fig 5).
Using the EWP series it was the driest April since at least 1766 when the series started (fig 6).
Sunshine was well above average in all western regions, but closer to average in eastern areas and the North of Scotland. The MWR says that Valentia Observatory had a total of 262 hours, the largest total for April in a record which started in 1880. It goes on to say that at Mallaranny, in County Mayo (notice how we didn’t exclude the Irish Republic back in 1938), they recorded 129.6 hours from the 8th-18th inclusive which is a daily mean of 11.8 hours for 11 consecutive days.
Temperatures were also above average in the west, but closer to average in eastern districts. Because I use the very warm 1981-2010 LTA to generate the anomalies for the April 1938 graphic (fig 1), all regions look rather cold, which just goes to show you just how misleading statistics can sometimes be.