New UK 20th Century low pressure extreme

Figure 1

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).

References

  • Burt S.D, (1982) New UK 20th Century Low Pressure extreme; Weather 38(7) pp. 209-213

The correlation between precipitation and air pressure

Figure 1

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).

Figure 2

A barograph for Stocking Farm, Shropshire

Figure 1 – Courtesy of Google Earth

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).

Figure 2

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.

April 2017 – Mean pressure

Figure 1 – Data courtesy of NCEP/NOAA Reanalysis (anomalies wrt 1948-2014 long-term average)

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 April been so dry & sunny?

Q: Just why has it been so dry and sunny and dry this April?

Figure 1 – Data courtesy of NCEP/NOAA reanalysis

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.

Figure 2 – Data courtesy of NCEP/NOAA reanalysis

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.

April 1938 – probably the most anticyclonic month on record

Figure 1 – Data and Images courtesy of The Met Office, CRU & Wetterzentrale

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.

(Courtesy of the Met Office © Crown Copyright)

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.

Figure 2 – Data Courtesy of NCEP/NOAA Reanalysis

Figure 3 – Data Courtesy of NCEP/NOAA Reanalysis

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.

Figure 4 – Data courtesy of CRU

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).

Figure 5 – Data courtesy of the Met Office

Using the EWP series it was the driest April since at least 1766 when the series started (fig 6).

Figure 6 – Data courtesy of the Met Office

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.

Mean pressure anomalies so far this February

 

Figure 1

Just a quick look at the latest pressure anomalies across our part of the northern hemisphere so far this month. The intense Scandinavian anticyclone that persisted for much of the month until recent days, has left a massive positive anomaly (+19) over Norway, this combined with a belt of lower anomalies (-8) stretching from the eastern central Atlantic to southeast Greenland (fig 1), have resulted in a mean south or southeasterly flow across western Europe (fig 2) for the first 16 days of February 2017. This has given us a rather mild and relatively dry February so far.

Angus set to run along the channel…

Looking at the latest pressure falls on the 2100 UTC chart, Angus looks set to run along the channel, at least till the early hours. Not a great night to be chugging up or down the English channel in a ship I would have thought, but of course they will be.

synops-2100-utc-on-sat-19-nov-2016

The strongest winds at 2100 were confined to the SW of England and the tip on northwest France.

highest-gust-in-the-last-10-minutes-in-map-area-lat-47-7-n-to-54-2-n-lon-9-5-w-to-11-5-e-2000-utc-on-fri-18-november-2016

Pretty wet at the moment from Cornwall through to Dorset.

rainfall-intensity-2125-sat-19-november-2016

Hear are the 0900-2100 UTC estimated rainfall accumulations with St Mary on Scilly the wettest place. To be fair, as soon as the showers gave out down here in Devon today a veil of cirrostratus was thickening up so quickly that the dry interlude didn’t last anytime at all.

estimated-rainfall-accumulations-0900-2135-utc-on-sat-19-november-2016

Lowest pressure in August

I got a little fed up with hearing every BBC weather presenter churn out the phrase “this are of low pressure is very unusual for the time of year” and decided to investigate the truth of the matter. Of course, it all depends on what you term “unusual”, I know personally that as I get older how easy it is to forget past events, and that the low now tracking into Ireland might not be that unusual at all, and that’s when gridded MSLP data and weather statistics drawn from that data come into their own.

I’ve used the NCEP reanalysis six hourly MSLP data back to 1948 to find the extreme mean sea level pressure [MSLP] range for August. I decided to look at a grid size that extended from 10W to 2.5E and from 50N to 60N a total of 30 grid points in total. I could go back farther but the grid size changes to a 2°x 2° grid from the 2.5° x 2.5° later grid and I need to write the code! There is one problem in the analysis in that it’s looking for a minimum pressure in the grid around the British Isles which won’t necessarily find a discrete low pressure circulation, but until I write an algorithm to do that with this will have to do.

Extreme MSLP August - British Isles [50N-60N 10W-2.5E] 1948 - 2016

Extreme MSLP August – British Isles [50N-60N 10W-2.5E] 1948 – 2016

As you can see the lowest MSLP in August in the vicinity of the British Isles was 972.3 hPa on the 24th August 2005, the one that’s affecting us at the moment is off the west coast of Ireland with a central MSLP of around 980 hPa and filling. Admittedly the lowest pressure then was just to the northwest of Scotland.

Extremw MSLP in August across the British Isles [1948-2016]

Extreme MSLP in August across the British Isles [1948-2016]

Here are just few of examples from the list of extreme low pressure systems across the British Isles that you may remember from the past.

Synops for Tue, 26 Aug 1986 at 0600 UTC

Synops for Tue, 26 Aug 1986 at 0600 UTC

Hard to say but the centre of this low over northern Scotland looks around 970 hPa on 30 August 1992.

Synops for Sun, 30 Aug 1992 at 1800 UTC

Synops for Sun, 30 Aug 1992 at 1800 UTC

This is the infamous low ‘Y’ that wreaked havoc in the Fastnet race in 1979.

Synops for Tue, 14 Aug 1979 at 1200 UTC

Synops for Tue, 14 Aug 1979 at 1200 UTC