A dry month apart from the last day?

Figure 1 – SYNOP data courtesy of OGIMET

Some of these plotted totals (fig 1) may be misleading due to missing SYNOP reports from some stations such as Edinburgh, but most of the others are more complete, but until the Met Office finally release their stranglehold on the daily NCM climate data, there is nothing I can do about it. Putting the Met Office monopoly on climate data in this country to one side (what ever happened to the free data campaign?), this may well end up being one of the driest April across some regions of the UK since 1938 (fig 2).

Figure 2 – Data courtesy of the Met Office

Having said that, Sunday is looking like a wet old day in the southwest, and may (excuse the slight pun) help redress the balance there (fig 3). I think a spell of heavy rainfall after a drought on dry cracked earth (like it is in our garden at the moment) can bring its own kind of problems with run off.

Figure 3 – Courtesy of OGIMET

Dry spell continues

The dry theme that started in late March continues into the last week of April, especially over central southern parts of England (where some stations are still in a state of partial drought), as this graph of precipitation illustrates from Benson in Oxfordshire since the start of this year (fig 1).

Figure 1

A big part of why it’s been so dry is the anticyclonic weather of the past month, as you can see from the Benson barograph (fig 2).

Figure 2

A state of type is now well under way, with cold air set to flood south across the country in a N or AN, but according to the latest NWP guidance that meridional type looks like it will be a relatively short-lived affair, before things become more mobile from Sunday, putting an end to the dry spell even across southern counties.

The driest Aprils since 1910 by region

I’ve spent a bit of time today creating an infographic of driest April’s. I’ve used the free data set maintained by the Met Office, which started in 1910 and is produced from gridded data for ten regions across the country. Hopefully one day the Met Office will extend this series to cover all the rainfall data that they inherited from the British Rainfall Organization in 1919 and just sat on for the last 98 years. I knew about the very anticyclonic and dry April of 1938 from a previous article that I had written earlier this month, but hadn’t realised that it was only driest in three out of the ten regions, even though it was the driest April in the EWP monthly series that started in 1766. The 1.0 mm in East Anglia in 2007 tops the list of driest region by region, which is something else I missed. I’ve borrowed the regional map from the Met Office, I’m sure that they won’t mind, let me know if you spot any issues.

Data courtesy of the Met Office

Later….

What’s slightly puzzling about these figures for April is why the UK value is the highest April value for all regions at 14.1 mm, when it’s made up of the value for Wales 8.8 mm, and the value for England 6.7 mm. Both values are for the same year 1938, I would have thought that the combined UK value should be some kind of mean of the two, but obviously not. It must have something to do with the gridding I suppose.

Latest April 2017 rainfall totals

You’re all probably all getting fed up to the back teeth hearing me prattle on about what a dry first half of April it’s been, so I’ll let the graphic do the talking.

Accumulations from available SYNOP – Data courtesy of OGIMET

Saturday 15 April 2017 – Dry spell hangs on despite overnight rain

 

Figure 1 – Data courtesy of the Met Office

The dry spell and absolute, or ‘meteorological’ drought as it’s known these days is just hanging on despite some overnight rain across the country (fig 1). St James Park in London last had >0.1 mm of rain 17 days ago, but as always the rainfall data in the SYNOP data can be omitted or the observation missing at times. Linton-on-Ouse in the Vale of York got away without any overnight rain, as did a few other stations including Exeter as far as I can tell (fig 2). Please post a comment if you know any different.

Figure 2

Figure 3

As far as I know, St James Park only reports 24 hour rainfall totals at 06 UTC each day, and it’s rarely missed so far this year. The accumulation for 2017 is currently standing at 135.2 mm (5.32″) for the last three and a half months. Looking more closely the last real rain was 7.2 mm on the 23rd of March, so I make it, barring an instrument malfunction, they’ve had 0.2 mm of rain in the last 22 days (fig 4). I’ve got a strong feeling that they won’t get away with it tomorrow, although Exeter might just get away with a trace.

Figure 4

 

Dry start to April continues…

Here are the number of consecutive dry days (<0.2 mm) across the southern half of the British Isles I’ve compiled from SYNOP reports. This is my best shot, some sites have missing observations, some have missing rainfall reports, and I have noticed that the AWS the Met Office employ are very sensitive and have a preponderance to push out a 0.2 mm report, when a trace or 0.1 mm might have been more appropriate. I reckon that’s the case at many stations, take Exeter Airport for example, some very good radiation nights in the last week could easily knock up 0.2 mm of dew in the gauge, and as this count is for days with less than 0.2 mm each day, so the total gets zeroed, and we have a count of 3 days instead of 8 or more.

Rainfall so far in 2017

Figure 1

At Wattisham in Suffolk (WMO #03590), I make it that there has only been 123 millimetres of precipitation this year (fig 1), that’s not a great deal for any three months period, but for the back-end of Winter in what are usually some of the wetter months that’s not very much at all. It’s been a little wetter in our part of Devon, with Exeter clocking up 186.4 mm this year (fig 2).

Figure 2

Despite the dry start to 2017, reservoir levels across England seem to be holding up very well, and some are at maximum at the end of March, according to the Environment Agency (fig 3), so there’s no need to panic quite yet Captain Mainwaring.

Figure 3 – Courtesy of the Environment Agency

The dry spell of early April 2017

The above plotted chart (fig 1) is of rainfall totals for stations since the 28th of March 2017. They show how dry it’s been recently in all areas, but particularly in eastern counties. The figures are as close as I can estimate them from the SYNOP data I download from OGIMET so don’t get upset if I get one or two of them wrong. I would prefer to use the 06-06 UTC totals because then you don’t have to do any maths or guess-work, but since the good old Met Office refuses to make NCM climate data available for the UK, this will just have to suffice. I estimate an 00-00 UTC total from the 6 and 12 hourly totals in the observations. I do this because not all countries, in particular the Irish publish their 06-06 totals. This is further complicated by missing rainfall reports from observations, or just plain missing observations. Why do government agencies such as the Met Office deliberately hide daily climate data that they collect on our behalf?

I was going to mention the word drought in the title, or rather meteorological drought, which as far as I know is still a period of 15 consecutive days with less than 0.2 mm of precipitation, but that can get rather complicated so I just called it a dry spell! Nevertheless, we are very close to meteorological drought conditions, in whats proved to be an exceptional start to April, and the latest NWP does seem to suggest that pressure will remain fairly high across the south, and especially the southwest, with anticyclones never that far away. Who knows, it may rival the April of 1938 but it will have to go some to beat that month, perhaps this article has already jinxed that possibility.

If anyone reading this in Scotland feels left out I can tag on a chart for the northern half of the UK without too much problem, just let me know.

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.

20 March 2017 – Line convection

Figure 1

I can vouch for the ferocity of rainfall in the last hour in Tiverton, it stands out clearly in the weather radar (fig 1) as line convection associated with the cold front that’s crossing central England at the moment. It almost occurred at exactly the same time as the Vernal Equinox, and who knows maybe a portent of things to come in this year’s weather – welcome to Spring!

Figure 2