The drought in the southeast

Figure 1 – Consecutive dry days (13 April 2017)

The overnight rain in some places put an end to any possible drought across the country, but as far as I can see both Wattisham in Suffolk, and St James Park in London have now gone 15 consecutive days with less than 0.2 mm of rain in any day, so are technically in drought, and when I say drought, that’s the old-fashioned meteorological type. It might not last very long though, because the GFS is forecasting rain from a cold front moving south during Friday, which might scupper the drought before it even gets going. Looking beyond Friday though, the same model predicts anticyclonic conditions returning and persisting till at least the 21st, so we could be in for a very dry April in some parts.

Yesterday’s rain fails to halt the drought

The Met Office have just issued their usual mid month report on the weather and say this about rainfall in the first half of August 2016:

“After the first day of the month, when an area of low pressure gave some large rainfall totals in the south, England, Wales and Northern Ireland have been much drier than average. By this stage in the month you would normally expect around 52% of the total month’s average to have fallen. However, many parts have seen less than 20%, with Anglesey only recording 5.6mm which is just 7% of the total month average”

Courtesy of the Met Office

Courtesy of the Met Office

This is a perfect example of how statistics in an anomaly map don’t always give you the full picture. I am not saying that the anomalies are wrong, they aren’t, but the anomalies are masking the fact that as in July, August at a good many stations across the south of England was a drought. So at first glance you would not know that the south although wetter than parts of Eastern England, had suffered a drought. There are a number of ways a meteorological drought can be defined, but I like the one that says 15 consecutive days with less than 0.2 mm of precipitation in any day. Even yesterday’s rainfall has failed to halt the drought in quite a number of locations across the south of England as you can see from the following map of ‘days of drought’ gleaned from the SYNOP observations.

rainBelow is a tabulated list of stations and the number of days of drought, and as you can, there are at least seven stations that have gone 15 days with less than 0.2 mm in any day, yesterday there were many more with 14 consecutive dry days.  Tomorrow’s rain will put paid to this very short-lived drought I’ve no doubt.

Estimated Accumulated Precipitation 00 UTC on 1 August 2016 - 12 UTC on 18 August 2016 WMO Block 03

Estimated Accumulated Precipitation 00 UTC on 1 August 2016 – 12 UTC on 18 August 2016 WMO Block 03

Please note that because the Irish stations don’t report a 24 hour total I calculate the above rainfall stats using a 00-00 UTC daily total, so the number of drought days at Bournemouth of 15 is based on this, but if you use the 06-06 (24 hour) total the drought ended yesterday. Rainfall statistics gleaned from SYNOPs are not at all straightforward!

The recent dry spell across southern England

I thought that I would revisit my application that processes rainfall statistics from SYNOP observations and look at the recent dry spell that as affected the south of England for so much of the summer. I’ve just added some extra code to calculate the total number of dry days and the current number of days since any meaningful rain. If you start looking into what constitutes a ‘meteorological’ drought on the internet I’ll warn you it’s very confused! There is a good Wikipedia article on the subject and I’ve decided to use the one in their that specifies 15 days or more with less than 0.2 mm of rainfall in a day. The programs flexible so I can set any threshold so let me know if you have a better definition.

I can’t remember a period when so many fronts have delivered so little rainfall. I know there has been no shortage of fronts, it seems to have been a summer like a lot of others, with continuous switching between tropical maritime and polar maritime air throughout, even if the number of fronts in the Met Office analysis does seems a little bit overdone at times. As you can see from the table below we are fairly close to another drought in a good part of central southern England, I say another, because there was probably another drought in places that occurred during July. The second column from the right shows how many dry days (<0.2mm) there have been in the last month (31 days) which also shows how seriously dry it’s been, with some places such as Odiham having 26 out of 31 dry days. These values are at best an estimate, because as good as OGIMET is there are always missing observations or observations without a rainfall group. This could be easily fixed by the powers that be, and then we wouldn’t be totally reliant on other countries providing the latest daily climate information for the UK, but I won’t go into that (again)!

Number of days since rain - 14 August 2016

Number of days since rain – 14 August 2016

Number of Dry Days

Current drought days count

Number of dry days in last month

Number of dry days in last month

Driest Places Estimated Accumulated Precipitation 00 UTC on 15 July 2016 - 12 UTC on 14 August 2016 WMO Block 03

Driest Places Estimated Accumulated Precipitation 00 UTC on 15 July 2016 – 12 UTC on 14 August 2016 WMO Block 03