Today’s precipitation totals and risk of flooding in Devon

Figure 1

Apart from the bright banding from the Dean Hill radar, the two interesting  areas of precipitation accumulations for today are one northwest of London which produced a lot of snow today at High Wycombe, and the other area across central Devon and the Blackdown hills on the Devon-Somerset border (fig 1). The estimates from the weather radar for that area are in excess of 50 mm from 06 UTC this morning. I imagine that most of this will have been rain, but it has been snowing for most of the afternoon a little further northeast at Bristol Airport, so you never know. The only thing that I can attribute the band of higher totals to is the occlusion that aligned itself across central Devon for most of today. A severe flood alerts has been issued by the Environment Agency for the rivers Clyst and the Culm in south Devon (fig 2), that’s according to the FloodAlerts.com web application.

Figure 2 – Courtesy of floodalerts.com

BBC: Yesterday’s flooding in Ceredigion

Figure 1 – Courtesy of BBC News

I was just about to start the twice yearly cutting of our hawthorn hedge yesterday when I saw the line of heavy rain or showers from West Wales extending south to Cornwall. It was about 11 am and I looked at the estimates which I thought then merited a yellow warning for heavy rain and duly posted this blog. Three hours later after doing the hedge I half expected a belated warning to have been issued, but no the totals had increased although the feature whatever it was had become fragmented and less intense. What I’m driving at with this story is if I can notice this happening and cut the hedge and generate the accumulations and write a blog surely someone who is employed to do this as is job at the Met Office would have been monitoring the situation, and could, and very probably should have issued a warning? What kind of impact do these events need to have before a warning is raised? Otherwise what is the point of them at all?

Figure 2 – Courtesy of the Met office
Figure 3 – Courtesy of the Met office
Figure 4 – Courtesy of the Met office

The analysis hasn’t got any less exotic over the last 24 hours as you can see (figs 2-4).  I see little way that you could justify the contorted occlusion in the midnight analysis, other than just running a line and linking all the discrete bands of showers embedded in the north northwesterly flow with it. Troughs appear and change orientation, seemingly at the drop of a hat. Interestingly, with regard to showers activity, the same thing seems to be going on today across Ceredigion as it did yesterday. Here are the estimated accumulations since 06 UTC yesterday (fig 5).

Figure 5

As you can see, the area of 50-75 mm estimated accumulations has grown bigger since yesterday (fig 5), and it’s no wonder that the BBC News item described it at flash flooding in New Quay and Talsarn. A repeat performance of yesterday could only exacerbate the situation, and I wonder what the Met Office mesoscale model rainfall accumulations are for today across this part of Wales and Cornwall, and whether we might even get a yellow warning for today to cover it?

Rain and flooding from Harvey continue

Figure 1 – Courtesy of NWS

The continuing heavy rain and resulting flooding from tropical Storm Harvey in Texas seem to have focus themselves on Houston, rather than Corpus Christi further down the coast to the southwest, with rainfall totals now well in excess of 20″ in the last four days (fig 1). The latest discussion from the NHC says it all (fig 2).

Figure 2 -Courtesy of the NHC

 

Has climate change shifted the timing of floods in the UK?

First off I will admit I haven’t fully read the study “Changing climate shifts timing of European floods” by Günter Blöschl and numerous other contributors from across Europe. The study has been picked up on by the BBC in this news article (fig 1), because they obviously see that it’s just another byproduct of AGW, rather than just climate change, and yes I do think there is subtle distinction. As far as I can see, none of the many contributors are employed by the UKMO, which I find unusual.

Figure 1 – Courtesy of the BBC

I have highlighted in yellow the areas in the news article that I have misgivings about, the first one is from Matt McGrath

The scientists believe this is due to changes in the North Atlantic Oscillation (NAO), the weather phenomenon that pushes storms across the ocean into Europe.

The North Atlantic Oscillation is simply an index of the pressure difference between the Azores and Iceland, as far as I know this index doesn’t push anything across any ocean, you might say that the Azores high or Icelandic Low are weather phenomena, but surely the difference between the pressure at two points is just a number?

Günter is the quoted as saying:

In southern England, it has been raining more, longer and more intensely than in the past. This has created a rising groundwater table and higher soil moisture than usual and combined with intense rainfall this produces earlier river floods

I refute that, unless the changes that the study is talking about have suddenly started to occur in the last five years. Cue some evocative pictures of rivers in southern England that have dried up in recent years that I’ve found on the Internet (figs 2 & 3). (N.B. to the BBC, two can play at that game!). Yes, I know these images are from 2012, but what about the River Derwent in the Lake District in May 2017 (fig 4). So it’s a well-known fact that river levels and groundwater tables do fluctuate, and can do so very quickly, that’s what they naturally do.

Figure 2 – The River Lavant, West Sussex in February 2012 courtesy of Press Associates and the Daily Mail.
Figure 3 – The River Pang, Berkshire in February 2012 courtesy of Press Associates and the Daily Mail.
Figure 4 – The River Derwent, Cumbia in May 2017 courtesy of Paul Kingston and Twitter

I can’t get any daily rainfall climate data for anywhere in the UK without paying loads of money to the Met Office, so I am stuck with the free 1910 monthly rainfall gridded data that they produce, I wonder if they used this kind of data or if the Met Office felt pity on their research and gave them the ‘real’ rainfall station data that they guard so jealously on our behalf? From that data here are some graphs with a simple linear trend for the southeast and central southern England region for winter, spring and summer (figs 5-7). I can’t see any discernible upward trend in rainfall in any of those three seasons, although I will admit that the 10 year moving average for summer is on the rise, and won’t be any lower after this wet summer.

Figure 5
Figure 6
Figure 7

Günter goes on further to say:

Half the stations recorded floods at least 15 days earlier than previously. A quarter of the stations saw flooding more than 36 days earlier than in 1960.

This one is a hard one to counter, especially without daily rainfall climate data and the dates of all fluvial flooding incidents since 1960, which I simply don’t have. But I don’t see that there is some kind of flood or monsoon season across the UK that starts at any precise date that you can readily identify, and if you can’t identify it, then how can you then go on to say that it’s starting 15 days earlier than it did in 1960? I do know from my interest in CET, that the spring is probably around 15 days earlier and it was in 1960, and that temperature is linked to increased convection and heavier rainfall, but I see little sign of it from the rainfall climate data that I can access.

Finally, here’s the last 12 months rainfall over southeast England (fig 8). It’s been a funny last 12 months as far as rainfall goes, up until mid May there was talk of an impending drought later in the year across southern and eastern areas, but the wet summer has put paid to that. There have definitely been some wet days in the last 12 months across the southeast of England, but they can occur in any month as far as I can see, and the accumulated rainfall is still only 84.7% of the annual average at the end of July 2017. What I really need is now is data from the environment agency, some kind of daily count on the number of alerts that they issue for rivers across the UK, a bit like the NAO, but not a phenomena, just a daily count.

Figure 8

PS I’ve just download the report to read – for a change it’s free to do so – and better still it’s only four pages long.

August 9 – rainfall

Figure 1

If they had only fitted a rainfall gauge to the AWS at Broadness on the Thames at Gravesend, they probably would have measured over 50 mm of rainfall in the 24 hours ending 06 UTC this morning (fig 2). I still can’t understand why this heavy rainfall event didn’t cause any pluvial or fluvial flooding issues yesterday in this part of the world.

Figure 2

BBC News: Yorkshire and Lincolnshire flooding caused by heavy rain

Courtesy of the BBC

A news item from the BBC concerning flooding caused by heavy rain in Yorkshire and Lincolnshire – http://www.bbc.co.uk/news/uk-england-40875248  The BBC were a bit late on this one as most of the heavy rain occurred on Tuesday in that area.

Met Office more reactive than proactive in the issuing of yellow warnings

Figure 1

I’ve been blogging ever since I retired and that’s now well over five years. I spend a good deal of my time watching how the Met Office performs through its forecasts and the various warnings that it issues, and recently, well in the last few weeks actually, I’ve been asking myself what is the exact purpose of the yellow warnings they issue.

What the colours mean

  • Yellow – Severe weather is possible over the next few days and could affect you. Yellow means that you should plan ahead thinking about possible travel delays, or the disruption of your day-to-day activities. The Met Office is monitoring the developing weather situation and Yellow means keep an eye on the latest forecast and be aware that the weather may change or worsen, leading to disruption of your plans in the next few days.
  • Amber – There is an increased likelihood of bad weather affecting you, which could potentially disrupt your plans and possibly cause travel delays, road and rail closures, interruption to power and the potential risk to life and property. Amber means you need to be prepared to change your plans and protect you, your family and community from the impacts of the severe weather based on the forecast from the Met Office
  • Red – Extreme weather is expected. Red means you should take action now to keep yourself and others safe from the impact of the weather. Widespread damage, travel and power disruption and risk to life is likely. You must avoid dangerous areas and follow the advice of the emergency services and local authorities.

More reactive than proactive

After the recent flash flooding in Coverack (18th July) and that at Okehampton yesterday (30th July), I now realise that many yellow warnings simply aren’t working, and that the Met Office have become more reactive than proactive in their issuing of them. I am convinced that yesterday’s late afternoon yellow ‘warning’ for 1750-2000 BST was prompted as much by Twitter, as it was by mesoscale NWP guidance or observational data from weather radar, river level gauges, automatic rain gauges or AWS.

I’m sure that they had reviewed the situation in the morning, and had duly issued a yellow warning for heavy rain for the north of Scotland, the reason that they gave for that warning of 20-40 mm was that the showers would be slow-moving.

What happened literally on their own door step at Okehampton seems to have caught them totally unawares, perhaps they thought the showers in Devon were moving quickly, and hadn’t anticipated that ‘peninsula convergence’ might just keep them coming along the spine of Devon for much of the day. In the end Okehampton received 84.2 mm in the 09-09 UTC period. I can’t believe that these accumulations were not picked up by any of the earlier mesoscale NWP model runs, and a warning not issued at the same time as the one for the north of Scotland was, I would love to see what the models were indicating.

The thinking must be at the Met Office, that a yellow warning must be in place to ensure that they are covered from any fallout in any severe weather, so no matter how late in the day it is, a yellow warning has to be issued. I recall a similar thing happened with the strong winds on the 6th of June, a warning for strong winds could have been issued with the one for heavy rain the day before, but for some reason known only to themselves wasn’t, two men were killed by falling trees on that day. It wasn’t a warning, it came into force as soon as it was issued, and that’s not much help to anybody, similarly yesterday, everyone in Okehampton knew it had been raining heavily all day, and by the time the warning had been issued the worst was over.

Figure 2 – Flooding yesterday at Okehampton courtesy of Twitter and Gillian Cross

I noticed that in the Coverack flash flooding on the 17th of this month, although there was a yellow alert in force, a red alert was never issued. It may be that localised nature of flash flooding does not meet the criteria of ‘widespread’ for the issuing of a red warning, and that’s why one was never issued. If that’s true, that would mean that if the NSWWS had been in place in the past for the flash flooding at Boscastle in 2004 and Lynmouth in 1952, both events would have only warranted a yellow warning.

In yesterday ‘peninsula convergence’ heavy rainfall event at Okehampton, the Met Office did have time to give an explanation to its Twitter followers of what ‘peninsula convergence’ was, but they only managed to issue a belated yellow alert from 1750 BST, when heavy rain had been falling for much of the afternoon (fig 1 & fig 3), and don’t forget that yellow means “Severe weather is possible over the next few days and could affect you“.

Here is my estimated total from low resolution weather radar images of what fell before the warning was issued from 0900 – 1650 UTC. As you can see from the inset hyetograph, there had already been, in the worst affected areas, around 61.7 mm of rain in less than eight hours (fig 3).

Figure 3

And here is my estimated total of what fell during the duration of the warning (1655-1900 UTC), and as you can see in that time a maximum of 25 mm fell in just over two hours in the same area (fig 4).

Figure 4