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).
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).