Uncharted Territory

May 1, 2012

The Wettest Drought in History

One of my responsibilities as a teenager was to keep the lawn under control. Flymos had presumably not yet been invented, and petrol-driven mowers were perhaps too much hassle, so ours was manual. If the grass got too long it was hard work and it could even become necessary to resort to shears, which was back-breaking work. But mowing was also difficult if the grass was damp. There was therefore a trade-off each spring. The first mow had to be done when it was mild enough for the grass to be reasonably dry, but couldn’t be put off until it was too long. And as the grass grew it dried out more slowly each day. So it was essential to make use of any opportunity to mow in case the weather turned wet again. It probably only happened once or twice, but it seems I was always caught out. I’d wait for one more dry day to make the job easier, but the skies would open and a week later the job would be twice as difficult.

Nowadays the internet and improved forecasting allows me to monitor the weather far more effectively. Thus it was I’d already been out with the mower in March, and, seeing the long-range forecast, made sure I got a mow in just before it started raining early in April.

The point is that the 5-10 day forecast is now fairly reliable.

Why, then, was the UK drought – declared in a few regions in March, with hosepipe bans from 5th Aprilofficially extended in mid April?

Yes, that’d be in the middle of the wettest April on record!

We’re now in the farcical situation of the “wettest drought in history”, with a succession of “experts” (and junior ministers) popping up on TV claiming the rain in April somehow doesn’t count. Apparently it’ll run off compacted ground. Yes, maybe for the first day or two, but not after a month. With the wettest April on record followed by significant rain already in May, and more forecast in a day or two, the drought risk is simply receding. We’re in one of those surreal situations where reasons are being invented not to contradict previous claims, in this case that the drought would last into next year.

What baffles me is why the drought was extended when wet weather was forecast. Surely – since most of the time it’s dry – the drought risk is receding as long as there’s significant rain in the forecast. And, as the 5-10 day forecast is fairly reliable and everything after that isn’t, you simply run the risk of looking stupid if you don’t wait until the forecast is for dry weather.

I wonder whether there’s a tendency to believe long-term forecasts more than short-term ones. But long-term forecasts only indicate a small bias one way or another, as Met Office modelling indicates:

“New three-month forecasts by the Met office suggest little respite with April, May and June expected to be drier than average. ‘With this forecast, the water resources situation in southern, eastern and central England is likely to deteriorate further during the period. The probability that UK precipitation for April-May-June will fall into the driest of our five categories is 20-25% while the probability that it will fall into the wettest of our five categories is 10-15%, it says.’ ” [my emphasis]

So 20-25% dry plays 10-15% wet plays (presumably) 60-70% around average. Not sure I’d have put a lot of money on the “expectation” of a dry spring this year (certainly wouldn’t now!). Even less after I’d looked at the Met Office report (scroll down to find PDFs) because the model runs are all over the place.

And are these “probabilities”, anyway? Isn’t the modelling signal swamped by the noise of uncertainty? It seems to me likelihoods based on model-runs are not the same as probabilities in the real world.

I’d say the Met Office and the media (the quote marks indicate the introductory sentence was written by the Guardian’s John Vidal) need to mind their language. How about “slightly more likely than not to be” rather than “expected to be”? And perhaps “indication” rather than “forecast”? And “x% of model runs gave…” rather than “the probability that…”? And definitely “might” rather than “is likely to”!

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2 Comments »

  1. […] first thought this morning was to write about the so-called UK drought again. Maybe I’ll post something on that […]

    Pingback by Gifts to Greece « Uncharted Territory — May 13, 2012 @ 12:51 pm

  2. […] It’s been nagging away at me to follow up my previous post which noted how much rain we started having as soon as drought was declared. […]

    Pingback by What Drought? « Uncharted Territory — July 12, 2012 @ 5:11 pm


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