Readers may recall how the Met Office botched yet another forecast, calling for drought but instead getting a month of deluge. Here’s the numbers.

By Paul Homewood

The UK Met Office have just released their weather data for April, which confirms just how wet and cold the month has been. With an average of 126.5mm of rain, this has been the wettest April on records which go back to 1910. It was also the coldest since 1989, 0.65C colder than the 1971-2000 average.

This, of course, is in stark contrast to the 3 month outlook the Met Office issued on 23rd March, which told us that :-

SUMMARY – PRECIPITATION: The forecast for average UK rainfall slightly favours drier than average conditions for April-May-June as a whole, and also slightly favours April being the driest of the 3 months.

and

SUMMARY – TEMPERATURE: The UK-average temperature forecast for spring (April-May-June) shows a range of possible outcomes that are warmer than the range observed between 1971 and 2000 (our standard climatological reference period), but quite similar to the last decade. For April the forecast also favours temperatures being warmer than the 1971-2000 reference period.

http://www.metoffice.gov.uk/media/pdf/p/i/A3-layout-precip-AMJ.pdf

http://www.metoffice.gov.uk/media/pdf/q/q/A3-layout-temp-AMJ.pdf

So far (unless I have blinked),the Met Office Chairman, Robert Napier (formerly Chief Exec of WWF-UK), has not been doing the round of TV studios to apologise for getting things so badly wrong. But what about the longer perspective, with parts of England still officially in drought?

Figure 1 shows annual rainfall trends up to the end of 2011. The last 2 years have been dry, but are similar to many earlier years in the record, while the long term trend is remarkably stable.

Figure 1

Further analysis on the 2011 numbers by season and also by region is available here. (Again, this analysis shows that there is not much happening with long term rainfall trends and also that there was nothing particularly unusual about 2011).

Figure 2 shows the cumulative rainfall totals in England & Wales for January – April, both for this year and the 1971-2000 average. In most parts of the country, rainfall has been close to normal. Only the West and Wales have had significantly low levels, but these areas have not been in drought and generally receive much higher amounts of rain than the rest of the country anyway. Figure 3 gives the cumulative amounts for the last 16 months. (The 1971-2000 average is also for 16 months, i.e. January – December PLUS January – April). The areas mainly affected by drought (as the map below indicates) are the Midlands, East Anglia and South East, and rainfall levels since January 2011 are down by 19%, 18% and 15% respectively, but close to normal this year.

Figure 2

Figure 3

Forecast for May/June/July

The Met Office have also recently issued their 3 month outlook for May to July. For temperatures, they have this to say.

SUMMARY – TEMPERATURE: The balance of probability, both for May and the period May-June-July 2012, favours UK-averaged temperatures above the 1971-2000 climate mean, but in line with those observed over the last ten years. However, predictability for both periods tends to be low, with current forecasts indicating greater-than-average uncertainty in UK weather patterns as early as the beginning of May. May is also a month where there can still be large swings in temperature depending on the prevailing wind direction and so cold spells are still possible despite the most likely scenario being for above-normal temperatures. The probability that the UK-mean temperature for May-June-July will fall into the coldest of our five categories is less than 5%, whilst the probability that it will fall into the warmest of our five categories is around 45% (the 1971-2000 climatological probability for each of these categories is 20%).

http://www.metoffice.gov.uk/media/pdf/3/d/A3-layout-temp-MMJ.pdf

And precipitation.

SUMMARY – PRECIPITATION: For UK-average rainfall, the predicted probabilities slightly favour above normal values during both May and May-June-July. However, confidence in this prediction is not high, and there is still a significant probability of below normal rainfall. Whilst the wet weather of recent weeks will have had a positive effect on soil moisture, with all that that implies for agriculture, it is unlikely to have had a significant impact on groundwater supplies. With the forecast for May and May-June-July not favouring a continuation of the current very wet spell, groundwater resources in southern, eastern and central England are very unlikely to recover during this period. The probability that UK-average rainfall for May-June-July will fall into the driest of our five categories is around 15%, whilst the probability that it will fall into the wettest of our five categories is around 30% (the 1971-2000 climatological probability for each of these categories is 20%).

http://www.metoffice.gov.uk/media/pdf/2/l/A3-layout-precip-MJJ.pdf

In other words, it will probably be hot and wet, but they don’t really know. Maybe, Robert Napier should go back to saving polar bears.

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