October 2011 Forecast Review

[Thundery wintry showers]

The original forecast ran as follows:

Following the exceptionally warm start to October, a generally anticyclonic month is expected, especially so around the middle part of the month, though with some changeable weather near both ends of the month. Although my prediction of the mean trough sliding east into Scandinavia during the first half of October looks set to happen, pressure will also be consistently high to the south of Britain and this will prevent a colder frosty northerly type from establishing.

It will be windy and changeable on the 4th-6th October with a north-westerly incursion on the 5th and 6th, with gale force winds in parts of north-eastern Britain and a fair number of showers on the 5th, though by the 6th most places will be dry and sunny with showers restricted to coastal areas due to the "wishbone effect" (high ground sheltering inland areas from showers in north to north-westerly airstreams).

The 7th-10th October are likely to be warm and moist across the country due to a tropical maritime westerly airflow on the northern flank of high pressure. Most parts of Scotland and Ireland will be wet, while most of England and Wales will be cloudy with the odd bit of drizzle, but some sunshine will break through at times in eastern and central England, where temperatures will hit 20-22C in the sunnier spots. Elsewhere highs of 15-18C can be expected with temperatures holding up into double figures overnight.

High pressure will assert itself from the south-west on the 11th/12th allowing a north-westerly incursion and then a spell of anticyclonic weather. There will be at least a few days of dry sunny weather with chilly frosty nights, but after midmonth banks of cloud are likely to become more of an issue as moist Atlantic air penetrates the anticyclone.

I expect that the high pressure will retreat eastwards during the last third of October, and this will promote an anomalous frequency of southerly winds across the British Isles with low pressure out in the Atlantic. As a result the weather will turn somewhat more changeable, and quite wet in western areas, but eastern areas will see comparitively little rain, with one or two late bouts of warm sunny weather delivered by southerly winds (though not as exceptional as the one we've seen recently).

October is likely to be a very warm month, and I am opting for a Central England Temperature of 12.5C- broadly speaking most of the UK will have positive temperature anomalies (relative to 1971-2000) of 2 to 2.5C.

Sunshine and rainfall are likely to show a west-east split this month, with much of Ireland and western Scotland having quite a wet month (rainfall excesses of 20-50%) and sunshine about 20-30% below average. Elsewhere it will be a dry month with much of England having less than 50% of average rainfall and some east-coast counties having less than 30%. Sunshine totals will be near average in the western half of the country, but 10 to 30% above in the eastern half.

I thought that the first third of this forecast went pretty well, with the north-westerly incursion pinpointed at short range for the 4th-6th October, and then the warm but often cloudy/drizzly spell after the 7th, and the brighter spell towards midmonth. I failed, however, to pinpoint the pair of north-westerly incursions in the third week of October, having instead predicted a continuation of the anticyclonic weather. The forecast then improved into the last third of October as the southerly type established as expected. I projected this based partly on educated guesswork (anticyclones often do drift east with time) and also the MJO composites, which pointed towards a mid-Atlantic trough during the last third of October.

I think I got the distribution of temperature anomalies pretty much right, but sunshine and rainfall showed more of a north-south split rather than the west-east split that I predicted.

Since the forecaster blogs aren't used these days I thought I would put this review in here, and then see how the November forecast goes, as November is traditionally the first month of "snow potential" and thus there is a lot more riding on it.