The UK rainfall total for 2001 was modestly below average (see Table 1) but spatial and temporal variations in rainfall patterns throughout the year were very marked. Provisional data indicate that 2001 was the third driest year on record for Northern Ireland (in a series from 1900) and some catchments in western Scotland reported less than 75% of average annual rainfall. By contrast, large parts of the English Lowlands, East Anglia in particular, reported well above average rainfall (Figure 1) - for the fourth successive year in many catchments. Counterbalancing below average rainfall to the west and north resulted in the 2001 rainfall total for England and Wales being close to the long term mean.
January excepted, the early months of 2001 - as during the last quarter of 2000 - were characterised by a remarkable moderation in the normal north-west to south-east rainfall gradient across the country. Within a few timespans the gradient was reversed, for example the January-April rainfall total for the Environment Agency's Southern Region were significantly greater than that for the Highland Region in Scotland. Thus, over a period when notable rainfall deficiencies were developing in parts of Scotland and Northern Ireland, many new rainfall records were being established across most of southern Britain.
After a respite in January, most Atlantic frontal systems continued to follow an unusually southerly track. The frequency and intensity of these systems contributed to the highest January-April rainfall for 50 years for England and Wales. More remarkably, the continuation of the exceptionally wet weather, which was a feature of late-2000, resulted in new maximum n-month rainfall totals being established for all timeframes between 5 and 32 months (15-17 months excepted); most of these outstandingly wet periods terminate in the first half of 2001. Generally the margin by which previous maxima were eclipsed is modest, but the length of the 2000/01 wet episode is very exceptional.
From late April, the synoptic pattern changed, rain-bearing frontal systems began to follow more familiar routes and below average rainfall characterised much of the remainder of the year. Significant rainfall deficiencies developed in most regions through the late spring and summer, culminating in exceptionally dry conditions during November and December - when rainfall was less than half the average in much of southern England. At year-end, rainfall deficiencies were of most water resources significance in the South-West which registered its driest May-December since 1921 in some catchments.
In contrast to the cyclonic conditions which dominated the last four months of 2000, the westward extension of a European high pressure cell produced colder, more settled conditions in January 2001. Notwithstanding significant snowfall in northern Britain, regional precipitation totals (the South-East aside), were mostly below average. However, the contrast in synoptic patterns between the north and south of the country in late 2000 re-asserted itself in February when some eastern catchments registered more than twice the average rainfall.
March was again very wet in the English Lowlands whilst rainfall totals were mostly <70% of average in western Scotland and Northern Ireland. Parts of northern Scotland were again dry in April but most regions of the UK recorded well above average rainfall. For the third time since 1997, April was notably wet across England and Wales. More remarkably, rainfall over the eight months ending in April 2001 exceeded the annual average by wide margins across most of southern Britain (Figure 2). May was a warm and, in most areas, a dry month - a few coastal districts in the North-East recorded less than 20% of the 1961-90 average rainfall and regional totals were mostly in the 40-75% range. More significantly, several catchments in the Scottish Highlands reported below average rainfall for the seventh successive month. Across much of Scotland, it was the driest January-May period in at least 20 years.
In most regions relatively settled weather continued into the early summer as frontal systems tended to follow tracks remote from the English Lowlands. Correspondingly, June rainfall totals were substantially below average in much of southern Britain. July weather patterns were much more variable - heatwave conditions alternating with more autumnal interludes. Thunderstorms, some of violent intensity, were common; on the 17/18th a precipitation total of 99 mm in 15 hours was recorded at Keyworth (Staffs) whilst Weybourne (Norfolk) registered 40.1 mm in three hours. As a consequence, spatial variations in monthly precipitation totals were large. A few catchments in western Scotland reported their first above average monthly rainfall total of the year - but accumulated deficiencies remained substantial. Provisional data indicate that Scotland and Northern Ireland registered their lowest January-July rainfall total since 1955 and 1953 respectively.
August continued the July theme with convective precipitation contributing a significant proportion of the monthly total; a 55.6 mm storm total was recorded at Northolt (west London) in 12 hours on the 9th - including 34 mm in less than an hour. Again, spatial coherence in monthly precipitation amounts was limited but, regionally, most August totals were in the 80-120% range. Summer (June-August) rainfall totals were also close to the 1961-90 average, with most regions registering within 10% of the mean - but many catchments in the south-west and north-west of England were significantly drier.
Rainfall in September was very close to the 1961-90 average for England and Wales as a whole - an unexceptional end to the wettest water-year (October-September) since 1876/77. The September rainfall strongly favoured eastern regions, much of northern and western Britain - and Northern Ireland - had a relatively dry month. Rainfall deficiencies continued to build in many western catchments. In the South-West, water was shipped to Lundy to replenish depleted stocks and the regional May-September rainfall total was similar to that recorded during the 1989 drought. Deficiencies extended over a longer period in Northern Ireland where rainfall for the first nine months of 2001 was the third lowest in a series from 1900.
Weather patterns became very much more unsettled in the final week of September. On the 25th, a rainfall total of 65 mm in just over an hour was reported for Oulton (Norfolk) - significant localised flooding resulted, some the result of rapid melting of large hail drifts. The passage of a very active frontal system on the 30th - which resulted in a 52.5 mm rainfall total in 24 hrs at Vyrnwy (north Wales) heralded a notably unsettled October with several exceptionally wet interludes. In East Anglia, many areas registered around the monthly average rainfall during the three days beginning on the19th; 90.4 mm was recorded in 18 hours at Cambridge (the corresponding return period exceeds 100 years); a similar total was reported for Stansted Mountfitchet (Essex) over the 20/21st. Britain as whole registered its third notably wet October in four years (both 2000 and 1998 produced substantially higher rainfall totals).
With soils at, or close to, saturation across much of the country in early November, the subsequent dominance of anticyclonic conditions was very helpful in moderating the flood risk. The low frequency of Atlantic frontal systems was reflected in the rainfall totals - England and Wales recorded its driest November since 1989. The high pressure intensified in December, diverting many early-winter Atlantic low pressure systems to the north of the UK. Temperatures declined steeply and a significant proportion of the December precipitation in northern Britain was as snow; some particularly heavy falls occurred in north- east Scotland. Nonetheless, monthly precipitation totals were modest across most of the country - well below 50% in much of southern Britain. Provisional rainfall figures indicate that the November-December precipitation total for the UK was amongst the three lowest since 1945; in parts of the South-East it was the second driest end to the year since 1933 (Figure 3).