Synoptic situation for 1800 GMT on Monday 16 August 2004. Source
Source
Graph below showing 15 minute rainfall totals for Lesnewth TBR (Tipping Bucket Rain Gauge). Source
 Google Earth Placemark - Lesnewth.
Source
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At midday, on the 16th August 2004, heavy, thundery showers had developed across the South West, these were the remnants of Hurricane Alex which had crossed the Atlantic. Bands of showers aligned themselves with winds that had converged along the coastal high ground around Boscastle, creating Cumulonimbus clouds 12192 metres high and kept them stationary for many hours. Source
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The chance of such heavy rainfall in any given year was around 1 in 400. Source
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A string of slow-moving thunderstorms caused localised extreme
rainfall over north Cornwall. The rain was extreme in both its intensity
and duration: up to 200 millimetres fell in 24
hours. Source
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The storms grew out of a large depression (low pressure)
area that dominated the eastern Atlantic that day. It had
sucked in pulses of warm, moist tropical air, including
the remnants of Hurricane Alex. Source
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The moist air blowing in from the sea in the prevailing
south-westerly wind and a morning of warm sunshine
combined to produce clouds over the north Cornwall coast. Some of these grew in size and spawned ‘stormlets’, individual small thunderstorms. Source
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The storms were small in size but produced very intense
rainfall. That intensity came from the storms’ high
moisture content and their ‘uplift’, the process of warm
moisture-laden air being driven upwards, causing rain
to fall as the air cools. Source
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The storms were channelled along the coast by what
weather experts call a 'strong line of convergence', a line
where a moving airflow meets an opposing one and the
air is forced upwards.
As the south-westerly wind blew over the land, friction
from the rough surface slowed the wind and swung it
from south-west to south-southwest. Source
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This south-southwesterly wind pushed up against the
prevailing wind at the coast and created strong uplift.
The uplift may have been reinforced by higher air
temperatures onshore, caused by the earlier sunshine
heating the land - air rises as it heats. Source
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Storm showers along the whole north Cornwall coast all
started at about the same time, 1pm. It is thought likely
that this synchronised timing was due to the
convergence along the coast.
The storms moved slowly up the coast, shedding their
load of thousands of tonnes of rain. Source
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It’s estimated that
two million tonnes of rainwater
flowed through Boscastle that day. Source
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