Storm Desmond - how and why it lead to UK floods

Question 1

Why and how did Storm Desmond cause floods?

Answer 1

• Hit Cumbria in Dec 2015 - caused by a low pressure system (-946 millibars) coming from the Atlantic bringing prolonged heavy rainfall and severe gales (through a mechanism called a warm conveyor) due to air being forced over the Cumbrian fells creating orographic + convectional rainfall to combine.
• this as well as already saturated ground (due to previous rain in Nov (was the 2nd wettest Nov since 1910) = record-breaking rainfall = severe flooding.
• long period of rainfall also due to the position of the jet stream - this determines the position of the depressions + their speed of movement and this band of fast moving air remained over NW longer than usual

Question 2

statistics of rainfall in Cumbrian regions:

Answer 2

• 5th dec - Honister Pass recorded 341.4 mm of rain in 24hrs
• 405mm of rain at Thirlmere in 38hrs