Storm Agnes struck Ireland and the UK yesterday. What made Agnes particularly interesting is how a tropical storm in the US helped to boost a fairly ordinary Atlantic depression, injecting it with extra energy that charged it up into a tempest that suddenly exploded into life with strong winds and heavy rains as it approached the British Isles — a phenomenon wonderfully described as explosive cyclogenesis.
The tropical influence began last weekend. A tropical storm named Ophelia drenched much of the east coast of the US with some torrential downpours and strong winds, and even though Ophelia soon lost its tropical storm credentials it dragged up hot and humid tropical air from the south that eventually collided with the jet stream and charged it up with an extra big dose of energy. That sent the jet stream racing across the Atlantic and drove a fairly innocuous-looking depression in the Atlantic, which rapidly turned it into a raging fury, and so Storm Agnes was born.
Tropical Storm Ophelia followed hot on the heels of the hurricanes Lee and Nigel that recently blew up in the Atlantic. When each of those two hurricanes died out their remains made a hefty punch on our weather by merging with Atlantic depressions and turbocharging them into big beasts, unleashing very heavy rainfalls as well as stiff winds. That has helped to boost this September’s rainfall way above average over much of the UK. With Agnes dropping even more rains, total rainfalls are rising even higher, although unlikely to set a record.
Another interesting feature of this month’s weather is how average temperatures have remained far above the norm. In large part that is thanks to the heatwave earlier in the month, but even after the heat subsided, some very mild days followed thanks to airflows coming up far from the south. This month’s average temperature is now almost certainly going to be warmer than both July and August, and may even break the record for the warmest September.
