Large parts of the US have been hit by severe weather this week as low pressure developed over the northwest coast. The weather system intensified into a phenomenon known as a bomb cyclone, which quickly intensified to an estimated central pressure of 942 hectopascals (hPa) – close to the lowest pressure recorded in that area.

To be classified as a bomb cyclone, the central pressure of a low-pressure system must drop by at least 24 hPa within 24 hours. Wind speeds exceeded 70 mph (113 km/h) near Seattle, while Canada had the strongest winds with gusts of about 160 km/h. Two people were killed and almost 500,000 people in the region were without power.

As the weather system stagnated, southwesterly winds brought warm, moist air from the tropical Pacific Ocean. This will continue to cause heavy rain and flooding in northern parts of California this week, with more than 200 mm (8 inches) expected to fall in some places. Rain has turned to snow in the Sierra Nevada Mountains, with more than 500mm of snow reported. The process that produced these large rainfall events is sometimes called an atmospheric river.

Meanwhile, parts of Europe, including Britain, are in the grip of an Arctic blast after weeks of dry and stable weather under high pressure. The Alps experienced their first period of snow this winter, with significant snowfall on Tuesday and Thursday. After a dry start to the month, the snow will be welcome news for those preparing to hit the slopes at the start of the ski season.

Some of the heaviest snowfall occurred on Tuesday in the western Alps, where the French ski season starts this weekend at the high resorts of Tignes and Val Thorens. Some higher elevations are likely to receive more than a meter of snow on Saturday.

Storm Caetano, which was responsible for Thursday’s wintry weather, not only brought snow to the Alps, but also snow, rain and strong winds to large parts of France. The first snowflakes of the season fell on Paris, with 10-50 mm reported. Although snow falls early in the season, this is not exceptional.