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Sunday 29 January 2017

The extreme weather behind the Abruzzo avalanche disaster

Wintry weather continues to grip Europe, though conditions have eased since those earlier in the month reported on AWN on 6 January, 9 January and 11 January. However, the most newsworthy was the phenomenal snow that fell in parts of central Italy, including Abruzzo, east of Rome. Over 2.5m of snow fell over a 3-day period and coincided on 18 January, with a sequence of four strong (magnitude 5.1 to 5.6) earthquakes, one of which is believed to have triggered an avalanche that evening that struck the Rigopiano Hotel, Gran Sasso, killing 29 people.

The snow itself was produced by an exceptionally powerful Bora wind in the Adriatic Sea producing a severe Stau wind as it reached the Apennines, which form the backbone of Italy. Put simply, the Bora is a local wind in the Adriatic which, in certain weather situations, can be stronger than hurricane force - it has been measured at 304km/h. Stau wind is the opposite of the better-known Föhn or Foehn wind. The Foehn effect occurs on the downwind side of mountain ranges, warming and drying the wind. The Stau effect occurs on the upwind side, producing cloud and rain or snow which in some places is among the heaviest measured on the planet.

Between 17 and 19 January, both winds conspired to focus on central Italy. The Bora, blowing from the NE, crossed the Adriatic from Croatia where wind gusts to 216km/h were measured on the 17th on the bridge to the Island of Krk. The pages of Severe Weather Europe for 17 January and 18 January give some idea of the massive snowfalls and appalling conditions facing rescuers in the Abruzzo, while this additional page for 18 January has videos that show the immense power of the Bora in the mostly very enclosed waters in NW Croatia and NE Italy. These same winds travelled across the Adriatic to cross the Apennines around Abruzzo. (NOTE that videos on these pages will not play in Chrome but will in Internet Explorer, Firefox and Opera.)

New Zealand's "summer that wasn't"

Kiwis have been complaining that this is the summer they never had as a procession of deep lows have passed across or close to the South Island. All have brought gales, very heavy rain and often wintry weather. Only in the past few days has more normal weather returned.

The most destructive weather came from the low that passed under NZ from 19 January. It brought over 300mm of rain to Arthur's Pass and winds gusting to over 150km/h at exposed places around the capital, Wellington, according to MetService, causing many flight cancellations. On the South Island, a media report said wind gusts of 172 km/h were recorded near Mt Hutt, 90km W of Christchurch. Major roads were closed by flooding in both the North and South Islands and by landslides in the South Island where rivers on the West Coast rose rapidly in a few hours, carrying trees and large debris washed down from the Alps. At one point, the Haast Pass road provided the only access to the West Coast.

Waikaia, 80km NE of Invercargill, was cut off and livestock had to be evacuated to higher ground. Cars on windswept roads were flipped over. There was also wind damage to property and power lines in both islands, and wind brought down trees onto power lines causing widespread blackouts, while in Auckland the following cold front on 21 January cut power to about 15,000 properties. Cardrona ski resort near Queenstown, South Island, saw over 20cm of snow and, even in the North Island, the top half of Mt Taranaki, 280km S of Auckland, was coated in rare summer snow when the freezing level dropped to around 1500m.

This "What you need to know" article from Radio New Zealand gives a feel for what it was like in the middle of the storm as well as some technical weather details. And there are some dramatic photos taken during the storm in this Guardian article.

Most of the lows to cause foul weather in NZ have "bombed", a meteorological term for them deepening very rapidly. The one that passed under the South Island on 24 January, for example, deepened 35hPa (or 35 millibars, they're the same) in just 18 hours. Five of these depressions have hit NZ in January, created by the contrast of very warm, humid air crossing the Tasman Sea from Australia meeting cold air being pushed north by the low pressure system.