WeatherBlog 19 2023/24 | Summer, sun, desert dust

Current situation and outlook

The Alps are currently experiencing a moderately strong westerly flow that is producing a rapidly changing sun-cloud mix. Today, Wednesday, there will be a small disturbance and a few drops or snowflakes. Towards the end of the week, a strong Atlantic trough will develop and the upper level flow will turn south-westerly. In susceptible regions in the north, it will be foehn again, although this time it will not be a hurricane and there will be no accumulating precipitation on the southern side. The controlling low-pressure centre is too far to the west for a repeat of the massive south to north foehn combination. Instead, there is likely to be plenty of sunshine and very high temperatures at the weekend. The 30 degree mark is within reach. Those who don't want to switch to summer mode will still find plenty of snow in the high mountains and, depending on the wind and clouds, perhaps some quite usable firn.

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500hPa geopotential and ground pressure, GFS forecast Wednesday, April 3: Roughly westerly flow direction, repeated small disturbances
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meteociel.fr

Easter storm and March heat

By meteorological definition, a "summer day" is a day with a maximum temperature of over 25°C. The first summer days of the still fairly young year were recorded in Austria at Easter. In Vienna, 28.5°C was measured on Monday - that is already close to a "tropical day" (>30°C) and very unusual for the beginning of April. In the Alps, we were more concerned with the days-long foehn storm in the north and the impressive amounts of fresh snow in the south, including widespread road closures and a high (level 4) avalanche risk. There was also a good load of Sahara dust. MeteoSwiss has a nicely illustrated summary of the storm situation, also at the SLF (thanks to Hannes for pointing out the station that registered almost 3 metres of "fresh snow" in 24 hours, with the "fresh snow" coming from an avalanche).

After the warmest February in recorded history, March has now joined the statistics. In Austria, the lower altitudes in particular were extremely warm. Geosphere Austria reports "since the start of the measurement series in 1767, there have never been two warmest months in a row. Now, with September/October and February/March, this has happened twice within a short space of time."

The year 2024 has so far been much warmer than the climate average.

Geosphere Austria

Sahara dust in the snow

When Sahara dust is covered with snow, it can later be found in snow profiles as a distinctive, reddish-brown layer. In recent years, there have also been repeated avalanches that looked as if they had occurred on the dust layer. In fact, the dust itself does not initially act as a weak layer. However, it can favour the formation of melt crusts, as the snow surface absorbs more energy when it is dusty and therefore darker, and therefore becomes damp more quickly. In addition, Saharan dust is already transported to the Alps by warm air masses anyway, which makes surface melting processes even more likely. Thick melt crusts, whether with or without dust, can stabilise the stability of the snow cover depending on the situation by "shielding" lower-lying weak layers from disturbances from above. On the other hand, temperature gradients that occur at melting ridges favour the formation of weak layers directly below or above the crust, which can become an insidious old snow problem. If you want to find out more, here is a scientific study that uses detailed snowpack modelling to investigate how Saharan dust affects the snowpack and under what conditions. The summary is, as so often, it depends.

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WeatherBlog 19 2023/24 | Summer, sun, desert dust

Calmer now after the storm at Easter, hot at the weekend

Current situation and outlook

Easter storm and March heat

Sahara dust in the snow

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