WeatherBlog 4 2023/24 | Pre-Christmas storm

Current situation and outlook

The calm, sunny weather of the last few days is over for now. The Azores high that has accompanied us since the weekend is retreating and today, Wednesday, a cold front will reach the Alpine arc from the northwest. This is not yet particularly spectacular, but serves to set the mood for the coming days. On Thursday, the north-westerly current picks up speed and brings the next front. The frontal zone will come to rest directly over the Alps and gale-force winds can be expected at high altitudes on Friday. The steering low over southern Scandinavia will push massive amounts of precipitation towards the Alps. The high wind speeds will also make it inhospitable south of the main ridge. The wind and precipitation will ease towards Sunday, although the exact development from the 24th is still uncertain. It is expected to be significantly warmer and less stormy again on the holidays with a persistently "changeable" weather character and a roughly westerly flow.

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500hPa geopotential and ground pressure, GFS forecast for Friday 22.12.23. Strong NW flow between Azores high and low in the northeast. Storm in the high mountains!

meteociel.fr

How much snow is there under the Christmas tree?

While our colleague the Oracle and the prediction game are wondering how much snow there will be, we turn our attention once again to the question: how much snow is there anyway? It's not that easy to find out because the amount of snow varies greatly, even over short distances. As an example, we quote our colleagues at ÖAV Wetter, who expect the following for next Friday: ".... dense clouds, poor visibility in the north, along the main Alpine ridge and significantly further south, and heavy snowfall in places, including "landscape-changing" drifts and a high risk of avalanches. Very different amounts are falling, mostly between 20 and 80 cm, which will hardly be measurable in any meaningful way anyway due to the massive loads in the terrain away from special, wind-calm locations."

Phrases such as "landscape-changing loads" and "hardly measurable in any meaningful way" indicate where the challenges of snow depth measurement lie. In this case, meteorology can be seen as a subfield of metrology ("the science of measurement" - rarely considered but very central to many areas of everyday life). In meteorology, as in metrology, there are no exact measurements. The best we can do are measurements where we can quantify the uncertainties and errors or at least estimate them to some extent. In the case of snow depths, these tend to be manual measurements (person measures with a meter stick or similar). Sources of error here are e.g: Meter stick is crooked, person squints) or automatic measurements at weather stations, e.g. by ultrasonic measurement of the distance between sensor and snow surface (sources of error, among others: inaccurate determination of sensor height, temperature dependence, dirt on sensor, ...). The uncertainties in the measurement are rightly ignored in the prediction game, as well as in the graph on the course of the snow depth at the weather station on the Pitztal Glacier, which shows us that there is a lot of snow at the location with 230 on December 18 for the time of year:

Snow depth at the Pitztal Glacier weather station compared with the minimum, maximum and average of the time series. Current season as thick green line.

Daten: ZAMG/Geosphere Austria

Apart from the pitfalls of the measurement itself, we also have the challenge of "landscape changes" caused by storms and the very different spatial distribution of the snow. Snow depth maps, such as the ICON maps in the new PG Weather, or SNOWGRID, which has a slightly better resolution for snow, only provide one value per map pixel. With ICON-D2, a map pixel is approx. 2kmx2km in size. In the PG map display we can see a yellowish color in the area of the Pitztal glacier, based on the color scale we are in the range of ~180-260cm snow depth (the WeatherBlog failed at the API of the DWD, but Sebastian may still be able to tell us exactly what value the yellow stands for in the Pitztal). The higher-resolution SNOWGRID recognizes a snow depth of 150cm for the location of the weather station in Pitztal (the data is available via the Geosphere Austria data portal).

ICON and SNOWGRID have very different methods of arriving at a snow depth value per pixel. Neither of them gets the station value in the Pitztal exactly, which does not mean that both are wrong or that one is necessarily more representative than the other. The station value applies (with the uncertainties mentioned) only to the station location. As we know, a few meters in front of or behind a terrain edge make a big difference to snow depth and the skiing experience.

And what does this tell us now? Basically nothing that we didn't already know: snow is complicated to measure and model. Weather station data and model maps provide clues, but are no substitute for visiting the field. If you know a region and the corresponding weather stations or maps well, it is easier to "translate" the information from the station graphics into the terrain.

There's always the potential for surprises, but that's what makes it exciting and is part and parcel of Christmas presents!