On the occasion of the fresh snowfall that storm depression Heike is bringing to large parts of the Alps, we take a look at a few subtleties of the precipitation forecast. Heike is a nice Italian low sitting over the western Mediterranean. As you can read from our colleague Orakel, it has therefore already snowed heavily in the western and especially the southern Alps, while the east and north have remained comparatively dry. The map shows the vertical movement of the air today (Wednesday), with negative values indicating lifting and positive values indicating sinking. The dark red spots are located on the Alpine arc and show us where Heike's air masses are flowing upwards. The water vapor it contains will turn into accumulated precipitation on the southern slope of the Alps.
Precipitable water
The black lines in the map indicate the precipitable water, a theoretical quantity that indicates how much water would fall if all the moisture contained in a column of air with a square meter base area and height of the atmosphere precipitated as rain. What actually falls as precipitation can differ greatly from this precipitable water. However, the size gives an initial indication and is useful, for example, as an indication of potential heavy precipitation. Precipitation forecast maps naturally provide more precise information on the expected amounts, although the accuracy here depends heavily on how well or poorly the weather model resolves the topography of the Alps.
Water and snow
The next step is to move from the precipitation forecast in liters or millimeters per square meter to the snow depth forecast: As the moisture content of the snow is known to change with the temperature, there are only approximate guide values and no exact conversion formulas. In figures, the difference between fluffy powder and fluffy snowman building snow corresponds roughly to 2 cm of snow per liter of dry, cold snow or 1 cm of snow per liter of precipitation in wet, heavy snow. For a conservative estimate, you can therefore assume 1 cm of snow per liter or millimeter of precipitation. Services such as snow-forecast use algorithms that adjust the conversion factor based on various criteria. How good or bad the result is still depends primarily on how good or bad the underlying numerical model is.
Outlook
Heike is moving slowly to the north-east, the flow is turning slightly and the focus of the precipitation will shift to the north in the coming hours as Heike's cold front crosses the Alps. After clearing up over the weekend, it should continue to be damp, with potential from the northwest.
