We begin at 1200 UTC (1300 CET) yesterday when an Atlantic cold front was sweeping across northern Europe and beginning to impinge on the western Alps. I have annotated a summary of the situation on the 850 mb (1500 m) wind and temperature analysis below. With southwesterly flow ahead of the front and associated trough, locally warm conditions predominated in the northern Alps and adjoining lowlands. This reflects the influence of the Foehn (see also the previous post), a warm, downslope flow that develops during periods of large-scale southerly and southeasterly flow.
The situation began to change quickly by 1800 UTC (1900 CET) as the cold front pushing into the western Alps, bringing with it cooler air from the northwest and a cessation of the Foehn. At this time, the warmest 850-mb air was confined to the northwest Alps and lowlands to the north where it had been transported to in the southwesterly flow. Note also the deceleration and splitting of the flow as it impinged on the western Alps. This is also very common and it reflects the stability of the low-level airmass, which is unable to move unimpeded over the formidable Alpine topography.
By 0000 UTC (0100 ECT) 11 February, cold air has encircled the western and the northern Alps. There is a sharp contrast in temperatures across the Alps, which reflects both the stagnation of low-level cold air to the north and the development of downslope flow into northern Italy, where temperatures have increased from the previous two analyses. Essentially, northern Italy is now experiencing Foehn conditions. In addition, note how rapidly the cold front has pushed eatward across Austria, Slovenia, and Hungary, which is a result of the blocking of the flow by the Alps, which rotates the flow to more westerly and allows the front to accelerate eastward.
And finally, the forecast for noon today shows cold air encircling the eastern Alps, downsloping the Dinaric Alps, and flowing over the Adriatic Sea.
The satellite image from this morning shows the sharp contrast in weather across the Alps. Note in particular the clear skies in the western Po Valley of Italy and the clouds banked up over the northern Alps and east of the Dinaric Alps.
European meteorologists have recognized these impacts of the Alps and other ranges for a long time. Below is an analysis from the great Swedish meteorologist Tor Bergeron, published in 1928. The flow is a bit more westerly, but you will see the similarities.
That analysis has always been one of my old-school favorites.