Between 30 April and 1 May a vast area of Atlantic low pressure, active off the coast of Ireland, extended towards the western Mediterranean, hooking onto a disturbance already active on the Strait of Gibraltar. This situation led to the formation of a low pressure minimum over the Gulf of Lion, which then moved towards the Ligurian Sea and central Italy. In Piedmont, between the evening of 30 April and 2 May, it caused extensive and prolonged rainfall, very heavy in particular in the middle-lower valley areas of the Alpine sectors. This was favored by the marked “stau” effect, i.e. lifting dictated by the Alpine reliefs, due to the intense and humid flow from the eastern quadrants. The areas less affected by this dynamic, such as the area far from the mountains in the far north of the region, saw less intense rainfall.
If the rains were more continuous on May 1st, on May 2nd they were more intermittent and characterized by a convective component, with episodes of showers or even intense thunderstorms in the Cuneo area and in upper Piedmont. The maximum precipitation peaks detected by the monitoring network were around 150 mm in 48 hours, in some stations in the middle-lower valley between the Po and Biella valleys: Piano Audi (167 mm), Praly (157 mm), Balme (154 mm), Camparient (154 mm), Piamprato (152 mm), Barge (137 mm). The data were released by ARPA.
The snowfall altitude generally stood at values higher than 2000 m with local and temporary fluctuations even at considerably higher altitudes (2300-2500 m). Overall, the greatest amounts of snow were recorded at altitudes close to or above 2300m: between the Pennine Alps and the Graian Alps the values reached 70-90 cm with peaks of 100 cm at Rifugio Gastaldi – Balme TO (2659 m), 40-60 cm on the Northern and Southern Cottian Alps, 20-40 on the Western Maritimes and values lower than 20cm on the remaining sectors of the Ligurian Alps and the Lepontine Alps. In the last 24 hours the snowfall has concentrated on the central-southern sectors (Cottian Alps and western Maritimes) with an accumulation of 20-30cm at around 2000m, lower values in the remaining sectors.
From the evening of May 2nd the snow level suffered a sudden drop which locally reached 1400m in the narrowest valleys and in conjunction with episodes of intense showers but in general it settled at 1500-1700m giving once again a wintry appearance to the Alpine valley floors of the Piedmontese valleys. The new, very humid snow, especially below 2000-2200m, quickly destabilized already during the snowfall, especially in the final phase of the precipitation, causing significant avalanche activity at high altitude which, in some cases, reached the valley bottom areas well below the snow line.
As regards the waterways, the phase of bad weather that hit the region caused a general increase in those of the main and secondary network without however reaching the warning levels. The most significant increases occurred on the Pesio a Carrù (CN), the Ellero a Mondovì (CN), the Corsaglia a Frabosa Soprana (CN) and the Orco a Spineto (TO) on May 1st while on the of May 2nd the Cervo in Quinto Vercellese (VC) and the Sesia in Palestro (PV).
The flood of the Po passed through Turin in the early hours of May 3rd while in the closing section of the Piedmontese basin, in Isola Sant’Antonio (AL), in the morning of May 3rd with levels that remained significantly below the level of guard. The intense rains on May 2nd caused slope phenomena with isolated triggers in the provinces of Turin and Cuneo.
This bad weather start to May has contributed to further increasing the water resources available to Piedmont in 2024.
The cumulative precipitation since the beginning of the hydrological year still presents a significant surplus of 38% compared to the norm (1991-2020), even if the month of April just ended was on average slightly below (-14%) the climatic norm of the period. The distribution of rainfall and snowfall in the last 30 days was uneven in space (positive anomalies on the Alpine reliefs of around 20%) and negative elsewhere (-25/30%), and also uneven in time, with the first two decades dry and the last decade very rainy).
The temperatures of the month were slightly higher than normal (+0.4°C), thanks to a very hot first half of April and a decidedly cold second half which balanced the monthly average. So far, however, this has been a very hot year, with a positive thermal anomaly since the beginning of the year of around 1.6° C, the largest in the last seventy years after 2007.
Again thanks to the abundant rainfall in February and the record-breaking rainfall in March, the SPI rainfall anomaly and SPEI rainfall and evapotranspiration anomaly indices highlight how at the 3-month time scales, Piedmont is in situations of severe and extreme rainfall while at those of the 6 and 12 months the region is in conditions of moderate rainfall and, only in lower Piedmont, the regime is the “normal” one.
The surface resources stored at the end of April are higher (+54%) than those normally expected at this time of year, thanks above all to the contribution of the SWE anywhere above 40% compared to the norm.
Record snow on the ground for the northern and western sectors of the Alps, above the 75th percentile in the south. The flow rates of the main water bodies are higher than the reference average with the exception of the Tanaro in Farigliano which has a slightly negative flow deficit. The most significant positive differences are highlighted in the Dora Baltea basin (+96%), Toce (+100%) and Stura di Demonte (+77%), while at the end of the Po basin there is a positive balance of + 91%.
Finally, as regards groundwater, a moderate rise in the groundwater level was recorded in all stations; in many piezometers the subjacent is lower than the 25th percentile and the average value of the reference historical series.
