The unusually long cold spell and the snowstorm Filomena over Spain in January 2021

In January 2021, large parts of Spain were affected by an unusually long cold spell and exceptional snowfall associated with the winter storm Filomena. According to the Spanish weather service AEMET, snow heights of nearly 50 cm were registered in and around Madrid. During the days after Filomena, record-breaking low temperatures were measured at many stations.

Already during the days before the arrival of storm Filomena, anomalously cold temperatures at 850 hPa and night frosts at the surface prevailed over large parts of Spain. During these days in early January, the air flow towards Spain was predominantly northeasterly and advected cold air masses from Central Europe, as revealed by backward trajectories that were initialised near the surface over Spain. The land surface progressively cooled down during the days prior to the heavy snowfall, which then prevented the snow from melting when reaching the surface. Therefore, this cold spell preconditioning seems to be very important for the extreme consequences of the snowfall event.

The storm Filomena affected Spain between 8 and 10 January. It developed from a precursor low-pressure system between the Azores and Madeira. The precursor low-pressure system itself developed on 2 January 2021 between the northeastern US and Nova Scotia, rapidly intensified along a potential vorticity (PV) streamer and propagated southeastwards. Between 4 and 6 January, the cyclone, now located near the Azores, was associated with a PV cut-off and eventually decayed into multiple centres. Out of this decaying low-pressure system, Filomena developed and reached Spain on 8 January.

The most intense snowfall occurred on 9 January and affected large parts of Spain, except for southwestern Spain, where temperatures were too high and all precipitation fell as rain. Filomena was associated with an intense air mass boundary, with dry and cold air in the north and warm and humid air in the south. Equivalent potential temperature differences at 850 hPa across Spain exceeded 20 K. Along the warm frontal part of this air mass boundary, strong ascending airstreams, intensified by the dynamics of Filomena, led to cloud formation. Due to the unusually cold lowermost troposphere, snow was not melting before reaching the land surface, and the surface snow layer could therefore easily grow.

Overall, the combination of the already cold temperatures near the surface, the optimal position of the air mass boundary, and the dynamical forcing for ascent at this intense baroclinic zone associated with Filomena were essential ingredients for this extreme snow fall event to occur.