Heavy snow in parts of Spain
8 January 2021 09:00 UTC-11 January 13:00 UTC
Parts of central Spain saw their heaviest snowfalls in decades in January 2021.
19 January 2021
12 January 2021
By Natasa Strelec Mahovic, Jochen Karl Kerkmann and José Prieto (EUMETSAT), Ivan Smiljanic (CGI) and Zsofia Kocsis (Hungarian Meteorological Service)
The snow was caused by Storm Filomena, which brought 30-40 cm on snow (about 50 mm of equivalent liquid water) to Madrid. This was the most snowfall in the area since 1971, although well behind a snowfall event in November 1904, when double in amount of snow was recorded. Filomena was also similar in precipitation to other such events in February 1907, February 1963 and March 1971.
On the morning of 11 January, as the clouds cleared, snow covering most of central Spain could be seen in satellite imagery (Figure 1). In the Snow RGB the orange-red colour representing snow is quite strong, meaning that the ground was covered by a thick layer of snow (more than 10 cm). In the Natural Colour RGB the snow covered areas appear as cyan, similar to the colour of ice clouds, so in this RGB snow cannot be so clearly distinguished from the ice clouds.
Snow cover could also be seen in southern Germany, all over Czech Republic, parts of France and Austria, the Alps and the Pyrenees in NOAA-20 VIIRS imagery (Figure 2), which shows a better spatial resolution view. In the future this view will be available from the Meteosat Third Generation Flexible Combined Imager (FCI) sensor.
The storm originated over the mid-Atlantic, where a fast-moving low, arriving from the American east coast, caught up with the shallow low-pressure centre south-west of Spain. The system then moved eastwards. The intensification began with strong cold air advection from the north, with the deepening of the upper-level trough, as seen in Figure 3, and the humidity being advected from the Atlantic.
Cold air advection in this case was quite exceptional, reaching as far south as the Canary Islands, due to a very deep (and very narrow) upper-level trough. The loop of Aimass RGB images from 8-10 January is shown in Figure 4. The heaviest snowfall occurred on 8 January.
A loop of coloured HRV images shows convective clouds embedded in the frontal cloud band during the period of the heaviest snowfall (Figure 5), whereas Figure 6 shows the movement and distribution of the coldest IR 10.8 pixel during that time.
Novel spectral view on the snow
Cloud Phase RGB
This RGB reveals notable gradient in blue shades of snow (most prominent gradients in the Aragon region, close to Zaragoza) (Figures 7 and 8). This is mostly due to the effect of the orography of the Ebro river valley, where the, probably melting, thinner, snow layer with bigger crystals in lower (and warmer) regions shows lower reflectance both in NIR1.6 and NIR2.2 spectral regions — this is also, normally, the case for clouds with big ice crystals. Snow in the higher elevations takes on brighter blue shades in the Cloud Phase RGB, and brighter cyan shades in the Natural Colours RGB. Smaller snow crystals show more reflection in spectral regions NIR2.2 and NIR1.6) and deeper, cleaner snow also reflects more in the VIS0.8 region.
Cloud Type RGB
In this RGB snow normally appears as green, since Sun's radiation is reflected much more in 0.6 µm spectral region than in the 1.3 µm or 1.6 µm regions. However, in the higher elevations, where there is less water wapour content in atmosphere above the snow, NIR1.3 channels start to detect snow. The spectral region around 1.3 μm is known for strong absorption of the Sun's radiation by water vapour molecules. Since the NIR1.3 channel is assigned to a red beam of this RGB, a joint signal from NIR1.3 (red) and VIS0.6 (green) beam renders snow in yellow shades. Higher elevations, or very dry total column water vapour content, can lead to snow appearing in yellow. One other possibility to detect 'yellow snow' in this RGB product is when there are (very) thin cirrus clouds (red) above snow covered ground (green) (Figure 9).
True Color RGB
Another product, which will be available in the future from MTG, is the True Color RGB, which utilises spectral channels with similar colour assignment to that of human eye, hence the 'true' look of this RGB, with snow and clouds in white shades (Figure 10). Due to cloud/snow ambiguity, this is not the best product when it comes to snow detection.
Quick guides to next generation RGB products can be found on the EUMeTrain pages.
Heavy Snowfall Blankets Spain (NASA Earth Observatory)
Storm Filomena: Spain races to clear snow as temperatures plunge (BBC News)
In Pictures: Madrid shovels out of record blizzard (Al Jazeera)
Madrid Is Buried Under Heaviest Snowfall in 50 Years (The New York Times)
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Cold wave along the Spanish Mediterranean coast
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