::: Snowfall 18/12/2009

The copious snowfalls in Tuscany between Friday and Saturday night (18-19/12/2009) have to be considered an exceptional meteorological event for our region. A lot of snow has fallen everywhere, both on mountain areas, on plains and on coasts. The south region is the only one where snow has not fallen due to too high temperatures and poor precipitations. Snow accumulations up to 40 cm on the Apennines, 20 cm on Pistoia and Prato city, 15 cm on Lucca, 10 cm on Arezzo, a few centimeter on Florence and Pisa; traces of snow on coasts and Viareggio's beaches.

These snowfalls have been caused to collision between cold air masses from Siberia - which carried very cold air in Italy the past days - and warm and humid ocean air currents. A low pressure area between Liguria and Corsica has been formed and rapidly has moved towards east. It has gave rise to heavy rainfall, thunderstorm and snow down to low altitude. Cold air masses had already been coming for different days in Central Europe, favoured by the presence of a blocking anticyclone extended in latitude from the Atlantic to Greenland. These cold air masses have affected more directly Italy only during the weekend . This baric configuration occurs rarely but usually it is more probable we can see that in January, in full winter, rather than in December.

In these circumstances it was difficult to forecast if it would be rain or snow in Pisa. An initial burst of snow has occurred around 16 pm; although temperature was about 4 ° C, the extreme cold at high altitude (-5 ° C to 850 hPa) and very low dew point have allowed the flakes to reach the ground without attacking. Precipitation has occurred again at 18 but temperature of 3 ° C, higher humidity in the air column and incoming warmer currents at high altitude has prevented snow could reach the ground and so precipitation was prevalently rain or very wet snow. However temperature has continued to decrease slowly to few less than 1 ° C and precipitation has intensified, so the air column was cooling. After 23 rain has turned to snow in the city. It has snowed heavily more than half an hour with very large and dry snow flakes, so the snowfall has been starting to cover pools of water and roads.

Unfortunately wet snow has been starting again to fall in the night and snow already on the ground has been partly dissolved and partly turned into mush. However, Pisa had a white awakening on Saturday morning because a bit 'of snow was remaining on cars, roofs, roads and gardens. A rare event in Pisa, since the last snowfall was in late December of 2005. The cold air following the perturbation (up to -9 ° C to 850 hPa) has meant that temperature were very low both Saturday and Sunday, so the snowfall has remained on the ground throughout the region. In Pisa high temperature just reached 1 ° C on Saturday while low temperature reached -5°C on Sunday morning (clear sky helped temperature to decrease), the lowest temperature since year 2000. The lack of wind and a more thickness of snow on the ground have decreased temperature down to -10 ° C in some places; these values are exceptional for our region. The lowest temperature value it has been measured is -18°C in Anghiari (AR) town.