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“Let it snow” in Frank Sinistra’s voice. “White Christmas” in Adriano Celentano’s. In the collective imagination, the Christmas of dreams is that of snow. A dream that has been cultivated since childhood, watching Walt Disney cartoons. But seeing snow in the city at Christmas is a dream. In the last 40 years the coveted Christmas Eve snow has never been seen; On Christmas Day it snowed a couple of times, but in the order of negligible sprays.
On a Christmas affected by the covid-19 health emergency, the snow will give it a touch of magic, painting the interior white and locally at lower altitudes. Only on December 25 there will be a change in weather conditions. To illustrate how the holidays will pass from the atmospheric point is Pierluigi randi, Certified Meteorological Technician and Ampro Meteorologist (Professional Meteorological Association).
It will be a different Christmas than in recent years not only because of the pandemic, but also meteorologically speaking. What Should We Expect?
After a fairly long period of gloomy, humid and very mild weather, which is still going on, there will be a rapid change on Christmas Day. Indeed, a rapid cold front will descend from northern Europe, digging a depression in the ground that will move very rapidly from the Gulf of Liguria to the southern Tyrrhenian, preceded by an even softer air call and chased by a mass of colder air that will burst from the north. -East. It will be the air of the Arctic sea, therefore very cold and unstable in height; much less on the ground, just enough, however, to cause a marked drop in temperature, although we cannot speak of a true “cold wave”, it will simply return to the most appropriate conditions for the season. In any case, the pace will be fast, and already on the day of Santo Stefano the weather will improve, in addition to the last phenomena of the dawn in the Apennines and Rimini.
This descent of cold air will be preceded by a soft call. What change in temperature should we expect?
In the areas that will be affected by the southwest winds that precede the cold front, therefore, particularly in the provinces of Forlì-Cesena and Rimini, between the afternoon and the night before, the temperature values will be quite high, locally up to 15-16 ° C. Then, on the morning of Friday 25 there will be a first rotation of the winds from the west and, from the afternoon, from the northeast with a rapid decline to 6/7 ° C in the low areas and around 8 ° C in the coastal strip. Therefore, the local temperature difference could be around 8-9 ° C. On Christmas night the temperatures will drop more except for the coastal strip, with values even below 5 ° C, while on the coast and the immediate hinterland will remain higher due to the Bora winds that will be mitigated by the transit of the air mass over the sea surface. still warm. The level of thermal zero will also drop rapidly, which on the afternoon of the 25th may reach 500 meters in the foothills.
Therefore, Christmas will see a fast and disturbed passage. As temperatures drop, can we expect snow too? Ability to flock even on the plains at the end of the event?
Snow will most likely reach most of the Apennine reliefs above 300-400 meters (between late afternoon and Christmas night); for the plains the risk seems decidedly less, if not absent, since it will start from a very soft air mass in the lower layers, and the cold, initially at least, will arrive mainly at high altitude. In any case, on Christmas night some choreographic appearances of snow cannot be excluded on the plains of the foothills of Faenza, Forlì and Cesena, indicatively with Via Emilia as a watershed between rain and snow. In the event of showers, a brief phase of snow or snow mixed with rain should not be excluded, even in the remaining low areas. Rain, however, on the coastal strip and the immediate interior.
Santo Stefano, on the other hand, will be sunny and cold. …
The improvement will be rapid, even if in the first part of the morning of the 26th, residual clouds and some phenomena may affect the Rimini area, in this case with possible short snow showers also on the coast, since on Saturday tomorrow the freezing level will be low throughout the region and the northeast winds will decrease. Then, in the second part of the day, the light episodes will prevail with a colder (but not icy) climate, dry and with excellent horizontal visibility. Nighttime and early morning frosts will occur between Saturday 26th and Sunday 27th.
How will the last days of the year pass?
There is still uncertainty about this, although it seems increasingly likely that there will be a new flow of marine air from the Arctic, but in this case with a target in Western Europe and with the formation of a deep depression over France. In this case we would be affected by a new deterioration but in the name of the mild temperatures again, especially as of Monday, that is, with some intermittent rains but with snow only on the reliefs around 1000 meters above the level of the river. sea. Temperatures could drop moderately towards the end of the year, but in this case the uncertainty increases significantly.
Will the beginning of 2021 be cold or in a late fall context?
The scenarios that are indicated for the first days of January 2021 show a greater probability of moderately unstable weather and with average temperatures slightly below the norm, and therefore with a prevalence of cold, although not intense. In essence, a condition prone to intermittent descents of polar or arctic sea air would be prolonged, alternating with shorter gentler phases. Let us remember that the true and intense cold is brought by the polar or arctic continental air masses, but at least in this first phase they do not seem to be able to intervene in the Euro-Mediterranean context.
Then there is talk of a sudden stratospheric warming. Could this have consequences for our country with the consequent cold waves?
A strong warming of the polar stratosphere is expected by the end of December, called in this case: Sudden Stratospheric Warming (Ssw), with a considerable weakening of the stratospheric polar vortex, mainly as a consequence of what will happen in the next few days in the troposphere. (the lowest region of the atmosphere) with amplification of waves and greater exchanges of heat in the meridians. West wind speeds in the middle and upper stratosphere show a drastic slowdown. In recent releases, the Ecmwf model increases sets with negative western velocities.
What does that mean?
That there is the possibility that stratospheric warming leads to a reversal of the thermal gradient, but also of the winds that would be oriented from the east, giving rise to a significant stratospheric event in early January. These types of SSW are the most incisive and capable of causing significant changes in atmospheric circulation by amplifying thermal and rainfall anomalies, both in a positive and negative sense.
Can we talk about probability now?
There is still uncertainty about this type of evolution, but, if so, it could affect the destination of January. The difficulty remains to frame well in advance the areas that would be subject to extremely cold or decidedly mild periods, but it is very likely that, at least in the first half of January, the polar vortex is weaker than normal, increasing the possibility ( but not the certainty) of strong meridian heat exchanges.
