By Marco Virgilio (authomaticaly translated by google)
Weather trend week n. 50 – 20-26 December 2021
Right in these hours a first cold break is directed towards the central-southern Italian regions. A more convinced attempt will also bring cold air to the Alpeadria macro-region at the beginning of next week. However, it seems to be a “hit and run” destined to quickly leave the field to wetter and milder Western currents during the Christmas holiday.
The premise suggests a rather lively evolutionary phase even if not exactly comforting for lovers of the cold and snow. For those, and there are many of us, that a white Christmas would be hoped for, disappointment is just around the corner even if hope is a friend of the decline in the reliability of long-term forecasts.
So what do the two main global models ECMWF and GFS tell us? A few days ago the hypothesis of a full-bodied cold outbreak of an Arctic-continental matrix was anything but a pilgrim for the Alpeadria. As the days went by, the subsequent modeling issues gradually reduced the possible event to the point of downgrading it to a rapid and fairly marginal episode.
The vast anticyclone, currently centered with its peaks between the British Isles and France, will tend to lose vigor, to narrow its range while expanding a little to the north with the axis of the promontory extending from Greenland to the western Mediterranean Sea within Monday 20. This will be due to the strong push of very cold masses descending from the Arctic towards Eastern Europe and the Balkans. However, the high pressure will remain strong enough on the central-western Mediterranean to repel the cold attack which will then move away to the east.
Synoptic evolution at the isobaric altitude of 500 hPa elaborated by the ECMWF model: the anticyclone expands towards the north but on the whole gradually weakens under the pressure of the icy arctic currents.
The cold blast, which will bring frost between Poland, Ukraine and western Russia with isotherms of -20 / -22 degrees at the isobaric surface of 850 hPa (in this case corresponding to about 1300 meters of altitude), will have much more marginal effects in Italy and in the macro-region Alpeadria, slightly more incisive in the eastern Slovenian areas.
The trajectory of the cold break is well traced by observing the movement of the jet stream: while in the current period the eastern branch crosses the skies of Scandinavia, passes through Moscow and then bends towards southwest and points to southern Italy with winds up to 140/160 knots, the new impulse in gestation on Sunday will hit the Balkans on Monday and then fade away and go rapidly towards the Black Sea, largely losing the meridian component of its motion.
Animation relating to the dynamics of the jet stream showing the current phase, the second icy pulse of the beginning of the week and the rapid change in general circulation that could lead to the reinforcement of the Jet Stream between the Azores and the Iberian Peninsula close to Christmas.
The cold in the Alpeadria area will be short but will still be felt between Monday evening and Wednesday as isotherms of -4 / -6 degrees are expected with peaks of -6 / -8 in the eastern Slovenia at an altitude of 1500 meters. Already on Thursday 23rd the weather will tend to be again milder due to the change in circulation which will favor the gradual return of weak Western flows progressively less cold.
Evolution of the thermal field at the isobaric surface of 850 hPa: highlight of the expected cooling between Monday evening and Wednesday followed by the progressive increase in temperatures in view of Christmas
Considering, as always, the decline in reliability that burdens the numerical calculation, it must however be said that the Ensemble scenarios show a fair agreement until Christmas.
Consequently, it appears quite probable that during the Christmas holidays a favorable scenario for humid and relatively mild southwestern calls activated by deepening depression fields between the Atlantic and the Bay of Biscay could actually materialize. It goes without saying that for the Alpeadria it would be a scenario of rain, winds between Libeccio and Scirocco and snowfalls only at medium-high altitudes in the mountains.
The expected synoptic is not generally favorable to rainfall for the Alpeadria macro-region, at least until Wednesday 22. The hypothesis of some rain between the lower Veneto, the coasts of Friuli Venezia Giulia, Istria and Croatia is more likely between Thursday 23 and Christmas Eve. If the subsequent and more organized deterioration between Christmas and Boxing Day is confirmed, then it can rain over the entire area of interest. Evolution of course to be confirmed.
Precipitation forecast for the week 50-2021. In green and yellow the rain, in blue the snow (model GFS).
The synoptic evolution clearly indicates that next week will be characterized by an appreciable decrease in the ground temperature in the first part and an increase in the second. On colder days, especially Tuesday 21st, extreme minimum values can be recorded up to -4 / -6 degrees in the plains between Veneto and Friuli Venezia Giulia, down to -10 / -12 at the lowest altitudinal levels in Slovenia.
Frosty climate on the Alpine hills and on the Karavanke. The values that will then tend to increase between Wednesday and Thursday returning closer to zero in the minimum values and even slightly positive at the altitude of 1500 meters at Christmas.
The forecast of the temperature anomaly at the isobaric surface of 850 hPa for week 50-2021 (ECMWF model).
The expected circulation is not conducive to snowy episodes. There should therefore be no new snowfall either in the mountains or in the plains until Christmas Eve. New amounts of snow could arrive between Christmas and Santo Stefano at altitudes that can be assumed between 900 and 1400 meters above sea level for now.
The possible snow phase of the Christmas holidays. At the moment it is difficult to establish quantities even if the most probable dynamics is associated with a southwestern wet recall (ECMWF model).
A trend line needs confirmations that we invite to deepen always following the updates of the agencies in charge, as well as the local weather alert services.
What has just been described represents in fact a simple look at the synoptic trend based on the model products freely available on the net, and does not constitute a weather forecast.
The maps shown and discussed in this article are taken from the portal tropicaltidbits.com. Ensemble maps, are extracted from https://www.wetterzentrale.de. To consult the model updates we suggest to connect directly to the mentioned portal.