After the jet stream had remained in the Mediterranean basin for several weeks, the Rossby wave has been dismantled. Now, the weather pattern in Europe tends towards rapid reorganization. The embryo of what will be a powerful blocking high will start to form in the coming days. While the NAO index is temporarily on the rise, simulations show it declining in December’s second-half. The possibility of widespread cold outbreaks across Europe in the Christmas week.
The deep wave that reaches the Mediterranean Basin is bringing cold weather to the region with frequent storms and widespread precipitation, as well as a lot of snow in Alps, is about to leave Europe, at the very least for a while.
The synoptic evolution for the week ahead is already set by the two major global weather models. Both ECMFW models and GFS agree that there is a strong blocking high emerging and rapidly strengthening between Monday and Wednesday.
This map has been modified from the original portal tropicaltidbits map
The low currently affecting mainly central and southern Italy and the Balkans will quickly move towards Albania or Greece. The last of the residual precipitation will be in the form snow in Balkans until Tuesday. The low will partially reorganize above Turkey in the second half of the week thanks to a new cold lift from the Baltic, which began Friday 18th.
Both the American GFS model and the European ECMWF are in agreement that the Atlantic High will evolve in its first phase. It will tend to grow rapidly as it moves towards the north-west, positioning itself between the UK, the Netherlands and northern France.
The strong push of the jet stream in Atlantic south of Greenland will produce winds between 300-350 km/h at the Geopotential height of 250 HPa (roughly 11 km). The deepening of this upper wave will create a powerful High over UK, peaking next weekend, Dec 17-19th.
The blocking High, which was established over France on Wednesday, Thursday, seems to be moving north slowly and is not likely to change its action range until at least the following week. This will allow for more cold to be delivered from the East towards the western North Africa, Greece, Turkey and the southern Balkans.
The Omega blocking High will also push Atlantic storm track from Scotland northward into western Norway. While the fronts will not affect Central Europe for a while, Scandinavia will feel the effects of a brief but intense positive NAO phase.
This will allow for the polar jet stream’s rapid drop in latitude to occur later next week, potentially invading again and again the Mediterranean with an east pulse from the Balkans.
The shape and geometry of the block High will be changed by the jet stream’s dynamic upper wave very soon. The main axis of the jet stream will move clockwise, from West-East Wednesday to North-South Friday. Although it will be less strong, the High will still be well developed to block the zonal flow.
It will be crucial that we know, as we move towards Christmas week, what happens to the High blocking cold air from East. A strengthening of the High towards the Faroe Islands would be the first step in a possible cold wave over Central and southern Europe towards Christmas.
Residual precipitation due to the stau will occur mainly early in Austria, while the deep low over Greece or Turkey will allow frequent rains in that region and major snowfalls across the central and southern Balkans.
Precipitation over Scotland is likely to end on December 16th. The upper High will move its centre to England and heavy rains and snow will affect southern Norway. After the 16th, Norway’s precipitation will move northward, while the Mediterranean Low will intensify again, although it will be less than before due to a cold lift from the East. It could drop to 1008 hPa in its center. It is expected to see more frequent precipitation and rains in the region.
On Sunday 19th, a deep low will likely form over western Russia, causing severe conditions in the region. Significant wintry conditions could eventually be expected as snow could be very heavy at times. Blizzard conditions are possible, especially as the deep lows will also contribute to the strengthening of winds. Travel disruptions and extreme wind chill are to be expected.
Temperatures will rise quickly this week, especially in high altitudes. The freezing level in the Alps will rise to 3000m by Wednesday, but thermal inversion will occur in the alpine Valleys. Because of the recent snowfalls, as well as the low temperatures that are favored by albedo feedbacks, fog and low clouds can likely cause persistent freezing conditions for several day.
For the entire week, the below-normal temperatures will continue in the Balkans and central-southern Italys, Greece, and western Turkey. Thermal inversions can cause South Norway to experience temperatures below normal at times.
The new East cold outbreak will eventually bring temperatures down to 8-10 degrees below the normal level in the Balkans and Greece, central and Southern Italy. Snow showers at low levels are possible. On the contrary, northern Europe will experience widespread positive temperature anomalies as high as +14°C in southern Scandinavia.
There is no snowfall forecast for many European countries and the Alps throughout the week. The only remaining orographic snowfalls that will affect central Austria in the early part of this week will be north of Karavanke. The forecast for snowfall in central Austria, Turkey, the Balkans and the mountain of Greece is quite impressive.
Due to the blocking of high deflecting north Atlantic fronts, heavy snow will befall western Norway and Iceland. The snow depth could be over 100 cm by Sunday 19. The next weekend will see the emergence of blizzard conditions in western Russia.
The rapid rise in the NAO (North Atlantic Oscillation), which is forecast for this week, confirms that synoptic configuration.
In the image below provided by NOAA’s Climate Prediction Center, the NAO index appears to return to negative values soon after the 16th and in the second half of the month. This situation further suggests that the blocking High could rapidly evolve into a more typical negative NAO Index phase. The likelihood of cold-related outbreaks in Europe over the next 10 days increases.
The most likely evolution of the powerful blocking high suggests that there will be a flow or very cold air on its eastern face, right around Christmas week. This specific weather pattern could be helped by the action from the two lows west- and east-European, which are going push from both directions at the same time as the blocking High above the UK.
Both the GFS- and ECMWF models see this pattern emerging starting at week 51-2021, December 21st.
The tropicaltidbits website contains the maps that are shown and discussed in this article.
Source: Severe Weather