As a blocking high is likely over and to the east of Greenland and Iceland, a key question becomes whether the very cold air moving into Scandinavia will reach the British Isles or remain locked to our east.
The winter of 2024/25 has so far gone rather against what most long-range predictions had suggested. Most La Nina winters turn mild and westerly-dominated, and often wet for many, during January and February, with December tending to be the month when we more often see colder weather. But this time around, December was mild and often anticyclonic, and January was colder than average.
Reflecting the warming climate, January's mean temperature was generally close to the old 1961-1990 average but a degree or more down on the 1991-2020 average, though it was cold for recent years. Preliminary data suggest that January 2025 was the warmest January on record globally, which is surprising as we are moving into a weak La Nina event, which normally suppresses global temperatures by at least a couple of tenths of a degree.
Scientists are struggling to understand why we continue to see this elevated global warmth, but one theory that has circulated recently relates ironically to clean air legislation for ships at sea. This legislation reduces the amount of aerosols that ships output, improving air quality but also reducing the output of certain gases that contribute to clouds reflecting heat away from the atmosphere. However, January, especially the cold snap early in the month, showed that even when the global average is very warm, individual regions can still be colder than average.
In mid-January, many signs pointed towards westerly winds and mild and wet weather developing into February, but it now looks as if much the opposite will happen. The current Scandinavian blocking high is likely to retrogress westwards and merge with an anticyclone developing over the Arctic, producing an impressive area of high pressure over and to the east of Greenland and Iceland. Low pressure will develop over Russia, sending a scoop of very cold air into Scandinavia and into eastern Europe.
What remains open to question is whether much of this cold air will make it as far west as the British Isles. We are set to have a ridge of high pressure to the north and north-east of Britain, keeping the weather generally cold and dry, but not necessarily exceptionally cold. If we pick up an easterly or north-easterly flow, this will send a lot of that cold air our way, and we could end up in a substantial cold, snowy spell. Eastern areas would be most likely to pick up significant snowfalls in that setup, due to the winds coming in off the North Sea and instability and snow showers being generated as the cold air flows over the comparatively warm North Sea.
There is also potential for us to stay in a mainly south-easterly airflow with the colder air being largely locked away to our east and north-east, maintaining cold dry weather and temperatures generally not quite low enough to support lying snow on low ground.
The large blocking high to the north will keep depressions and fronts coming in from the North Atlantic away from the British Isles. These will be largely bottled up to the west of Britain, and shallow lows will run from west to east through southern Europe, probably resulting in unusually wet weather for the Mediterranean.
Longer-range guidance from the forecast models suggests that in around 10 days' time, the blocking high to the north will probably start to weaken, and we will likely see frontal systems and milder air attempt to push in from the south-west. This is uncertain, as the forecast models often struggle with handling these sorts of blocking highs, but there is quite high confidence among the model ensembles for the block fading starting in about 10 days' time. Thus, there is essentially a limited window of opportunity for this very cold air out to the east to make it over to the British Isles. However, confidence is quite high that much of Scandinavia and eastern Europe will see a notable cold spell due to the blocking high to the north.
In my previous article, I noted that the Februarys of the 2020s have often been particularly mild, more so than we would expect in line with the overall warming of the climate. February 2021 was the only one with a substantial cold spell, and even that was offset by a very mild second half. While there is potential for the last third of February 2025 to be much milder, which is uncertain, it is looking increasingly probable that the first two-thirds of the month will be colder than average, and so overall we can expect a substantially colder February than we have become used to in recent years.
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