Turning cold late November; but how long for?

It’s been a much milder than average Autumn.. so far. That includes the first two months, September and October, as well as the first two weeks of November. The weather is about to take a change for a much colder pattern, though, thanks to the development of a “blocked” pattern. Such patterns are responsible for driving colder air into the UK during winter months.

Often in the UK cold patterns are slow to develop, taking several days or even weeks to build to a magnitude enough to bring cold into the UK. These spells are fraught with uncertainty to, especially when the cold spell is still a few days away. Questions such as how cold will it get, cold, very cold or severe, and how much snow and where, are questions which will remain relevant until quite close to the cold snap. This is why definitive forecasts for surface conditions can not be made until closer to the upcoming cold spell.

With that in mind, let’s take a look at the model output for the UK. It’s very much indicative of this “slow developing” blocked pattern mentioned above, and is also fraught with some uncertainty regarding details, also as mentioned above. Slow developing blocking systems can also be slow to unwind, too, meaning that their effects linger on for longer periods of time. This potentially means a more prolonged cold spell could be on the time, perhaps lasting between 7 and 10 days. This is no guarantee of a prolonged spell, however it does seem to be in scope given the strength and durability of the modeled blocking.

Much of the computer output is on roughly the same page, now, regarding the upcoming cold spell. Yes there are differences in the depths of cold, and therefore snow. These differences only need to be very slight to produce large differences in surface conditions, i.e. the difference between rain and sleet and wet snow, is only a difference of 1 degrees C. However, getting the cold in is fairly well supported throughout the model output. Let’s take a look at the ECMWF (European weather model). It starts us on a mild week with showers in the west, before turning cold later into the weekend. Sunday and Monday look to be the coldest days of the “initial” northerly, with highs between 4 and 7C across the UK, lows below freezing in places leading to a risk of ice. There may also be some wintry showers on the high ground across Scotland, and North East England. Say above 250-350m in these areas.

The chart below for Sunday shows high pressure to the west, and low pressure to the east. This is responsible for the “weak” northerly. Only a sign of things to come.

The air isn’t particular cold nor thicknesses deep so any snow that does fall will be light and restricted to hills. As we head through Monday and into Tuesday the high sinks south from Iceland to take up a position directly over the UK. This puts the flow into the east – a chilly easterly but too mild for snow to lower levels, so instead, just a cold, windy and grey picture across the south of the UK. Frost can be expected overnight, potentially moderate frosts in the north where winds are lighter, closer to the center of the high. Some winter sunshine is possible, but also some fog in places due to the light winds.

Thursday 24th and Friday 25th of November is when things really start to get going cold wise. The ECMWF shows a screaming northerly head south on Thursday bringing with it cold air. The depth of cold shown on the ECMWF would produce day time temperatures of 0 to 5C UK wide, with overnight temperatures of -2 to -5C. These temperautres are not highs or lows, instead they respresent a typical temperature for the periods of day mentioned away from the extremes. It is evident that the air is cold enough for snow for most areas. Coastal areas, especially in the east would see frequent wintry showers with snow falling readily to an altitude to 200ft, granted respectable distance from the coast. Below this altitude and nearer the coast, temperatures would rise above freezing between showers allowing for a thaw of lying snow, however the heavy precipitation would quickly allow for snow to fall again.

ECMWF chart for next Friday showing cold air engulfing the UK, bringing with it a risk of snow. Notice the slightly less cold air to the east of the UK, potentially delivering more of a sleety mix to low ground eastern areas.

In the west, conditions would be significantly drier with a good deal of winter sunshine. Some showers would get through, however the risk of snow would be restricted to a trough feature. On western coasts, however, showers would likely fall, potentially the “Pembrokshire dangler”, for example. A trough feature, or low pressure, could pop up anywhere in the cold northerly flow and would exacerbate the snow risk. Of course, these details are merely speculations for now, however are suggestive of potential conditions if the northerly was optimal in terms of bringing cold air into the UK.

There is still a change, as has been suggested on some model output, that the coldest air could push both east and west of the UK. This would increase the risk of marginality, i.e. more of a wintry mix rather than snow, and is still a valid risk. As mentioned before, only time can tell as to which outcome will verify, the coldest option, which would likely give some places a memorable week of winter weather. Or, the less cold options, which would give quite a few more areas a wet week, cold though, with enough cold air about for a risk of a snow event or two.