Is an arctic blast on the way?

October snow in the south is rare but not unknown

In the UK snow is most likely in January, February and March. Therefore, to see it falling and settling away from high ground in the north during late October is very unusual. In relatively recent time the best example was on 28th and 29th October 2008.

A plunge of very cold Arctic air had covered the UK and a band of precipitation moved southwards. Initially the precipitation started off as rain in central and southern Britain. However, as it turned heavier it increasingly fell as sleet or snow, with even central London having a covering as the photograph from Regent's Park shows.

In the Chilterns accumulations of several cms of snow were recorded. Prior to this event the last time lying snow had been recorded in this area in October was in 1922.

The cold spell was notable, with some places setting new October records for low maximum and minimum temperatures. New minimum record were set at: Coningsby -4.2C, Lyneham -3.6C and Tiree -0.3C.

Any sign of an arctic blast?

There is a relatively high degree of confidence that high pressure will have a lot of influence on the weather during the next two weeks or more. That suggests an increased chance of dry periods, but temperatures will be very dependent on where the high pressure becomes centred.

For an Arctic blast to develop the high pressure needs to be centred to the west and north of the UK. That would allow a very cold air mass to move southwards, leading to the possibility of sleet or snow.

Alternatively, if the high pressure is centred close to the UK, calm and clear conditions could lead to temperatures falling at the surface, even if the air aloft is warm. While it would be cold, any precipitation would fall through warm air, resulting in rain rather than snow despite the chill at the ground level.

Where will the high pressure go?

At the moment most computer models are positioning the high pressure close to the UK. However, there is growing support for a plunge of cold Arctic air to sweep southwards into much of continental Europe. As is often the case, the UK may well be on its western edge.

The forecast chart above is for 3rd November. It shows the core of the cold plunge moving southwards over continental Europe. The blue shading over eastern Britain shows cold air, with green and yellow in the western and over Ireland indicating much milder air.

The chart above is just one prediction. Every day multiple computer models provide forecasts and when looking more than a few days ahead they usually offer a wide range of possibilities. Nonetheless, the general theme is quite widely supported.

Cold weather possible but not certain

With high pressure centred near the west of the UK mostly settled weather could be expected. Showers would be most likely in northern and eastern coastal counties of Britain. With the air aloft below -5C they could bring a wintry mix, so a few flakes of snow wouldn't be out of the question, even locally in the southern half of the country.

Despite that, the main features of the weather would be dry conditions along with the possibility of fog and frost becoming widespread. If that happened and the fog lingered through the days, it could stay very cold.

Cold conditions are not assured. Very small changes to the positioning of the high pressure could lead to a mild air mass bathing all regions and breezier conditions allowing more mixing of the air at low levels. The mixing would stop temperatures falling close to the surface, resulting in a continuation of the mild weather in the last days of October and the early part of November.

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