Friday 29 January 2010, 15:49
The big freeze of January 2010 released its grip mid month with a thaw. The period January 15th to the 18th was mild in comparison with daytime temperatures as high as 8 to 11 Celsius.
But January is going to finish on a cold note with northerly winds from the Arctic this weekend bringing a mixture of sunshine, frost and wintry showers.
Most of the snow will be in Carmarthenshire, Pembrokeshire and Ceredigion with accumulations likely, especially on higher ground e.g. The Preseli and Cambrian Mountains.
Although the month is not over yet, it looks like being the coldest January in Wales since 1987 with a mean temperature of 1.0 Celsius.
The coldest ever January was in 1963 with a mean temperature of -2.5 Celsius.
The big question on many peoples lips now, though, is have we seen the worst of the cold this winter or are we in for another freeze sometime in February?
Well predicting what is going to happen more than 5 days ahead is not easy. There are no guarantees but there are signs that a forecaster can look for.
The very cold spell earlier in January was due to a combination of factors. One factor was a warming of the stratosphere above the North Pole which began in late November and December 2009.
Normally low pressure sits over the pole but when warming takes place, the low begins to weaken and change direction.
This led to an anti-cyclonic (clock-wise) circulation developing high in the atmosphere. The reversal took about 3 weeks to find its way down to the surface with high pressure forming over the pole.
Air in high pressure systems flows outwards and the cold air began to filter southwards into northern and central Europe.
The extra warmth in the stratosphere can cause a blocking high pressure to form and if it develops in the right place in winter it can mean very cold winds for Britain.
What is of concern now is that more stratospheric warming started last week. This suggests that another blocking high is likely to develop during February.
And the Arctic oscillation (AO)
Both the NAO and AO are connected and affect our weather patterns. They have both gone into a negative phase recently, having temporarily turned positive around the middle of this month - and are forecast to stay negative into early February.
This suggests that the jet stream will move to the south of us once again, and high pressure is likely to build over Scandinavia and Greenland with Britain exposed to very cold air from continental Europe.
Of course, this is a forecast and things could change - but at the moment there is a strong signal for another very cold spell with an increasing risk of snow after February 6th although the finer details will obviously become clearer nearer the time.
A real winter for a change!
Join the discussion...
Friday 29 January 2010, 13:24
Monday 1 February 2010, 14:52