January 2015 summary for Reigate, Surrey
Average temp 4.5C (UK av 3.7C, CET 1C above long term average)
Tmax 14.7C (12.2C 2014)
Tmin -4.9C (-1.5C 2014)
Total precipitation 69mm (183mm 2014)
Max wind gust 42mph (52mph Jan 2014)
snowfall was recorded on 3 days, snow lying on 2 mornings
Total sunshine 96 hours (98hrs 2015)
January was mostly under the influence of an occasionally strong westerly and NW airstream with some stormy weather for the NW of the UK but, predictably, Reigate and the SE was sheltered from most of the weather action and our rainfall total was about on the long term average for the South East at 69mm. This was, of course, considerably less than the 183mm rainfall last January 2014!
Overall, January was just about average temperature in the UK. This average hides the variation though… the start of January was extremely mild with some very warm Tmax temperatures early in the month reaching nearly 15C in Reigate. The end of January was considerably colder than average although nothing extreme.
The end of January cold snap ran into the first week of February. During this period Reigate saw temperatures drop modestly below average and we experienced 3 spells of modest wet snowfall, albeit lying snow from night time falls rapidly melted by early morning and the lowest temperatures we a mere -4C. Reigate experienced one notable but brief “thundersnow” event on the afternoon of 29 Jan. This caused some local traffic disruption.
The cause of our swings in weather a neatly summarised in the mean monthly sea level pressure and 500hPa anomaly chart from the JMA. Here you can see the building high pressure over the Azores towards Iceland and the low pressure to the North and NE nudging towards Scandinavia. It is this configuration of building ridge in the Atlantic and LOW over Scandinavia that eventually brought our modified “Arctic blast” through the last week of January. As is normal for an Arctic airmass the SE of England away from the North Sea coast of Kent rarely sees any prolonged snowfall and this was the case. An interesting feature developed on the 29/01/2015 within the Arctic airmass: a polar low may have spun up and moved South through the Irish Sea. This was controversial and not accepted as a true polar low by everyone but it seemed to have many of the characteristics. A post on this polar low can be found here.
The evolution of the January 2015 cold snap was interesting because it was initiated by an unusually cold and unstable NW airstream on the back of a deep low that crossed Scotland. The NW airstream was unstable enough to bring the thundersnow event to Reigate. Oddly the original NW “blast” from Greenland pushed through so quickly that a lot of polar maritime returning and tropical maritime air was secluded in the low core over Scandinavia. It was this secluded / occluded warmer airmass that a) probably contributed to the polar low feature and b) modified the Arctic blast and , at least initially, made it much less cold than is usual for such an airmass direction. It took several days for any truly cold air to reach the SE of England and, even then, 850hPa temps never fell below -6 or -7C.
The CET for January was nearly 1C above average for the long term average, quite a lot more than the MetOffice. As can be seen from the chart below this January was not exceptionally warm, being moderated by the cold snap at the end of the month. Only 6 of the last 20 years have come out colder than the long term CET average.
Globally January was +0.35C above the long term (30 year) average temperature. The northern hemisphere was +0.55C above the long term average.