Its been the snowiest December weather since 2010, here in Watford. After not having any snow for the last 5 years we have had 5 days of snow cover, and 8 days of falling snow. Which is also more than the last 5 winters put together.
The remarkable thing is that it hasn’t been that cold. The snow has been of the wet variety which means that the snow flakes have been bigger but melt quicker. We have always been on the border line of any snow, the upper air temperatures have only just been cold enough for a snow flake to make it all the way to the ground.
Atlantic fronts
Our snow has come from fronts moving in from the Atlantic and bumping onto cold air, which originated from the Arctic. The rain has only turned to snow because of the intensity of the rainfall, by evaporative cooling. The snow flakes that are falling through the air would normally turn to rain when an Atlantic front moves in as the air close to the ground is normally too warm, above 0c. But the snow flakes that are falling, are cooling the air as it falls, with the intensity this allows more of the air to cool. Thus bringing the boundary layer where snow would turn to rain closer to the ground. Heavy wet snow can accumulate quite quickly which is why we ended up at 15cm.
Jet Stream
We have also been on the cold side of the jet stream for most of the month. Bringing low pressure systems close to the UK or going across the southern part of the UK and into France keeping the UK on the cold side of the jet. This looks like continuing into the new year.
Watford Stats
As mentioned above it has been our snowiest month for 5 years with 8 days of falling snow, our highest depth has been 15cm back on 10th December. The coldest temperature has been -3.8c on the 12th with the wettest day being the snow to rain event on 26th giving 15mm in the 24 hour period. With 10 dry days it hasn’t been a particularly cloudy month we have had quite a bit of sunshine and 8 air frosts.
UK Stats
Being on the cold side of the jet stream for most of the month has resulted in lots of snowy days across the UK. The highest snowfall was on the 11th with 31cm at Sennybridge in Wales.
2017 round up
Its been another above average year by 0.55c, I thought it was going to be much warmer after the first 6 months but it has gradually cooled down. The final figure for the year came out at 10.58C. Only 4 months were below average with all the others above and some very significantly. I have added the list from the Met Office below, all in Celsius, its was the 5th warmest year since UK records began in 1910.
The most notable weather of the year was in September when ex Hurricane Ophelia dragged up Sahara dust and made the sun disappear at midday.
The wettest place in the UK was Ennerdale with 168mm of rain in 24 hours on 15th May, the highest temperature of the year was at Heathrow with 34c on 21st June. The snowiest place with 31cm was Sennybridge in Wales on 11th December.
January 4.0 [-0.5]
February 6.1 [1.7]
March 8.7 [1.1]
April 8.9 [0.3]
May 13.2 [1.5]
June 16.0 [1.5]
July 16.8 [0.1]
August 15.6 [-0.7]
September 13.5 [-0.2]
October 12.4 [1.65]
November 6.8 [-0.3]
December 4.8 [0.2]
Looking ahead to January
The first part of the month is looking mild with lots of rain and wind, the jet stream has picked up in strength recently which has been as a result of the very cold weather in the USA. This has increased the temperature differences helping to strengthening the speed with lots more low pressure systems travelling across the Atlantic towards Europe. After the 5th there looks like being a cold northerly plunge which could result in lots more snow over the UK and possibly a longer cold spell. The rest of the month is looking unsettled.
Watford Stats – Summary for December 2017
Temperature (°C):
Mean (1 minute) 5.3
Mean (min+max) 5.4
Mean Minimum 2.6
Mean Maximum 8.2
Minimum -3.8 day 11
Maximum 13.7 day 30
Highest Minimum 8.3 day 30
Lowest Maximum 2.0 day 10
Air frosts 8
Rainfall (mm):
Total for month 64.5
Wettest day 12.0 day 11
High rain rate 14.4 day 25
Rain days 21
Dry days 10
Wind (mph):
Highest Gust 22.8 day 29
Average Speed 1.4
Wind Run 1034.6 miles
Gale days 0
Pressure (mb):
Maximum 1044.9 day 22
Minimum 979.7 day 10
Days with snow falling 8
Days with snow lying at 0900 5
Links
Met Office – https://www.metoffice.gov.uk/hadobs/hadcet/cet_info_mean.html
Real time Watford Weather – https://weather.andrewlalchan.co.uk/
Flickr – httpss://www.flickr.com/photos/alalchan
Weather Outlook – httpss://www.theweatheroutlook.com/twocommunity/
November 2017 – https://blog.andrewlalchan.co.uk/watford-weather-november-2017-statistics/
