Reigate May 2016 summary statistics
- Tmax 27.4C
- Tmin 0.3C
- Tav 13.8C (UK 11.3C)
- total rainfall 42mm (town) 45mm (Hartswood)
- max gust 36mph
- average wind direction NNE
- sunshine 181.7 hours (May 2015 161 hours)
Whilst there were fortunately no severe weather events in Reigate and few across the UK in May, the weather we experienced more widely could be linked tenuously to climate change. Of course, caution is required with such speculative statements but attribution studies on the May floods in Paris, not so far away, have concluded that they were made 90% more likely due to climate change. The same stalled low pressure system delivered our easterly winds so we were influenced, albeit on the edges, by the same blocked weather pattern.
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- more sunny than usual Wales
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- easterly winds with some sunshine but heavy showers Surrey
People attempting to climb Snowdon in North Wales in May were lucky to experience sunnier-than-usual conditions for much of the month (south wales had more thunderstorms which reduced the sunshine totals there). Meanwhile, in Surrey, we experienced occasionally warm conditions with an unusual mean monthly wind direction from the NNE.
Reigate pressure rose hesitantly mid-month
Pressure fell across the UK to start May but then rose mid-month, especially to the North, bringing a relatively unusual easterly flow into Reigate and the south. Whilst there were few severe weather events during the month, this post briefly explores some of the wider factors that may have contributed to this Easterly flow and the possibility of it being linked to climate change.
Whilst mostly dry for the UK as a whole, occasional showers, some thundery, brought Reigate rainfall totals to just above average at around 42-45mm as recorded from our two weather stations respectively in and out of town. SE England as a whole recorded rainfall at 111% of normal rainfall, mostly falling in thundery showers, more common in SE wind regimes.
With relatively dry Easterly winds, sunshine totals for the UK were accordingly above average given the relatively high pressure overall. Reigate experienced 182 hours of sunshine in total for the month.
Unusually, the sunniest places in the UK were in the North and West given the easterly winds bringing occasionally cloudier conditions off the North Sea to the south and east. May 24 shows a typical scenario with the higher pressure to the North dragging in E/NE winds across the southern part of the UK with cloud across eastern areas and clearer conditions to the west.
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- May 24 2016 synoptic chart
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- May 24 2016
Some great sunny days were recorded in the mountains of Wales, Cumbria and Scotland!
Sunny Snowdonia with kelvin-helmholtz type wave clouds trying to form over Ogwen Valley
The higher than average rainfall patches shown below in the south were associated with showers on occasionally unstable warm and humid SE winds. Reigate reached a Tmax of over 27C in this warm flow.
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- Rainfall anomaly May 2016
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- Temperature anomaly May 2016
The wider pressure pattern across the northern hemisphere was characterized by anomalously high heights over the Arctic and LOWER than normal pressure in mid-latitudes including Europe. This situation is called “northern blocking” and in winter could cause cold conditions in mid-latitudes. In Spring, as the continent rapidly warms up in stronger sunshine, easterly winds can be warm or even hot for the UK.
northern blocking over Pole
Northern Hemisphere pressure patterns are measured by the Arctic Oscillation which, as can be seen below, remained unusually negative through much of April and May showing high pressure persisting over the Arctic relative to low pressure in the mid-latitudes. This pressure pattern turned winds from the usual westerlies into easterlies in the UK and Europe.
negative arctic oscillation Spring 2016
The causes of this reversal of the usual mid-latitude zonal westerly wind set-up have been linked to low sea ice extent in the Arctic, especially the Kara and Arctic Gateway seas. Warmer influxes of air into the Arctic builds air pressure which then links to higher chances of Easterly winds in mid-latitudes.
low Arctic sea ice cover March 2016
The very low sea ice extent this year was brought about by much warmer-than-usual conditions during the Polar winter, where monthly average temperatures in the Arctic (>60N) were at times 3.5C or more above average during the cold season of 2015-16. This Arctic amplification is widely accepted as being caused by human induced climate change.
It turns out that Spring Arctic sea ice extent is some of the lowest recorded in the 38 year satellite series.
So, unusual sunshine in North Wales, a warm NNE mean wind direction in Reigate and cloudy conditions on the east coast can be linked to the above tele-connecting weather patterns which, in turn, can be linked to climate change in the far flung Arctic.
Meanwhile, the strong 2015-16 El Niño declined rapidly through May and ENSO conditions were neutral by early June. Models suggest the chance of La Niña (cool Pacific) conditions by Autumn 2016 are as high as 60%. Some forecasters bring La Nina through the summer. La Nina, and the warmer SSTs of the tropical Atlantic, are associated with more frequent hurricanes in the Atlantic basin. In turn, high hurricane accumulated energy transfered to the North Pole during such seasons can build Polar heights in Northern Hemisphere winters, warming the Arctic and further melting sea ice. Whilst this is just outrageous long term amateur speculation, it is nevertheless interesting to ponder the potential for feedbacks to accelerate further climate change in the near future.
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- ENSO CFS 2016
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- ENSO ecmwf 2016
The turning down of the vast heat engine of the El Nino might be linked to the slightly lower May global average temperature, though confirmation from expert sources has not verified this as yet.
Local data for May and all months stretching back to 2012 can be found on our data page here
http://www.metoffice.gov.uk/climate/uk/summaries/2016/may
Arriving from high altitude, stingjet winds are unsurprisingly cold with low dew points. So you’d expect a dip in temperature at the surface. Parts of Wales and Shropshire had snow in this airstream wrapping round the back of the low as it moved into the North Sea. Stingjets match the mature stage of the Shapiro-Keyser cyclogenesis model. In all respects, they are aptly named as the “sting in the tale”.
The chart shows the persistent warm sector gales proceeding from midnight on 28 March through to about 5am. These gales do not exceed 80kmh but they modestly peak just before the passage of the front, an expected pattern. Fronts then pass through Reigate at about 5am and temperatures fall as expected, as do gusts. However, from 7am gust strength sharply increase, this matches the timing of the sting jet cloud feature on satellite photos. This increase in wind speed had no front associated with it and therefore suggests evidence of a stingjet process: arriving out of the blue!
For comparison shown above is a “normal” cold front passage from the previous weekend when a cold front squall line of some note passed through. Note there is no dip to minimum temperature associated with the maximum gusts and temperatures remain cold after the passage of the front because of the insurgence of cold polar air. This is more typical behaviour when it comes to frontal passage.
sirgeogy
Reigate Grammar School Weather Station 