19 April 2018
It was an April day to remember in old London. The UK Met office reported a temperature in St. James’s Park in central London today of 25.7° C. That is a very warm day for the UK in July or August and this was the hottest April day in London since 1949. Even in North Wales, at Gresford, the temperature reached 25° C (77°F). The reason for the heat is a highly unusual upper-level high-pressure system over France. This dome of warm air has blocked the normal progression of Atlantic cold fronts that keep the UK mainly in the 10-15° C range in later April.
The map below shows the 500 millibar heights over western Europe. This chart shows how high up the 500 millibar pressure level is. The warmer the air beneath, the higher you have to climb to reach 500 mb, so this is a good chart to quickly look at how warm the bottom half of the atmosphere is. This is the initial analysis for the European ECMWF model from the Noon GMT today in the UK.
Tomorrow will not be as warm, as the high pressure begins to weaken, but it will still be unusually mild with temps. over southern England at 21°C and maybe 22°C in central London. In Paris, the temp. reached 28° which is 82°F. There is growing research indicating that these blocking highs are becoming more frequent as greenhouse gases rise in the atmosphere, and there may also be a connection to the drop in Arctic ice.