September 2016 heatwave
After an unusually warm start to September, there was a remarkable heatwave in the middle of the month - that resulted in the hottest temperature of the year and rounds of severe thunderstorms developing from the 12th - 15th. The 13th was the hottest September day in the UK since 1911, while some areas in the south saw over half a month's rainfall in under an hour from the downpours.

Whilst it is not unusual for heatwaves to occur during September, and for temperatures to exceed 30˚C, it is very unusual for the hottest temperature of the year to be recorded outside of the summer months. The heatwave brought with it three consecutive days (from the 13th - 15th) when temperatures exceeded 30˚C - the last time this happened was in 1929. On the 13th, the maximum temperature was 34.4˚C at Gravesend (Kent) - the hottest day of the year. For the 14th, the maximum was 31.1˚C at Marham (Norfolk) and 30.1˚C was reached at Gravesend on the 15th. 

The high temperatures were brought about by a weather set-up that regularly brings warm or hot weather to the UK in the summer months. A blocking area of high pressure over Scandinavia forced the jet stream to the north of Europe - an omega block - and prevented areas of low pressure and their associated weather fronts from moving across the UK. Meanwhile, across much of southern Europe, the heat had been building for several days previously, with heatwave conditions across Spain and Portugal. Here, temperatures reached over 40˚C. 

A warm south-easterly airflow resulted from the 12th September over central areas of the UK. As a result, hot tropical continental air was advected towards eastern parts of England for the next few days. The strong advection of this hot air helped to offset the weakening solar insolation that is available to heat the ground during mid-September. Along with sunny conditions, this is what allowed temperatures to soar. The surface analyses below illustrate how warm air was imported into the UK:

The preceding day to the heatwave saw temperatures into the middle-20s˚C in the south-east, with a warm southerly airflow. 

The Atlantic front remained quasi-stationary for the next few days over western parts of the UK, as hot air filtered into more eastern areas. As a result, not all places had the hot weather - some locations in Scotland reached no higher than 14˚C on the 13th, while it was 20˚C higher in the south-east of England! Maximum temperature 34.4˚C.

By the 14th, much of the UK was within the hot tropical continental airmass. However, subtle changes in the conditions meant that the maximum temperature was not quite as high. Notably, the wind took a more easterly course, meaning it had a longer fetch over the cool North Sea before it reached the UK. Also, slightly higher winds dampened temperatures at the surface. Maximum temperature 31.1˚C.

By the 15th, the air becomes more stagnant, meaning there is little advection of hot air. The air mass began to cool, resulting in slightly lower temperatures than on the previous days. Maximum temperature 30.1˚C.

Overnight into the 16th signalled the end of the hot spell, as a cold front swept eastwards across the country, bringing with it fresher Atlantic air. However, numerous thunderstorms broke out across southern-central England, producing frequent lightning and flash flooding. 

*** There has been some debate in the meteorological community over the accuracy of the weather station at Gravesend. It is a known hot-spot and holds the all-time UK temperature record of 38.5˚C (101.3˚F), recorded on 10th August 2003. It is located in a meander in the river Thames to the east of London, where there are tidal mud flats exposed at low tide, which can locally increase the temperature. The station is in a sheltered location with slight grassy banks located on two sides, and there is an outbuilding with an air conditioning unit pumping out warm air within the observation site. Sudden jumps in the air temperature of several degrees cast further doubt over the reality of the measurements there. However, the Met Office verifies it as an official station and so the record stands. The next highest temperature recorded was 33.5˚C at Kew Gardens in London. ***

UKMO surface pressure analysis for 0000 UTC on 12th September 2016. Credit: Met Office.
UKMO surface pressure analysis for 0000 UTC on 13th September 2016. Credit: Met Office.
UKMO surface pressure analysis for 0000 UTC on 14th September 2016. Credit: Met Office.
UKMO surface pressure analysis for 0000 UTC on 15th September 2016. Credit: Met Office.
UKMO surface pressure analysis for 0000 UTC on 16th September 2016. Credit: Met Office.