The maximum temperature at this time in September usually occurs around 13 UTC or possible 14 UTC. The hottest place in the SYNOP observations in the UK on the 13 September 2016 was at Gravesend. Its maximum temperature of 34.4°C made it the warmest September day since 1911. Here’s a snippet from the Monthly Weather Report for September 1911 (courtesy of the Met Office) about that warm spell. Since 94°F is 34.444°C so the Gravesend reading just falls short.
The thermograph based on hourly SYNOP data was quite peculiar. The temperature instead of peaking at 13 or 14 UTC as you might have expected on such a hot day just plateaued.
The temperature hit 32.0°C at 1300 UTC and again at 1600 UTC, but between those times (at 1400 and 1500 UTC) it dropped slightly and levelled off at 31.4°C. In that time period the temperature must have peaked to produce the 34.4°C maximum, so it must have climbed and fallen by at least 2.4°C in the space of an hour. It certainly can fall and rise very quickly at Gravesend because in the morning it rose 4.7°C in an hour (between 09 and 10 UTC) and later it fell 6.5°C in an hour (between 17 and 18 UTC). I would have said that the latter may have been down to a sea breeze, but there had been a light flow from the east for most of the day which confuses matters. The one minute data that the Met Office collect from Automatic Weather Stations like the one at Gravesend would clarify matters, but I doubt that this data would ever be released.
A similar hiatus in temperature occurred at Heathrow that day too, at 1200 UTC the temperature was 32.1°C but by 1300 UTC instead of being a little higher at 1300 UTC it had fallen by 2.2°C to 29.9°C. The temperature recovered a little by 14 UTC and recovered again at 15 UTC to reach 31.8°C. Somewhere between these times a maximum of 32.8°C occurred.
A drop in temperature at either 1300 or 1400 UTC on the 13th is also noticeable at a lot of other sites across the southeast, I suppose it could have been caused by some medium or thick high level cloud, but there certainly was little evidence of it as far as I can see from the visible satellite images apart from a okta or two of cirrus or medium level castellanus.
As far as I can deduce this is the location of the meteorological enclosure and Stevenson screen at Gravesend, and yes that it is some kind of communications mast next to it!
And here is a zoomed out view of its location with a little more perspective of where it lies, east of the city of London on the south bank of the Thames which of course is tidal – but I don’t intent to dig around for those details! If you are interested the Talkbloke blog has an interesting article about the location of the site.
In conclusion all I can say given its location is that Gravesend seems a most unlikely place for a hot spot. Why higher temperatures were not recorded at places relatively nearby like Heathrow, Northolt or St James Park on a day when there’s a light flow from the east and sitting as it does jutting out into the Thames with the open sea no more than twenty miles to the east beats me, perhaps it’s all down to its proximity to mainland Europe. The 13th of September was not a one-off, Gravesend is a well-known hot spot and has done it before and come under scrutiny, and now it ‘s done it yet again.