I have read reports that this month will end up being the warmest May on record thanks to a heat wave in the last 10 days of the month. Going by the latest provisional values of the Central England Temperature [CET] series, so far this month (up until the 20th) that looks a total impossibility. A quick bit of calculation to see just how hot the last 11 days of the month would have to be to beat May 1833 reveals that the daily mean would have to exceed 20.1°C for every remaining day of the month. The people who are making these predictions obviously didn’t realise just how exceptionally warm May 1833 was, +4.64°C above the long-term average. Currently May 2016 is tracking just inside the top twenty warmest at #19, with a mean anomaly of +1.97°C, which in itself is very high. Even if the short cold spell between the 14-16th hadn’t occurred it still wouldn’t have been anywhere close to beating 1833.
Remember you can keep an eye on the latest CET values in these special static pages dedicated to them.