How unusual are deep lows in August?

The word unusual seems to be cropping up in every weather forecast on the TV when describing low Yap* at the moment. But how unusual are deep lows in August across the British Isles? Well I ran an analysis on the reanalysis data from 1948 to find the lowest MSLP from the 2.5° x 2.5° grid of six hourly data from across the UK and this is a table of the lowest values that I found.

Raw data courtesy of NCEP reanalysis

As you can see the lowest pressure of 972.8 hPa occurred on the 30th of August 1992 at 18 UTC. Here is a plotted chart for that date and time, notice that’s when we still received observations from places such as Cape Wrath and RAF Finningley. I mentioned in a recent blog about the similarities between 2019 and the Fastnet race storm of 1979 which you’ll notice features at #10 in the table.

The month of August can throw up some surprisingly cyclonic situations but usually the lows are fairly shallow. Here is a graph of extreme pressures across the British Isles for August since 1948.

Raw data courtesy of NCEP reanalysis

And here is a more detailed look at some of the most vigorous lows in the table. But don’t forget because of the coarse nature of the gridded data the cyclones will probably have been more intense features.

Raw data courtesy of NCEP reanalysis

If the NWP forecast is correct about Yap having a minimum central pressure of 981 hPa as it crosses the country, it will no doubt feature in the top twenty lowest August pressures, which indeed will be unusual.

*Low ‘Yap’ named by the Institute of Meteorology Berlin

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