July continues it a mood of being mobile and often quite cyclonic at times, in fact that’s how it’s been since the end of May across the British Isles, as this graph of zonality shows (fig 2). The simple answer of why has it been so mobile is that mean pressure in the first three weeks of July 2017, has been 8 hPa below average across Baffin Island, and there has been a band of lower than average mean pressure extending westward across Iceland, before arching around Scandinavia and into eastern Russia. South of that the mean pressure across much of the central Atlantic has been higher than average (+3 hPa), and between the two the W’SW gradient has been tightened.
It’s very likely that this is all driven by temperature of the atmosphere at all levels. The SST is well below average around Baffin Island (-4°C), and the central mid Atlantic remains generally cooler than average (-1.5°C), although there is now a band of warmer water (+1.5°C) extending westward from America at ~38° north. It’s interesting to see the SST down the east coast of Greenland being colder than average, probably from fresh water from the summer melt of the glaciers.
This may help explain the very intense lows for July’s that we are seeing. Here’s the forecast for this Wednesday (26 July) from the Met Office, which shows an intense low of 973 hPa low at 19° west, throwing a frontal system across the British Isles. This low in turn, will spawn a series of secondary lows that will dumbbell around it before the week is out maintaining the mobility.
Finally, I thought that I would look at a virtual barograph that I would sit at 57.5° north and 22.5° west (where hopefully it won’t get too wet). As you can see (fig 4) the anomalies there have been mainly negative there for long periods, and this is in an area of low pressure anyway and generally close to where the Icelandic low is found on mean pressure charts.