Monday’s storm: cyclogenesis
Back | HomeMonday’s storm is an excellent example of what has several names in meteorology: rapid cyclogenesis (“RACY”), explosive cyclogenesis, or even “bombing”. All refer to the much faster and deeper development of a depression than is normal.
I spent a bit of time this evening creating some videos with the IDV to illustrate this event, but when I uploaded them to Youtube the results were, not to put too fine a point on it, crap. Instead I’ve uploaded them to box.net and will link to them directly—-you’ll be downloading Quicktime videos if you choose to view.
The first video shows the jetstream piling out of North America at over 140 knots (over 190 knots in the core) and crossing the Atlantic (time is from midnight Saturday until midday Monday), with sea level pressure below. If you look at the front left of the jet you’ll see, about half way through, the isobars south of Greenland start to tighten up. In the space of just 18 hours they go from a gentle U-shaped trough to a tightly-wound depression.
The second video shows the depression as the height of the 925 hPa level, this time viewing the North Atlantic from the northwest looking southeast (so the UK is in the corner closest to the viewer). You can see the deep depression suddenly appear half-way or so through and then drive towards Ireland.
The final video shows the results. This is a short series of four Meteosat IR images through Sunday showing the developing storm, overlaid with sea level pressure and surface wind barbs—-you can see the isobars tighten and become circular picking the wind up to over 60 knots. The effect can be dramatically seen in the data from an Irish bouy a few hundred miles out into the Atlantic:
While that graph is the combination of not just the deepening of the depression but also the depression moving in, the Met Office analysis charts show that the storm’s central pressure dropped 30 mb in just twelve hours.
Bernard Burton, who operates his own satellite image receiving station, has produced a couple of excellent visible-light views of the developing storm. The first is from a polar orbiter at 2pm on Sunday, while the second is from MSG1 at 6pm as it looks down on the storm in the evening light. Pretty dramatic!
The reason all this occurs is basically quite simple—-the front left of a jetstream (the “left exit) sucks air up, cooling it and making it become unstable, lowering the surface pressure. The opposite end (the “right entrance”) does the same thing, and you can actually see another deep depression coming off the eastern US seaboard behind the jet. (You can read a more detailed explanation in a previous post.) This particular jetstream is unusually fast, and so the depressions it creates are unusually deep.
The result is going to be a day with two very windy periods for southwestern and southern Britain, with occasional gusts of up to 80 mph and lots of 60 mph ones, no doubt doing a fair amount of damage. The first period will occur as the depression moves in and the pressure rapidly drops early Monday morning, while as you can see from the animations there’s a tight squeeze to the isobars on the rearward southwestern flank of the depression, which will bring another bout of windy weather as it crosses in the afternoon.
The low pressure is also arriving during some of the highest spring tides of year, and the resulting storm surge may well lead to flooding in a few susceptible places on the Cornwall and Devon coasts.
Perhaps the most amazing thing about this very-well-forecast storm is that from last Tuesday at the earliest until Sunday morning it only existed as the mathematical imaginings of a few big supercomputers. Now, in the space of less than a day, it’s become the very real violent movement of many millions of tonnes of air and water. And sadly, by this time tomorrow, it may well have killed several people.
Review and early outlook some time on Monday, hopefully.
