Explosive Cyclogenesis Explained

A basic and simple introduction to 'bombing depressions', and how to spot them using satellite
imagery, by Brendan Jones.

Disclaimer: This isn’t a detailed, scientific report about how explosive cyclogenesis actually occurs, but rather, handy hints on what to look for when it does! Many of the satellite images within this report have been extracted from the internet, but all are fully credited, with an associated online reference at the end of this report.

What is cyclogenesis?
This is the obvious starting point. Basically, the term ‘cyclogenesis’ means the creation of cyclones (or depressions, as we better know them). This is in the same way that ‘frontogensis’ is used to describe the creation of weather fronts. All Atlantic depressions have some form of cyclogenesis, ranging from wave depressions, to the modifications of ex-hurricanes. In the interests of this discussion, I’m going to concentrate on those depressions which develop from frontal waves, as I reckon they’re the ones which are the most captivating.

What is explosive cyclogenesis?
This is basically the same as normal cyclogenesis, but it happens very rapidly, and very vigorously. That is, a depression can seemingly form from nothing in a very short space of time, often becoming a very vicious storm in a matter of hours.

The general term used for these rapidly deepening depressions, is a ‘bomb’. The strict definitions of this, is a low pressure centre which deepens by 24mb in 24hrs, or less. These happen frequently throughout the Atlantic, and several times a year, we see this happening pretty close to the UK. It’s only in recent years that such things have been better understood, and indeed, before the 1987 storm across southern England, meteorological models hardly had the signature of explosive cyclogenesis incorporated within them.

The 1987 storm in southern England was, of course, the classic example of such phenomenon, with a monster storm exploding onto the scene, when just 24hrs earlier, it hardly existed. Other noteworthy events include the Christmas storm of 1997, which brought large amounts of damage to Northern Ireland, northern England and north Wales, and also the impressive storm around 10th October 2004, which flooded parts of the south-west of England and the Isles Of Scilly.

How to spot explosive cyclogenesis
There are many ways available to the forecaster, which are used to spot the rapid development of nasty depressions. These range from the obvious model outputs, to more short-range use of satellite imagery. Personally, you can’t beat using satellite images to watch a storm which is beginning to develop, but that’s of little use if you need to forecast the event long in advance.

This is the most obvious method for a ‘heads-up’ of such damaging storms, simply by using model output. For illustration, I’m going to use the 1997 Christmas ‘bomb’ as an example, and although I’m using charts which have been analysed SINCE the event, what you see can be mirrored exactly by using forecast charts instead. As mentioned earlier, the 24mb drop in central pressure in 24hrs is the indicator of a bomb, although this can obviously be greater.

The first chart below (taken from the Wetterzentrale re-analysis product) displays the Atlantic pressure pattern on midnight of the 24th December 1997:


The obvious thing to note is the huge low pressure complex in the Atlantic, comprising several centres. The one to note, is the little trough-like low on the southern side of that complex, to the west of Spain. Note that the current pressure reading for that small, developing low is around 1000mb.

The next chart is for midnight on the 25th December 1997, 24 hours later:


Believe it or not, that nasty beast of a low exiting eastern Scotland is the same low I was talking about just 24hrs earlier. The central pressure on this chart is 980mb. Whilst this doesn’t quite reflect a 24mb drop in 24hrs, it’s worth remembering that this low actually reached its most intense near south-west Scotland at around 21:00 on Christmas Eve, and I believe the central pressure was around 976mb at this time.

Therefore, the pressure fell from 1000mb at 00:00, to 976mb at 21:00. This represents a deepening of 24mb in just 21hrs – definitely into bomb category!

For a couple of news reports on what this low did, see the links below:




In this storm, as with most violent depressions, the strong winds were on the southern side of the depression, for reasons that will be discussed later in this report.