S is for Storm
How many storms is normal for the UK and Ireland?
A pilot scheme is underway in the UK and Ireland to name storms that are forecast to have substantial impacts. Storms have been named since October 2015, with the aim of increasing public awareness of severe weather, and improving safety – the theory being that a named storm makes it easier for the public to tracks its progress, and to share information on social media.
UK Met Office says that ‘for a storm to be named, it must be forecast to have a medium or high impact from strong winds in the UK’ (see UK Met Office storm page ). Met Éireann go into more detail: for an orange or red weather warning to be issued, the forecasts need to be for wind gusts between 68-80 mph and >80 mph respectively (see Met Eireann storm warning explanation).
Since November 2015, nine storms have been named and the UK media have started asking if naming the storms makes us think they are worse and/or more numerous than ‘normal’ (e.g. BBC article). Using historical data (assimilated archived observational data) and very short range (up to 9 hour) forecasts, we take a look at what a ‘normal’ storm year looks like for the UK and Ireland.
First we define the area in which we look for storms, as shown in Figure 1 below
Next, we consider the storm criterion of 10m wind gusts > 68mph. The ECMWF historical and forecast data set we are using is at a lower resolution than the observations UK Met Office and Met Éireann have access to, which is likely to make the maximum wind gusts we find slightly lower than actually observed (see EWEA pdf presentation). We therefore adjust the criteria slightly, to >60mph 10m wind gusts. These criterion picks out the storms named between Oct-Dec 2015 (and only the named storms), as shown in figure 2 below.
The onset of storms according to our criteria is often a day earlier than the Met Office reports, likely due to our search region extending further west and out to sea. This does not, however, impact the number of storms.
Figure 2 also shows that forecasts of maximum wind gusts are not that reliable. Mean sea level pressure (mslp) in a low pressure system associated with a storm is usually lower than normal. Looking at the named storms so far, then requiring mslp < 980mb looks to be a reasonable additional criterion.
Historical ‘normal’ storm frequency
Defining a storm year as September through August, and using data from September 1980 through August 2015, we identify how many times there are wind gusts > 60mph and mslp < 980mb. Storms are classified as independent if more than 48 hours elapses between these criteria being met. A total of 349 storms have reached the LSC ‘storm’ criteria for the UK and Ireland over these 35 ‘storm years’, averaging at just under 10 storms a year.
94% of the storms occur between September and March, with figure 3 showing the month within year profile.
Figure 4 below shows that the storm frequency by storm year. 2015-2016 is, so far, by no means out of the ordinary.