National Severe Weather Warning Service

An attempt to explain what all the various warning categories are

National Severe Weather Warning Service (NSWWS) (UK service only)

Within the United Kingdom, the UK Met Office is responsible for the issue of various classes of warnings under this service. These are funded by contributions from central & regional governments, and are issued when the weather is expected to be of sufficient severity to cause significant dislocation / disruption to transport and / or loss of life (or in the case of the Heatwave warnings, extreme stress to vulnerable groups of people). (The criterion used to be 'widespread' dislocation: in practice, this was always difficult to define, and in recent years, any dislocation, no matter how small the area, has been targetted; similarly, population density is now not a critical determinant when warnings are issued - so you will see warnings for sparsely populated areas that in years past we would never have considered issuing).

Warnings - public (UK service only):

Under the NSWWS the UK Met Office issues various levels of warnings of weather conditions sufficient to cause significant disruption to communication (road, rail etc.), and / or loss of life (or stress due to high temperatures). There are strict criteria observed which are 'weather related' (i.e. amount of rain in a given time), but some latitude may be employed when considering the "end-effect". As an example, 24hr of light but steady snowfall in the Hebrides is unpleasant & unwelcome, but would not generate anything other than a low-level local advisory: 2 or 3 hr of similar snowfall in the English Home Counties on a Monday morning would cause chaos!

The various types of warnings cover the following weather conditions:
Severe Gales, Storms, Heavy Snow, Very heavy snow, Blizzards, Severe blizzards, Heavy Rain, Dense Fog, Glazed Frost/Widespread Icy roads & Heatwave.

The two classes of warnings are: Early and Flash.

"EARLY" warning (UK service only)

These warnings (for all except 'Heatwave' in the list above**) are issued by the Met Office (HQ only) ahead of an expected event, ideally 3 or 4 days before, but sometimes with less than 24hr lead-time. They are only issued when there is a 60% or more chance of the expected disruption occurring: this means that given 10 similar synoptic outcomes (usually determined by inspection of NWP ensemble output - see entries elsewhere), on at least 6 of them, populated locations in the UK should experience conditions sufficient to cause significant disruption to communication, or there is a high risk of loss of life. It should be carefully noted that this means that on 4 or fewer occasions, the expected disruption may NOT occur: this is not a failure, but is the standard agreed between the Met Office & the users of the system (i.e. local authorities, emergency services, public utilities etc.) [ For more on the use of probabilities in this scheme, click HERE.]

( ** Heatwave warnings have a different text-based format - the initial warning is put up on the Met Office home page, with a link to a 'Heat Health-watch' page, which divides the country up into regions, with a % prob. risk of exceedence of the critical temperature and explanatory text. )

An Early warning is usually issued (when required) around 0900 - 1000 each day, but it can be issued at any time when the situation warrants. It is updated each morning, (with revised regional risk assessment if necessary). An early warning will be updated until either it expires or until it is superceded by Flash messages, or, it may be cancelled ahead of time if the overall (UK-wide, NOT the individual regional value) probability falls too low for it to be valid.

These warnings prompt the issue of a 'Weather Watch' by the BBC Weather Centre: see below.

"FLASH" warning (UK service only)

These are issued in the hours leading up to the expected event, when confidence is very high that the conditions will cause disruption etc. They are issued by HQ MetO at Exeter, after consultation with the regional Met Offices. Exceptionally, "Emergency Flash" warnings are issued by HQ when 'extreme' conditions are expected (e.g. exceptionally high winds or heavy, prolonged blizzard conditions).

[ The "FLASH" tag belongs to the days of the precedence categories used to relay signals over closed military/diplomatic communications networks: The Meteorological Office was much more closely tied to RAF & MOD signals protocols than it is now, and would often use the same (or parallel-bearer) links to relay meteorological warnings. ]
These warnings will generate an appropriate alert on the Met Office web site & also prompt issue of a 'Weather Warning' by the BBC Weather Centre: see below.
[ NB: as of September 2005, 'Advance' & 'Motoring' warnings are no longer issued.]


BBC Weather Centre warnings:

These are based upon the warnings issued by the Met Office, but unfortunately, different terminology is used. It is probably best to visit the BBC Weather Centre site to get the full picture, but in essence:

 Met Office category  BBC Weather Centre category
 Early Warning  Weather Watch
 Flash Warning  Warning of Severe Weather

For more on warnings issued by the UK Met Office, visit their site: navigate via ' Products and Services', then 'Public Sector', or follow the appropriate links from the warnings listing on the home page - much more of interest on the categories etc.
For more on warnings issued by the BBC Weather Centre, visit their site: navigate via 'UK & Warnings' links.
For some general points surrounding Warnings to the general population (not just weather related), visit: the "UK Resilience" site, which also has information on other warnings for public safety.

Early warnings and probabilities

Although it doesn't always come across very well in broadcast presentations, when an 'Early Warning' is issued for a particular meteorological event, forecasters are issuing what is known as a 'probabilistic' forecast not a 'deterministic' one. It is important to understand the distinction between the two.

When a forecast says something like ... " strong winds and heavy rain will sweep north from SW England, across Wales reaching SW Scotland this evening ", then that is a 'yes/no' (or deterministic) forecast - that is, the expected evolution of the weather with time.

When a NSWWS 'Early Warning' is issued, the probability of those conditions occurring somewhere within the UK must be 60% or higher. That is, on the balance of probabilities, the event is more likely than not to occur (> 1 in 2) .. BUT it is not a certainty.

Increasingly, the level of risk will be set using objective techniques based upon NWP ensemble output - some very sophisticated routines are being developed which will take such output and, based on past cases and the knowledge of current meteorological & infrastructure conditions, produce a probability - which of course can be modified manually if required.

So, what does a probability figure mean? Well, if a figure of 60% probability is issued, it means that given 10 situations with the same set of precursor ( & forecast ) indicators, on 6 of those, 'something' extreme (meteorologically) will occur which will lead to a significant impact on the community - with consequences for the emergency services, local authorities, public utilities etc. However, this also means that on 4 occasions, nothing 'extreme' (in terms of public impact) is likely. The primary customers understand this - unfortunately the general public is not too well educated about the latter point. Even in the dizzy heights of 90% probability (not often used for events more than 12 hours ahead), then there is still a slim chance that nothing untoward will occur.

Within the individual warning, the probability for any particular area may be even lower. It may be that the 'threat' to Wales, SW England and SW Scotland is 60%, but for adjacent areas, such as NW England, the West Midlands etc., perhaps 40%. In this latter case, it is even more likely that nothing untoward will occur (i.e. 60% probability of a 'non-event').

This gives rise to the charge ... "well, they can't get it wrong, can they!" Well, believe me if forecasters continually issued warnings and nothing happened time after time, then the 'powers-that-be' would soon get involved. Each warning is carefully monitored after issue to find out how well (or otherwise) both the forecasters & the end-users coped. Central (UK) government also gets involved directly and indirectly and would view over-use of these warnings with concern. In fact, it turns out that forecasters, being human beings, actually try to get each one exactly right - which is not the idea of the scheme!

So, why use probabilities then? If you are a county planner, or a team leader with the emergency services, or responsible for having 'stand-by' resources available to support the public utilities (electricity supply, rail transport, airport management etc.), then the idea of attaching a % level of risk is supposed to help in the provision of stand-by/back-up services: it allows planners to avoid over-preparing by bringing expensive resources 'on-stream' for a low-threat event (say 20% or one-in-five possibility), but have plenty of back-up available for a 80% (or roughly four-in-five probability) event. Even in this last occasion though, remember that the wind may blow, the snow may fall, but there is still that slim chance that little 'disruption' will occur.

For the general public, the probabilities tend to get 'smeared-out' into very broad "low", "moderate" or "high" risks of disruption on radio and television presentations. But, even when a "high" risk is indicated, it must not be assumed that trouble will ensue. The information is presented to enable YOU the user to decide whether to carry on with your planned journey, building project etc., or to delay perhaps by 24 hours, setting this against the cost in monetary and other terms of either the delay, or carrying-on regardless and 'taking a chance'. This is what 'probabilistic' forecasting is all about - you, the user have extra information to assess the likely outcome of the event, and the costs/risks associated.