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.
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.]
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.