What are the National Severe Weather Warning Service Impact tables?

Heavy rain warning

The Met Office National Severe Weather Warning Service (NSWWS) warns of the impacts caused by severe weather. Met Office weather warnings include a level of impact that the forecast weather is expected to bring and the likelihood of those impacts occurring.

The tables below provide an indication of the types of impacts that could occur for each of the weather types we warn for. They also provide a relative scale of impacts i.e. what is meant by 'low', 'medium', and 'high' levels of impact. The tables are a guide to the types of impact that can be expected as a direct result of the forecast weather conditions. They are not exhaustive and do not include secondary impacts, such as school closures, staff absence or the ability to deliver social/health care, which are usually a consequence of direct impacts.

  • Generic impact levels for ALL SEVERE WEATHER
  • Impact levels associated with RAIN
  • Impact levels associated with WIND
  • Impact levels associated with SNOW
  • Impact levels associated with ICE
  • Impact levels associated with FOG

NSWWS impact tables

In using these impact tables please note that:

  • On many days of the year, the weather has the potential to impact our lives. Most of the time these impacts are so small that we don't notice them. These are the days we often describe as 'typical weather' in the UK. These types of weather days are often assessed as having a 'very low' impact. The Met Office doesn't send out warnings for these days but there could still be some impacts caused by the weather. However, impacts would be expected to be short lived and not extensive over any geographic area.
  • The impacts listed in the columns are illustrative of the range of impacts that may be experienced for this warning level, but not all the impacts listed may occur in any particular weather event.
  • In any particular weather event, not every location in the warning area may experience the impacts indicated.
  • In an event where a given level of impacts is expected, impacts at lower levels are also likely to be experienced.

Note that the level of impacts can be affected by previous conditions. For example, modest amounts of rain may cause flooding during a prolonged wet spell, or relatively modest gusts of wind may produce some structural damage where structures have been weakened by an earlier storm.

Some definitions

  • "Localised" implies there will be a few instances of the impact across the warning area
  • "Widespread" implies the impact is expected to be experienced across much of the warning area
  • "Prolonged" implies the impact could persist for several days
  • "Short-lived" or "short-term" implies the impact could last for a few hours
  • "Utilities and services" may include electricity, phone, mobile phone, internet services, water and gas supplies.

Generic impact levels of ALL SEVERE WEATHER

The impacts in this table are generic impacts applicable to all weather types.

Very Low
LowMediumHigh

On the whole, day to day activities not affected but some localised, small scale impacts occur

A few transport routes affected.

Some short lived disruption to day to day routines in affected areas

Incidents dealt with under 'business as usual' response by emergency services

Some transport routes and travel services affected. Some journeys require longer travel times.

Injuries with danger to life

Disruption to day to day routines and activities.

Short-term strain on emergency responder organisations.

Transport routes and travel services affected. Longer journey times expected. Some vehicles and passengers stranded.

Disruption to some utilities and services.

Damage to buildings and property.

Danger to life

Prolonged disruption to day to day routines and activities

Prolonged strain on emergency responders organisations.

Transport routes and travel services affected for a prolonged period. Long travel delays. Vehicles and passengers stranded for long periods.

Disruption to utilities and services for a prolonged period.

Extensive damage to buildings and property.

Specific impact levels associated with RAIN

The table below indicates the kind of impacts which may be experienced as a result of rainfall.

The main impact from rain is flooding. Assessment of the potential flood impacts are carried out in consultation with the UKs authoritative flood agencies Our partners are: Environment Agency (covering England), Natural Resources Wales, Rivers Agency Northern Ireland and Scottish Environment Protection Agency.

Very Low
LowMediumHigh

Localised flooding of low-lying land and susceptible roads.

A few transport routes affected.

Road conditions affected with localised spray and some standing water.

Localised flooding of homes and businesses and susceptible roads

Some transport routes and travel services affected. Some journeys require longer travel times.

Road conditions affected by spray and standing water. 

Localised and short term disruption to utilities and services

Flooding of homes and businesses.

Danger to life from fast flowing/deep water.

Damage to buildings/ structures.

Transport routes and travel services affected. Longer journey times expected. Some road closures.

Difficult road conditions due to spray and standing water

Interruption to utilities and services.

Some communities temporarily inaccessible due to flooded access routes.

Widespread flooding of homes and businesses.

Danger to life from fast flowing/deep water.

Extensive damage to and/or collapse of buildings/ structures

Transport routes and travel services disrupted for a prolonged period. Long travel delays.

Widespread road closures.

Dangerous driving conditions due to spray and standing water.

Prolonged disruption to or loss of utilities and services

Communities become cut off for a prolonged period, perhaps several days, due to flooded access routes.

Specific impact levels associated with WIND

The table below indicates the kind of impacts which may be experienced during wind storms.

Wind storms tend to last for several hours in any one location although the impacts may continue for longer. However, if a major wind storm is followed by subsequent windy spells, recovery from the impacts of the original storm can be hampered and the impacts can become prolonged.

Strong wind and storms often result in tree damage and trees being brought down. Where trees fall is critical to the type and severity of the impact caused. One tree falling across a road, railway or falling onto property can have a very different level of impact to several trees falling in a field. Trees can often be the catalyst for the impacts listed below, such as loss of power (when a tree falls onto power lines) or delayed travel times (when a tree blocks one or several trees block a number of road or rail routes).

Strong wind and storms can lead to impacts around the coastline of the UK. This is particularly the case when strong winds and storms coincide with high tide. Large waves are generated by strong winds and when these break, they can throw beach material and sea water on to transport routes, walkways and into properties along the coastline. Strong wind and storms can occur at the same time as coastal flooding is expected, but not always. Coastal flooding impacts are covered by the Flood Warning Services of the Environment Agency (covering England), Natural Resource Wales and Scottish Environment Protection Agency. In Northern Ireland, awareness raising in relation to coastal flooding is facilitated through Rivers Agency.

Very Low
LowMediumHigh

Loose debris blown around.

A few transport routes affected by difficult driving conditions.

Instances of spray and large waves affecting coastal routes, sea fronts and coastal communities

Some transport routes and travel services affected. Some journeys require longer travel times. Some disruption to road, rail, air and ferry transport.

Localised problems for high-sided vehicles on prone routes e.g. due to cross winds on exposed high level roads.

Localised power interruptions.

Coastal routes, sea fronts and coastal communities affected by spray and/or large waves

Injuries and danger to life from flying debris.

Some structural damage, such as slates dislodged from roofs.

Transport routes and travel services affected. Longer journey times expected. Disruption to road, rail, air and/or ferry transport.

Closure of some susceptible and key routes (e.g. some vulnerable bridges)

Interruptions to power and/or other utilities and services.

Casualties and danger to life from large waves/beach material being thrown onto coastal routes, sea fronts and coastal communities

Widespread danger to life from flying debris.

Widespread structural damage e.g. roofs blown off, mobile homes overturned, power lines brought down.

Transport routes and travel services affected for a prolonged period. Long travel delays.

Closure of main bridges, road and rail networks in many areas, and significant disruption to air and ferry transport.

Widespread and prolonged disruption to power, and/or other utilities and services.

Danger to life from large waves/beach material being thrown onto coastal route, sea fronts and coastal communities.

Specific impact levels associated with SNOW

The table below indicates the kind of impacts which may be experienced as a result of snow.

The impacts of snow on the transport network, particularly on road and air transport can be mitigated by winter maintenance actions taken by the appropriate authorities (i.e. gritting, ploughing). The authorities take specialised daily forecasts to help them make decisions on treatment strategies.

When accompanied by strong winds, snow can drift, which can cause large variations in snow depth and make it difficult to carry out successful mitigating action.

Exposure to snow and cold temperatures can increase the chance of hypothermia, cold related injuries and health complications.

Very Low
LowMediumHigh

A few transport routes affected.

Some transport routes and travel services affected. Some journeys require longer travel times.

Transport routes and travel services affected. Longer journey times expected.

Some stranded vehicles and passengers, with disruption to rail, road and air services.

Interruptions to power and/or other utilities and services

Some rural communities temporarily inaccessible due to deep snow or snow drifts

Transport routes and travel services affected for a prolonged period. Long travel delays.

Large numbers of stranded vehicles and passengers with widespread disruption to rail, road and air services.

Widespread and prolonged interruptions to power and/or other utilities and services

Rural communities cut off for a prolonged period, perhaps several days, due to deep snow or snow drifts.

Specific impact levels associated with ICE

The table below indicates the kind of impacts which may be experienced as a result of ice.

Ice is a common problem during the winter months. The impacts of ice on roads are usually mitigated by treatment i.e. the application of salt/grit. However it should be noted that not all roads, cycle paths and pavements will be treated. The appropriate authorities take specialised daily forecasts to help them make decisions on treatment strategies.

As a result of appropriate treatment actions most impacts from ice will be at the low or medium levels. However, some extreme occasions of freezing rain, where rain freezes immediately on contact with very cold surfaces, such as roads and windscreens, can lead to very dangerous icy conditions for a few hours resulting in high impacts on emergency services and normal daily life for that period.

Very Low
LowMediumHigh

A few transport routes affected by ice on some untreated roads, pavements and cycle paths.

Limited travel disruption with difficult conditions mostly confined to a few prone routes.

Some injuries from slips and falls

Some transport routes and travel services affected with some ice on untreated roads, pavements and cycle paths, but road networks generally open. Some road traffic collisions.

Casualties with risk to life. Injuries from slips and falls

Transport routes and travel services affected by widespread black ice. Some road closures and some only passable with care. Untreated pavements and cycle paths impassable. Some travel disruption with longer journey times and road traffic collisions.

Widespread risk to life. Casualties and injuries from slips and falls

Transport routes and travel services affected by sudden formation of black ice across whole communities with roads pavements and cycle paths becoming instantly impassable. Widespread disruption to road, rail and air transport with frequent road traffic collisions.

Widespread interruptions to power due to power line icing, leading to impacts on telecommunications.

Specific impact levels associated with FOG

The table below indicates the kind of impacts which may be experienced as a result of fog.

Fog rarely causes any impacts other than on the transport network. Where fog is patchy, drivers will tend to slow down or speed up as they move into and out of areas of fog. This can present a greater risk of road traffic collisions than widespread fog, when drivers are more likely to drive more slowly and appropriate to the constant low visibility conditions.

Dense fog lasting for several days is likely to have an initial impact on the first day, but organisations and people tend to adjust to persisting fog and therefore the impact beyond the first, or perhaps second, day is often lower. High impacts from fog are therefore not expected.

Very Low
LowMedium
High

Limited travel disruption with difficult travel conditions mostly confined to a few prone routes.

A few road traffic collisions.

Difficult driving conditions with slower journey times.

Some road traffic collisions

Passengers delayed with short-term closure of airports.

Difficult driving conditions with long journey times.

Road traffic collisions

Passengers delayed and/or stranded at airports and/or ferry terminals.

N/A

Last updated: 14 June 2016

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