What are the National Severe Weather Warning Service Impact tables?

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.

Weather warning impacts and what they mean

The Met Office issues weather warnings, when severe weather has the potential to bring impacts to the UK.  These warnings are based on a combination of the level of impacts the weather may cause and the likelihood of those impacts occurring. Yellow and Amber warnings represent a range of impact levels and likelihoods. This means it is important to read each warning to know what level of impact you can expect for your chosen warning area – and how likely those impacts are to occur.

An indication of the types of impacts that could occur for each of weather types we warn for are shown in the tables below. They also provide a guide to impacts for what is included at 'low', 'medium', and 'high' levels of impact. The tables provide some of the types of impact that can be expected as a direct result of the forecast weather conditions. These tables 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.

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 quite small so we do not 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 does not send out warnings for these days but there could still be some impacts caused by the weather. However, these impacts would be expected to be short-lived or fairly localised.
• The impacts listed in the columns are illustrative of the range of impacts that may be experienced for this warning level, but not all of the impacts listed will necessarily occur with any particular severe weather event.
• In any particular weather event, not every location in the warning area may experience the same level of impacts indicated.
• When severe weather is expected, impacts at lower levels are also likely to be experienced.
• The level of impacts can be affected by previous weather conditions. For example, small amounts of rain may cause flooding during a prolonged wet spell, or gusts of wind may produce some structural damage where structures have been weakened by an earlier storm, even though the gust speeds are not particularly strong.

Each weather warning will have an impact level of either:

• very low
• low
• medium
• high

The tables below provide a guide to the types of impacts that could occur, for all of the weather warnings we issue, including general impacts that could be expected in any type of severe weather.

There are also guides for some of the weather impacts that could be expected when warnings are issued for:

• Rain
• Thunderstorms
• Wind
• Snow
• Lightning
• Ice
• Extreme heat
• Fog

Some definitions used in our impact tables:

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

As our weather warnings are based on the likelihood of impacts occurring because of severe weather, several factors have to be taken into account and these will include:

• Time of year  
• Time of day
• Time of week
• Location
• Underlying wet or dry ground conditions
• Any local events taking place

Throughout the year, the weather could have a very low likelihood to bring very localised or short-lived impacts to our daily routine, even when a weather warning has not been issued.

These are general impacts associated with the relevant impact level for any type of severe weather.

Very Low	Low	Medium	High
On the whole, day to day activities not affected but a few places may see 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 responder 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.

Impacts resulting from rain

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

The main hazard 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)
• Northern Ireland Department for Infrastructure
• Scottish Environment Protection Agency
• Natural Resources Wales

Flood Warning Services of the Environment Agency (England), Natural Resource Wales and Scottish Environment Protection Agency provide warnings and alerts of flooding for specific rivers. In Northern Ireland, awareness raising in relation to river flooding is through the Department for Infrastructure.

Very Low	Low	Medium	High
A few places will have flooding of low-lying land and susceptible roads. A few transport routes affected.  Road conditions affected with spray and some standing water in a few places.	Some 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. Short term disruption to utilities and services in some places.	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.

Impacts resulting from thunderstorms

Impacts from thunderstorms can occur at any time of the year and are most common during the summer months – when areas of intense thunderstorms can move across the UK and heighten the risk of disruption to transport networks and damage property.

It is usually during warm and humid weather in the summer, when thunderstorms in the UK are most likely to produce large hail, gusty winds and torrential downpours.

One of the most notable aspects of thunderstorms can be the localised nature of the impacts they could bring. This in particular can be the case with rainfall amounts, with big differences in amounts that fall from one place to another – and over only a very short distance.

Very Low	Low	Medium	High
A few places will have flooding, usually lasting an hour to a few hours at most. A few local transport routes may be affected with difficult driving conditions. Very short-term disruption to power and/or other utilities and services in a few places.	Some flooding of homes, businesses and susceptible roads lasting several hours in places. Some damage to buildings/ structures from flooding and/or lightning. Some transport routes and travel services affected. Some journeys require longer travel times.  Road conditions affected by spray and standing water and/or hail. Short-term disruption to power and/or other utilities and services in some places.	Flooding of homes and businesses. Danger to life due to sudden deep/fast flowing water. Damage to buildings/ structures from flooding and/or lightning, hail, strong winds.  Transport routes and travel services affected quickly by flooding. Longer journey times and cancellations.  Difficult road conditions due to spray, standing water and/or hail, sudden gusty winds. Interruption to power and/or other utilities and services. Some communities temporarily inaccessible due to flooded access routes.	Widespread flooding affecting homes and businesses. Danger to life due to sudden fast flowing/deep water. Injuries from hail. Casualties and danger to life from lightning strikes. Extensive damage to buildings/ structures from flooding and/or lightning, hail, strong winds. Transport routes and travel services affected by flooding for a prolonged period with long travel delays and rapidly changing/deteriorating conditions.  Dangerous driving conditions due to spray, standing water and/or hail, sudden gusty winds. Prolonged disruption to or loss of power and/or other utilities and services. Communities become cut off for a prolonged period, perhaps several days, due to flooded access routes or damage to road infrastructure.

Impacts resulting from wind

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 winds 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. A single tree falling across a road, railway or falling onto property can have a very different level of impact - compared to several trees falling in remote countryside. 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 trees block road or rail routes.

Strong winds and storms can lead to impacts around the coastline of the UK, particularly when they coincide with high tides. Large waves are generated by strong winds, which can throw beach material and sea water on to transport routes, walkways and into properties along some coasts. Coastal flooding impacts are covered by the Flood Warning Services of the Environment Agency (England), Natural Resource Wales and Scottish Environment Protection Agency. In Northern Ireland, awareness raising in relation to coastal flooding is through the Department for Infrastructure.

Very Low	Low	Medium	High
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. Difficult driving conditions for high-sided vehicles on prone routes, such as cross winds on exposed or high level roads. A few 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.

Impacts resulting from snow

When snow falls in the UK it can quickly lead to impacts on transport networks. Road and air transport organisations help mitigate possible disruption by taking winter maintenance actions, such as gritting and ploughing. Daily forecasts, specifically focussed on gritting and ploughing requirements, are used by some organisations to help them make decisions on treatment strategies to reduce impacts caused by snow.

When accompanied by strong winds, snow can drift and this can cause large variations in snow depth and make it difficult to clear roads, railways and runways.

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

Very Low	Low	Medium	High
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.

Impacts resulting from lightning

Lightning can occur at any time of the year, although it is most common during the summer months – when frequent lightning can move across large areas of the UK and heighten the risk of impacts. It is during the summer when ‘dry storms’ can develop and produce frequent lightning in clouds very high up in the sky. However, any rain has to fall such a long way to reach the ground and through very dry air – it evaporates before reaching the surface. 

Lightning can strike the ground many miles away from where it forms high up in the sky. However, just a single strike of lightning can be enough to cause disruption or damage buildings.

Of course, lightning impacts do not always occur in isolation – when they can be accompanied by large hail, gusty winds and torrential rain – so please see thunderstorm impacts for these details.

Very Low	Low	Medium	High
Disruption to power and/or other utilities and services in a few places. Damage to buildings/ structures from lightning strikes in a few places.	Some disruption to power and/or other utilities and services. Some damage to buildings/ structures.	Injuries with danger to life due to lightning. Interruptions to power and/or other utilities and services. Damage to buildings/ structures from lightning strikes.	Injuries with danger to life due to frequent lightning strikes. Prolonged disruption to or loss of power and/or other utilities and services over a widespread area. Extensive damage to buildings/ structures from lightning strikes over a widespread area.

Impacts resulting from ice

Impacts from ice on roads can be mitigated by treating roads with salt or grit. However, not all roads, cycle paths and pavements will be treated. Some organisations take specialised daily forecasts to help them make decisions on treatment strategies.

On some rare occasions, freezing rain can fall. This is when rain freezes immediately on contact with very cold surfaces, such as roads and windscreens. This can lead to very dangerous icy conditions, resulting in high impacts for emergency services and normal daily routines.

Very Low	Low	Medium	High
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. Interruptions to power and/or other utilities and services.	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.

Impacts resulting from extreme heat

Whilst many of us like to enjoy the sunshine and hot weather, we should make sure we do it safely and remember certain groups of people are more vulnerable than others to heat.

Hot weather places a strain on the heart and lungs, for that reason, the majority of serious illness and deaths caused by heat are respiratory or cardiovascular. Older people, those with pre-existing health conditions and young children are particularly at risk.

Extreme heat not only affects us but can also place strains on water and energy utilities, road and rail transport and the health and fire services.

For summer 2021 the Extreme Heat warning will be for Medium and High likelihood of Medium and High impacts. For year one of the service this will enable the Met Office and Public Health England Heat Health Alert service to be aligned on colour of warnings. In 2022 and following some extreme heat weather events this will be reviewed and may change.

Very Low	Low	Medium	High
N/A	N/A	Adverse health effects experienced by those vulnerable to extreme heat.   Some changes in working practices and daily routines may be required   Some heat-sensitive systems and equipment may fail, leading to power cuts and the loss of other services to some homes and businesses   Some delays to road, rail and air travel, with potential for welfare issues for those who experience long delays.   More people visiting coastal areas, lakes and rivers leading to risk of water safety incidents.	Adverse health effects experienced by all, not just limited to those most vulnerable to extreme heat, leading to serious illness or danger to life.   Changes in working practices and daily routines will be required.   Failure of heat-sensitive systems and equipment with loss of power and other essential services, such as water, electricity, gas or mobile phone services.   Delays on roads and road closures, along with delays and cancellations to rail and air travel, with significant welfare issues for those who experience even moderate delays.   Significantly more people are likely to visit coastal areas, lakes and rivers leading to risk of water safety incidents.

Impacts resulting from fog

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 or so, predominantly to the transport network. Dense, persistent fog can lead to delays at airports and flight cancellations.  As organisations and people adjust to persisting fog, we often see that the impact of the fog lessens beyond the first day or so, even though the fog itself is still present. High impacts from fog are therefore not expected and not catered for on the impacts table.

Very Low	Low	Medium	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