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Key to flash warning criteria

Warnings are issued using a set of fixed weather criteria for the whole of the UK
Weather and its possible effects
  Met Office criteria Possible effects
Severe gales Repeated gusts of 70 m.p.h. or more over inland areas.
  • High-sided vehicles at risk of being blown over
  • Some trees uprooted
  • Tiles, slates and chimneys dislodged from some buildings
Storms Repeated gusts of 80 m.p.h. or more over inland areas.
  • Cars blown out of lanes on roads
  • Widespread removal of branches from trees; many trees uprooted
  • Tiles, slates and chimneys dislodged from many buildings; some structural damage

Where wind is forecast above 90 m.p.h. the following may occur.

  • Collisions whilst driving
  • Widespread uprooting of trees
  • Injury due to flying debris
  • Widespread damage to buildings; some buildings collapse
Heavy snow Snow falling at a rate of 2 cm/hour or more expected for at least two hours.
  • Increased journey times
  • Minor accidents
Very heavy snow Snow falling at a rate of 2 cm/hour or more expected for at least two hours, accumulating to 15 cm or more.
  • Local routes impassable
  • Local loss of power and telecommunication lines
Blizzard Moderate or heavy snow accompanied by winds of 30 m.p.h. or more, with visibility reduced to 200 m or less; or drifting snow giving rise to similar conditions.
  • Major routes impassable
  • Local loss of power and telecommunication lines
Severe blizzard Heavy Snow accompanied by winds of 30 m.p.h. or more, reducing visibility to near zero.
  • Transport infrastructure paralysed
  • Regional loss of power and communication lines
Heavy rain Rain expected to continue for at least two hours and to give at least 15 mm within a three hour period or, following previous heavy rain events, 25 mm/day.
  • Aquaplaning
  • Flooding
Warning may be triggered by thunderstorms (warnings will state this if expected).
  • Squally winds or tornadoes may remove roof tiles or chimneys
  • Power surges
Fog Visibility below 50 metres (restricted to heights where major roads occur).
  • Difficulty identifying distances and speed of movement leading to increased journey times
  • Road traffic accidents
Widespread icy roads, glazed frost, freezing rain

When rain falls onto surfaces with temperatures at or below zero; or condensation occurs on surfaces at or below zero; or already wet surfaces fall to or below zero. The ice is usually clear and difficult to distinguish from a wet surface. It usually forms in sheets.

Warnings are issued when any depth of ice is expected over a widespread area.
  • Damage to power and telecommunication lines
  • Driving difficulties
  • Difficulty when walking
Heatwave (NHS Heat-Health Watch) Expectation of significantly higher than average temperatures in one or more regions of England and Wales; thresholds are pre-determined regionally via the Heat-Health Watch system.
  • Heat exhaustion and heat stroke; this can lead to organ failure, brain damage or death

Those most vulnerable to these effects include:

  • older people, especially those over 75 years old;
  • those suffering from mental ill health;
  • the bed-bound;
  • those on certain medication;
  • babies and young children, especially those under four years old.