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Weather Elements And When They Are Included In The Forecast

Note: These are the general rules for including weather elements in the forecast.  Slight variations of these rules do exist, however they are not included in this document.

Sky Condition

Included in the forecast for Day 1 & 2? - Yes
Included in the forecast for Day 3 to 7? - Yes

Indicates the degree of cloudiness of the sky

Days One and Two: Sky Condition is replaced by a precipitation term when the Chance of Precipitation (COP) is equal or greater than 80 per cent. It is also replaced by the following terms when they are expected to occur: Fog, Ice Fog, Blowing Snow, Local Blowing Snow, Blizzard, or Snow Squalls.

Days Three to Seven: Sky Condition is replaced by a precipitation term when the Chance of Precipitation (COP) is equal or greater than 80 per cent. It is also replaced by the following terms when they are expected to occur: Fog, Blowing Snow, Blizzard, Near Blizzard Conditions, or Snow Squalls.

No-transition phrases such as: “Becoming a mix of sun and cloud late this afternoon,” are used in Days Three to Seven.

Chance of Precipitation

Included in the forecast for Day 1 & 2? - Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - Yes, if it meets the criteria.

The Chance of Precipitation (COP) is the chance that measurable precipitation (0.2 mm of rain or 0.2 cmof snow) will fall on “any random point of the forecast region” during the forecast period.

Whenever the COP is expected to be between 30 and 70 per cent inclusive, it is indicated in the forecast. The COP values are stated in increments of 10 per cent.

The term Risk is used in association with the terms Thunderstorm(s), Thundershower(s),Hail, Freezing Rain and Freezing Drizzle, when there is a 30 or 40 per cent chance of occurrence of these phenomena. In these cases, the percentage value is not stated.

The use of 50 per cent is not permitted.

Precipitation

Included in the forecast for Day 1 & 2? - Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - Yes, if it meets the criteria.

Precipitation is included in the forecast when the Chance of Precipitation (COP) is equal to or greater than 30 per cent.

Days Three to Seven:  Only the following types are permitted: Rain, Snow, Drizzle, Showers, Flurries, and Thunderstorms. Additionally, the term Freezing Rain is used when the freezing rain amount is forecast to be greater than 5 mm There are no precipitation type changes indicated in the extended forecast bulletin.

Precipitation Amounts

Included in the forecast for Day 1 & 2? - Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - No.

Days One and Two: Rainfall amounts for the full length of the forecast (Day One, Day Two, and Day Two night) shall be reported when they are expected to be:

The following table indicates Precipitation Amounts by Region

Region(s)Amounts must be equal to
or greater than
Coastal British Columbia, Ontario, and Quebec25 mm.
British Columbia interior, and Yukon forecast regions10 mm.
Manitoba, Saskatchewan, Alberta, Northwest Territories, Nunavut5 mm.
New Brunswick, Nova Scotia, Prince Edward Island,
Newfoundland and Labrador
2 mm.

OR

The snowfall amount, ice pellet amount, or a combination of snowfall and ice pellet amounts are expected to be equal or greater than 2cm for the full length of the forecast (Day One, Day Two, and Day Two night).

Days Three to Seven:  Precipitation amount is NOT forecast.   

Wind

Included in the forecast for Day 1 & 2? - Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - Yes, if it meets the criteria.

Days One and Two: 

Wind is included when the mean speed is greater than or equal to 20 km/h.

The terms Light or Calm are only used in a diminishing wind situation. For example: “Wind west at 20 km/h becoming light this evening.”

A wind speed change is indicated only when the average sustained speed is expected to change by at least 20 km/h. (i.e. “Wind south 20 km/h increasing to 40 this evening.”)

Wind is always included when wind chill is mentioned in the forecast. If the mean wind speed is less than 20 km/h and the wind chill criteria are met, then the phrase: "Wind up to 15 km/h," is used.

The following are terms used to describe wind speed in British Columbia and the Yukon only:

  • Locally windy: For wind conditions that are local in nature.
  • Gusty winds: Mean wind speed less than 30 km/h and gusts between 31 and 64 km/h.
  • Windy: Mean wind speed 30-45 km/h and gusts less than 65km/h.

Days Three to Seven: The term Windy is used when the mean wind speed is expected to be equal or greater than 40km/h or when gusts are equal or greater than 50 km/h, for a period of six or more hours.

The term Very Windy is used in British Columbia and the Yukon when winds meet local wind warning criteria.

Temperature

Included in the forecast for Day 1 & 2? - Yes
Included in the forecast for Day 3 to 7? - Yes

A single value of temperature is used. The following applies to both the maximum and minimum temperature:

  • Temperature equal to 0ºC, write the term "zero"
  • Temperature below 0ºC, add the qualifier "minus"
  • Temperature from 1 to 5ºC, add the qualifier "plus"
  • Temperature above 5ºC, no qualifier added

Days One and Two: In cases where temperatures are anticipated to remain steady or undergo an abnormal trend for the time period (i.e. the temperature drops during the day or rises overnight by at least 3°C), this will be indicated in lieu of a forecast maximum or minimum.

A second temperature can be given if the conditions exist in a way that there is a difference in temperature within a forecast region. These conditions usually exist in a forecast region which has shoreline and/or large elevation changes. 

The difference must be at least 1°C when the temperature is from plus 5°C to minus 5°C, otherwise the difference has to be at least 3°C. 

An example would be: “Friday..Mainly sunny. High 24 except 12 along the coast.”

Days Three to Seven:  The low and high temperatures respectively, reflect extreme values during the previous night (usually the early morning temperature) and daytime hours (usually early afternoon).

Frost

Included in the forecast for Day 1 & 2? - Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - No.

When the minimum temperature is forecast to be at, or close to, the freezing point, the term Frost may be used to describe where freezing is expected. The usage of this term is dependent on the extent of the freezing conditions.

For the chance of local frost, the following phrases are used: "Risk of frost," "Patchy frost," "Frost in low lying areas," and "Frost in valleys."

For extensive frost or when frost is likely, the following phrases are used: "Frost," "Widespread frost," "Extensive frost," and "Frost likely."

The inclusion of frost in the forecast is regionally dependent on the time of season.

Obstructions to Visibility

Included in the forecast for Day 1 & 2? - Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - Yes, if it meets the criteria.

Day One and Two:

Obstructions to visibility include:

  • Fog
  • Haze
  • Blowing snow
  • Blowing dust
  • Smoke
  • Snow squalls
  • Blizzard
  • Near blizzard conditions.

The phrase: "Areas of morning low cloud and fog," is used in the mountainous terrain of British Columbia and the Yukon, when different areas within a forecast region have low stratus clouds, fog, and clear sky conditions at the same time.

Days Three to Seven:

Obstructions to Visibility include:

  • Fog
  • Blowing Snow
  • Snow Squalls
  • Blizzard

Near blizzard conditions

Wind Chill

Included in the forecast for Day 1 & 2? - Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - No.

Days One and Two: Wind Chill values are included in the forecast bulletin when the wind speed is greater than 10 km/h AND the wind chill satisfies the criteria specified below. In general, only the lowest wind chill is indicated.

The following table indicates the minimum wind chill value required for wind chill to be mentioned in the forecast.

Region(s)Value
Coastal B.C.-15
Southern Interior, Central Interior and Northern B.C., Prairies, Ontario,
Atlantic Canada, Yukon, NWT and Nunavut
-25
Western, Central and Eastern Quebec-28
Northern Quebec-35
Extreme Northern Quebec-42

Further information on wind chill can be found here

The thresholds for a Wind Chill Warning to be in effect are different. 

Humidex

Standards pertaining to this topic are currently under review and will be posted upon approval. At present, humidex is reported in hourly weather observations and advisory bulletins when applicable only.

UV Index

Included in the forecast for Day 1 & 2? -Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - No.

The Ultraviolet Index (UV Index) is included in the forecast bulletin when it is forecast to be greater than or equal to three. The value plus the associated UVI category will be included.

For example:

Today.. A mix of sun and cloud. High 27. UV index 7 or high.

The 5:00 a.m. and 11:00 a.m. morning forecast bulletins include the UVI for the first day.

The 4:00 p.m. afternoon forecast bulletin includes the UVI for the next day.

A chart showing the exposure risks of the UV Index

UV CategoriesUV Index Range
Extreme11 or higher
Very high8 to 10
High6 to 7
Moderate3 to 5
Low2 or less

Note: UV Index values above three should involve use of different sun protection measures.