METEOROLOGICAL SERVICES

GRENADA, CARRIACOU & PETITE MARTINIQUE

Terms

  • Brief: Short duration (Not exceeding 10 minutes).
  • Intermittent: Precipitation that does not exceed one hour per spell. It can be light, moderate or heavy.
  • Occasional: Precipitation that occurs infrequently, over an extended period of time and is usually intermittent in nature. For example, 15 – 20 minute precipitation spells every 40 – 60 minutes.
  • Frequent: Showers occurring often (Persistent). E.g. in excess of 2 showers within an hour.
  • Continuous: Precipitation that exceeds one hour per spell. It can be light, moderate or heavy.
  • Periods of Rain/Showers: Rain/showers are expected to fall most of the time, but there will be breaks. i.e. A series of episodes of precipitation.
  • Sharp: Brief, abrupt showers. Could be moderate to heavy with gusty winds.
  • Brisk Showers: Fast moving showers, usually the low level winds are strong.
  • Isolated: Showers which are well separated in space (i.e. generally covering less than 10% of the total area) during a given period.
  • Widely scattered: Means the precipitation areas are displaced a considerable distance from each other

    (Covering 10 – 30% of the total area)

  • Scattered: Irregularly distributed over an area (i.e. generally covering 30 to 60% of the total area). Showers which, while not widespread, can occur anywhere in an area. Implies a slightly greater incidence than isolated.
  • Widespread: Occurring extensively throughout an area (i.e. generally covering over 60% of total area).
  • Fine/Clear: 1/8 or less cloud cover, bright, good visibility at least greater than 15 km, no precipitation expected, stars easily visible at night.
  • Fair: Refers to less than 3/8 cloud cover, no precipitation or a light shower of short duration (less than 5 minutes) is possible, and no extremes in temperature, visibility or winds.
  • Mostly/Generally Fair: Fair conditions for more than 50% of the forecast period but a few light/brief showers possible.
  • Partly Cloudy: No rainfall or only brief light showers. Total low and medium level clouds less than or equal to 4 oktas. May or may not have high level clouds.
  • Mostly/Generally Cloudy: Five (5) – seven (7) oktas low and medium level clouds for 50 – 80% of the forecast period. Despite the fact that precipitation usually accompanies cloudy spells, this term can also be used for forecast period where likelihood of precipitation is nil.
  • Cloudy: Five (5) – seven (7) oktas low and medium level clouds for more than 80% of the forecast period. Despite the fact that precipitation usually accompanies cloudy spells, this term can also be used for forecast period where likelihood of precipitation is nil.
  • Cloudy to overcast: Five (5) oktas or more low and medium level clouds for more than 80% of the forecast period. Indicates a greater chance of precipitation but does not mean that precipitation is a definite.
  • Overcast: Eight (8) oktas low and medium level clouds for more than 80% of the forecast period. Indicates a greater chance of precipitation but does not mean that precipitation is a definite.
  • No probability used: Over 85% probability of occurrence
  • Likely: 50 – 85% probability of occurrence
  • Slight chance: 5 – 15% probability of occurrence
  • Chance of: 15 – 50% probability of occurrence
  • Low chance: Approximately 30% probability of occurrence
  • Medium chance: Approximately 40% probability of occurrence

A small craft advisory must be issued whenever the state of the sea is expected to be “Rough or greater” (8 feet or more) within the forecast period.

  • Windy: Sustained wind speed of 15 – 21 knots (17 – 24 mph) for more than 50% of the forecast period.
  • Very windy: Sustained wind speed of 22 – 27 knots (25 – 31 mph) for more than 50% of the forecast period.
  • Near gale: Sustained wind speed of 28 – 33 knots (32 – 38 mph). Source: Manual on Codes WMO No. 306 (2011 edition) – Beaufort Scale
  • Gale: Sustained wind speed of 34 – 40 knots (39 – 46 mph). Source: Manual on Codes WMO No. 306 (edition 2011) – Beaufort Scale
  • Strong gale: Sustained wind speed of 41 – 47 knots (75 – 88 mph). Source: Manual on Codes WMO No. 306 (edition 2011) – Beaufort Scale

Flash flood: A flood of which peak appears within, in general, six hours from the onset of torrential rainfall and confined to relatively small and poorly drained areas.

Landslide: Down slope movement of soil mass under the influence of precipitation, ground water and other causes.

 

METEOROLOGICAL SERVICES

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