Sky Conditions

Clear Skies

No cloud cover

Fair

Cloud cover is 2 oktas or less of low and or medium clouds for most of the time with isolated light rainfall or no rainfall. Term can be used both for day and night time

Mainly Fair

Generally fair (2 oktas or less) interspersed by brief periods of partly cloudy to cloudy skies (3 oktas or more) with isolated light rainfall or no rainfall. Term can be used both for day and night time

Sunny

Cloud cover 2 oktas or less being sunny for prolonged periods with isolated light rainfall no rainfall

Partly Cloudy

Cloud cover for the greater part of the time averages 3 to 4 oktas of low and or medium clouds or dense high level clouds of 5 oktas or more, interspersed with brief periods of fair or cloudy skies. Periods of isolated light or moderate showers are likely or no rainfall

Mostly Cloudy

Cloud cover of 5 oktas or more interspersed with brief periods of fair to partly cloudy skies. Shower activity likely to be light to isolated moderate

Cloudy

Cloud cover is greater than 4 oktas and shower activity likely to be light to moderate and isolated heavy

Overcast

Cloud cover of 8 oktas and shower activity likely to be light to moderate interspersed by heavy periods

Increasingly Cloudy

Cloud cover generally increasing from fair or partly cloudy to cloudy or overcast

Decreasingly Cloudy

Cloud cover generally decreasing from overcast or cloudy to partly cloudy or fair

Showers

Precipitation, often short-lived and heavy, falling from convective or towering clouds. A shower is characterized by its sudden beginning and ending and generally by large and rapid changes of intensity

Rain

Precipitation of liquid water particles in the form of drops of more than 0.5 mm in diameter

Spatial Coverage

Isolated – Rainfall occurring across 10% or less of the forecast area
Few scattered – rainfall occurring over 10% to 30% of the forecast area
Scattered – rainfall occurring over 30% to 60% of the forecast area
Showers (no description) – rainfall covering greater than 60% of the forecast area

Temporal Coverage

Brief – Shower event that occurred for 10 minutes or less

Periods of showers/rain – several instances of shower events throughout the forecast period

Conditions Associated with Wind

Breezy

Wind speeds averaging 12 – 18 kts (22-33 km/h)

Windy

Wind speeds averaging 19 kts (35 km/h) and higher

Low Level Jet

Wind speed in the lower levels of the atmosphere (surface-10,000ft) averaging 25kts (46 km/h) or higher

Upper Level Jet

Wind speed in the upper levels of the atmosphere of 50 kts (93 km/h) or higher

Moisture Convergence

Moisture convergence – Is the advection or transport of moisture into a fixed region such as the Lesser Antilles that usually promotes an increase in moisture levels and the potential for shower activity. Moisture advection is common along cold fronts or any other low-level boundary that causes general convergence such as the land mass of mountainous islands

Advection

Transport of water or air along with its properties (e.g. temperature, moisture) by winds or currents

Subsidence

Slow descent or sinking of a mass of air, over a wide area, generally accompanied by horizontal divergence or air moving away from a point, in the lower layers. The subsiding air is compressed and warmed usually increasing stability or lessening the potential for shower activity

Weather Systems

High Pressure System

An area of higher pressure relative to the surrounding region identified with a clockwise circulation in the northern hemisphere and a counterclockwise circulation in the southern hemisphere. Also, defined as an anticyclone

Ridge

An elongated area of relatively high pressure that is typically associated with an anti-cyclonic wind shift

Low Pressure System

An area of low pressure relative to the surrounding region identified with counter-clockwise circulation in the northern hemisphere and clockwise in the southern hemisphere. Also, defined as a cyclone

Trough

A trough is an elongated area of low pressure. Troughs develop at varying levels of the atmosphere and can be separated as:

• Low level trough – up to 10,000ft
• Middle level trough – up to 18000ft
• Upper level trough – above 18000ft

Tough System

Term used to describe a trough that is manifested from the surface through to the upper levels of the atmosphere

Tropical Wave

A trough or cyclonic curvature maximum in the trade-wind easterlies. (A tropical wave may be differentiated from the normal trough based on where and how they are usually formed and their movement)

Tropical Disturbance

A discrete tropical weather system of apparently organized convection originating in the tropics or subtropics that maintain its identity for 24 hours or more

Potential Tropical Cyclone

A term used to describe a disturbance that is not yet a tropical cyclone, but which poses the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours

Tropical Depression

A tropical cyclone in which the maximum sustained surface wind speed is 33 kt (38 mph or 62 km/hr) or less

Tropical Storm

A tropical cyclone in which the maximum sustained surface wind speed ranges from 34 kt (39 mph or 63 km/hr) to 63 kt (73 mph or 118 km/hr)

Tropical Storm Watch

An announcement that sustained winds of 34 to 63 knots (39 to 73 mph or 63 to 118 km/hr) are possible within the specified area within 48 hours

Tropical Storm Warning

An announcement that sustained winds of 34 to 63 knots (39 to 73 mph or 63 to 118 km/hr) are expected somewhere within the specified area within 36 hours

Hurricane

A tropical cyclone in which the maximum sustained surface wind is 64 kt (74 mph or 119 km/hr) or more

Major Hurricane

A hurricane that is classified as Category 3 or higher

Hurricane Watch

A Hurricane Watch means that hurricane conditions are possible within the watch area. A watch is typically issued 48 hours before the anticipated first occurrence of tropical-storm-force winds, conditions that make outside preparations difficult or dangerous

Hurricane Warning

A Hurricane Warning means that hurricane conditions are expected somewhere within the warning area. A warning is typically issued 36 hours before the anticipated first occurrence of tropical-storm-force winds, conditions that make outside preparations difficult or dangerous. Preparations to protect life and property should be rushed to completion

Eye

The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud. The eye usually experiences little or no precipitation

Rapid Intensification

An increase in the maximum sustained winds of a tropical cyclone of at least 30kt (56km/h) in a 24-h period

Feeder Bands

This term is used to describe spiral-shaped bands of convection surrounding and moving toward the center of a tropical cyclone. Same as inflow bands

Inter-Tropical Convergence Zone

A zonally elongated axis of surface wind confluence of northeasterly and southeasterly trade winds in the tropics

Fronts

The interface or transition zone between two air masses of different density. Density depends on temperature and moisture content. Fronts either lie along shear lines or can lag behind them

Shear Line

It is an area of directional wind confluence (coming together) along the tail end of a surface front or a line or narrow zone across which there is an abrupt change in the horizontal wind component parallel to this line

Flood

The term flood is generally defined as a rise in the water level of a stream or water body to a peak from which the water level recedes at a slower rate

Flash Flood

A flash flood is a sudden local flood of great volume and short duration which follows within a few (usually less than six) hours of heavy or excessive rainfall, or due to dam or levee failure. Saturated soils, steeply sloping highland terrains and narrow valleys or ravines which hasten runoff also promotes flash floods

Flood/ Flash Flood Watch

A Flash Flood Watch means that flash flooding is possible

Flood/ Flash Flood Warning

A Flash Flood Warning means that flooding is already occurring or will occur during the warning period

Small Craft Warning

A small craft Warning means that winds-speeds of 25 knots (46km/h) or higher and/or seas of 3m (10ft) or greater are forecast to affect the marine area

High Surf Advisory

A High Surf Advisory is issued when breaking wave action poses a threat to life and property within the surf zone

Marine Conditions (Waves/Swells)

Slight: 0.5 -1.3m / 1 to 4 feet
Moderate: 1.3 – 2.5m / 4 to 8 feet
Rough: 2.5 – 4m / 8 – 12 feet
Very Rough: 4.0 – 6.0m / 13 to 20 feet
High: 6.0 – 9.0m / 20 to 30 feet
Dangerous: >9.0m / 30.0 feet

Storm Surge

An abnormal rise in sea level accompanying a hurricane or other intense storm and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide

Landslide

Generally describe the downslope movement of rock, debris or soil. Landslides can be initiated in weak or compromised slopes by heavy or prolonged rainfall, stream erosion, changes in ground water, earthquakes, volcanic activity and disturbance by human activities

Rockfall

Rock falls are abrupt, downward movements of rock or earth, or both, that detach from steep slopes or cliffs

Vigilant

Keeping careful watch, be alert, be on the lookout for deterioration in weather conditions that may affect you or create difficulties