The Anatomy of a Forecast The Met Service issues forecasts for sky condition, precipitation probability, wind, seas state and temperature on a routine basis. Because the weather is always changing, the terminology used in these forecasts is also quite variable. Listed below are descriptions of regularly used weather terms that should give you a better understanding of each forecast. SKY CONDITION The sky condition describes the predominant/average sky condition based on eighths of the sky covered by opaque (not transparent) clouds. If a high probability of precipitation (60% or greater) is expected, then the sky condition may be omitted since it is inferred from the precipitation forecast. Sky Condition Opaque Cloud Coverage Overcast 8/8 Cloudy 7/8 to less than 8/8 Mostly Cloudy / Considerable Cloudiness 5/8 to less than 7/8 Partly Cloudy / Partly Sunny 3/8 to less than 5/8 Mostly Clear / Mostly Sunny 1/8 to less than 3/8 Clear / Sunny Less than 1/8 PRECIPITATION PROBABILITY The Probability of Precipitation (POP) is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. In this case measurable precipitation is equal to or greater than 0.01 /0.2 mm (the least about the models can forecast). Unless specified otherwise, the time period is normally 12 hours. The forecast area, or zone, is generally considered to be a county. Such categorical terms as occasional, intermittent, or periods of to describe a precipitation event that has a high probability of occurrence (80% +) but is expected to be of an on and off nature. POP Percent Expression of Uncertainty Equivalent Area Qualifier (convective use only) 10 percent None used (Isolated / Few) 20 percent Slight chance (Widely scattered) 30, 40, & 50 percent Chance (Scattered) 60 & 70 percent Likely (Numerous or none used) 80, 90, & 100 percent None used 1
WIND The wind describes the prevailing direction from which the wind is blowing, with speeds in knots (times 1.15 to get miles per hour). In addition to the wind velocity, descriptive terms may also be used/given for emphasis. Sustained Wind Speed Descriptive Term 0 Calm Less than 5 knots Near calm / Light / Light & variable wind Other ranges Beaufort Scale for guidance SEA STATE The sea state in a forecast is used to describe the forecast of the mean or average height of the highest one third of all waves in a swell train or in a wave generating region. Sea State Description Calm (glassy) 0 Calm (rippled) 0 0.1 Smooth (wavelets) 0.1 0.6 Slight 0.6 1.2 Moderate 1.2 2.4 Rough 2.4 4 Very Rough 4 6 High 6 9 Very High 9-14 Phenomenal Over 14 Forecast Meaning (metres) TEMPERATURE The temperature in a forecast is used to describe the forecast maximum and minimum temperature, or in some cases, the temperature expected at a specific time. Temperature Description Forecast Meaning Around 30 / Near 30 / About 30 A range of temperatures from 29 to 31 Lower 30s Temperatures of 30, 31, 32, 33 Middle 20s Temperatures of 24, 25, 26 Upper 20s Temperatures of 27, 28, 29 20s A range of temperatures from 20 to 29 30 to 35 Temperatures of 30, 31, 32, 33, 34, 35 2
TIME PERIODS Normally, forecasts are divided into periods (12 hours/day and night). In some instances, the forecast may be divided into subperiods (6 hours). Forecast Time Period Today This Morning Early Morning Late Morning Near/Around Midday This Afternoon Early Afternoon Late Afternoon This Evening Early Evening Late Evening Near/Around Midnight Overnight After Midnight Fore-day Morning Tonight Tomorrow Day of Week (e.g. Tuesday) Definition (Local Standard Time) 6AM 6PM (Sunrise Sunset) 6AM Midday (Sunrise Midday) 6AM 9AM (Sunrise to 9AM) 9AM Midday 11AM 1PM Midday 6PM (Midday Sunset) Midday to 3PM 3PM 6PM (3PM Sunset) 6PM Midnight (Sunset Midnight) Sunset 9PM (6PM 9PM) 9PM Midnight (9PM Midnight) 11PM to 1AM Midnight 6AM (Midnight to Sunrise) Midnight to 3AM 3AM 6PM (3PM Sunrise) 6PM 6AM (Sunset Sunrise) 6AM 6PM (Sunrise Sunset) the following day 6AM Tuesday 6AM Wednesday (Sunrise Tuesday Sunrise Wednesday) unless context suggests otherwise Some Details Areal qualifiers of showers and chance (probability of precipitation POP) of precipitation: o Isolated showers: showers that produce measurable rainfall over no more than 10% of the forecast area in usually 12 hour period (day or night).this is equivalent to a 10% chance of showers or a slight chance of showers. o Widely scattered showers (Local rain): showers (rain) that produce measurable rainfall over no more than 25% of the forecast area in usually 12 hour period (day or night). This is equivalent to a 20% chance of showers or a slight chance of showers. o Scattered showers (Areas of rain): showers (rain) that produce measurable rainfall over 25 to 54% of the forecast area in usually 3
12 hour period (day or night). This is equivalent to a 30, 40 or 50% chance of showers or there is a chance of showers. o Numerous showers/showers Likely (Widespread rain): showers (rain) that produce measurable rainfall over more than 54% of the area in usually 12 hour period (day or night). This is equivalent to a 60 or 70% chance of showers and showers are likely. o Showers/Showers Expected (Rain): (occasional, intermittent, frequent, periods of, developing): showers (rain) that produce measurable rainfall over at least 80% of the area in usually 12 hours (day or night). This is equivalent to 80, 90 or 100% chance of showers. Note that a day may be subdivided into smaller periods or not at all according to the conditions expected. Chance of precipitation (probability of precipitation): The chance that there will be measurable rainfall (greater than or equal to 0.2 mm) at your location. E.g.: 10% means a one in 10 chances; 20% means a two in 10 chances; 30% means a three in 10 chances etc. Marine Cautions and Warnings: o Sea bather caution (SBC) o Sea bather advisory (SBA) Should avoid sea bathing o Sea bather warning (SBW) Should avoid sea bathing, extremely dangerous o Small craft caution (SCC) o Small craft advisory (SCA) should stay in or near port o Small craft warning (SCW) should stay in port, extremely dangerous Measurable rainfall: Rainfall greater or equal to 0.2 millimetre Seas: The mean height of the highest 1/3 of the waves, consisting of sea waves or swells only or a combination of both. Sky Conditions: o Overcast: The sky is expected to be completely covered by opaque clouds. o Cloudy: Expected predominant or averaged opaque cloud coverage of 7/8 to less than 8/8. o Mostly cloudy: Expected predominant or averaged opaque cloud coverage of 5/8 to 7/8. 4
o Partly cloudy (Partly sunny): Expected predominant or averaged opaque cloud coverage of 3/8 to 5/8. o Fair: Expected predominant or averaged opaque cloud coverage of less than 3/8. Also, no precipitation and no extreme conditions of visibility, wind or temperature. Generally pleasant weather conditions. o Mostly clear (Mostly sunny, Mostly fair): Expected predominant or averaged opaque cloud coverage of 1/8 to 2/8. o Clear (Sunny): Expected predominant or averaged opaque cloud coverage of less than 1/8. Ultraviolet Index (UVI): UV index at local solar noon for clear-sky conditions. Wind description: o Wind direction: The predominant or average direction the wind is expected to blow from. E.g. East (90 degrees) wind means that the predominant or average direction is east, however a maximum wind direction spread of 45 degrees about the average can be expected i.e. the direction could range between 67 and 113 degrees or eastnortheast and east-southeast. o Wind speed (e.g. 10 knots): The predominant or average 10- minute wind speed. o Wind speed range (e.g. 7 to 13 knots): The predominant 10- minute wind speed range. o Calm: Wind speed less than 1 knot. o Near calm (light and variable L/V): 10-minute wind speed of less than 5 knots. o Strong wind (windy): 10-minute wind speed of 22 to 33 knots. o Gusty wind: Wind gusting at times to or in excess of 25 knots (lasting for a few seconds). 5