Every day people check the weather forecast to plan their daily activities, but do they really understand what the meteorologist is telling them? This document is intended to develop a better understanding of those forecasts. It is designed to help people make informed decisions about travel plans, sports practices and events, campouts, or other outdoor activities before hazardous weather develops.
The National Weather Service (NWS) is the federal agency charged with providing weather services to the nation. It is the official source of watches, warnings and advisories for hazardous weather. Weather forecasts and warnings can be received directly from the NWS through its nationwide network of NOAA Weather Radio.
This document will use terminology as defined by the NWS; forecast products issued by countries other than the US have different criteria. Forecasters outside of the NWS (such as on radio and television) may use different terms or place a different meaning on weather terms.
WEATHER FORECAST TERMINOLOGY
The basic weather forecast includes the following weather elements: precipitation, probability of precipitation, sky condition, temperature and wind. Forecasts describe the weather in 12-hour increments such as "today", "tonight" and "tomorrow." Forecasters often use descriptive terms to convey the forecast message. While these terms may be subjective, the NWS attempts to standardize them. Details on precipitation probabilities, sky condition and wind are provided in subsequent sections.
PRECIPITATION PROBABILITY
The probability of precipitation forecast is one
of the most least understood elements of the weather forecast. The probability
of precipitation has the following features:
..... The likelihood of occurrence of precipitation is stated as a percentage
..... A measurable amount is defined as 0.01" (one hundredth of an inch)
or more
(usually produces enough runoff for puddles to form)
..... The measurement is of liquid precipitation or the water equivalent
of frozen precipitation
..... The probability is for a specified time period (i.e., today, this
afternoon, tonight, Thursday)
..... The probability forecast is for any given point in the forecast area
To summarize, the probability of precipitation is simply a statistical probability of 0.01" inch of more of precipitation at a given area in the given forecast area in the time period specified. Using a 40% probability of rain as an example, it does not mean (1) that 40% of the area will be covered by precipitation at given time in the given forecast area or (2) that you will be seeing precipitation 40% of the time in the given forecast area for the given forecast time period.
Let's look at an example of what the probability does mean. If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time.
This point probability of precipitation is predetermined and arrived at by the forecaster by multiplying two factors:
Forecaster certainty that precipitation will form or move into the area
X
Areal coverage of precipitation that is expected
(and then moving the decimal point two places to the left)
Using this, here are two examples giving the same
statistical result:
(1) If the forecaster was 80% certain that rain
would develop but only expected to cover 50% of the forecast area, then
the forecast would read "a 40% chance of rain" for any given location.
(2) If the forecaster expected a widespread area
of precipitation with 100% coverage to approach, but he/she was only 40%
certain that it would reach the forecast area, this would, as well, result
in a "40% chance of rain" at any given location in the forecast area.
The following terms of duration imply a high probability (80-100%) of occurrence: brief, periods of, occasional, intermittent, frequent.
Terms of uncertainty are listed:
Probability of
Descriptive Terms
Precipitation
Used
----------------------------------------------------------------
0%
None
10%
Slight Chance, Isolated
20%
Slight Chance
30-50%
Chance, Scattered
60-70%
Likely, Numerous
80-100%
Categorical ("Rain this afternoon")
SKY CONDITION
The predominant or average sky cover for the forecast period is also presented in the forecast. This percentage of the sky is the amount expected to be covered by opaque clouds, the type that do not allow other clouds, or blue sky to be visible through or above them. The following table lists descriptive terms for sky conditions:
TERMINOLOGY
AVERAGE SKY COVER
-----------------------------------------------------------------
Cloudy
90-100%
Mostly cloudy
70-80%
Partly Cloudy/Partly Sunny 30-60%
Mostly Clear/Mostly Sunny 10-30%
Clear/Sunny
0-10%
Fair
Less than 40% cloud cover, no
precipitation and no extreme weather
WIND
There are several descriptive terms often used to better define wind conditions in weather forecasts:
SUSTAINED WIND
SPEED
TERMINOLOGY
--------------------------------------------------------------
0-5 miles per hour
Light/Light and Variable
5-15 miles per hour
---No terminology used---
15-20 miles per hour (mild)
Breezy
15-25 miles per hour (cold)
Brisk
15-30 miles per hour
Windy
25-40 miles per hour
Very Windy
40 + miles per hour
Strong, High, Damaging
(Editors Note: This information is taken from the National Weather Service Brochure, "Is it Going to Rain Today? Understanding the Weather Forecast.")