Jargon --> Risk vs Chance

Jun 10, 2004
Kitchener, ON, Canada
When reading forecasts, the words risk and chance are used often to describe thunderstorm activity. In the met world, is there any difference in the meanings of these words?

Be well, TR
I would think chance and risk would basically mean the same thing. Unless the word risk, was used for example a slight risk of severe thunderstorms, etc.

Some background stuff on the term chance and how the National Weather Service defines it and also included PoP.

National Weather Service Definition of Chance:
Chance: A National Weather Service precipitation descriptor for 30, 40, or 50 percent chance of measurable precipitation (0.01 inch). When the precipitation is convective in nature, the term scattered is used. See Precipitation Probability (PoP).
National Weather Service Definition of Precipitation Probabilites:
Precipitation Probabilities (PoP): It 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.

Measurable precipitation is defined as equal to or greater than .01 inch or .2 mm. Normally, the period of time is 12 hours, unless specified otherwise. The forecast area, or zone, is generally considered to be a county. In some geographically unique areas (mountains), the forecast area/zone may consist of portions of a county or two counties.

At times, some NWS forecasters will use occasional or periods of to describe a precipitation event that has a high probability of occurrence, i.e., they expect any given location in a forecast zone area to most likely have precipitation, but it will be of an on and off nature. Usually, away from the mountains, each and every county is a forecast zone area itself.

The following table shows precipitation probabilities used in National Weather Service forecasts and a brief explanation of each. Precipitation Probabilities (PoP) used in National Weather Service Forecasts

PoP Percent Expressions of Uncertainty
10 percent: none used
20 percent: slight chance
30-40-50 percent: chance
60-70 percent: likely
80-90-100 percent (categorical): none used

Equivalent Areal Qualifiers (convective only)
10 percent: isolated, or few
20 percent: widely scattered
30-40-50 percent: scattered
60-70 percent: numerous (or none used)
80-90-100 percent (categorical): none used
Character of the Precipitation
Terms used to describe the character of the precipitation are:

Brief: short, abrupt showers
Frequent: persistent short intervals of precipitation
Occasional: precipitation occurring at irregular or infrequent intervals
Periods of:: series of episodes of precipitation
Intermittent: precipitation starting and stopping at intervals; not continuous.
Source: National Weather Service

This is probably number one on the things that give me fits as a TV weather weenie. How does someone properly explain to a non-weather audience (your average viewer) slight risk?

I usually don't. I know I should, but when you say "Slight risk of severe storms"... most folks will discount that as being similar to a "slight chance of rain." They don't pay attention to it.

It has taken time, and we've been successful explaining to our news managers that slight risk does mean we should have heightened awareness (i.e. I'd like more time for my show)...

but as for the viewer, I typically just make a map bannered "Severe Risk" or something like that. I use yellow, orange and red.

"High risk" everyone gets.


NZ Meteorological Service likes to use the word 'Isolated' to cover a slight risk of a shower or thunder. I think isolated is one of their favourite words. It covers them both ways. :D
Isolated and scattered are faves here in the States too. I have had to explain to folks the diff between the two. Most people are not weather inclined (or savvy) so both words mean the same to them.
I love to go to work and say "it's a tornado day folks, if you see me heading to the janitor closet, y'all better be right behind me". No one understands unless I put it in those direct words. While studying a PDS watch, I get "what's that?" from family. They can't remember it correctly but one kid has the jist of the idea.........he says "pretty damn serious". :D

If a storm comes up, I am called by the manager to assess the situation at the front door. It cracks me up. Not allowed to cruise the 'net at work but if it looks bad, I am told to go "check your weather sites". I have told my manager that to be more sure of what is going on, I need to get in my car and go check around. She doesn't buy it. Damn, I tried.