14 November 2006

It has been raining here for a few days and it looks like it will be raining here for quite a few days longer. I am worried that the Leonids Outburst will be rained out this Saturday night and the last crescent Moon on Sunday morning, not to mention public observing at Columbia University on Friday night. That has me listening to NOAA on the amateur radio and thinking about just what they might mean when they say how likely it is to rain. So I looked it up on the Web.

Probability of Precipitation

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 0.01" or 0.2 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 a portion of a county or two or more 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.

Expression Uncertainty	Equivalent Area Qualifiers (Convective Precipitation Only)	Probability of Precipitation (PoP)
none used	Isolated or few	10%
Slight Chance	Isolated	20%
Chance	Scattered	30% - 50%
Likely	Numerous	60% - 70%
none used	none used	80% - 90%

Convective Precipitation = Thunderstorms

Text quoted and table adapted from Jetstream: An Online School For Weather