“If a forecast gives the chance of rain (‘probability of measurable precipitation’ is the correct term within the science) as 70%, it is describing the chance of rain occurring—1) at any point selected in the area 2) during the period specified—as 70%.
Clouds drifting over a Hawaiian trail. (REI photo: Damon Parrish)
“In other words, if the area covered by the forecast is King County (in which Seattle lies) from, say 4am to 4pm, then it means there's a 70% chance of measurable rain occurring at any point in the county during that time span.
“In fact, the statistic is normally given for a bigger area, and that's why it's rarely used (or at least I rarely use it) on television forecasts.
“In Seattle we know weather varies considerably over short distances because of the interaction of incoming ‘weather’ and terrain. What might be a 70% chance for downtown Seattle may be a 90% chance for Issaquah." Note: Issaquah is a suburb east of Seattle (elevation 108 feet) where precipitation gets an assisting boost due to the city’s proximity to nearby mountains, the so-called Issaquah Alps: Cougar, 1,595 feet; Squak, 2,024 feet; Taylor, 2,600 feet; and Tiger, 3,004 feet.
“At the same time, the chance of rain might be 40% over the north end of Lake Washington (a large lake that separates Seattle from its eastern suburbs), which may happen to fall in the 'rain shadow' of the Olympic Mountains.
“Some days Seattle might be under the influence of a Convergence Zone (see left). That’s where winds in the upper atmosphere are split by the Olympic Mountains west of Seattle, then re-converge over Puget Sound. At that convergence point, winds can cause updrafts that stir up rain showers. On those days downtown Seattle may have a 20% chance, but the north end of the city may have an 80% chance.
“That's why I only use percentage probabilities when I'm forecasting for a specific location, such as for Mount Rainier.”
So of the points presented at the top of this post, No. 3 is our winner. Jeff’s technical explanation of what chance-of-rain percentages mean pretty well matches what I guess is the general layman’s understanding of the concept.
Like me, you’ve probably seen printouts of 7-day NOAA forecasts taped to windows or counters at national park visitor centers. (Thanks to rangers who go to that effort.) NOAA forecasts routinely include daily chance-of-rain estimations, and if I see a 70% chance predicted on a day at the place when I plan to hike, that’s my hint to double-check that I packed my rain gear.