By far the most accurate weather forecast I've found is at:
http://forecast.weath...
This is the National Weather Service site.

Though I look at many variables on this site, my main concern is precipitation. I closely look at:
1- Precipitation Potential % (brown graph)
2- Rain - which gives % as a bar graph and the amount (green bars with amount)

The amount of rain is just as important as the %. If there is a 100% chance of rain and the amount is only .1 inches over a 6 hour period, this would not be much of a deterrent. A 70% chance of rain with the amount being .5 inches would be much more of a deterrent. I noticed that it rarely rains when the precipitation potential is under 100%.

If the temperature for rainy day is under 50, I consider this very nasty weather. I am likely to reschedule a hike if the rain is significant. For the same rain forecast with temperatures over 65, the hike may be a go. Thunderstorms will cancel a hike if the % is near 100.

It's always a judgement call. About 98% of the time I think I'm right. Have never gone on a rainy day hike that I regretted and I hate mud and thunderstorms. I will change a hike path to be more on terra firma on a rainy day. For some reason, rainy day hikes are real nice and calming, quite different and definitely more memorable than non-rain hikes.

Of course I have proper rain gear and clothing which includes: all non-cotton quick drying or waterproof-breathable outerwear. I stopped wearing jeans, which I used to wear about 99% of the time, after buying several North Face hiking pants. What a difference!

For rainy day forecast hikes I also always carry a hiking umbrella. It's sturdy, extremely light weight, small when closed, comfy to carry, and opens/closes easily. Campmor sells a terrific one that can't be purchased elsewhere (about $25 at the time of this writing): Euroschirm Light Trek Umbrella. Link on the day I wrote this: http://www.campmor.co...

Snow without a blizzard is great for hiking. The problem is driving, getting there and getting home again. Often I'm real happy to hike in the Chestnut Hill/Wissahickon area from Hill House in a storm or right after a storm. You can get there by the Chestnut Hill West train or Chestnut Hill East train, last stop in either case.

FYI: If an inch of rain falls on a square mile of land, then 17 million gallons of water have fallen from the skies according to this article: http://ga.water.usgs.... Philadelphia according to Wikipedia is about 143 square miles, so by multiplying this number by 143 for each inch of rain we get into the billions of gallons. For larger regions and more rain the numbers get astronomical. There's a lot of water coming down from up there!

However, most of us are more interested in the amount falling on our own heads or body. I'm not doing these calculations as a friend suggested, just because I have too much time on my hands. I do believe that we are staying indoors because we are afraid of a few ounces of water per hour.

Assuming your body, looking down from the sky, is 1 square foot in area, you would get plummeted with about 2.5 quarts per inch of rain. Of course this rain does not instantaneously pour down like a shower. It rains over a period of time. Let's say the time period for the one inch of rain is 24 hours. You would be plummeted each hour with 3.3 ounces of water (less than half a cup). Not a helluva lot of water and certainly no reason to run for cover unless you are from Puerto Rico.

For at least one year now, I've been closely looking over hourly US Weather Service forecasts. Usually the rainfall amount during a hiking period is quite small. Of course the rain intensity regularly changes during a storm and the US Weather Service forecast can not account for every second, just the average rainfall. They can warn about downpours.

One equivalent that you should keep in mind if you look at a detailed hour by hour weather report:
.1 inches of rain per hour means 1 cup of water per hour on your body.
Rarely does a US Weather Service forecast show that much rain each hour. Hardly an amount that can effect your day unless you walk in mud or there is a downpour. I avoid both.

Bill supplied this info from Wikipedia: (slightly edited for simplicity)
Rainfall intensity is classified according to the rate of precipitation:
• Light rain — when the precipitation rate is < .10 in per hour (.10 in/hr = 1 cup on body/hour)
• Moderate rain — when the precipitation rate is > .10 in per hour
• Heavy rain — when the precipitation rate is > .30 in per hour,
• Violent rain — when the precipitation rate is > 2.0 in per hour