AccuWeather Garden Weather
Gardening Planner Maps, How-To's, and more. You'll need to enter your location (just enter your Zip code in the field at the top).
Home & Garden Weather on The Weather Channel
Get your "Gardener's Local Forecast" and even a "Garden Care Calendar" (a monthly gardening activity guide) for your Zip code.
National Weather Service
The NWS provides extremely detailed weather information. They even have weather watches, warnings, and advisories by state available as RSS feeds (for a free news reader that can read these feeds, try Awasu Personal Edition, FeedDemon, Google Reader, NetNewsWire for Mac/iPhone/iPad, or Gruml: Google Reader for Mac). You can also sign up for email alerts.