Creating nature photographs that really stand out takes more than just leaving the house on a nice day and hoping to get lucky. Dedicated nature photographers must research and plan from a host of variables that will make or break a great shot.
Weather is the biggest thing new nature photographers miss out on, they will wait until the weather is 'nice' to go out shooting. I will let you in on a secret, dramatic weather creates dramatic photographs. If there are thunderstorms get out there, if there's snowstorms get out there (as the storm is coming in or just after the storm clears you will have dramatic light) on overcast days go shoot waterfalls or macro, white puffy clouds make for great black and white, about the only time I don't go out is on clear sky days. While most people love clear skies, it is the most despised thing of nature photographers, these are great days for sitting inside and editing images...or just be like a normal person for a day, leave the gear behind, and go out to enjoy it...yeah, that's probably not going to happen. If I'm on a road trip and the skies turn clear for a week I will refocus my energy on night photography since clear skies are desirable to see the stars.
The NWS is the resource for weather prediction in my opinion, they have recently cleaned up their site to make it more user friendly, it used to be quite bad and why most people would gravitate towards other services. The NWS has some features that the others do not provide, first up is point forecasts, this means you can actually drill down to a specific spot on a map to get a forecast for an area. Go to the main site above, click on your region which brings up a regional map, click in the general area you want which will bring you to a forecast page, now to really get specific look for the google map in the right column and find the exact location you want, click that spot on the map and forecast page will change to this location. The NWS actually takes elevation into consideration which is critical if you're shooting in the mountains, I've found these forecasts to be surprisingly accurate.
The second feature they have is the hourly weather graph, when you're on the forecast page look for the hourly weather graph
This graph gives you a plethora of easy to digest information, take some time to learn how to read this and it will pay off bigtime. I've found the sky cover % to be especially useful
Clear Sky Chart - This is a great resource I mainly use for night photography to determine if the skies will be clear enough to see the stars at night, I've found it to be fairly accurate and use it during the day as well, but for the opposite reason; to make sure the skies will not be clear! This also has a great predictive satellite feature that many people probably miss; when you're on your forecast page you can click on one of the boxes for the hour your investigating and it will take you to a satellite image of the region, checking this 24 hours before you go out may completely change your plans!
Visible Satellite - This is a great detailed satellite view to help you see where the clouds are, scattered clouds are generally the best, but this really depends on what you're shooting. I also use this when storm chasing to spot overshooting tops which are an indication of a supercell that may develop a tornado.
Surface Analysis Map - This map shows current temperatures, dewpoints and wind speed/direction, which is good general information but I mainly use it for predicting where storms will be developing based on the dewpoints, Jeff Haby has a nice simple article explaining this in his Severe Thunderstorm Cookbook
Storm Prediction Center - The SPC is the resource to look to for severe weather, I check this site everyday to see if there will severe weather near my area
Graphical Forecasts - Another useful tool from the NWS, it gives good forecasts for a region rather than a specific area
Dark Sky Finder - This is specific to night photography to find dark skies which is incredibly important to actually see the stars!
Stellarium - Again night specific, I use this to determine when the milky way will be out and in the right position
The Photographer's Ephemeris - Last and certainly not least is TPE, this incredibly valuable tool will help you determine where the sun and moon are in relation to the landscape you're trying to photograph, much has been written on how to use this tool so I won't go deep into here.
This post turned out to be much longer than I thought it would, I have another post planned just for all the iPhone apps I use when in the field