Many people go to the National Weather Service's site, http://forecast.weather.gov first, but I prefer http://www.wunderground.com/ because they get their information about current conditions closer to my home. The same thing was true where we lived before. However the Weather Service does predict things a few more days in the future.
http://www.weather.com/ gives ten day forecasts. Of course the farther ahead the forecasts are, the less likely they are to be correct, but at least they give a general idea of what to expect. The fifteen day predictions on http://www.accuweather.com are even less likely to be accurate.
There are lots of other sites online that predict weather conditions, including local TV news channels, but these four are the ones I use most often.
When planning a trip or event I usually look at the 15 day forecast when thinking about what to bring, then go to the 10 day, and, finally, to the more dependable short term predictions.
Of course there are no guarantees because weather conditions can change unexpectedly. Once the day before my grandson was coming to visit all four forecasts said there was 100% likelihood of snow the next day. But they were all 100% wrong. We had a little bit of light rain, then clear skies and a disappointed kid.
Oh, well, he had fun in the sun anyway.