The weather outside is frightful but we have a movie to make! Unless you're incredibly lucky, every film production has to deal with crazy weather days, i.e. high winds, rain, intense heat or cold, etc. You need to be prepared to keep on filming despite nature's rebellion.
Each day the weather should be monitored by a film production. There are companies that offer up-to-the-minute weather reports for a fee, such as CompuWeather. On a micro-budget film, you may only be able to afford to use a free weather tracking service on the Web. These free services are okay but if you are working on a film with a large budget, you are best working with a service that can provide more detailed weather reports. No matter how you get your weather, you need to be aware of it and plan for how to deal with it so you don't lose too much shooting time.
Most productions can keep on filming during poor weather conditions. If it's sprinkling, just pull out the rain gear for both crew and equipment and keep on filming. Put a canopy over your actors so their hair and clothes do not get wet. One consideration with filming in the rain is that your production sound will probably be unusable (the rain will be heard beating down on the tarp protecting the actors). But having to re-record those scenes in post production is usually much less costly than not filming. Please note: Never film in lightening. People and copious amounts of metal equipment make great lightening rods. And for obvious reasons, filming in high winds is not practical. A breeze is fine if you weigh down your equipment but unless you want to worry about injuries to your cast or crew, running after your fly-away props, blown over equipment, and wind noise then you are best waiting out the wind storm or moving to another set.
If it's hot, make sure there are cool areas for the cast and crew to have a break from the heat. An air conditioned area is ideal plus tents, i.e. E-Z Ups, for shade. And provide lots of water. If you have a larger budget, you may even want to rent or buy a few misters so everyone can periodically get cooled off by the mist. If it's cold, rent or buy some space heaters and set them up so people can stand in front of them to warm up periodically. You can even provide hand and toe warmers. It's also wise to have a heated room available and extra hats and gloves on hand.
If it looks like the weather may ruin the shot that is planned, a cover set should be considered and simultaneously planned for so that at a moment's notice, the entire production can be switched to the cover set. Having a cover set can sound like a lot of extra work. Why not just cancel the shoot for that day? It can be extremely costly to cancel a day of filming. Each day film productions are burning through their budget with equipment and location rentals, insurance, and crew and cast salaries. In addition, the schedule is like dominoes -- you make one small move and if you aren't careful, the entire thing can fall apart.