For full portrait shoots I check Weather Underground daily starting ten days before our shoot. It's San Francisco and lot's can go wrong.
When the cloud cover goes over 60%, the light looses direction and intensity. Reflectors do not work well. We can hold our shoot, but we should expect flat images.
When the temperature drops below 65F, models often show goosebumps. And, when coupled with wind, eyes tear and become red.
When the wind speed exceeds 10 miles per hour (mph) shooting on the street starts to become difficult. At that speed, reflectors blow over in their stands. At 15 mph, reflectors can be blown out of the hands of grips. Even worse, models' eyes begin to tear. Once this process starts, it cannot be stopped. We should reschedule the shoot for a less windy day.
With rain, we can embrace it and shoot in the rain or wait. I offer the choice to models. They have all turned down shooting in the rain.
Fog can be wonderful. The light becomes diffuse but keeps direction and renders rich skin tones. Usually, however, fog comes with wind.
So, if it is too cloudy, too cold, or too windy, we should consider rescheduling the shoot. I will be following the weather daily and we will discuss our options.
WeatherUnderground uses data from hundreds of thousands of home weather stations to make micro-climate predictions. I have found the 10 day neighborhood forecasts to be very accurate, even in San Francisco. I find myself frequently checking Lexington & 21st, Palo Alto, or Sacramento & Kearny.