SAN FRANCISCO — If the sky looks a little more rain-like than an ordinary gray San Francisco morning Friday, there is actually something behind it.
The National Weather Service’s early-morning forecast gives San Francisco a 20% chance of showers before 11 a.m. Friday, Aug. 28. After that, the forecast calls for the clouds to gradually give way to sunshine, with a high around 65 degrees. Winds could gust to about 23 mph. NWS forecast.
Twenty percent is worth putting in perspective. It does not mean San Francisco is in for a rainy day, and it does not even mean most neighborhoods will see rain. The odds still favor a dry morning at any particular spot. But for late August, it is enough of a chance that someone leaving home early may want to look outside before deciding the umbrella is definitely unnecessary.
Why is there a chance of rain in San Francisco?
The unusual ingredient is moisture arriving from farther south. In its overnight forecast discussion, the National Weather Service said remnant moisture from Tropical Storm Iselle was moving over the region. Forecasters were watching for showers and high-based thunderstorms through Friday, although the greater thunderstorm concern was focused on the Central Coast rather than San Francisco itself.
San Francisco’s own point forecast is much less dramatic: a slight shower chance through 11 a.m., not a thunderstorm forecast. This is not a tropical storm arriving in the city. It is a modest chance that some of the extra moisture produces a brief shower.
When should the clouds clear?
The useful dividing line is 11 a.m. The NWS forecast puts the shower chance before then and expects skies to gradually become sunnier afterward. Friday evening should be partly cloudy, with temperatures falling into the mid-50s and gusts again potentially reaching the low 20s mph.
So if you are commuting, walking the dog or waiting outside Friday morning, a light outer layer that can tolerate a stray shower is not a silly idea. By afternoon, the forecast becomes considerably friendlier.
What does the weekend forecast look like?
Saturday is expected to be mostly sunny, with temperatures generally in the 60s in San Francisco. Sunday also looks predominantly dry and mostly sunny in the current forecast.
That is useful timing for a busy weekend. The city has Pokémon Worlds activity, Giants baseball at Oracle Park and several Muni changes to contend with. Our Aug. 28–30 weekend guide collects the larger picture.
So, will it actually rain?
Maybe. At a 20% probability, plenty of San Franciscans could go through the entire morning without feeling a drop. Others could get a brief shower. The forecast is not calling for a washout, prolonged rainfall or a wet weekend.
If you are leaving home before 11 a.m., check the sky. If you are making Saturday or Sunday plans, the current forecast gives you considerably less reason to worry about rain.