Welcome and the Weather

My first blog entry! I’m going to try to keep these entries mostly to weather/climate, but don’t be surprised if I stray off topic once in a while.

First off: The weather has been gorgeous in and around San Francisco for the past couple days. Today's (Tuesday) high was 82º at SFO and 76º downtown, both several degrees above normal. The reason? High pressure has built over California. This has compressed the marine layer, keeping the low clouds/fog offshore. This weather (wx) pattern will continue into Wednesday, with highs in the city near 80º. As most of you probably already know, September is on average the nicest month of the year in SF. I'll talk about why this is in a future post.

Secondly: Big changes may be in store beginning this weekend and especially early next week. The high pressure system over us now will break down, resulting in cooler temps and a return of the marine layer as early as Thursday. There may be a few showers this weekend as a weak storm system moves through, but nothing heavy. It's early next week that looks especially interesting. The computer models, which forecasters rely heavily on for medium and longer range forecasts, have been showing a much stronger storm system moving into the West Coast. But, these models have been inconsistent in the strength and location of this storm. Meteorologists usually look to two major computer forecast models: the Global Forecast System (GFS), which is run by the U.S.; and the European Centre for Medium-Range Forecasts (ECMWF), which is run in London. There are many other models as well, each with their strengths and weaknesses. When the different models are in agreement, we have more confidence in the forecast.

Next week, the GFS brings a strong cold front with heavy rain across central California on Tuesday, while the ECMWF takes this system into Washington and Oregon. This is obviously a big difference, so there is considerable uncertainty with this forecast. What we do know is that it is unusual to have such a strong system move across California this early in the season (this would be much more common Nov-Feb). But, that doesn't mean it hasn't or can't happen. So, stay tuned.