As I think I've mentioned, I have some interest in the science of weather forecasting and in observing how the folks who do it do it. We have a fascinating (to me) situation looming for Southern California for Tuesday into Wednesday. Here, the main method of figuring out if it's gonna rain or shine is by use of two computer models that track satellite data in various ways and project what's going to happen. One is the NAM and one is the GFS. NAM stands for "North American Mesoscale Model" and GFS stands for "Global Forecast System." Others are used but 95% of the time, those two tell the story.
Ordinarily, they are in rough agreement and as they get closer to the time period in question, they converge on one another. If the NAM says it's going to be 70 and the GFS says it's going to be 74, the forecast will generally be for 72…though sometimes, human beings look at the projections and decide that one or the other is the more accurate and will give it more weight. Usually, they're pretty close when you're looking 24-36 hours ahead.
Not this time. In a rare lack of convergence, the NAM is currently projecting a huge rain storm beginning late Tuesday and lasting into Wednesday with rainfall totals in the 1-2 inch category. That's a lot for Southern California this time of year and it would probably cause mudslides and flooding, especially in the burn areas of the mountains and outlying areas. On the other hand, the GFS is saying that the storm in question will turn into a cut-off low and remain off-shore for a few days, dumping most of its energy out there, spinning off small impulses. That would lead to a small (20-30%) chance from late Tuesday through about Friday afternoon.
To make matters more confusing, the two models have swapped points of view in the last few days. On Saturday, the GFS was projecting what the NAM now projects and vice-versa. Lately, instead of coming together, each is stubbornly sticking to its guns.
As of this moment, the National Weather Service is going with the GFS projection but cautioning other folks in the weather business that confidence is low and that a sudden change could be necessary. Let's see if they made the correct call. Despite the way we all wail and curse their profession when they're wrong, they're usually right.