Weather models, which to laypeople like me are inscrutable mathematical machines with a magical ability to look into the future, are suggesting that we just might see rain in Northern California late next week.
Among the many, many reasons that would be welcome news: It would be a blessing to firefighters who have struggled with intractable wildland blazes for the past six weeks. It would give us a break from smoky, borderline unbreathable air. And it might hasten an end to evacuation orders that some people in the northern half of the state have lived with for weeks.
But here's the cautionary note: Next week is a long, long way off, and the people who pore over forecast model data say rain is no sure thing.
The main factors driving the potential for precipitation are twofold.
First, some models have forecast a trough of low pressure to arrive on the Northern California coast next Friday or Saturday (Oct. 9-10) in the form of a front. With a supply of moisture, that could be the setup for a storm.
Second, models have suggested that Hurricane Marie – a Category 4 storm with sustained winds of 130 mph about 1,100 miles southwest of Cabo San Lucas – will spin off moisture as it dissipates, and that the moisture may feed into the trough approaching our coast.
Very early Friday, the area forecast discussion for the National Weather Service's Bay Area office noted that a blend of weather models called an ensemble "is showing a 15-20% chance for measurable rain." Furthermore, the forecasts from some individual models suggested the possibility of "impressive rainfall amounts" ranging from a half-inch to an inch. The discussion added, though, that "obviously a forecast at over 180+ hours out will change a lot."
The very next new forecast discussion from the Bay Area office gives a hint of how changeable that long-range forecast can be. With a nod to the growing legion of civilian weather geeks who are studying forecast models themselves, the discussion said:
"Long-range forecast is most interesting in how the remnants of Hurricane Marie may get entrained in the long-wave trough off the Pacific Northwest coast. Those following along at home have likely seen some of the operational models showing some wet solutions for NorCal by next weekend. Cluster analysis shows there is still about a 30% solution that dry ridging verifies and the latest operational ECMWF keeps things completely dry with ridging as the main trough/front stays well to our north. So not surprisingly out in the Day 8 time frame there lies lots of uncertainty. ..."
A bit of a translation: A further look at a collection of model outputs shows a 30% chance that a high-pressure ridge will keep us dry. The ECMWF — one of the key global models, from the European Center for Medium-Range Weather Forecasts — is also projecting bone-dry conditions.