Weather Change

Finally, it looks like we will see our fire season end sometime tonight or tomorrow, but at least this week.

From Weather West aka Daniel Swain:

The low making landfall near the OR/CA border will be pretty impressive in its own right, technically meeting the meteorological definition of a “bomb cyclone” as it will deepen by more than 24mb in a 24 hour period. (Now, to be clear: the term “bomb cyclone” is a real scientific term that has been widely misused and misunderstood in the past couple of years in the media. It refers specifically to rapidly-strengthening low pressure systems, which tells us little about the absolute strength of the storm or its potential impacts. In this case, hurricane force winds (sustained at or above 74mph) will occur over the open ocean west of the far NorCal coast, but winds over land will be much less extreme (with gusts of perhaps 65mph along the far North Coast, which occur multiple times per winter in that part of CA.) This low pressure system and its associated cold front will bring a burst of fairly heavy rain and gusty winds (perhaps as high as 50mph or so as far south as the Bay Area) to most lower elevation parts of NorCal late Tuesday, and heavy snowfall to the mountains. The weakening cold front will likely hold together to bring at least light rainfall to SoCal, as well.

Snow at relatively low elevations and thunderstorms possible

In some ways, the most interesting weather of the week will arrive after the cold front passes through. On Wednesday, the entire state will be under the influence of a very anomalously cold airmass for the time of year. As CA will remain within the broader trough axis, the atmosphere will be quite unstable, with widespread convective showers and isolated thunderstorms statewide. Given the unusually cold airmass for late Nov, with 850 mb temperatures as low as -3 or -4C nearly statewide, accumulating snowfall is likely down to 2,000 or 2,500 feet nearly statewide–which includes essentially all of the major travel corridors in California (on Thanksgiving week, no less!). In far NorCal, snow levels will be even lower–perhaps down to 1,500 feet locally. In fact, the far northern end of the Sacramento Valley could even see a bit of light snow (around Redding, between 500-1000 feet in elevation).

This is the kind of pattern that could bring some significant snow accumulation to well-traveled roads between 2000-3000 ft that don’t typically experience it. Additionally, the cold and unstable airmass will be conducive to locally strong thunderstorms dropping accumulating small hail even at sea level, so that could cause additional travel disruptions. At at the highest elevations of the Sierra Nevada, snow accumulations may be measured in feet rather than inches.