A moderately strong storm will impact mostly NorCal tonight and CentCal tomorrow. Frontal band is fairly moist but not very wide, and although rain rates may be briefly heavy would expect less than 1.5 inches nearly everywhere except far NorCal where up to 4 inches could fall. Iso. tstms are poss. along front and then behind it in very cold and unstable airmass, mainly north of Monterey. 850mb temps of -6 C in the far north will produce 500-1000 foot snow levels near Eureka tomorrow, and some wet snow/grapuel should fall even on the beaches. This may be a taste of things to come.

The control runs of the GFS have stayed remarkably consistent with their depiction of the upcoming major pattern change. The change is now 7 days from impacting NorCal/ CentCal, and it still looks like it may be quite impressive. As mentioned before, a powerful and elongated meridonal (north-south) blocking pattern will develop over the Eastern Pacific about 1000-1500 miles west of CA/OR (see illustration on our homepage). This will allow Arctic and even polar air to dive almost due southward into WA/OR/CA and also offshore. As this occurs, a retrogressive (west-moving) low will develop offshore of CA and remain nearly stationary for at least 3-5 days, allowing cold and unstable air to pool north of the front on its eastern side and drawing up warmer and very moist (and also unstable) air from the subtropics south of and along the front. This will have several effects. 

For Oregon and Washington: precipitation will be less frequent and lighter up here than in CA, but temperatures will be much colder. I would expect all precip. to fall as snow north of Eureka during this period as 850mb temps will be -8 degrees C or below (as low as -18 degrees C in the north). Significant lowland snows are possible or even likely.

For NorCal between San Francisco and Eureka: convective precipitation will be likely during this period, possibly producing some heavy rainfall amounts. The air aloft won’t be quite so cold this far south, but it will still be very interesting to see how cold it can actually get around here. Most recent GFS does drop 850mb temps as low as -12 C south to Mendocino County, and as low as -9 or -10 in the Bay Area. This would indicate snow levels essentially at sea level in the north, and possibly to sea level in the south. Do feel that these temps may moderate some if future runs by 1-3 degrees, but that’s still really cold. Also, the long duration of cold temps aloft will allow temps at the surface to cool more than during a brief event. So…snow showers are still conceivably possible on the beaches in San Francisco, but highly likely in the surrounding hills above 1000-1500 feet. For CentCal and SoCal: substantial precipitation and a possible subtropical connection may limit the magnitude of cold air advection from the north. However, it is still possible that the -3 C isotherm at 850 mb could make it down to the Mexican bordern, bringing accumulating snows down to 2000 feet near San Diego. Near Los Angeles and the northern and western suburbs, snow levels could be in the 1000-1500 foot range.    

Thunderstorms are certainly possible, given the very steep lapse rates, and any strong shower would likely contain hail/wet snow. So even if accumulating “snowfall” may be limited to areas just above sea level, frozen precip, such as hail or sleet, could actually accumulate at sea level. Thunderstorms could be quite strong at times, esp. in SoCal where sea surface temperatures are still relatively warm and lapse rates will be especially steep.

In short, a very interesting and dynamic pattern appears to be evolving for the western United States. The strong MJO currently propagating across the Pacific seems to be contributing to this dramatic amplification of the jet stream. It may well be the case that the powerful Asian jet will eventually undercut the block in the East Pac., bringing wet, warm, and stormy weather to the West Coast immediately following the wet and cold pattern. Stay tuned…