An unusually strong (for October, in any case) Gulf of Alaska storm system is currently moving ashore in Northern California. The associated 500 mb trough is remarkably well-defined for such an early-season storm, and a powerful 130 kt jet is currently rounding the base of the trough off of the NorCal coast this evening. Strong upper-level diffluence, a very moist airmass (PW values approaching 1.5 inches), and an abundance of cold air behind the cold front will lead to a pretty solid soaking across much of the state over the next 24 hours, especially from the Monterey area northward.

Water vapor imagery of the incoming Gulf of Alaska storm stystem (NOAA)

Several inches of rain could fall in the favored locations in the mountains and along the coast, while even some low elevation inland areas could see upwards of an inch before all is said and done. Though surface pressure gradients are fairly steep at the moment and 800 mb winds will approach 50 kts near the time of cold frontal passage, I would expect these stronger winds to stay mostly above the surface (except possibly in some stronger convective elements embedded in the frontal precip band). Lapse rates near and behind the cold front are quite steep for early fall in California, and there is indeed already a large post-frontal cumulus field visible on satellite imagery. Isolated to scattered thunderstorms are possible across much of the state by late tomorrow as this unstable airmass slowly moves through the region. Even Southern California will get in on some of the action–it won’t be anything too spectacular, in all likelihood, but it certainly won’t be too bad for the first week in October. After the present storm, though, I expect conditions to clear out and warm up nicely once again by the weekend.

On a related note, the recent and anticipated rainfall in NorCal will probably bring an end to fire season north of about the Monterey Bay Area this week. SoCal is certainly going to get wet, but since we still have the peak of Santa Ana season to get through, it is still too early to call for more than a temporary reduction in fire risk. Enjoy the rain!

© 2011 WEATHER WEST

After what has thus far been a remarkably quiet summer in California, a significant and potentially very noticeable pattern change is currently underway. A Rex Block has developed over the far Eastern Pacific and the West Coast over the past day or so, forced (at least in part) by blocking induced by the remnants of Tropical Storm Lee current wreaking havoc in the Eastern states. The upper-level low currently located over northern Nevada will retrograde very slowly over Northern California over the next 24 hours and eventually set up shop off of the Central California coast this weekend, staying nearly stationary until at least early next week. As the low begins to move offshore, counterclockwise circulation will induce southeast and easterly flow over the northern 2/3 of the state, advecting monsoonal moisture into Northern and Central California from the Desert Southwest. Most of this moisture will be at mid and upper-levels, and any convection that develops on Friday or early Saturday will likely contain little or no rain, leading to a potentially widespread threat of dry lightning. By later in the weekend and possibly continuing into early next week, this flow will probably contain enough moisture to result in wetting rains in most thunderstorms as lofted moisture from diurnal storms serves to moisten the lower levels with time.

GFS forecast of retrogressive upper-level low on Friday

Not only will there be the threat of dry lightning this weekend, but easterly flow will result in downsloping, offshore winds in the Sierra foothills, Central Valley, and possibly the Bay Area. The triple threat of hot, potentially gusty offshore winds, low surface humidity, and possible dry lightning will probably cause some fire-related problems this weekend. This system also has the potential to bring multiple rounds of lightning to unusual areas, including the Central Valley and the Bay Area. Much of Southern California may miss out on the deep convective activity as it will remain in a pronounced dry slot for much of the event, though some potential for dynamically-forced elevated convection does exist over almost the entire state at some point during the next 5 days.

The numerical models are currently in disagreement regarding the evolution of the pattern after next Monday. It does not appear that this cutoff will be in any hurry to leave, however, and the risk of at least mountain thunderstorms (and possibly elsewhere) may continue straight through the next week. Stay tuned!

© 2011 WEATHER WEST

First, I just wanted to thank everyone for sticking with the site through this period of transition! The extended downtime was frustrating for me as well, but ultimately the changes that have been implemented will make the site easier to maintain and navigate.

As many of you have already noticed, the URL of the California Weather Blog has changed from http://www.weatherwest.com/current_weather to http://www.weatherwest.com . All Weather West content will now be accessible from the blog on the main page, and this change should make Weather West easier to find in web searches. If you have not done so already, please update your bookmarks!

I am pleased to introduce a brand new Weather and Climate Links page (accessible using the tab near the blog header). This page will be a work in progress, but is eventually intended to serve as a comprehensive directory of all things California weather (and geophysical!)-related. Please check it out!

Moving forward, I plan to update the site less frequently but hopefully more comprehensively than in the past. I will no longer provide updates for every storm or pattern of interest affecting California, but will instead try to focus on the really big or unusual events that occur or are anticipated to occur in the near future. For day-to-day weather events, I’ll be more likely to make brief posts in the Comments section to keep things up-to-date when possible.

Finally, as the site has lost a great deal of traffic during the transition and since it is possible that some longtime users were lost in the flurry of server errors, please feel free to promote the new Weather West when and where appropriate. I truly appreciate your help!

After an extended delay, I’ve finally gotten the backed-up database installed and all old blog content has been successfully restored. All your old comments (and username/password combinations) should have been restored as well!

I still need to make some tweaks and additions to the site, but the present version is now a stable one, so the site should not go down during this final part of the process.

Thanks for your patience!