Powerful winter storms to slam CA this week

Filed in Uncategorized by on December 31, 2007 43 Comments

The long-advertised major pattern change for the end of the week still appears to be on track to bring heavy rainfall and strong winds to most of the Golden State Thursday into the weekend. The first system will be weakened significantly by the persistent ridge it will dissolve as it moves ashore and upper-level dynamics will be lacking, but some locally heavy rainfall will still be possible from the Oregon border south to the Los Angeles area (and possiblly even San Diego) by late in the day. Rainfall totals of 1.0-3.0 inches are possible statewide. Some gusty and possibly strong winds, with gusts over 50 mph, could impact portions of NorCal on Thursday. The second and much stronger storm system will follow close on the heels of the first, moving into NorCal by midday Friday and bringing with it very heavy rainfall and very strong to potentially extremely strong winds. The left front quadrant and exit region of the jet will be located over the northern half of the state just as the 958 mb low reaches its deepest point off the Oregon coast, and so the jet dynamics will be ideal for copious precipitation production and powerful surface winds. The surface pressure gradient between San Francisco and Santa Barbara will be in excess of 20 millibars, and baroclinic instability at the 850 mb level will lead to spectacular mixing of these strong winds to the surface. I would fully expect, given current model forecasts of 70 kt 850 mb winds over the Bay Area, for parts of NorCal to see wind gusts in excess of 80 or 90 mph. The high Sierras will almost certainly see winds above 100 mph at times on the most exposed ridgetops. Significant wind damage, including widespread power outages and downed trees, is certainly possible. Precipitation could fall at a rate of 1-2 inches/hr for a time near cold frontal passage, leading to widespread precipitation totals of 3-6 inches in NorCal for this second event and possibly some totals as high as 8 inches (SW facing slopes of Coastal Range, hills east of Redding, West Slope of Sierras). As the cold front slides south, it will maintain most of its strength, and will bring strong winds to coastal and mountain parts of SoCal (possibly in excess of 50-60 mph in places that do not normally experience Santa Ana winds of this magnitude. Very heavy rainfall is a near-certainty in at least parts of Southern California, as well. Extremely strong flow aloft impinging orthogonally on the Transverse Range will lead to huge rainfall totals in some mountainous parts of SoCal, and some hefty totals nearly everywhere else (except, possibly, for rain-shadowed deserts). This second storm system could easily bring 2-4 inches of liquid to the SoCal mountains, and some really favored slopes could actually see double these amounts. All of SoCal should see at least 1.5-3.0 inches of rain with this second storm, and flooding will become a great concern, particularly near burn areas. After this second storm moves through, cold air aloft will move into the northern 2/3 of the state on Saturday, leading to less continuous but still occasionally heavy precipitation in a cold convective regime. A few strong thunderstorms could develop across interior Northern California on Saturday as moisture will be copious, lapse rates will be steep, and the vertical atmospheric profile will remain cyclonically curved. The third system…due in late Saturday into Sunday, currently looks somewhat less impressive than it did 24 hours ago and certainly less impressive than the very ominous Friday system, but it will bring heavy precipitation and possibly some more strong winds nonetheless. This system will be developing very quickly (undergoing rapid cyclogenesis) as it approaches the coast on a strong 180 kt jet, and so it is definitely possible that the models are underestimating the degree to which this system will have time to strengthen before it reaches the coast. Expect another 1.5-3.0 inches from this system statewide, with locally higher amounts. Following the Sunday system, very cold modified Arctic air aloft will overspread CA from the north and northwest late Sunday into Monday, bringing cold surface temperatures and widespread convective activity. Strong to potentially severe thunderstorms could devlop over the Sacramento Valley on Monday, and snow levels will drop to 1000 feet or less. It is possible that a dusting of snow will again be possible near sea level towards the very end of this event. This is a very dynamic scenario, and things will likely change as time progresses. Needless to say…stay tuned.