A series of major winter storms is approaching California this evening. The first storm will arrive late tonight on the far North Coast and will drop some moderate to heavy rain and produce strong winds mainly across the northern half of the state. Rainfall totals in NorCal with this first system will range from 0.5-1.5 inches in most places to as much as 3 inches in orographically favored areas. Winds will gust as high as 55-60 mph as far south as the Bay Area, potentially initiating some power outages and minor wind damage as early as Thursday afternoon. Rainfall in SoCal with this first system will be mostly light (locally moderate in the mountains), generally 0.25-0.75 inches. The second and much stronger storm, however, will move into NorCal early on Friday. This system will be anchored by a 962 mb low off the OR/WA coast and fueled by a powerful 150 kt jet stream and significant subtropical moisture. Heavy rain and strong winds will occur statewide as a result of this second system. Rainfall rates will be quite impressive in the Coastal Mountains as a powerful low-level jet impinges perpendicularly to the topography–perhaps on the order of 1-2 inches per hour for several hours. Expected rainfall totals from this second system are quite impressive: in NorCal, 2-5 inches in the lowlands, with 5-10 inches in higher terrain and locally up to 12 inches in favored mountanous parts of the Bay Area and the northern Sierra foothills. In SoCal, widespread rainfall totals of 1-4 inches can be expected on the coastal plain and in the valleys, with more than double these amounts (4-8 inches) expected in most mountain areas. Favored slopes on the Transverse Range and elsewhere in SoCal will probably see 36 hour totals in excess of 10 inches from this system. Needless to say, rainfall of this magnitude and intensity may lead to significant flooding across parts of the state. Although most mainstem rivers are forecast to crest below flood stage due to very dry antecedent conditions, creeks, streams, and rivers in smaller drainage basins in CA will be at risk of flash flooding during the peak of the storm. Southern California is particularly at risk in situations such as this one because of the increased hazards posed by bare land in the recent wildfire burn scar areas. The October 2007 fires in particular burned very large tracts of open land very near major housing developments (Rancho Bernardo, land near Julian, and in the mountains near Big Bear), and so these areas will likely experience damaging debris flows or mudslides by Saturday. The other and possibly even more impressive aspect of the Friday storm will be the powerful winds it brings to nearly the entire state. In Northern California, the winds associated with the Friday system will likely be the strongest since December 11, 1995, when a similar system brought 110 mph wind gusts to parts of the Bay Area and 80 mph winds to the Sacramento Valley (with 140+ mph winds over the Sierra ridges), leading to four deaths and days-long power outages along with significant wind damage throughout much of the northern half of the state. Currently, it appears that sustained winds of 30-50 mph (strong tropical storm force) will impact the Bay Area and the Central Valley in addition to parts of coastal SoCal. Wind gusts over higher terrain in the Bay Area will easily reach upwards of 80-90 mph, potentially hitting 100 mph at places such as Mt. Tamalpais, Mt. Diablo, Mt. Hamilton, and Angel Island. Gusts over 70 mph are possible in the Sacramento Valley, and over 60 mph in the San Joaquin Valley. Across the SoCal coastal plain, wind gusts of 60-70 mph are certainly possible. In the SoCal mountains, wind gusts in the 80-90 mph range are likely, though several Santa Ana events as of late have brought winds of similar magnitude. Wind gusts in the high Sierras will be well in excess of 100 mph, possibly reaching over 140 mph as the jet stream essentially slams directly into the highest mountain peaks. After the cold front passes through, cold and very unstable air will move into NorCal on Saturday, ushering in an extended period of convective showers and thunderstorms. Intense rainfall and significant accumulations of small hail can be expected will all thunderstorms that develop, along with wind gusts of 40-50 mph as strong winds aloft translate down to the surface. Some strong to locally severe storms are possible, especially in the Central Valley as wind shear in the lee side of the Coastal Range adds to the already veering vertical atmospheric column. The “third system” will actually be a strong shortwave making its way through this cold and unstable atmosphere, and will likely manifest itself as a period of increased shower and thunderstorm activity culminating in a strong squall line with torrential rainfall, lightning, and strong wind gusts of up to 55 mph some time on Sunday. In addition to the widespread convective precipitation, cold air aloft will drop snow levels into the 1000-2000 foot range (locally lower in the north) by Sunday, so some measurable snow accumulations are possible in higher parts of the Bay Area and Sierra foothill communities by Monday morning. One final note: this powerful storm will create incredible blizzard and total whiteout conditions in the Sierras beginning on Thursday afternoon and continuing with only short interruptions through Sunday afternoon. 4-8 feet of snow is likely to fall in many places, with over 10 feet possible in favored locations. Needless to say…travel across the Crest (and almost certainly elsewhere) will become extremely dangerous–and, in some cases, impossible–by Friday afternoon. Stay tuned…
