2008

A very interesting and extremely dynamic weather pattern has developed over the far eastern Pacific and will lead to a powerful and unusual storm system over CA over the next 48 hours. A deep and moderately retrogressive cutoff low has continued to spin 700-800 miles west of Los Angeles. The deep southerly flow aloft ahead of the cold front brought flooding rains in excess of 6-7 inches to parts of NorCal (mainly the northern Bay Area yesterday) before retreating offshore. The low has dug far enough south such that the extremely powerful southerly flow aloft (in excess of 50-55 kts at 850 mb south to the subtropical Pacific) is beginning to draw up an enormous amount of subtropical moisture-rich air. As strong divergence and other large-scale forcing acts on the moist plume, cloud tops are rapidly enhancing and cooling as they race north and begin to pivot towards SoCal. In addition, strong baroclinic instability exists in the horizontal plane, as cold temps aloft at the center of the low (-3 C at 850 mb) are in stark contrast to the balmy temperatures over the state of CA (approaching 10 C by tomorrow afternoon). High clouds and temperatures in the 60s over most of the state today (after a week of highs barely reaching 40 over much of NorCal) are a welcome reprieve, but also portend the dynamic nature of the storm system to come. The evolution of the storm is still slightly uncertain, but right now this is the most likely outcome:

1. Heavy/torrential rainfall

An extremely moist atmospheric column combined with deep southerly flow will lead to some incredible rainfall totals in the Transverse Range and on all south-facing slopes. Rainfall will exceed 12 inches in these areas. Widespread heavy rainfall will occur elsewhere as there will be efficient warm-rain processes involved in addition to warm-conveyor convection. These convective elements are likely to be quite intense (discussed in more detail below), and could produce rainfall rates of 1 to locally 3 inches per hour under the most intense cells. These cells will have a tendency to train over relatively narrow areas, leading to a very high risk of flash flooding in all areas, but an especially strong risk of dangerous flooding in favored areas and burned slopes.  Due to the combined orographic and convective nature of this system, there will be quite a bit of variation in rainfall totals, but generally a 2-4 inch minimum is expected, and some lowland areas under strong/severe thunderstorms may see up to 8 inches of rainfall. This is especially true in Ventura and Los Angeles County.  Widespread flooding of creeks and urban areas will occur, and some major rivers will approach or exceed flood stage (particularly the Ventura River).

2. Strong/gusty winds

Much like the rainfall, the strongest winds will be somewhat localized with this storm (but not necessarily in the usual locations). Convective elements will cause strong and potentially damaging gusts in excess of 55 mph to mix to the surface locally just about anywhere. Strong sustained winds can be expected over water and in mountain areas.

3. Strong/severe thunderstorms

This storm will bring an unusually high risk of severe weather to SoCal. Warm-conveyor events have a history of producing their fair share of severe thunderstorms in SoCal, most recently in 2005. In these types of events, the main risk from severe storms is torrential rainfall and damaging winds, but with this particular event the progged degree of wind shear and veering with height would indicate the potential for tornadic activity. I fully expect to see at least several reports of waterspouts tomorrow, some of which may move onto the coast as tornados. Because of the risk of relatively long-lived rotating low-topped supercells (for CA, anyway), I wouldn’t even be suprised to see a fairly significant tornado (perhaps F1) develop out of one of these bands. If warnings are issued tomorrow, please take them seriously. In any case, could be a good light show in many places…

This storm will hit SoCal the hardest, but NorCal will see some more heavy precip and possible thunderstorms tomorrow as well as the low begins to move inland. There appears to be a series of very cold and moist disturbances progged to affect mainly NorCal next week, with snow possible below 1000 feet once again. First, though…let’s get through the big one…