2010

The stormy pattern that has been in place for the past week across California and much of the southwestern United States was record-breaking in some rather surprising ways. The series of very strong storms brought widespread heavy rainfall and strong winds to the entire state, with prodigious snowfall over higher elevations (and some appreciable snow accumulation as low as 1000-1500 feet in some places). Rainfall totals were actually fairly uniform north to south, though the usual topographical gradient was still substantial. Totals of generally 3-6 inches at lower elevations (with 5.6 inches here in Davis) and 5-10 inches at higher elevations were observed, though some mountain areas did receive higher totals . The Northern Sierra at resort level received anywhere from 50 to 100 inches of snowfall, and the highest peaks in the Southern Sierra likely received significantly more than that. Parts of the Southern California mountains received upwards of 30-40 inches of snowfall. Widespread flooding was reported around the state, especially in the south, and a few rivers (such as the San Diego River) approached or slightly exceeded flood stage. For the most part, the flooding that was observed was rather local and flashy in nature (though it was significant in some places). Pressure-gradient winds, too were significant throughout the state, leading to downed trees and power line at the lower elevations and blizzard conditions in the mountains.

The truly remarkable aspect of this past week’s weather was the widespread and persistent strong to severe convective activity that accompanied it. Though California does receive more severe weather than is commonly believed, those events that do occur are usually fairly localized and limited in magnitude. In this case, severe thunderstorms were reported every day from Monday through Friday of this past week. A preliminary (and unofficial) tally:

_______________________________________________________

Damaging Wind/ Large Hail/ Tornado

Monday 1/18

1                             0                          1

Tuesday 1/19

5                            0                          2

Wednesday 1/20

13                          2                          0

Thursday 1/21

10                          2                          3

Friday 1/22

0                             0                          1

_______________________________________________________

These statistics are even more impressive when one considers the fact that numerous locations received significant hail (and even accumulating hail up to several inches in depth) between 0.5 and 0.75 inches in diameter that did not officially qualify as “large” (1.00 inch is now the threshold for severe hail as defined by the NWS). What really stands out here, however, are the several tornadoes that occurred in the southern half of the state this week. Most of these actually did occur in populated areas and caused at least light damage. In some cases, the tornadoes were strong enough to roll vehicles off the highway and snap trees at the base. Especially notable are the 92 and 93 mph wind gusts reported at Huntington Beach and Newport Beach, respectively, as a very intense line of thunderstorms moved ashore on Tuesday. Since a large waterspout had been been confirmed immediately offshore at that time, it is certainly possible that these extremely high wind speeds were associated with the vortex as it moved ashore. Two tornadoes occurred near Santa Barbara (one in Goleta), one in Ventura, one in Blythe (which may have been an EF1-EF2, according to preliminary damage reports), and another weak one near Fresno. Late reports are beginning to come in that the extensive wind damage reported in central Los Angeles (Compton) today may have been the result of a tornado, as well. Regardless, all of this severe convective activity is very impressive for California in January, if not unprecedented.  If any readers have pictures of any of these events, please submit them for publication on Weather West via the form on the homepage!

One more item of interest here is the extremely low barometric pressure experienced across the entire Southwestern United States over the past 48 hours. Nearly every recording station in the state of California, Arizona, Utah, and much of Oregon and Nevada broke their all-time lowest barometric pressure records this week, and some by a significant margin. Though these records are not especially important in terms of sensible weather impacts, the depth and southerly location of the low pressure center does demonstrate just how impressive this week’s systems were.

After all is said and done, rainfall totals statewide did end up towards the lower end of initial estimates in most areas. This was extremely fortunate, as significant flooding was reported in some areas with the rainfall that did occur, and any additional heavy rainfall would have caused much more serious problems. The soils are now saturated, however, and renewed heavy rainfall in the next several weeks could become problematic.

At the moment, however, it does not appear that there is any serious danger of that occurring in the immediate future. In the very short term, scattered showers (some still containing small hail) will continue statewide tomorrow before ending on Sunday. Then…a new and warmer storm moves into the state from the northwest just in time to start the new work week. This storm will be much weaker than any of the storms this past week and will not have nearly as much dynamic support or cold air aloft to work with. However, with a rather moist subtropical plume, some significant precipitation could occur (mainly over higher elevations in NorCal). Some minor flooding problems are possible, but no major issues are anticipated at this time. A second system will follow the Monday storm and dig a little further south on Wednesday. This one will have more cold air associated with it and may have the potential to be a little more convective in nature, bringing a bit more intense rainfall to some areas. Still, significant impacts are not expected. After that, is does appear that the pattern over the far Eastern Pacific will quiet down considerably. A week ago, the models were showing a significant subtropical moisture tap feeding into California and leading to a heavy warm rain event this coming week. Obviously, this no longer appears to be the case. During late week 1 into week 2, ridging is expected to develop over western Canada and form another block over the West Coast. This blocking ridge will induce split flow over the Eastern Pacific, though it does appear that the southern stream will remain fairly robust through week 2 and beyond. This is a pattern consistent with a strong El Nino event. While dry and generally quiescent weather is currently anticipated for this period, undercutting of the block can occur quickly and with relatively little warning with a strong subtropical jet so close to the state. Eventually, I do anticipate that the jet will undercut the ridge and bring a renewed period of active weather to California. The timetable for this to occur, however, is highly uncertain. Stay tuned.

[ad#post-ad-w]