*Very stormy pattern for California; flooding possible*

Filed in Uncategorized by on January 15, 2010 74 Comments

The long-advertised major shift to a much more active pattern over the far West Coast has arrived. Storms are lining up off the coast, and a powerful 200+ kt zonal jet will slam into California beginning Sunday and persisting for all of the coming week (at least). There are strong indications that precipitation may be very heavy in many places, especially across Central and Southern California. As has been widely noted in recent days, the upcoming pattern is analogous to the mid-winter flow regimes in both 1995 and 1998. Some of you may remember the very large (and destructive) storms that occurred during both of those years, causing widespread flooding and wind damage (especially in Northern California). While it is not immediately likely that the upcoming series of storms will be quite as severe as those historic events,  the potential does exist for a very significant event. One difference between 1995/1998 and the present is that the focus of the most intense precipitation is expected to be over Southern California, at least initially. Heavy rain and high winds will impact literally every corner of the state over the next 7-10 days. Rainfall totals statewide through next Saturday will likely be extremely impressive. Here are some estimated ranges for total liquid accumulations through next Saturday:

  • Far Northern California (Eureka area northward): 3-7 inches lowlands; 6-12 inches higher terrain.
  • Middle-Northern California (Eureka south to the central Bay Area): 6-12 inches lowlands; 12-20 inches higher terrain.
  • Sacramento and San Joaquin valleys: 3-7 inches south; 5-9 inches north.
  • The Sierra and higher foothills: 8-15 inches, locally more than 20 inches (remember–this is liquid equivalent).
  • Central California (South Bay Area south to Santa Barbara): 5-12 inches lowlands; 12-25 inches higher terrain (locally more)
  • Southern California (Santa Barbara south to Mexico): 5-10 inches lowlands; 12-20 inches higher terrain (locally higher)

Given such large liquid equivalent, snowfall will be extremely heavy in the high Sierra, with some local areas possibly seeing 10-15 feet of new snow (maybe even more). Temperatures aloft will become progressively colder as the week passes, and snow levels will eventually drop below 3500-3000 feet, especially in the north. This could air aloft, combined with periodic waves of energy and dynamic instability, will lead to significant convective activity at times across the state. Thunderstorms will likely be associated both with the main cold frontal bands and also in the cold and very deep trough that settles over California towards the end of the week. Some of these storms may become rather strong, with the main threat being damaging winds and torrential rainfall. Later in the week, there may be some hail threat as 850 mb temperatures drop below -2 C and 500-1000 mb thicknesses below 530 dm with continued instability.

Strong and possibly damaging winds will accompany most or all of the storms this week, as well. A very deep low is expected to approach the coast by Wednesday/Thursday. The GFS currently pegs this low at near 965 mb, which would be very impressive for this neck of the woods. One mitigating factor may be that the low is too large–the pressure gradient may not be quite as steep as it would have been otherwise. Regardless, several notable wind events are likely to occur across the entire state this week.

GFS 120 hour MSLP and 6 hr. precip. Note sub-970 mb low of CA coast. (NCEP)

The models are in remarkable agreement through about day 7/8 regarding this longwave pattern shift. After that, however, there is significant divergence. All the models bring significant precipitation back to the state by days 11/12, but the GFS solutions would imply a bit of a break for 3-4 days in the interim as the powerful zonal jet lifts briefly to the north. The ECMWF has no such break, and brings significant or even heavy precipitation into California straight through next weekend and beyond. Either way, there is likely to be a renewed flood threat whenever this second series of storms ride the persistent zonal flow into the state.

GFS spaghetti ensemble. Note the remarkably consistent zonal flow through week 2. (CPC)

Very large rainfall deficits can be eliminated rapidly in the sort of scenario (which is a good thing), but there exists the fairly obvious caveat of flooding. Flash flooding and earth movements (mudslides) are–unfortunately–essentially unavoidable in Southern California during a pattern such as this one. If the higher end of the rainfall estimates come to fruition, there could even be some mainstem river flooding. There is also a risk of flooding in Northern California, though this may be a little more dependent on higher-end rainfall estimates to occur. All bets are off, however, if the scenario depicted by the ECMWF pans out. And even the GFS solutions would bring an eventual flood risk to all of CA; the risk in NorCal would simply be reduced until the second (and warmer) wave arrives in 10+ days. This is a pattern to watch closely, in any case. Potential impacts may be large,  and there are no indications that a significantly drier regime will take over anytime soon. Stay tuned!

Interesting side note: the NOAA Gulfstream IV will be investigating the upper-atmospheric conditions over the north Pacific for combined research and operational purposes this week. The data sampled by the aircraft will be ingested into the numerical models, and there may be even greater confidence than usual in forecasts due to the enhancement of knowledge of initial conditions during this storm event…

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