One of the defining characteristics of California’s Mediterranean climate are its well-defined wet and dry seasons–corresponding to meteorological winter and summer, respectively. One can almost always expect a bone-dry summer outside of occasional summer thunderstorms in the higher mountains, and increasingly heavy winter rains and mountain snows beginning some time during the late fall or very early winter.

The observed weather in 2011, however, didn’t follow these time-honored expectations. The rainy season in Northern California lasted until early July, and fresh snow on the ground greeted July 4th revelers in the Sierra Nevada. The wet season seemed to be getting off to a respectable start, with a fairly strong system bringing significant rain and wind in mid-October. After a few more minor precipitation events in early November, though, the proverbial spigot shut off abruptly in Northern California. In Southern California, an unusual series of “outside slider”-type systems brought some significant (and in some cases anomalously heavy) rainfall to coastal areas through November. By the beginning of December, however, even the outside sliders tapered off, and December 2011 turned out to be one of the driest Decembers on record for almost the entire state. In fact, most of the state (with the singular exception of the far northwestern corner) received essentially no precipiation at all–and what little did fall was hydrologically insignificant.

(National Weather Service/NOAA)

The question, of course, is how long this present very prolonged dry spell will continue. The answer, in short, is that it will last at least another 1-2 weeks. There are not, at present, any compelling indications of a major longwave pattern shift in the near future. In fact, it appears that the persistent East Pacific ridge will strengthen in the coming days, bringing balmy conditions to much of California with continued dry and stable weather. The MJO–which in most winters is a fairly significant indicator of impending westerly jet incursions–appears to be rather unimportant this year in forcing pattern changes (which is not extremely surprising given prevailing La Nina conditions).

(NCEP)

The only interesting feature in the extended forecast models at the moment is some early indication of a potential retrogression of the ultra-persistent block over the Pacific. If this actually does occur, it could shift the mean trough axis towards the West Coast, though because flow would be highly meridional and northerly over California conditions could become dramatically colder and unsettled but ultimately result in little precipitation due to the cold continental origins of the source airmass. In any case, this possible development is still far in the future–and extremely dry patterns such as the present one are notoriously persistent. For now, we can only hope that the pattern in early 2012 becomes more promising than the one that closed out 2011…

© 2011 WEATHER WEST

Much of the fall has been very dry across large portions of California, and what precipitation did fall came relatively early in the season and was generally not sufficient to generate meaningful runoff. The past week has seen one of the most powerful Santa Ana/offshore flow wind events in recent memory, with reports of widespread damage and wind gusts well in excess of 100 mph on some mountain peaks. Unfortunately, the prevailing pattern is not showing many signs of change in the near future.

GFS 00Z Analysis 500 mb heights, 12Z 12/3

A large, powerful and persistent blocking ridge has set up in the Eastern Pacific, creating high amplitude meridional flow upstream and downstream of the block (poleward in the Central Pacific, equatorward over the Great Basin and Intermountain West). Occasional disturbances riding down the backside of the block have generated strong pressure gradients and powerful offshore winds over California and other Southwestern states in recent days, and will most likely continue to do so for the next 1-2 weeks as the blocking ridge holds strong.

Prospects for a pattern change in the immediate future are pretty slim. These sorts of blocks can be difficult to break down, usually requiring an easterly extension of the East Asian jet to displace the Central Pacific ridge. Two other options are for the block to retrogress and the equatorward flow on the east side to shift westward, or for the block to slowly weaken with time. The former option brings the possibility of Arctic intrusions along the West Coast, and the latter brings the possibility of very slow-moving cutoff systems forming southwest of California. One caveat is that the MJO does appear to be fairly active in the Western Pacific, and the GFS is indicating that the signal may begin to propagate eastward over the next 1-2 weeks. If this does indeed occur, there is some hope for the block to break down and for more zonal flow to take over along the West Coast. We’ll just have to see about that…

Despite early fall rains in Northern California, very dry offshore flow with continuing strong winds in the higher elevations is beginning to lead to fire weather concerns. Though December Red Flag Warnings are not particularly rare in Southern California, they certainly are in the northern half of the state. The possibly very strong wind event for later next week could result in critical fire weather conditions from the Sierras to the Bay Area, which would be extremely unusual, to say the least.

We have now entered the critical period for Northern California rainfall. Each day that passes without precipitation adds to the deficit that must be made up later in the season if we are to avoid a significantly below average water year. Since the first half of December appears unlikely to produce that sort of pattern, we may have to hope for wetter conditions in the new year.

© 2011 WEATHER WEST

An unusually strong (for October, in any case) Gulf of Alaska storm system is currently moving ashore in Northern California. The associated 500 mb trough is remarkably well-defined for such an early-season storm, and a powerful 130 kt jet is currently rounding the base of the trough off of the NorCal coast this evening. Strong upper-level diffluence, a very moist airmass (PW values approaching 1.5 inches), and an abundance of cold air behind the cold front will lead to a pretty solid soaking across much of the state over the next 24 hours, especially from the Monterey area northward.

Water vapor imagery of the incoming Gulf of Alaska storm stystem (NOAA)

Several inches of rain could fall in the favored locations in the mountains and along the coast, while even some low elevation inland areas could see upwards of an inch before all is said and done. Though surface pressure gradients are fairly steep at the moment and 800 mb winds will approach 50 kts near the time of cold frontal passage, I would expect these stronger winds to stay mostly above the surface (except possibly in some stronger convective elements embedded in the frontal precip band). Lapse rates near and behind the cold front are quite steep for early fall in California, and there is indeed already a large post-frontal cumulus field visible on satellite imagery. Isolated to scattered thunderstorms are possible across much of the state by late tomorrow as this unstable airmass slowly moves through the region. Even Southern California will get in on some of the action–it won’t be anything too spectacular, in all likelihood, but it certainly won’t be too bad for the first week in October. After the present storm, though, I expect conditions to clear out and warm up nicely once again by the weekend.

On a related note, the recent and anticipated rainfall in NorCal will probably bring an end to fire season north of about the Monterey Bay Area this week. SoCal is certainly going to get wet, but since we still have the peak of Santa Ana season to get through, it is still too early to call for more than a temporary reduction in fire risk. Enjoy the rain!

© 2011 WEATHER WEST

After what has thus far been a remarkably quiet summer in California, a significant and potentially very noticeable pattern change is currently underway. A Rex Block has developed over the far Eastern Pacific and the West Coast over the past day or so, forced (at least in part) by blocking induced by the remnants of Tropical Storm Lee current wreaking havoc in the Eastern states. The upper-level low currently located over northern Nevada will retrograde very slowly over Northern California over the next 24 hours and eventually set up shop off of the Central California coast this weekend, staying nearly stationary until at least early next week. As the low begins to move offshore, counterclockwise circulation will induce southeast and easterly flow over the northern 2/3 of the state, advecting monsoonal moisture into Northern and Central California from the Desert Southwest. Most of this moisture will be at mid and upper-levels, and any convection that develops on Friday or early Saturday will likely contain little or no rain, leading to a potentially widespread threat of dry lightning. By later in the weekend and possibly continuing into early next week, this flow will probably contain enough moisture to result in wetting rains in most thunderstorms as lofted moisture from diurnal storms serves to moisten the lower levels with time.

GFS forecast of retrogressive upper-level low on Friday

Not only will there be the threat of dry lightning this weekend, but easterly flow will result in downsloping, offshore winds in the Sierra foothills, Central Valley, and possibly the Bay Area. The triple threat of hot, potentially gusty offshore winds, low surface humidity, and possible dry lightning will probably cause some fire-related problems this weekend. This system also has the potential to bring multiple rounds of lightning to unusual areas, including the Central Valley and the Bay Area. Much of Southern California may miss out on the deep convective activity as it will remain in a pronounced dry slot for much of the event, though some potential for dynamically-forced elevated convection does exist over almost the entire state at some point during the next 5 days.

The numerical models are currently in disagreement regarding the evolution of the pattern after next Monday. It does not appear that this cutoff will be in any hurry to leave, however, and the risk of at least mountain thunderstorms (and possibly elsewhere) may continue straight through the next week. Stay tuned!

© 2011 WEATHER WEST