Well, it does certainly appear that the remarkably quiescent and dry weather pattern that has persisted nearly throughout California since November is finally coming to an end. After more than two months of extremely dry conditions during what is normally the height of the rainy season, nearly the entire state has accumulated some truly impressive precipitation deficits. Most of Northern California is experiencing hyrdological drought according to NOAA’s Palmer Drought Severity Index, with a good portion of the Sierra Nevada squarely in the “severe drought” category. Fortunately, though, these metrics are based primarily on short to medium-term soil moisture, which (as those who have lived in this state for more that a few years already know) can change quite quickly under the right circumstances. And it starting to appear that conditions may be right for some very significant precipitation over the next 10 days.

(NOAA/NWS)

First things first, though: a very cold Arctic airmass has invaded the Pacific Coast of North America over the past few days, bringing snow all the way down to sea level in Washington, Oregon, and even briefly on the beaches of the far north coast of California. The entire state of Washington, and much of Oregon, are expected to receive heavy and possibly record-setting lowland snowfall over the next 3-4 days, with accumulations exceeding 12 inches possible all the way down to sea level near Seattle. While California is unlikely to see any extraordinarily low snow levels this week (outside of the possibility of a few more flurries along the far North Coast early Tuesday morning), a long-duration (and possibly record-setting) deep freeze is already underway across much of NorCal this evening. Temperatures by 11 PM had already dropped into the 22-27 degree range across large parts of the Sacramento Valley and were already below freezing across much of the Bay Area. With dewpoints mostly in the low 10s, record low temperatures are likely across large portions of NorCal on Tuesday as surface temperatures drop into the low to mid 20s in most locations and even as low as 12-17 degrees in the coldest valleys, even near sea level. In short, tomorrow morning will be very cold by NorCal standards, and there will probably be some agricultural damage and perhaps even some burst pipes as a result.

These cold temperature, though, will not last long. By Wednesday, warm and very moist air will initially override and eventually displace the cold and dry Arctic airmass over California as a strong zonal jet–yes, finally, a strong zonal jet!–noses into the far northern part of the state. Rain will begin slowly over far NorCal on Wednesday and spread southward through Thursday, reaching the Monterey Bay Area by late in the day. PWs over 1.5 inches are associated with these incoming systems, and there will be a moderate low-level jet acting to squeeze out some fairly high precipitation rates over the Coast Ranges and the Sierra Nevada. This sort of precipitation will be highly beneficial, since rain rates will not be high enough to cause any flooding problems but accumulations will be rather significant, perhaps on the order of 2-4 inches in the higher elevations, with 1-2 inches elsewhere. A similar storm is expected to arrive by Friday and drop a similar amount of precipitation across NorCal. The first storm probably won’t make too much headway into Southern California, perhaps bringing some light rain, but the second system will probably bring at least some precip to the entire state.

Probably of greater interest is the possible evolution of the pattern days 7-10. The extended forecast models–and the GFS in particular–are showing early indications of what could be a very significant precipitation event on Day 8. It appears that a large and deep cyclone off of the Washington and B.C. coasts may set up a long and deep band of subtropical moisture convergence somewhere along the Pacific Coast. With the presence of a very strong zonal jet (and also a low-level jet enhancement near the coast), this is a setup that has the potential to produce heavy precipitation where the moisture plume hits the coast. The 00z GFS focuses this plume on Central California, bringing very heavy 48-hour precip totals to the Bay Area next Tuesday. This is still over a week out, and very much subject to change, but it ‘s worth keeping an eye on considering the soil-saturating rains expected over the next 7 days over the northern 2/3 of the state.

(NOAA/NCEP)

In any case, one statement can be made with very high certainty: winter is, at last, on our doorstep. Stay tuned.

© 2011 WEATHER WEST

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