The wide geographic scope of the very dry conditions across the Western United States is indicative of the type of prevailing weather pattern that has led to the present situation: rather than a long train of “near-misses,” whereby numerous storm systems miss California either to the north (bringing copious rains to the Pacific Northwest) or to the east (in the form of “inside sliders,” bringing widespread rain and snow to the interior West), there has been a remarkable dearth of storm activity across the entire Western third of the country. The most immediate cause of this impressive dryness has been the persistence of an anomalous ridge over the Northeastern Pacific for essentially the entire winter.

Teleconnections between California wintertime precipitation and tropical convection in the Western Pacific are well-known, and on intra-seasonal timescales the Madden-Julian Oscillation tends to be a good indicator of the potential linkage at a given point in time. This winter, the MJO has been very quiet–regions of suppressed/enhanced tropical convective activity have been small and longitudinally stationary. Such behavior is not particularly unusual during a La Nina event, but there are usually some other aspects to the hemispheric pattern during Eastern Pacific cold events that usually bring at least modest (often cold) precipitation to much of the American West. This year, we haven’t really had any amplified meridional flow over the U.S., which has prevented the kind of sub-polar incursions that might otherwise have boosted our presently meager precipitation totals.

The all-important question, of course, is whether we might finally receive some late-season precipitation that could at least mitigate the effects of the very dry season thus far. While I see little hope of significant precipitation anywhere in California through the end of February, there are some recent changes in large-scale conditions worth considering. In the past 1-2 weeks, a rather strong MJO signal has emerged in the Western Pacific and is slowly propagating eastward. The first question is whether this signal can hold itself together and continue to propagate into a position favorable for forcing an amplified Pacific jet stream. The second question, of course, is whether we would even see any rainfall out of a strengthened zonal jet given the magnitude of the entrenched Pacific ridge. Only time will tell on this front, though it’s worth paying attention to the large-scale pattern in the 2-3 week range.

Secondly, La Nina appears to rapidly be on its way out of the picture. SST anomalies in the traditional Nino 3/Nino 4 regions have actually gone positive in the last few weeks, and there’s even a subsurface pool of warm water around ~300 m. In addition, there are presently strong westerly wind anomalies in the Eastern Pacific, which will only contribute to additional surface warming. While this is probably a positive development with respect to long-term rain prospects, it’s unclear if the transition to ENSO-neutral conditions will occur soon enough to have a significant impact on water year 2011-2012. It’s still too early to be talking seriously about El Nino prospects, though I will say that some of the dynamical models are indicating positive SST anomalies by mid-summer.

As always…stay tuned.

© 2011 WEATHER WEST

(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

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

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

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

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

As many of you have already noticed, the URL of the California Weather Blog has changed from http://www.weatherwest.com/current_weather to http://www.weatherwest.com . All Weather West content will now be accessible from the blog on the main page, and this change should make Weather West easier to find in web searches. If you have not done so already, please update your bookmarks!

I am pleased to introduce a brand new Weather and Climate Links page (accessible using the tab near the blog header). This page will be a work in progress, but is eventually intended to serve as a comprehensive directory of all things California weather (and geophysical!)-related. Please check it out!

Moving forward, I plan to update the site less frequently but hopefully more comprehensively than in the past. I will no longer provide updates for every storm or pattern of interest affecting California, but will instead try to focus on the really big or unusual events that occur or are anticipated to occur in the near future. For day-to-day weather events, I’ll be more likely to make brief posts in the Comments section to keep things up-to-date when possible.

Finally, as the site has lost a great deal of traffic during the transition and since it is possible that some longtime users were lost in the flurry of server errors, please feel free to promote the new Weather West when and where appropriate. I truly appreciate your help!

I still need to make some tweaks and additions to the site, but the present version is now a stable one, so the site should not go down during this final part of the process.

Thanks for your patience!

The blog is moving today and if all goes well should be ready by tonight. Stay tuned…

A very busy schedule and further technical setbacks have prevented me from working more on the site since it was originally compromised a couple of months ago. I will at some point this summer have the blog back up and running with occasional updates. I still plan to move the blog to the main site address by the end of summer–that is, you will eventually have to adjust your bookmarks to www.weatherwest.com to read the blog rather than the current url (weatherwest.com/current_weather).

I apologize for the extended delay, and I promise that the Weather West blog will be back up and running in the not-so-distant future…