It has been an odd rainy season this year in California. Things got off to an early start in October, but rainfall and overall storm activity over the far Eastern Pacific dropped off dramatically during the traditionally wet months of November through February. Since approximately the end of February, however, storms returned to the state of California, bring much-needed rainfall and substantially boosting the previously anemic Sierra snowpack. Still, while recent precipitation has greatly reduced the potential for any serious or widespread water concerns this summer, water-year precipitation remains substantially below average for most of the state.

Source: NWS/NOAA

At the present time, a fairly complex and increasingly active pattern is evolving over the far Eastern Pacific. A cutoff low is currently circulating several hundred miles off of the California coast, which triggered mountain thunderstorms this evening in the moist, warm, and slightly divergent southerly flow on its eastern side. As this low begins to approach the SoCal coast over the next 48 hours, I expect the coverage of showers and thunderstorms to increase. Tomorrow afternoon, a weak perturbation and associated vorticity maximum will lift north over NorCal, likely triggering scattered convection in many places (especially over the mountains). Similar activity could occur on Tuesday, though there doesn’t appear to be a specific focusing mechanism on that day so shower/thunderstorm coverage could be somewhat more limited.

On Wednesday and Thursday, the cutoff low will move inland near the Los Angeles area, bringing a fair bit of rain and possibly thunderstorms to a wide swath of the state. At the same time, a modestly stronger and colder low will begin to approach the state from the northwest. The confluence of these two systems will bring unsettled and potentially active weather over most or all of the state into late Thursday.

Source: NCEP.

Thunderstorm potential is difficult to assess at this point given the complexity of the developing pattern, but there may be lightning observed somewhere in the state each day between now and Friday. Right now, the mountains, the Central Valley, and the coast from the L.A. Basin sourthward appear to have the highest thunderstorm potential this week, but this may change. Stay tuned!

© 2011 WEATHER WEST

The very long-anticipated–and nervously hoped for–pattern change has finally arrived. After nearly a full winter of extremely dry and stable conditions over California, particularly the northern half, an extended period of cold, wet, and unsettled weather is finally headed toward the state.

GOES water vapor imagery depicting a deep trough developing off of the West Coast (NOAA)

The first in a series of storm systems is currently moving onto the coast in far northern  California, and is expected to bring periods of heavy rain and gusty winds to the northern 2/3s of the state through Wednesday. Rain totals will be pretty hefty in favored NorCal locations–probably in excess of 5 inches on south-facing slopes. Significant flooding is not anticipated due to the extremely dry antecedent conditions.

Later this week, a very deep trough, fueled by a powerful jet streak on its back side, will deepen off West Coast. Very cold Arctic air will be ingested into the trough, and at present it looks like a pretty strong storm system will develop on the leading edge. More heavy rain and gusty winds are likely in NorCal, and given the heavy rainfall expected earlier in the week this could result in some minor hydrological issues.

Probably of more interest, though, is the very cold and unstable airmass expected to move in behind the primary cold front over the weekend. This trough will be very slow-moving, and California will be in a very favorable position for convective activity for at least 48 hours from late Friday to late Sunday. In addition, snow levels could be very low in convective precipitation, especially if the latest models solutions pan out.

GFS depiction of deep and very cold West Coast trough (NCEP)

Thunderstorms and hail are quite likely across most of the state Saturday and Sunday, and snow levels will plummet below 2000 feet in NorCal and perhaps as far south as the Mexican border before all is said and done. North of Point Conception, it is certainly conceivable that snow levels could drop locally below 1000 feet, given recent model forecasts of 500 mb thicknesses approaching 520-523 dm over San Francisco with 850 mb temperatures of -5 to -6 C. This could be a fairly widespread convective setup, and some stronger/organized thunderstorms will be possible given favorable wind shear, strong jet-level winds, and very steep lapse rates.

In short…the next week will bring more active weather to California than the entire winter has cumulatively to this point. This is much-needed rain and snow, and will probably be enough to at least mitigate the impacts of an otherwise extremely dry winter.

© 2011 WEATHER WEST

As has been case for the entirety of the traditional “wet season” thus far, California continues to experience very dry conditions. Though the length of absolutely dry periods has decreased in recent weeks, the dry spells have been punctuated only by brief periods of light or very light precipitation (with a couple of highly localized exceptions). Cumulative precipitation totals for the water year, winter season, past month, and–despite some recent precipitation–even for the past week are all well below normal across nearly the entire state (and, for that matter, across most of the Southwestern U.S.). In fact, the most recent update from NWS/CPC indicates that most of California (and Nevada, and Utah, and Arizona) has received less than half of typical precipitation so far this winter, and over a third of the state has received less than 25% of average during the three months that usually make up the peak of the rainy season in our region.

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

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