A classic cold front associated with a cold upper level low and Pacific trough pushed through Southern California overnight Sunday, producing rain at the lower elevations and snow at the higher elevations. Scattered convective showers and isolated thunderstorms followed in the wake of the front as the low and trough slowly moved onshore. TV news reports showed marble sized hail produced by a strong cell in the eastern San Fernando Valley.

Downtown Los Angeles (USC) recorded 0.91 inch of rain for the storm, bringing the water year total to 16.17 inches, which is more than an inch above the 1971-2000 annual climate norm of 15.14 inches*. Here's an archived copy of a NWS Public Information Statement with some rainfall totals from around the area.

The additional rainfall puts Downtown Los Angeles at 1.51 inches for the month of April, well above the climate normal of 0.83 inches. The normal amount of precipitation for May is 0.31 inches, and for June is 0.06 inches. Although the current El Nino appears to be in decline, convection is still enhanced in a broad area of the western and central equatorial Pacific, and the GWO, MEI, and ONI all indicate the continued presence of El Nino. This could result in more active Spring weather than usual. We'll see!

More information concerning Southern California weather can be found using our WEATHER LINKS page.

*The average annual rainfall for Los Angeles of 15.14 inches is computed on a calendar year basis for the 30 year period 1971-2000. Technically it is not a water year average, but by convention it is used as a reference for water year rainfall. For details about how normal temperature and precipitation values are computed, see CLIMATOGRAPHY OF THE U.S. NO. 81 - Monthly Station Normals.

After peaking at a standardized relative AAM anomaly of 2.21 around February 5, 2010, the Global Wind Oscillation (GWO) has plunged 3 SD in relative AAM to a value of -0.90. This is the lowest value of relative AAM anomaly since mid October, 2009. Much of the loss in AAM was from the mid latitudes of the NH. From the week of February 22-28 to March 19-25, 7-day averaged 250 mb Zonal Mean Zonal Winds decreased from about 43 m/s at 30°N to 33 m/s at 30°N-45°N. The decrease in relative AAM reduced the likelihood of an extended and southward displaced North Pacific Ocean jet, and associated Southern California El Nino impacts.

In fact, there has been no measurable precipitation at Downtown Los Angeles (USC) since March 6, 2010. Los Angeles rainfall is now 2.23 inches below normal for the month of March, and only 1.15 inches above the water year norm for the date.

In the last week there has been an increase in relative AAM from about 40°N to 50°N. The GWO and AAM appears to have bottomed out and cyclical subseasonal processes may be working to revitalize Western Pacific convection and El Nino related forcing. If the GWO continues its current orbit, relative AAM would be expected to increase in the 6-10 day period.

The GFS and ECMWF have been showing a strong Pacific jet and trough affecting the West Coast next week. There are significant differences in the GFS and ECMWF model solutions, and although 5-day model performance has been good recently, performance often suffers during seasonal transitions. This morning's 12z GFS puts the initial focus of the event in the Pacific Northwest early in the week. Precipitation is forecast to spread into Northern and Central California, and finally Southern California, as the week progresses, and the trough deepens and moves onshore.

It's way too early to say how this system will affect Southern California. Many processes are in play that could affect the amount of precipitation here, pro or con. Yesterday afternoon's 00z ECMWF appeared to project a wetter solution for Southern California than this morning's 12z GFS, with the upper low and trough evolving a little more to the west. It does look like there is the potential for heavy snow in the Sierra. We'll see!

Update 04/03/10. The frontal band and upper low associated with the trough that moved through the area overnight Wednesday into Thursday produced some showers and some spectacular clouds in the Los Angeles area. The bulk of the precipitation occurred from Central California north into the Pacific Northwest. The upper low stayed offshore as it moved down the coast and past the Los Angeles basin. Avalon and some mountain locations recorded more than 0.1 inch of precipitation, but generally amounts south of Pt. Conception ranged from a trace to few hundredths. Here's an archived copy of a NWS Public Information Statement with some rainfall totals from around the area. A similar system is forecast to produce some rain in Los Angeles area Sunday night. BUFKIT analysis of 18z NAM data for Van Nuys projects about 0.4 inch of precipitation. The 09z SREF shows a high probability of measurable rain, but a sharp decrease in the probability of more than 0.1 inch/day south of Pt. Conception. We'll see!

There was an interesting lenticular wave train northwest of Los Angeles earlier this month. The situation was peculiar because the wind at nearly all levels at that time was from the northwest, and the wind forming the wave clouds appeared to be from the north-northeast. The tops of the wave clouds were being sheared by winds blowing from the northwest (left to right). An ARL NAM-12 based wind profile for the area on March 10 for 06z, shows a possible source of the winds that produced the waves, as well as the shear. Here's an animated series of NRL satellite photos showing the complex wind and wave pattern at the time.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

As forecast, an energetic upper low skirted the Los Angeles area yesterday on its way south into northern Baja, Mexico. As the low moved south, a vorticity lobe spinning around the low arced into the Southern California coast, producing rain, thunderstorms, waterspouts, and blustery winds.

Rain amounts were variable, generally ranging from about 0.25 inch to 0.75 inch around the Los Angeles area. Indicative of the convective nature of the precipitation, Downtown Los Angeles (USC) recorded 0.42 inch, while a few miles away LAX recorded only 0.18 inch for the storm. The water year total for Downtown Los Angeles now stands at 14.66 inches -- only 0.48 inch less than the annual norm of 15.14 inches. Here is an archived copy of NWS Public Information Statement with some more rainfall amounts for this rain event.

Reflecting the enhancement of El Niño by the MJO at the end of January, the Multivariate ENSO Index (MEI) for January-February has increased to 1.5. As discussed by Klaus Wolter, MEI originator, this is the highest value of the Jan-Feb MEI since the strong El Niño of 1997-1998, and the fifth highest MEI value for Jan-Feb since 1950. This places the El Nino of 2009-10 in the top 10% of MEI rankings for the season since 1950, and above the "strong" El Niño threshold, as measured by the MEI. Here is a chart comparing the current El Niño to others since 1950.

The increase in the MEI corresponds to the increase in relative AAM since mid January and underscores the potential role of the Global Wind Oscillation (GWO) in assessing the response of the atmosphere to El Niño. It also reinforces the notion that the El Niño phenomena is more than just warm SSTs in the central equatorial Pacific. As we've seen several times in recent years, warm SST anomalies in the Niño 3.4 region alone are not sufficient for evaluating an El Niño and its potential impacts.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

A weakening frontal band produced some light rain in Southern California yesterday afternoon and evening. Precipitation amounts in the Los Angeles area ranged from a trace to a few hundredths of an inch. Amounts were higher to the north, closer to the surface low. A few stations in Santa Barbara and San Luis Obispo counties recorded more than 0.5 inch and some recorded more than an inch. Here is an archived copy of a NWS Public Information Statement with some precipitation totals from around the area.

Today, a much stronger Pacific storm system begins to impact the West Coast, with the highest precipitation totals forecast for Northern California and the Sierra Nevada. BUFKIT analysis of this morning's 12z NAM data for Van Nuys generates about 1.3 inch of rain, beginning late Friday night and continuing into Saturday afternoon. The 18z NAM run starts the rain a little earlier Friday night, and extends the period of precipitation into Saturday evening. It produces about 1.8 inch of rain at Van Nuys over the period. Today's 09z SREF indicated a probability of about 50%-70% that precipitation in the Los Angeles area would exceed 0.5 inch for the 24 hr. period ending Saturday afternoon at 4:00 p.m. Precipitable water values approaching an inch and strong southerly low level inflow could produce higher rain rates and precipitation totals on south to southwest facing foothill and mountain slopes.

Since being reinforced by a strong MJO at the end of January, El Niño convection has remained active in the Central Pacific (animation). In terms of relative atmospheric angular momentum (AAM), the atmosphere's response is the strongest since the El Ninos of 1997-1998 and 2004-2005. Average AAM for our rain season (November to date) has increased from -0.046 at the beginning of the year to 0.341 as of February 23. This El Nino Comparison Chart shows how this El Niño compares to others since 1950.

So far this rain season, Downtown Los Angeles (USC) has recorded above average rainfall every month, except for November. And November's rain came early -- in October.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

Yesterday's cold upper level low behaved about as expected, following a track just off the coast. Because of the convective nature of the system, rainfall amounts varied from under 0.25 inch to over 1.0 inch in some foothill and mountain locations. About a foot of snow was reported at the mountain resorts. Here is a NWS Public Information Statement with precipitation totals from around the area. Downtown Los Angeles (USC) recorded 0.44 inches for the storm, increasing the water year total to 13.2 inches. The is 4.78 inches above normal for the date.

Compared to the AHPS 90 Day Precipitation map from about 40 days ago, a fairly typical El Niño precipitation pattern has emerged in the West. Medium range models are projecting about a 7-10 day break in what has turned out to be a busy rainy season in Southern California. Assuming these forecasts verify, such a break could not come a better time for those threatened by mudslides and debris flows. The recent enhancement of El Niño convection by the MJO, and the ongoing phase 6-7-8 transition of the GWO might have resulted in a wetter pattern, and this has occurred in similar circumstances in past El Ninos.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

Wet antecedent conditions and heavy rain early this morning combined to amplify the amount of runoff from our latest Winter storm. In addition to localized street flooding, mud and debris flows have occurred in the vicinity of the Station Fire burn area. The trough associated with the system, and a second frontal band are moving onshore this afternoon, and are producing some additional rainfall in the Los Angeles area. Please refer to www.weather.gov/losangeles for the latest warnings and weather information.

Yesterday's runs of the NAM/WRF did a pretty good job of forecasting the area of enhanced precipitation that developed overnight in Southern California. BUFKIT analysis of 12z NAM data from yesterday generated about 2.4 inches of precipitation at KLAX for the 24 hr. period ending early this morning. According to preliminary NWS data, as of 10:00 a.m. LAX had recorded 2.31 inches for the storm, and Downtown Los Angeles (USC) had recorded 2.84 inches. The water year total rainfall for Los Angeles is now about 5 inches above normal. Here is a NWS Public Information Statement with rainfall totals from around the area. (Link will be updated as revised totals become available.)

The recent enhancement of El Nino convection in the equatorial Pacific by the Madden-Julian Oscillation (MJO) has triggered a strong atmospheric response. A Global Wind Oscillation (GWO) phase space plot shows large increases in relative atmospheric angular momentum (AAM) and AAM tendency. As a result of this increase, the average relative AAM anomaly for the rain season to date is now positive. As mentioned in this post from December 2009, relative AAM is correlated with rain season precipitation in Southern California. This suggests an increased likelihood of wet weather in Southern California in the medium range outlook period.

So what happens next? The ECMWF and GFS projections have not been particularly consistent. At the moment, it looks like a shortwave trough could affect Southern California in the Tuesday evening or Wednesday timeframe and then again Friday. We'll see how the week develops.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

The active phase of the Madden-Julian Oscillation (MJO) is enhancing El Nino convection in the equatorial Pacific, increasing the probability of above average rainfall in California over the next two weeks. Here are today's 6-10 day outlook and 8-14 day outlook periods from the Climate Prediction Center (CPC). Once again, a number of the CPC's best fitting 6-10 day outlook analogs and 8-14 day outlook analogs are from wet El Nino years.

Today's Global Wind Oscillation (GWO) phase plot reflects the atmosphere's response to the MJO, and enhanced central equatorial Pacific convection. ECMWF-based 250 hPa streamfunction anomalies from 01/29/10 00:00 GMT appear to be consistent with the ERSL/PSD phase 7 MJO streamfunction composite and phase 6-7 GWO streamfunction composite centered on January 27.

Another strong Westerly Wind Burst (WWB) is also occurring, and appears to be one of the mechanisms associated with MJO enhancement of deep El Nino convection. This enhancement and resulting feedback could slow the decline of, or even increase, equatorial Pacific SST anomalies in the weeks ahead.

Both the ECMWF and the GFS are projecting a return to a wet pattern in California next week. The first skirmish occurs in central and northern California over the weekend, and then a transition to a much wetter, and possibly persistent, pattern is forecast to occur during the week. We'll see!

More information concerning Southern California weather can be found using our WEATHER LINKS page.

After nearly two weeks of idyllic days with temps in the 70's and 80's, it rained in Southern California last night and into this morning. But today's rain is just a precursor. Just a little reminder that it's the rainy season in Southern California, and more rain is on the way -- possibly a lot more rain.

Skies will start to clear later today, and temps are forecast to be back into the 70's tomorrow. But the dry weather won't last. For several days forecast tools have been projecting a major pattern change -- to a wet pattern more like what might be expected in January of an El Nino year.

Today's Climate Prediction Center 6-10 day precipitation outlook indicates a 70% probability that precipitation will be above normal in Southern California for the period January 19-23, and the 8-14 day precipitation outlook indicates a 60% probability that precipitation will be above normal for the period January 21-27.

As part of its analysis, the CPC now uses historical analogs with 500 hPa patterns similar to the current forecast. In the 6-10 day forecast period the 10 most similar analogs produced above average precipitation in Southern California 9 out of 10 times. In the 8-14 day period the 10 closest analogs produced above average precipitation 6 or 7 times out of 10. Also of note, many of the analog cases with the most similar 500 hPa patterns are from wet El Nino years, such as 1958, 1978, 1983, 1992, 1995, and 1998.

Late in the weekend, around Sunday evening, the first of these potentially strong systems is expected to move onshore. Additional impulses and systems are forecast to follow in quick succession, driven by a fast moving zonal flow and strong jet stream. If current computer projections validate, the wet pattern could continue through the end of January. If precipitation totals approach what is forecast over the period, a host of rain related impacts are a possibility. We'll see.

Update January 15, 2010. PDF of slides from NWS Los Angeles/Oxnard Weather Briefing "Significant Storm Impacts For Southern California Next Week" by Eric Boldt Warning Coordination Meteorologist. Presented on January 14, 2010. Please refer to www.weather.gov/losangeles for the latest information.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

Related post: How Does the El Nino of 2009-10 Compare to Other Warm ENSO Episodes Since 1950?

In terms of typical El Nino impacts, Southern California is still waiting. If you squint your eyes and have a good imagination, there is a hint of an El Nino rainfall pattern on the West Coast, but rainfall over much of our area ranges from a little below normal to a little above.

Including Wednesday's meager rainfall, Downtown Los Angeles is now about 1.1 inch above normal for the water year. That sounds great, but is a little deceptive. Some stations are well below normal and some well above. For example, Camarillo is 1.4 inches below normal, and Santa Barbara 2.3 inches above. According to the AHPS precipitation map, some mountain areas of Santa Barbara have had 200% of normal rainfall the past 90 days.

As expected, the Global Wind Oscillation (GWO) did orbit up into the positive AAM side of the phase space; but as feared, Indian Ocean/West Indonesian convection appears to have (once again) quashed the positive response. The average relative AAM anomaly for the period November 1 to December 29 remains slightly negative, and total global relative angular momentum is just about where it was at the beginning of November.

Updated January 2, 2010. Recent runs of the GFS show the Pacific jet extending and then collapsing into another trough off the West Coast during the week. This seems a little late to be directly related to the recent GWO phase 8-1 transition. A continued orbit of the GWO to phase 2 and 3 would be expected to shift the jet north and could result in another highly amplified wicked ridge of the west. Today's 18z run of the GFS plows the trough into a resistant ridge with most of the energy going up and over the ridge. The 12z ECMWF splits the trough and attempts to bring part of it in under the ridge. We'll have to see how things develop during the week.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

Following several weeks of dry weather, a shift to a wetter pattern brought much needed rain and snow to California this past week. The pattern change was apparently induced by MJO enhancement of El Niño convection in the equatorial Pacific.

Downtown Los Angeles recorded 1.78 inches of rain from Thursday to Sunday, bringing the water year total to 4.81 inches. This is about 2.20 inches above normal for the date. Rainfall totals generally ranged from 2 to 4 inches in the basin and valleys, and 2 to 6 inches in the mountains. Here's an archived copy of a NWS Public Information Statement with rainfall totals from around the area.

Will the current El Niño produce the expected seasonal impacts in Southern California? A new tool that can help gain some insight into the linkage of climate and weather is the Global Wind Oscillation (GWO) phase space plot. Ed Berry repeatedly demonstrated the usefulness of this tool in his blog Atmospheric Insights. Although the blog has been discontinued, its content remains a valuable resource.

The GWO is a recurring subseasonal phenomenon that involves the transport and interchange of momentum in the earth-atmosphere system. It encompasses the MJO and occurs on a similar timescale. Analogous to the MJO phase space plot, but based on a framework of atmospheric angular momentum (AAM), the GWO phase space plot is a measure global relative atmospheric angular momentum and it's tendency. For details, see Weickmann & Berry, 2008.

Relative AAM is generally positive during an El Niño and negative during a La Nina. Relative AAM is correlated with rain season precipitation in Southern California*, and can be helpful in assessing potential El Niño impacts. The following November to March GWO phase plots show the distinctly different behavior of the GWO during the strong El Niño of 1997-98, and the strong La Nina of 1973-74:

GWO El Niño 1997-1998	GWO La Niña 1973-1974
Click for larger image	Click for larger image

In a decade characterized by quirky El Niños, the El Niño of 2009-10 has been acting like another odd one. From a Southern California perspective, the concern has been that it might be like the El Niño of 2006-07 when Downtown Los Angeles recorded only 3.21" of rain over the water year. In the early stages of the El Niño of 2006 strong convection developed in the Indian Ocean during November, but an MJO did not develop until a second round of Indian Ocean convection occurred in mid December. Relative AAM remained negative, and in terms of the atmosphere, the 2006 El Niño didn't make it to 2007.

This year, Indian Ocean convection did spawn an MJO which eventually enhanced El Niño convection near the dateline. Significant momentum was added to the mid-latitudes of the Northern Hemisphere, energizing the westerlies and contributing to the pattern change that resulted in our recent wet weather. However, the increase in mid-latitude AAM has been mostly offset by negative anomalies at higher latitudes. Following are the GWO phase space plots for the current rain season to date, and the quirky El Niño of 2006-07.

GWO El Niño 2009-2010	GWO El Niño 2006-2007
Click for larger image	Click for larger image

So where do we go from here?. For at least a few days the storm track has shifted north. It appears the GWO might be starting an orbit that will take it farther into the El Niño side of the phase space. Several MJO forecasts show the next MJO cycle shortcutting it's orbit back into the Western Pacific. In the 6-10 day period, the GEFS ensemble MJO forecast indicates enhancement of equatorial convection around 90E, and suppression (by the MJO contribution) of El Niño convection near the dateline. This could modify any short term response of the El Niño signal and the GWO, once again reducing AAM.

Below are the GWO phase plots for two El Niños with ONI and MEI values comparable to the current El Niño. In the 1986-87 El Niño, November to March AAM averaged 0.02, and it wasn't until February that the GWO climbed out of the AAM doldrums. That water year Los Angeles recorded 7.66 inches of rain. On the other hand, in the 1994-95 El Niño, November to March AAM averaged 0.76. In mid December the GWO began to reflect an ocean-atmosphere coupled El Niño response, and Los Angeles went on to record 24.35 inches of rain for the water year. At the moment it looks like Southern California might not be as dry as 1986-87, but probably not as wet as 1994-95. We'll see!

GWO El Niño 1986-1987	GWO El Niño 1994-1995
Click for larger image	Click for larger image

*1958-2008 Precipitation/AAM correlation plot generated using US Climate Division Dataset Seasonal Correlation Page.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

Drier than average weather in the Southwestern U.S., and wet weather in the Pacific Northwest in recent weeks has some folks wondering if the 2009-2010 El Niño is going to be another one of the quirky, underachieving El Niños we've been seeing this decade.

For much of September and October, the primary area of equatorial convection in the Eastern Hemisphere was in the Central Pacific, just west of the dateline. Convection was suppressed in the Indian Ocean. Such a pattern is consistent with El Niño.

Enhanced convection centered at about 160E in early October may have helped to extend the Pacific jet following an East Asian mountain torque event around October 5. This in turn may have contributed to an atmospheric river precipitation event in California October 13-14, 2009.

At that time it looked like we might finally be off and running with a "real" El Niño event. Particularly because during October equatorial Pacific SSTs and heat content anomalies increased dramatically. Theses increases were the result of of a strong Westerly Wind Burst (WWB), an associated downwelling Kelvin wave, and diminished tradewinds,

However -- and I think I hear Ed Berry's Rottweiler growling -- beginning in mid October, strong convection developed in the Indian Ocean. This essentially neutralized El Niño, and perhaps contributed to the La Nina like precipitation pattern we've been experiencing along the west coast of the U.S.

Indicative of the ocean-atmosphere issues, relative Atmospheric Angular Momentum (AAM) has been negative since early September, and the Global Wind Oscillation (GWO) has remained on the La Nina side of the GWO phase space.

Does this mean we've had an El Niño false alarm?

I wouldn't put away my rain gear just yet. Although November rainfall for Downtown Los Angeles is about 0.6 inch below normal, we usually only get about an inch of rain in November. And don't forget, because of the big storm in October, we are still about 0.6 inch ABOVE normal rainfall for the water year.

Update November 24, 2009. Current equatorial heat content anomalies are about the same as in 2006, not greater. See this composite TAO/TRITON section plot comparing equatorial heat content in 2006 and 2009.

And there are some positive signs. Equatorial Pacific SST and heat content anomalies are much greater than in the short-lived 2006 El Niño, and are more along the lines of those observed in the 2002-2003 El Niño. In addition, a strong MJO has developed. It has remained coherent, and is propagating eastward into the Western Pacific. It already appears to be helping to enhance convection in the Western and Central Pacific, and could kick-start the El Niño engine over the next week or two. We'll see!

More information concerning Southern California weather can be found using our WEATHER LINKS page.