Woodland Hills (Pierce College) topped the 90° mark for the first time this year Memorial Day weekend, and after cooling a few degrees during the week, temperatures are back into the 90's this afternoon. A quick look at temps around the area shows temps near 70 on the coast; 80's to 90's in the valleys, and triple digits in the deserts.

Reflecting the transition to ENSO neutral conditions, the Multivariate ENSO Index (MEI) for April-May declined to 0.54. According to the CPC's Weekly ENSO Update, issued June 1, the latest weekly SST departures are -0.6°C (Niño1+2), -0.4°C (Niño 3), -0.2°C (Niño 3.4), and 0.4°C (Niño 4).

In his May 2010 MEI discussion Klaus Wolter projected the probability of a La Nina event emerging by the end of 2010 at roughly 50%. In it's May 2010 ENSO Quick Look, the International Research Institute for Climate and Society (IRI) put the probability of La Nina conditions from the August-October season through the remainder of 2010 at 42%. We'll see!

Downtown Los Angeles (USC) has recorded 16.36 inches of rain since the water year began July 1. This is more than an inch above the 1971-2000 annual climate norm of 15.14 inches. The water year ends June 30. Climatology and current forecasts suggest it is unlikely that this total will change by more than a hundredth or two before the end of the month.

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

During the last month the 2009-10 El Niño has essentially transitioned to ENSO neutral conditions. Over the course of the event, the Multivariate ENSO Index (MEI) peaked in JAN/FEB at 1.5, and the Oceanic Nino Index (ONI) peaked in NDJ at 1.8. The mean 5-day averaged November 1 to March 31 relative AAM anomaly was 0.304. The strongest El Niño forcing appeared to have occurred in late January after the active phase of the Madden-Julian Oscillation (MJO) enhanced El Nino convection in the equatorial Pacific. Here's a chart comparing the 2009-10 El Niño to 17 other warm ENSO episodes that have occurred since 1950.

Dynamical and statistical ENSO model forecasts suggest the possibility that equatorial Pacific SSTs will continue to decline, leading to weak to moderate La Niña conditions by late Summer or Fall. A return to El Niño conditions does not appear likely, but cannot be ruled out. In his May MEI discussion, Klaus Wolter points out that the 1957-1958 El Nino, an analog to the 2009-2010 El Niño, rebounded after dropping into neutral territory.

Downtown Los Angeles (USC) has recorded 16.31 inches of rain since the water year began July 1. This is more than an inch above the 1971-2000 annual climate norm of 15.14 inches. Recent runs of the GFS and ECMWF show a negatively tilted trough deepening and moving onshore early next week. At this point it looks like there's a pretty good chance of showers north of Pt. Conception, but only a slight chance down here. We'll see!

Update 05/18/10. Stations in the Los Angeles area generally recorded under a tenth of an inch from this system. Some isolated totals of about 0.25 inch were recorded in Ventura and Santa Barbara county, and Rocky Butte in San Luis Obispo recorded 0.48 inch. Here is an archived copy of a NWS Public Information Statement with some rainfall totals from around the area.

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

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.