Sunday evening through Wednesday afternoon, another energetic cutoff upper level low spun showers and thunderstorms into Southern California, increasing water year rainfall totals for many stations to near or above normal. Precipitation amounts varied widely, with some stations recording a few hundredths of an inch or less, and some as much as an inch or more. As was the case with the upper low at the end of September, this system produced a number of strong thunderstorms. A severe thunderstorm warning was issued as a result of a cluster of strong thunderstorms in the Antelope Valley Tuesday evening.

Downtown Los Angeles (USC) was one of those stations that didn't have its rain bucket in the right place. The station recorded only 0.08 inch from Sunday evening to Wednesday afternoon, bringing its water year rainfall total to 0.51 inch, which is about 0.16 below normal. Here is an archived Public Information Statement (PDF) from the NWS with some rainfall totals from around the area.

The Aug/Sep value of the Multivariate ENSO Index (MEI), released October 6, dropped another 0.18 sigma to -1.99. This is the lowest value of the MEI since Jul/Aug of 1955, and ranks the current La Nina as the strongest in the MEI record for the Aug/Sep season. Equatorial SST have continued to cool, and this plot of 5-day TAO/TRITON SST shows anomalies of -2.0°C or greater extending west to nearly the dateline.

Cooling of SSTs in the Central Pacific appears to have been enhanced by strong 850mb easterly winds associated with an MJO which propagated into the Western Pacific over the past 2 weeks and slowly diminished in amplitude. Divergence aloft associated with the MJO appears to have enhanced typhoon and tropical storm development in the Indian Ocean and Western Pacific. CIRA TPW loops show a lot of tropical moisture being injected into an extending Pacific jet. This could amplify rainfall amounts along the west coast of the U.S. the next few days.

There's been some drizzle and light rain in the Los Angeles area this morning. BUFKIT analysis of 12z NAM data for Van Nuys and LAX generates about 0.1 inch of rain late Sunday afternoon and evening. The bulk of the precipitation is forecast to occur in the northern two-thirds of California and into the Pacific Northwest. We'll see!

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

Updated October 6, 2010. Replaced 1975 with 2007 in composite of climate division precipitation for eight transitional moderate to strong La Ninas, and other revisions.

Monday at 12:15 p.m. PDT the temperature at Downtown Los Angeles (USC) reached 113°F (45°C). This is the highest temperature recorded downtown since weather recordkeeping began in 1877. All-time and daily records were broken at several location. Here's an archived NWS Public Information Statement with a list of the records that were broken.

A moist south to southeasterly flow associated with a high over the Great Basin and an upper low off the coast resulted in some thundershowers (and rainbows) in the Los Angeles area yesterday, and there's a chance of more thundershowers today. According to the NWS zone forecasts, a chance of thunderstorms will continue in the mountains through Saturday evening. (See update below.)

Since the peak of last Winter's El Nino, there has been an unprecedented drop in the Multivariate ENSO Index (MEI). MEI bimonthly (e.g. Sep/Oct) seasonal ranks range from 1 to the total number of years for the season in the record, currently 60 or 61. A rank of 1 represents the strongest La Nina for the season, and a rank of 60 or 61 the strongest El Nino. Based on MEI seasonal ranks, in six months ENSO conditions switched from the fifth strongest El Nino for the Feb/Mar season to the second strongest La Nina for the Jul/Aug season. As Klaus Wolter pointed out in his September 3rd MEI discussion, the only stronger La Nina in the MEI record for the Jul/Aug season was in 1955, and the last time the MEI was lower in any season was 1975.

In all 11 cases since 1950 in which the MEI was -1.0 or less in the Jul/Aug season, La Nina conditions persisted through the Nov/Dec season. In all but two cases, 1950 and 1964, the MEI remained below -1.0 from the Jul/Aug season through the Nov/Dec season. Most ENSO assessments project that La Nina conditions will persist through at least the end of 2010.

As they say, past performance does not guarantee future results, but in the cases of well established La Ninas and El Ninos, historical composites can provide some insight into what is likely. CPC's ENSO Temperature & Precipitation Composites page now includes composites which incorporate recent precipitation trends. Frequency of occurrence maps are also included for each composite. These composites indicate coastal Southern California has been trending drier in the rain season during the past 15 years. The composite plus trend plot for Jan-Feb-Mar precipitation anomaly is particularly dry in coastal Southern California, indicating negative anomalies in excess of 3 inches, with a high frequency of occurrence.

To get an idea of how US precipitation anomalies might differ in a strong La Nina versus the more general La Nina case, the ESRL/PSD US Climate Division Dataset Mapping Page was used to construct maps of US composite precipitation anomalies. The two cases can be compared by moving the mouse cursor over the image. The initial image is a composite based on eight moderate to strong La Nina years which were preceded by El Nino or neutral conditions. The "mouse over" image is a composite of 16 cases* in which the MEI was in the lower 30%-tile of MEI ranks.

Another consideration in assessing potential ENSO impacts is the likelihood that a La Nina (or El Nino) will result in either extremely dry or extremely wet conditions. Here are maps showing the risk of precipitation extremes for the periods Nov-Dec-Jan, Dec-Jan-Feb and Jan-Feb-Mar during La Nina. These were generated using the ESRL/PSD page Risk of Seasonal Climate Extremes in the U.S. Related to ENSO. According to these maps, Southern California isn't likely to have an extremely dry 2010-2011 rain season, although CPC ENSO composites suggest there is at least some possibility of a dry rain season, particularly in the period Jan-Feb-Mar.

CPC's current operational precipitation outlook for Nov-Dec-Jan, issued September 16, 2010, shows roughly equal probabilities of above average, near average, or below average precipitation in the Coastal Southern California climate division. As the rain season progresses, these probabilities become more skewed toward below normal precipitation. In the period Jan-Feb-Mar the probability of below normal precipitation is calculated to be about 40%, while the chances of above normal drop to around 25%. The skew become slightly more pronounced in the period Feb-Mar-Apr.

All of this suggests below average precipitation in the Coastal Southern California climate division for the period November 2010 through March 2011. How much below average? Perhaps on the order of 3 to 6 inches. Over the 16 years used in the La Nina climate division composite, rainfall at Downtown Los Angeles (USC) has ranged from 7.17" (1971) to 16.00" (1955), with an average value of 10.95". This is about 72% of the normal 15.14". If only the lowest 8 MEI ranked (strongest La Nina) years are included, the average rainfall for Los Angeles works out to 11.08", or about 73% of normal.

Although it appears unlikely, average to above average seasonal rainfall could occur. Also, damaging storms with heavy or prolonged precipitation are still a possibility.

*The 16 years in the La Nina composite were taken from the 19 lowest MEI ranks for Sep/Oct. This includes 1950, 1954, 1955, 1956, 1961, 1962, 1964, 1967, 1970, 1971, 1973, 1974, 1975, 1988, 1995, 1998, 1999, 2007 and 2008. The marginal La Nina years of 1961, 1967 and 1995 were excluded in the 16 year set. An Oceanic Niño Index (ONI) based selection of cold episodes for SON would include 1984 and 2000, and exclude 1961, 1967 and 2008.

Update October 1, 2010. Intellicast.com composite radar shows some strong thunderstorms in Southern California this afternoon, with hail reported, and tops indicated at 35,000 to 40,000 ft. The moist subtropical flow is forecast to continue through Sunday, and along with it, a chance of showers and thunderstorms.

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

Following a 26 day stretch in which the temperature at Downtown Los Angeles was below normal 25 of the days, and several lowest maximum temperature records were set around the area, temperatures have soared, and record high temperatures for the date have been recorded at several locations.

On June 30, Downtown Los Angeles ended the water year about 8% above normal. Some stations in the Los Angeles area recorded more than this and some less. There was a wide range of values. For example, Long Beach Airport was about 20% above normal and Santa Barbara Airport about 23% above; but Camarillo Airport reported 31% below normal rainfall. Here's an AHPS graphic showing the percent of normal precipitation in California and the Southwest for the year ending July 1 at 12z.

Equatorial Pacific SSTs have generally continued to cool. According to the CPC's Weekly ENSO Update, issued July 12, the latest weekly SST departures were -1.3°C (Niño1+2), -1.0°C (Niño 3), -0.8°C (Niño 3.4), and -0.4°C (Niño 4). According to the CPC, La Niña conditions occur "when the monthly Niño3.4 OISST departures meet or exceed -0.5°C along with consistent atmospheric features. These anomalies must also be forecasted to persist for 3 consecutive months."

Reflecting the transition from El Nino to La Nina conditions, the Multivariate ENSO Index (MEI) for May-June experienced an unprecedented drop for the time of year, decreasing nearly 1 standard deviation to a value of -0.41. In addition, the Global Wind Oscillation (GWO) has orbited about -2 SD down on the La Nina side of the phase space plot, and with the atmosphere in a generally low momentum state, may be in that neighborhood for a while.

The most recent IRI ENSO Update, released July 15, projects "an approximately 80% probability for continuing La Niña conditions, and a 20% probability for returning neutral ENSO conditions. "

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

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.

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.