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.

Temperatures today at Los Angeles Pierce College in Woodland Hills appear to be running about a degree hotter than yesterday, when a new record high for the date of 109°F was recorded at the campus weather station.

A BUFKIT analysis of 12z data for Van Nuys shows temps peaking today at about 107°F, followed by a slow cooling trend into next week, with high temps dropping down into the high eighties. In this graph of temperatures, the red line is the 2-meter temperature and the blue line is the surface "skin" temperature.

Equatorial Pacific SSTs continue the transition to neutral conditions. This TAO section plot of 5-day Depth Average Temperature clearly shows the change in state with a marked increase in sub-surface temperatures, particularly in the far eastern and western equatorial Pacific.

It looks like Downtown Los Angeles (USC) will end the July 1, 2007 to June 30, 2008 water year with 13.53 inches of rain recorded. This is 91% of the 1921-2006 average of 14.87 inches.

Update Monday, June 23, 2008. On Friday, Los Angeles Pierce College in Woodland Hills eventually reached a record-setting high of 111°F! Here is an archived copy of a NWS Los Angeles/Oxnard Record Event Report listing record highs from around the area for Friday, June 20.

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