Just a week after Downtown Los Angeles (USC) set a new all time temperature record of 113°F, an unseasonably strong cutoff upper level low set up shop over Southern California, cooling temperatures and producing record rainfall over much of the area. Precipitation was recorded in many locations Monday and Tuesday, but Wednesday was the record setting day.

According to the NWS Los Angeles/Oxnard, rainfall records for the date were set at numerous locations. Downtown Los Angeles (USC) recorded 0.30 inch, breaking the record of 0.22 inch back in 1916. Just a few miles away, LAX recorded 0.62 inch, smashing the old record of 0.16 set in 1945. Here's a preliminary record report (PDF) from the NWS.

Cumulative rainfall totals recorded over the three days varied widely, ranging from 0.10 inch in Lancaster to over 2.0 inches at some locations in Ventura county. Downtown Los Angeles (USC) recorded 0.40 inch for the storm. This is slightly above the October monthly norm, but because July, August and September had no measurable rain, Los Angeles remains 0.12 inch below normal for the water year. Here is a Public Information Statement (PDF) from the NWS with some rainfall totals from around the area.

The 12z NAM and GFS show temperatures rebounding quickly, with highs reaching the 80's and 90's across much of Southern California by Sunday. Next week the GFS projects a gradual cooling trend. 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.

This year it wasn't until July 14th that Downtown Los Angeles topped the 90°F mark, and Woodland Hills the 100°F mark. Through mid August we'd had only one short (four day) heatwave. But it's Southern California, and you knew sooner or later temps would soar.

Since the middle of the month temperatures have been above seasonal norms, and particularly so the last couple of days. Monday and Tuesday, Downtown Los Angeles recorded highs of 98°F and 94°F, and Pierce College in Woodland Hills hit 104°F and then a scorching 111°F. This afternoon temperatures downtown have been a few degrees cooler than yesterday, but at Pierce College temps have been just about as hot as yesterday. The good news is an upper level trough is forecast to move onshore in the western U.S. this weekend. In Southern California this will result in a deeper marine layer, and dramatically cooler temperatures.

Intellicast composite radar shows some strong thunderstorm activity in Southern California this afternoon. In some cases hail is indicated, with tops reaching 50,000 ft.

The rapid cooling of equatorial Pacific SSTs took a bit of a breather at the the beginning of August, with SST departures leveling out in Nino regions 3.4 and 4, and even warming somewhat in Nino regions 1+2 and 3. But subsurface temperatures remained cool, and in the last ten days surface temperatures have once again been on the decrease. Here is a comparison of TAO 5-day SST anomalies for August 14 and August 24. TAO data from August 24 shows SST anomalies of -2°C, or greater, extending from 95°W, nearly to the dateline.

Update 08/27/10. Thursday the high at Downtown Los Angeles (USC) was 87°F. Pierce College in Woodland Hills topped out at 107°F. In the 9:00 hour this morning Pierce College temps are running 11°F to 12°F cooler than yesterday.

Update 08/26/10. Wednesday the high at Downtown Los Angeles (USC) was 90°F. Pierce College in Woodland Hills topped out at 109°F. Today's temps should be a near repeat of yesterday's. Big cool down kicks in on Friday, and continues into Sunday. Looks like inland highs will drop around 10°F (give or take) each day. A warming trend is forecast for next week.

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.

The frontal band associated with the first Pacific upper level low system and trough of Autumn 2009 is producing some showers in Central California. Some snow showers could occur at the higher elevations of the Sierra. A strong onshore flow has dramatically cooled temperatures throughout the state.

Today's cool temps are a welcome respite from several days of hot weather. Wednesday and Thursday of last week, Downtown Los Angeles (USC) recorded a high of 100°F; and Saturday, Woodland Hills (Pierce College) reported a high of 107°F. Temperatures are expected to rebound Wednesday, Thursday and Friday, but are forecast to cool again over the weekend.

Similar to what occurred in July there has been little change in equatorial Pacific SST anomalies, the Southern Oscillation Index (SOI) has switched from negative to positive, the Aug-Sep Multivariate ENSO Index (MEI) value is not expected to significantly increase, and the Global Wind Oscillation (GWO) has dropped its orbit to a lower relative AAM state.

On the plus side, a large area of enhanced convection has redeveloped in the West Central Pacific from about 150E to the dateline, and another of a series of Westerly Wind Bursts (WWB) is occurring -- the strongest observed so far during this ENSO transition. This WWB will likely be followed by another eastward propagating downwelling Kelvin wave, which could help increase upper-ocean heat content anomalies in the equatorial Pacific.

Moderate El Niños come in many flavors and have varying impacts. Under the guise of such El Niños Los Angeles experienced its second wettest water year on record in 2004-2005, when 37.25 inches of rain was recorded; then in 2006-2007 had its driest water year on record, when only 3.21" was recorded.

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

Including today, Los Angeles has enjoyed an unprecedented ten straight January days with highs in the eighties. In the yin yang of weather, the western half of the U.S. has been enjoying unusually warm temps, while the eastern half of the country has shivered.

Since the start of the water year on July 1, Downtown Los Angeles (USC) has recorded 4.66 inches of rain. Just 10 days ago, 4.66 inches was almost exactly normal rainfall for the date. However, as is so often the case in Southern California, this apparent normality was the sum of offsetting wet and dry periods. December's precipitation was generally well above normal, but January has been dry, dry, dry. Today, the Los Angeles rainfall total is about 1.10 inch below normal, and every day it doesn't rain, our deficit increases by about 0.10 inch.

Our warm temperatures and dry weather have been the result of a high amplitude ridge, pushed up over the West Coast by a very strong and extended Pacific jet stream. Big upper level ridges such as this are consistent with La Nina, and have been a recurring theme this Fall and Winter. Much of our rainfall and cold weather this season has occurred when an extended Pacific jet collapses or contracts -- as is occurring now -- and the blocking ridge shifts westward, opening the door to cold storms plunging down the backside of the ridge from the north.

This time there is a wildcard in the mix. One of the reasons the Pacific jet has been extended is the active phase of the Madden-Julian Oscillation (MJO) has recently propagated from west to east, across the Pacific. The system that is forecast to affect Southern California Wednesday night into Thursday looks like it might be enhanced by an inflow of moisture from an area of tropical convection that may be associated with the MJO.

How much might it rain? A BUFKIT analysis of 12z NAM data generates 1.3 inches of rain at Van Nuys for the period Wednesday evening through Thursday afternoon. The HPC 2 Day forecast is not so bullish, indicating about 0.50 inch to 0.75 inch over the area. We'll see!

Medium range models suggest the weather in the West could remain unsettled into next week and beyond. Of particular interest is this morning's 12z ECMWF forecast for mid-week next week, which projects the retrogression of the West Coast ridge to 150-160°W, with a cold, wet pattern similar to what we saw in December. That would be quite a change!

Update 01/22/09. It seems the NAM and GFS models can't get a handle on the system currently affecting our area. The 12z NAM generates 1.6 inches (!) of rain at Van Nuys and the 12z GFS 1.2 inches at LAX from this morning into Saturday afternoon. On the other hand the 09z SREF Ensembles puts the probability of more than 0.25 inch of rain for the 24 hr. period ending mid-morning Friday at about 50%, and then only about 10-30% for the following 24 hr. period. Upslope enhancement may produce higher totals on south facing foothill and mountain slopes. The GFS continues to advertise the possibility of a significant rain event Monday afternoon into Tuesday or Wednesday, but given recent model performance, we'll wait and see!

Update 01/21/09. The 12z models now extend the rainy period for the first system into Saturday. The 12z NAM generates about 1.0 inch of rain at Van Nuys, beginning Thursday morning and ending Saturday midday. The 12z GFS also produces about 1.0 inch, beginning Wednesday evening and ending Saturday afternoon. The GFS adds another 0.9 inch from Sunday morning to Monday night. Somewhat higher amounts would be expected in some foothill and mountain locations. As always, we'll see!

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

Downtown Los Angeles recorded a trace of rain, and a few other stations in Southern California recorded meager, but measurable, precipitation Monday. Where it did rain, amounts of a few hundredths of an inch or less were the norm. A good cell must have developed over the southwestern San Gabriel Mountains -- a NWS technical discussion mentioned that 0.5 inch was recorded at Big Tujunga Dam.

Following our "welcome to Summer" heatwave back in June, summertime temperatures have generally been seasonable. Yesterday, Los Angeles Pierce College recorded a high of 102°F, and inland temperatures today appear to be running a little warmer.

This weekend temperatures are expected to drop dramatically as a Pacific storm system and it's unseasonably strong 165+ kt jet affect the west coast this Friday into Saturday. If this morning's 12z GFS verifies, as much as 3-4 inches of precipitation could occur in the mountains of Northern California, Oregon and Washington. At high elevation some of this precip could be in the form of snow.

The front associated with the trough is currently forecast to hold together south of Pt. Conception and could produce some showers in the Los Angeles area on Saturday, particularly in the mountains. A BUFKIT analysis of 12z GFS data generates about 0.3 inch of rain at Van Nuys, but given the time of year, and the ample time remaining between now and Saturday, we'll have to see!

Ed Berry first mentioned the possibility of an "anomalous extended North Pacific Ocean jet collapsing into a western USA trough" in a September 6, 2008 post on Atmospheric Insights, and refined the projection, including a timeframe, in subsequent posts.

Update 10/06/08. Saturday's trough and front behaved about as expected with scattered light rainfall around the area. Precipitation amounts generally ranged from a trace or less to about 0.1 inch or so in some foothill and mountain locations.

Update 10/03/08. Focus of the incoming storm system continues to be Northern California and the Pacific Northwest. A BUFKIT analysis of 12z NAM data shows the expected north to south gradient in projected precipitation. About 0.5 inch is projected at Monterey, 0.2 inch at San Luis Obisbo, 0.1 inch at Santa Barbara, and 0.06 inch at Van Nuys. The 09z SREF Ensembles suggest a high probability, low precipitation event in the Los Angeles area with about a 90% probability of more than 0.01 inch of rain, but less than a 10% probability of more than 0.1 inch. We'll see!

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