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.

An unseasonably strong Pacific storm system, primed with the remnants of Typhoon Melor, and powered by an extended Pacific jet, is expected to pound California early next week with high winds, heavy rain, and heavy snow at the higher elevations of the Sierra.

Medium range model forecasts suggest that the northern two-thirds of the state will see the highest rainfall totals, but over the last couple of days, the GFS has been trending wetter in Southern California. Significant rainfall in the Los Angeles area, and recently burned areas in the San Gabriel Mountains, appears to be a possibility. Forecast details should become more clear as we move closer to the event and into the forecast window of the high resolution models.

The Aug-Sep Multivariate ENSO Index (MEI), decreased from 0.978 to 0.754, however other El Niño signals appear to be rebounding. In the last two weeks low level equatorial westerly anomalies have increased significantly. The reduction in the strength of the trade winds, and a downwelling Kelvin wave resulting from a very strong westerly wind burst already appear to be increasing upper ocean heat content in the central equatorial Pacific. The 30 day moving Southern Oscillation Index (SOI) is still positive, but is dropping, and should return to negative territory over the next week or so.

The foci of recurring equatorial westerly wind bursts and enhanced west-central Pacific convection has been slowly migrating eastward. The most recent round of enhanced convection was centered at about 160E. This is consistent with a developing El Niño, and may have helped to extend the current Pacific jet following an East Asian mountain torque event. However, total and relative AAM remain negative, and are lower than is generally the case during a developing El Niño.

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.

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.

After spending the last week of May and most of June on the positive relative AAM (El Niño) side of the Global Wind Oscillation (GWO) phase space, the GWO slipped back into negative relative AAM territory the last week of June and stayed there most of July. Following the lead of the atmosphere, development of the current El Niño also slowed in July.

During July there was little change in equatorial Pacific SST anomalies; the Southern Oscillation Index (SOI) became strongly positive; and the June-July Multivariate ENSO Index (MEI) value, reported August 5, increased by only 0.05.

However, during July, enhanced tropical convection shifted from the western Indian Ocean to the west central Pacific, extending from about 140E to the dateline. This was followed by the development of a strong Westerly Wind Burst (WWB) in the western Pacific. Coincident with the WWB was a dramatic reduction in the SOI, and an an orbit of the GWO to a somewhat higher global relative AAM state. These events are consistent with a developing El Niño.

According to the El Niño/Southern Oscillation (ENSO) Diagnostic Discussion issued August 6 by the Climate Prediction Center/NCEP/NWS, "current conditions and model forecasts favor the continued development of a weak-to-moderate strength El Niño into the Northern Hemisphere Fall 2009, with the likelihood of at least a moderate strength El Niño (3-month Niño-3.4 SST index of +1.0°C or greater) during the Northern Hemisphere Winter 2009-10."

How often are Summer temps hotter in Seattle than the San Fernando Valley? From July 26 to August 3, 2009, the Seattle area suffered through a heat wave that broke numerous records, including several "all-time" temperature records. On July 29, 2009 Sea-Tac reached 103°F, Bellingham 96°F, and Seattle WFO (Sandpoint) 105°F, the highest temperature ever recorded at these stations.

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

Los Angeles Temps Continue Below Normal.

Downtown Los Angeles (USC) ended the water year on June 30th having recorded 9.08 inches of rain, which is 6.06 inches below the 1971-2000 norm of 15.14 inches. Some additional water year rainfall totals:

According to the El Nino/Southern Oscillation (ENSO) Diagnostic Discussion issued July 9 by the Climate Prediction Center/NCEP/NWS, "During June 2009, conditions across the equatorial Pacific Ocean transitioned from ENSO-neutral to El Niño conditions." According to the report, El Nino conditions are expected to continue to develop, and last through the NH Winter.

The May-June Multivariate ENSO Index (MEI) value, reported July 6, has increased by 0.51 to +0.85. As was the case last month, the 3-month rise of the MEI (since February-March) is the 4th highest on record for this time of year, and was last exceeded by the strong El Niño of 1997. According to the MEI's originator, Dr. Klaus Wolter, "the combination of already border-line moderate El Niño conditions along with such a big rise in the MEI at this time of year has always been followed by continued El Niño conditions through the remainder of the calendar year, at least in the modern MEI record (since 1950)."

However, as the truncated 2006-07 El Niño event demonstrates, an El Niño is more than just warm Pacific equatorial SSTs. Through complex forcing and feedback mechanisms, the atmosphere and oceans have to cooperate on a global scale. As climate scientist Ed Berry cautions in his July 3rd Atmospheric Insights post, "Should total AAM departures become comparable to that observed during this past January and February, my concerns of an El-Nino 'false alarm' for the weather-climate dynamical system will be significantly raised."

Generally speaking, the momentum of the atmosphere increases when there is an El Niño, and decreases during a La Niña. Over the past 40 days the Global Wind Oscillation (GWO) has retreated from the higher AAM (El Niño) side of the GWO phase space to the lower AAM (La Niña) side. One concern is that two areas of tropical forcing (Indian Ocean and Pacific) may interact in such a way as to quash the El Niño engine. We'll see!

Closer to home, although temps in Southern California have recently been more seasonable, particularly in the Valleys, the last time the average daily temperature in Downtown Los Angeles was above normal was back on May 21 -- 48 days ago.

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

Instability and moisture associated with a very late season upper level low produced more rain in Southern California Friday, setting a record for June rainfall at Santa Barbara Airport.

Downtown Los Angeles (USC) recorded a total of 0.13 inch of rain Friday. The rainfall increased the water year precipitation total for Los Angeles to 9.08 inches, which is 6.05 inches below normal. Santa Barbara Airport recorded 0.51 inch, setting not only a new record for the date, but for any day in June, as well as for the total amount of rain in June. Here's an archived copy of a NWS Public Information Statement with some preliminary rainfall totals from around the area as of 5:00 p.m. Friday.

The April-May Multivariate ENSO Index (MEI) value (released today) has increased by 0.54 to +0.34. As discussed by MEI originator, Klaus Wolter, the 3-month rise of the MEI since January-February is the 4th highest on record for this time of year, exceeded last by the strong Niño of 1997. According to Dr. Wolter, if next month's MEI rank is at least the same as this month (37th), "it would be unprecedented for it to drop below that high-neutral ENSO-phase range by the end of 2009, virtually excluding a return to La Niña, based on the MEI record since 1950." We'll see!

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