Following a circuit through Arizona, Mexico and the Eastern Pacific that started in Southern California on October 5, a moisture-laden upper low moved into SoCal for a second time Thursday, October 15.

On the second go-round the upper low packed an even bigger punch, producing strong afternoon thunderstorms with very high rain rates that resulted in severe flash flooding and debris flows in northern Los Angeles County and southern Kern County.

CIMSS Morphed Integrated Microwave Total Precipitable Water Imagery from October 12 shows the upper low entraining moisture from the sub-tropics and tropics as it retrograded into the Eastern Pacific.

Below are some NEXRAD Level-III/Google Earth composites of northern Los Angeles County and southern Kern County for the afternoon of October 15:

Overview

- PPS Storm Total Precipitation for the period ending 6:00 pm PDT. (KSOX)
- QPE Storm Total Precipitation for the period ending 6:00 pm PDT. (KSOX)
- PPS Storm Total Precipitation for the period ending 6:00 pm PDT. (KEYX)
- QPE Storm Total Precipitation for the period ending 6:00 pm PDT. (KEYX)

Lake Hughes - Elizabeth Lake Area

- One Hour Precipitation ending 3:30 pm PDT. (KSOX)
- Instantaneous Precipitation Rate at 3:20 pm PDT. (KSOX)
- Instantaneous Precipitation Rate at 3:30 pm PDT. (KSOX)

Leona Valley

- One Hour Precipitation ending 4:42 pm PDT. (KSOX)
- Instantaneous Precipitation Rate at 4:25 pm PDT. (KSOX)

Hwy 58 - Cameron

- One Hour Precipitation ending 5:39 pm PDT. (KEYX)
- Instantaneous Precipitation Rate at 5:21 pm PDT. (KEYX)
- Instantaneous Precipitation Rate at 5:30 pm PDT. (KEYX)

Here are NWS tabulations of some rainfall totals around the area for October 15 and some totals for October 16.

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

Recall that storm a week ago Monday that blasted down the West Coast and into the Southland? The Los Angeles County mountains received as much as two inches of rain; Downtown Los Angeles (USC) recorded 0.45 inch of rain; and the Sierra got a good shot of snow. (Here are some precipitation totals for that storm from the NWS Los Angeles/Oxnard and the NWS San Diego.)

Well, surprise, surprise that upper low isn't done with us yet! After a multi-day circuit into Arizona, Mexico and the Eastern Pacific, the low is now sitting off the coast of Southern California and forecast to move over the area on Thursday. The result is a chance of showers and thunderstorms today, Thursday and possibly Friday, particularly in the mountains. We'll see!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

Downtown Los Angeles (USC) ended the 2014-2015 rainfall year (July 1 - June 30) with 8.52 inches of rain; well below the normal of 14.93 inches. It was the fourth consecutive year of below normal rainfall in Los Angeles and much of Southern California.

To kick off the new new rainfall year two waves of moisture and instability associated with ex-hurricane Dolores, other tropical sources, and a strong monsoonal flow from Baja resulted in record-setting rainfall in Southern California from Saturday July 18 to Monday July 20.

Constructive interference of the El Nino base state by the active phase of the MJO resulted in negative 200-hPa velocity potential anomalies and enhanced convection in the Eastern Pacific during the first half of July. This appears to have contributed to the rapid development of Dolores from a tropical depression on July 11 into a Category 4 hurricane July 15. Anomalously warm SSTs in the tropical and sub-tropical Eastern Pacific also played a role, helping to maintain the strength of Dolores and increasing the amount of water vapor entrained by the system and transported into Southern California.

Many stations set new records, not only for the date, but for any day in July. Downtown Los Angeles (USC) set rainfall records for the date on Saturday and Sunday and tied Monday's record. Downtown Los Angeles recorded 0.36 inch of rain Saturday. This is more rain than any day in any July since recordkeeping began in 1877. That one day of rainfall even broke the monthly record for July in Los Angeles! Prior to this event the wettest July on record was in 1886, when 0.24 inch was recorded.

There was very heavy rain in the mountains on Sunday, with rain rates exceeding an inch a hour. From 5:15 p.m. to 5:25 p.m. a CBS Radio weather station on Mt. Wilson recorded a half-inch of rain in just 10 minutes!

Though the rain created its own problems -- including flash floods, debris flows and rock slides -- the soaking rains helped quell the Pines Fire near Wrightwood and the North Fire near Cajon Pass. Over the three day period from Saturday to Monday the Big Pines Remote Automated Weather Station (RAWS), near the Pines Fire, recorded 3.12 inches of rain. Several stations in the San Gabriels recorded more than three inches of rain, including Clear Creek and Opids Camp. Here's a NWS compilation of some rainfall totals in the Los Angeles forecast area and the San Diego forecast area.

After dawdling around for several months our on again, off again El Nino is finally firing on all cyclinders and could reach ONI and MEI levels not seen since 1997-98 and 1982-83. The Multivariate ENSO Index (MEI) for May-June was 2.06. This was the third highest value for the season, exceeded only in 1983 (2.2) and 1997 (2.3). It is the second highest for the season during the development phase of an El Nino event. The 2015 April-May-June Oceanic Nino Index (ONI) value of 0.9 was higher than than in 1982 (0.6) and 1997 (0.6). Several dynamical models in the IRI/CPC ENSO Predictions Plume of forecast Nino 3.4 SST anomaly, released July 16, project Nino 3.4 SST anomalies in excess of 2.5°C this fall.

Analysis of correlations of CMAP Precipitation with globally integrated atmospheric angular momentum using ESRL/PSD's Linear Correlations in Atmospheric Seasonal/Monthly Averages tool suggests that precipitation in the southern half of California is more strongly correlated with atmospheric angular momentum (AAM) than with Nino 3.4 SST. For example, compare the correlation of CMAP Precipitation to AAM and to Nino 3.4 SST for Dec-Jan-Feb 1980-2012. Cyclical increases in relative atmospheric angular momentum are often associated with El Ninos. This can be seen in the plots of the Global Wind Oscillation in my El Nino Comparison Chart.

After being negative for 3 1/2 years, the PDO Index has been positive since January 2014. December's PDO value of 2.51 was the highest for that month on record since 1900. June's value of 1.54 was the 13th highest since 1900.

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

After being negative for 3 1/2 years, the PDO Index has been positive since January 2014. December's PDO value of 2.51 is the highest for that month on record (since 1900), exceeding December 2002's value of 2.10 and December 1940's value of 1.96.

Even so, the highly anticipated 2014-15 El Nino continues to dawdle with little ocean-atmosphere coupling and an emphasis on an higher amplitude meridional flows. Due in part to the upwelling phase of a series of oceanic Kelvin waves, equatorial Pacific heat content and SST anomalies have diminished. After having been at or above 0.5 °C for the past twelve weeks OISST.v2 Nino 3.4 SST Anomalies dropped to 0.4 °C for the week centered on January 7, 2015 and as of the week of January 14 was at 0.5 °C.

The Multivariate ENSO Index (MEI) for NOV/DEC has decreased 0.13 SD to +0.58, but has maintained its historic rank (since 1950) at 47. A rank of 46 is the threshold for weak El Nino conditions in the context of the MEI. The FNL Global Wind Oscillation for the past 90 days continues to reflect the lack of definitive atmospheric coupling.

So far this rain season Northern California and the coastal areas of Central and Southern California have fared much better in terms of precipitation than last year. As of today Downtown Los Angeles (USC) precipitation for the water year (beginning July 1) is at 90% of normal and Downtown San Francisco is at 127%.

As this AHPS plot of precipitation since October 1 shows, many areas of California have recorded at least 75% of normal precipitation. Unfortunately the Sierra Nevada is not one of them. While better than last year's dismal 14% of normal on this date, this year's snowpack is currently well below average, and was last reported at 36%. Here is an AHPS plot of precipitation departure since October 1 that shows the deficits in the Sierra Nevada.

At the moment it doesn't look like the snowpack will get much help in the short term. After fending off a little shortwave the blocking ridge is forecast to rebuild to new heights, bringing unseasonably mild weather to much of the West Coast. There does appear to be the possibility of a wildcard low latitude low developing under the block and that could result in some precipitation in Southern California next week. If such a system develops, given the warm state of the Eastern Pacific, it could be wetter than expected. We'll see!

In the early morning hours of Friday, December 12, 2014, a very strong cold front, enhanced with moisture from an atmospheric river, produced a line of strong storms that produced rain rates in the Springs Fire burn area as high as 2 inches per hour. This resulted in widespread flash floods and debris flows in the burn area, much of which is in Pt. Mugu State Park. Mud and debris flows originating from the burn area inundated homes below Conejo Mountain and closed Pacific Coast Highway. This slideshow includes photos of the aftermath of the flash floods and debris flows in Blue Canyon, Sycamore Canyon and Upper Sycamore. The photos were taken on a trail run on December 14, 2014. Also included are some NWS Los Angeles/Oxnard tweets and some additional meteorological images and info.

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

After being negative for 3 1/2 years, the PDO Index has been positive since January 2014. October's PDO value of 1.49 is the 6th highest for that month since 1900 and comparable to PDO Index values in October 1997, 1957, 1993, and 1987.

PDO/AMO-based Precipitation composites suggest a warm Pacific generally produces more Winter precipitation in California than a cold Pacific. This makes sense. Among other effects a warm Pacific increases evaporation, air temperature, and the amount of water vapor transferred to the atmosphere. This in turn can increase precipitation. Here is a comparison of Pacific SST anomalies at the beginning of December 2014 and December 2013. Mouse over the image to switch to the December 1, 2013 image.

A warm Pacific may have contributed to the amount of precipitation produced in California by a large Pacific upper level low this past week. Two periods of rain were observed in Southern California -- one on Sunday and the other on Tuesday and Wednesday.

Sunday an embedded short wave in the moist southerly flow produced more rain than expected in many areas of Los Angeles. Surprisingly high rain rates and amounts in the Santa Monica Mountains, resulted in debris flows in the Springs Fire burn area that closed PCH. Here's a NWS compilation of some rainfall totals in the forecast area.

Even with the parent low lifting to the north on Tuesday and the 570 mb contour near Santa Barbara, the very moist southerly flow orographic lift and sufficient dynamics produced widespread precipitation in Southern California with some impressive precipitation totals. Tuesday record rainfall for the date occurred at both Downtown Los Angeles and Downtown San Francisco. Yucaipa Ridge in the San Bernardino Mountains recorded a whopping three-day rainfall total of 14.6 inches. Following are three-day rainfall compilations from the NWS Los Angeles/Oxnard and NWS San Diego.

Here are graphics from the CNRFC with gridded precipitation totals for the four day period from early Sunday morning to early Thursday morning for Southern California, the Sierra Nevada and the San Francisco Bay Area. Some preliminary precipitation totals of 5 inches or more are noted.

Equatorial Pacific SSTs continue to rebound with OISST.v2 Nino 3.4 SST Anomalies above 0.5 °C for the past seven weeks and now stand at 1.0 °C for the week centered on November 26, 2014. The Multivariate ENSO Index (MEI) for OCT/NOV has increased 0.35 SD to 0.71, increasing its historic rank (since 1950) from 42 to 47, just within the MEI's weak El Nino threshold. However as noted by the CPC in their December 4, 2014 ENSO Diagnostic Discussion, "the overall atmospheric circulation has yet to show a clear coupling to the anomalously warm waters." The FNL Global Wind Oscillation for the past 90 days continues to reflect the lack of definitive atmospheric coupling

For now the Pacific storm door appears to be open. Most medium-range guidance is suggesting another significant system will be affecting California in the Thursday-Friday timeframe. We'll see!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

One thing is very clear; we have a lot to learn about the atmosphere's response to anomalously warm equatorial Pacific SSTs. It's difficult to imagine a better scenario for El Niño development than the conditions seen in the equatorial Pacific earlier this year.

After being negative for 42 out of the previous 43 months, the Pacific Decadal Oscillation (PDO) index turned positive in January 2014 and has remained positive through the summer. Beginning in January and ending in late June the downwelling phase of a very strong Kelvin wave propagated across the Pacific, with upper ocean area-averaged heat content anomaly between 180 and 100W peaking at the end of March. Since January there have been westerly wind bursts and periods of increased low-level westerly zonal wind anomalies of variable duration and extent.

The Oceanic Niño Index (ONI) jumped from -0.5 in the FMA season to -0.1 in MAM, and then to +0.1 in AMJ. During that same period the Multivariate ENSO Index (MEI) jumped from a rank of 35 in MARAPR to 59 in APRMAY -- a value that put it on the doorstep of a strong El Nino ranking. Which all looked supportive of at least a moderate El Nino developing.

But it didn't. After climbing up to 0.6 °C in late May, Nino 3.4 region SST anomalies dropped to below 0.0 °C in late July. Since then a more modest downwelling Kelvin wave has restored some of the basin heat content and Nino 3.4 anomalies have rebounded back to 0.6 °C.

So what's next? While El Nino development still appears to be possible this Winter, it would be one of the two latest developing El Ninos in the record from 1950 to the present -- the other being 1952-53. One possibility is that this year's vacillations are the precursor to the EARLY development of an El Nino event next year.

One worrisome detail is that the Global Wind Oscillation (GWO) has been behaving similarly to the failed El Nino of 2012. Compare this plot of the GWO from June 1 to October 31 of this year to the plot of the GWO from June 1 to October 31, 2012. In both 2014 and 2012 the GWO has shown a neutral or weak La Nina-like response. The GWO for the period June 1 to October 31, 1997 is an example of a definitive atmospheric response to strong El Nino conditions.

On a more positive note, a relatively strong, but fast-moving Pacific cold front and trough resulted in rain and snow in California over Halloween. In the Los Angeles area rainfall amounts generally varied from around 0.3 inch to 0.75 inch or so with isolated amounts as high as about 2.0 inches in the mountains. Here are some tabulated rainfall amounts from around the area from the NWS Los Angeles/Oxnard and NWS San Diego.

The 1981-2010 climate normal average rainfall for Downtown Los Angeles in November is 1.04 inches. Month to date we're about average for the date, and water year to date we're ahead of last year, but about 0.6 inch below normal.

Based on the current GFS and ECMWF forecasts those deficits are probably going to increase over the next two weeks, but it is way too early in the rain season to attribute the dry weather to a continued dry pattern. El Niño or not; dry November or not; the switch from a cold Pacific to a warm Pacific is a significant change and one that some guidance suggests should increase our rainfall. We'll see!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

The "PDO minus AMO" composite precipitation anomaly maps prepared by Dr. Klaus Wolter of CIRES as part of an experimental seasonal forecast for the California DWR last year were consistent with the below average precipitation observed in California in the water years Oct 2012 - Sep 2013 and Oct 2013 - Sep 2014.

As referenced in Dr. Wolter's 2013 DWR presentation, Schubert et al. (J. Climate, 2009) found that five global climate models produced the least precipitation in the continental U.S. when the Pacific is cold (Pc) and the Atlantic warm (Aw). Conversely four of the five models produced the wettest conditions when the Pacific is warm (Pw) and Atlantic cold (Ac). As shown in this figure, all combinations with the Pacific cold (PcAw, PcAn, PcAc) produced below average precipitation and all combinations with the Pacific warm (PwAw, PwAn, PwAc) produced above average precipitation.

While the development of the 2014 El Nino has waxed and waned over the past several months, there has been a definitive change that might significantly impact the weather in California and the U.S. -- the Pacific has warmed. For five of the past six years the JUL-SEP PDO has been negative. This year the PDO has been positive since January.

To get an idea of how the change to a warm Pacific might affect precipitation in the U.S. the PDO index and AMO index values for JUL-SEP for the past 115 years were ranked and then divided into tercile classes, producing nine PDO/AMO states.

The NOAA/NCDC Climate Division Mapping and Analysis Web Tool was then used to generate composite precipitation maps for October to September and December to January for the six combinations of Pacific warm and Pacific cold states: PwAw, PwAn, PwAc and PcAw, PcAn, PcAc.

The PwAw composites show much more precipitation in Southern California and the Southwestern U.S. than the PcAw composites. It might be argued that it is the El Nino years in the PwAw composites that produce the wet signal in Southern California. However, if the El Nino years are removed from the PwAw composites for Oct-Sep and composites for Dec-Feb, a wet signal persists.

Of course there are many factors that can influence the amount of precipitation in a particular locale in a particular season or water year. Composites are not forecasts any more than monthly climate normals are forecasts; but both can provide useful guidance. In the PwAw case 7 of the 12 selected years in the composite were wet in Southern California and two-thirds of the years had near normal or above average precipitation. We'll see what happens in 2014-2015.

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

Downtown Los Angeles (USC) finished the water year (July 1 to June 30) with 6.08 inches of recorded rainfall. This is about 41% of the 1981-2010 normal of 14.93 inches. It was the seventh driest water year since recordkeeping began in 1877. When combined with last year's water year total of 5.85 inches, the back-to-back water years from July 1, 2012 to June 30, 2014 are the driest on record for Los Angeles. The total rainfall deficit over the two year period was about 18 inches.

El Nino has been knocking on the door, but until very recently the atmosphere has only been responding in spits and sputters.

The weekly OISST.v2 Nino 3.4 SST temperature anomaly has been vacillating in the range 0.4 to 0.6 beginning with the week centered on April 23, 2014 and now stands at 0.5. Nino 1+2 and Nino 3 anomalies have generally been increasing and as of the week centered on June 25 are at 1.6 and 1.0 respectively. Here is a plot of the Nino regions SST anomalies from the CPC. Following a period of strengthened tradewinds, another westerly wind burst has developed in the Western Pacific. There have been several WWB this year and the most recent WWB could lead to additional warming of equatorial SSTs.

In response to the upwelling phase of an oceanic Kelvin wave there has been a decrease in subsurface Pacific equatorial heat content and some associated cooling at depth.

The Global Wind Oscillation (GWO) can be a useful tool for evaluating the degree to which the atmosphere is responding to El Nino or La Nina conditions. As this plot of the GWO for the period March 1 to July 31, 1997 shows, during the spring and summer of 1997 El Nino conditions were already resulting in a definitive atmospheric response. Here is a plot of the GWO from March 1 to July 1 of this year. Over most of this period the GWO has shown a neutral or weak La Nina-like response. Recently the GWO has exhibited a positive shift in AAM anomaly that, if sustained, could be indicative of a coupled ocean-atmosphere response.

The Plume-based and Consensus Forecasts in the June 19 IRI/CPC ENSO Quick Look (PDF) show an increasing chance of El Nino conditions developing over the NH summer, with an approximately 80% chance of El Nino conditions being established by the OND season. A July 2 run of the CFSv2 forecasts Nino 3.4 anomalies to briefly decline, then increase substantially from July into October. We'll see!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

Extended by the active phase of the MJO, a strong Pacific jet provided the impetus for two Pacific storm systems to undercut a persistent ridge over the West Coast and bring much-needed rain to parched California.

Beginning Wednesday evening (Feb 26) and continuing into Sunday (Mar 2), the storm systems produced the most rain over five days in Los Angeles since December 2010, ending a nearly 14 month period with record-setting dry weather. Los Angeles experienced the driest calendar year on record in 2013, and until Friday had recorded less water year rainfall than in 2006-07 — the driest water year (July 1 - June 30) since recordkeeping began in 1877.

According to preliminary precipitation data, Downtown Los Angeles (USC) recorded 4.52 inches of rain over the course of the storms, increasing its water year total from a dessicated 11% of normal to a not-too-bad-considering 50% of normal. Downtown Los Angeles' water year rain total now stands at 5.72 inches. This exceeds last year's cumulative precipitation total on this date by more than an inch, but still leaves us with a deficit of nearly six inches. The storms increased February's rainfall total to near normal, and jump-started March with nearly half its normal amount of rain. Prior to these storms the most rain recorded at Los Angeles in a day this water year was 0.29 inch back in November!

Orographically favored foothill and mountain areas that faced into the storms' moist southerly flow recorded some impressive rainfall totals. According to this compilation of preliminary rainfall totals from the NWS Los Angeles/Oxnard, Opids Camp near Mt. Wilson recorded nearly 11 inches of rain, and several stations in the Ventura Mountains recorded double-digit rainfall totals. Here are a CNRFC map of Gridded QPE for the 7-day period ending March 3 at 4:00 am and a CNRFC map of 7-day Gridded QPE and 120 hr raw precipitation for stations recording over 4.0 inches.

With this recent rainfall 2013-14 will not be the driest water year in Los Angeles, but one good storm, or even two, "does not a rain season make." In the short term these storms have dramatically reduced the fire danger, provided crucial relief to plants and animals, and increased groundwater and reservoir storage. What happens in the longer term we'll just have to see. Over the next several days a series of systems are forecast to produce additional rain from Central California north into the PNW. While no rain is forecast in Southern California over the next week or so, and the 8-14 day outlook is for below average precipitation, as long as the Pacific weather pattern remains progressive there should be additional opportunities for rain in the weeks ahead.

It looks like El Nino is beginning to knock more loudly at the door. The third and strongest of a series of oceanic downwelling Kelvin waves continues to significantly increase subsurface equatorial heat content in the Pacific basin and another strong Westerly Wind Burst has occurred in the equatorial Pacific. The CFSv2 forecasts Nino 3.4 anomalies to reach El Nino thresholds in the May-June 2014 timeframe, however the IRI/CPC Plume-based and Consensus Forecasts released February 20 are less bullish, forecasting about a 40% chance of El Nino conditions developing in the MJJ season. We'll see!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

From July 1 to February 14 Downtown Los Angeles has recorded only 1.20 inches of rain. This is less than the 1.92 inches of rainfall recorded over the same period in 2006-2007, making 2013-2014 the driest water year to date since recordkeeping began in July of 1877. Downtown Los Angeles averages 14.93 inches of rain in a water year, July 1 through June 30.

Los Angeles recorded only a trace of rain in January, and so far this February has recorded only 0.23 inch. At the moment neither the GFS or ECMWF show any rain south of Pt. Conception through the morning of February 25. At that time both models have another high amplitude ridge over the West Coast, so unless something changes the chances for additional rain this month do not look good. The 1981-2010 normal rainfall for January is 3.12 inches and for February 3.80 inches.

Active weather in Central California culminating in a strong atmospheric river event February 8-9 produced double-digit precipitation totals in the coastal mountains and Sierra. Over the 5-day period ending Monday, February 10 at 3:15 pm precipitation totals as high as 23.51 inches were observed. Since February 12 another atmospheric river has been feeding moisture into Northern California and Oregon.

It looks like El Nino will be knocking at the door in the next 2-3 months and we'll have to see if the atmosphere and ocean respond. The third and strongest of a series of oceanic downwelling Kelvin waves is propagating into the Eastern Pacific and is increasing subsurface equatorial heat content in the Pacific basin. The CFSv2 forecasts Nino 3.4 anomalies to reach El Nino thresholds in May, however the IRI/CPC Plume-based and Consensus Forecasts are far less bullish, forecasting about a 30% chance of El Nino conditions in the AMJ season. We'll see!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.