For the past two years Los Angeles has recorded above normal rainfall over the first six months of the water year (Jul. 1 to Dec. 31). So far this year is following suit.

After a record-setting storm in early October in which Downtown Los Angeles (USC) recorded over an inch or rain, a series of upper level troughs have continued to bump up the rainfall total. The systems have tended to be amplifying upper level troughs that split, typically transforming the southern half of the trough into a difficult-to-forecast cut-off upper level low. The resulting cut-off lows have then tracked over, along, or off the Southern California coast producing varying amounts of rain.

Sunday's system was the fourth to produce measurable rain in Los Angeles this November. A strong cold front produced very heavy rain, resulting in flooded streets and highways and resulting in a host of other weather-related problems. In the middle of it all runners in the PCTR Santa Monica Mountains 9K, 18K, 30K and 50K were running distances up to 31.5 miles on the trails of Pt. Mugu State Park.

Downtown Los Angeles (USC) recorded 0.90 inches for the storm, bringing the water year total to 2.75 inches. This is 1.11 inch above the new 1981-2010 normal for rainfall from July 1 through November 22. Here are some additional (preliminary) precipitation totals from around the area, compiled by the NWS Los Angeles/Oxnard. More than an inch above normal sounds like a lot, but as of today only guarantees Los Angeles precipitation will be above normal through December 12.

Back on July 1 NOAA released the new 1981-2010 climate normals, replacing the 1971-2000 normals that have been used this past decade. Normals serve as a 30 year baseline average of climate variables such as monthly and annual maximum, minimum, and mean temperature; and monthly and annual total precipitation. For example, Downtown Los Angeles' new normal annual precipitation total is now 14.93 inches, down from the 15.14 inches we've been using.

After dropping to -0.503 for July/August, and then to -0.772 for August/September, the September/October value of the Multivariate ENSO Index (MEI) has dropped a bit more to -.968 sigma. This is well within La Nina territory, but almost one sigma less than last year's September/October value. Plots of the Global Wind Oscillation (GWO) for July 1 to November 19, 2011 and the same period last year illustrate the year-to-year difference in the atmosphere's response to La Nina conditions. So far, this year's response is consistent with a more active MJO and a shift toward a more neutral ENSO state.

While there's still a chance of rain in Southern California Thanksgiving Day, the forecast is looking better than it did a few days ago when rain was considered likely. The difficulty in the forecast is yet another upper level trough splitting into a cut-off upper level low. The 12z NAM forecasts the low to remain offshore on Thanksgiving Day, and move into Northern Baja by the late afternoon. The 09z SREF puts the probability of more than 0.01 inch of rain in coastal Southern California for the 12 hours ending 4:00 pm Thursday at 10% - 30% and the chance of more than 0.10 inch of rain at around 10%. A shift in the track of the low toward the coast would significantly increase the chance of rain. We'll see!

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

An unseasonably deep upper level low, unusually strong 170+ kt Pacific jet, and associated cold front combined to produce record-setting rainfall in Southern California Wednesday, October 5. Rainfall totals exceeded 1.0 inch in many areas.

Downtown Los Angeles (USC) recorded 1.15 inches, setting a new record for the date, and kick-starting the area's rainy season to 0.78 inch above normal. Rainfall records for the date were also set for LAX, Long Beach, Camarillo, Santa Barbara and several other locations. It was the first measurable rainfall at Los Angeles since June 17. Here are some preliminary precipitation totals from the NWS Los Angeles/Oxnard,NWS San Joaquin Valley/Hanford and NWS San Diego.

Enhanced convection in the Western Pacific associated with active phase of the MJO, and an extension of the North Pacific Jet caused by a positive East Asian mountain torque event appeared to have contributed to the unseasonable amount of rainfall. As observed by Ed Berry (Atmospheric Insights, Dec. 30, 2007), "...the MJO itself does not generate enough extratropical westerly wind flow to allow the East Asian jet to impact the USA west coast. A strong positive East Asian mountain torque needs to be involved, on average."

The increase in relative AAM shown by the GWO not withstanding, La Nina conditions appear to be consolidating in the equatorial Pacific. This four month sequence of Pacific Ocean Equatorial Temperature anomaly cross sections shows substantial subsurface cooling from July 11 to September 11. The Multivariate ENSO Index (MEI) has dropped from -0.5 for July/August to -0.8 for the August/September season. This drops the MEI from a rank of 16th last month to 13th this month, just above the quintile definition of a moderate La Niña ranking. Last year the MEI for August/September ranked 1st in the record since 1950. This TAO time-longitude plot of SST and SST anomaly shows less cooling than last year on this date.

To get an idea of what the impact of a second year La Nina might be on 2011-12 Winter precipitation in the continental U.S., the ESRL/PSD US Climate Division Dataset Mapping Page was used to construct a map of US composite precipitation anomalies for November to March for year two La Ninas since 1949. The years included were based primarily on MEI rankings, and include 1950-51, 1955-56, 1962-63, 1971-72, 1974-75, 1999-2000 and 2008-09. The base period was 1971-2000.

For this selection of years the coastal Southern California climate division recorded about 5 to 6 inches less precipitation than normal for the period November through March. The percent of normal water year rainfall recorded at Downtown Los Angeles (USC) ranged from a low of 47% (1971, 7.17"), to a high of 106% (1955, 16.00"). The average rainfall at Los Angeles for these years was 70.5% of normal, or 10.7".

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

A dynamic cutoff upper low centered over Southern California produced numerous thunderstorms in Southern California and a broad area of the southwestern U.S. Saturday. The thunderstorms produced thousands of lightning strikes, some heavy downpours, hail and strong winds.

According to an article on Yuba.net over 15,000 lightning strikes were recorded in California over the 24 hour period ending 0600 PDT Sunday. This plot of lightning detections was produced by Tahoe National Forest. (Typically lightning detections include both cloud-cloud and cloud-ground strokes.)

Thunderstorms were widespread in Southern California and several strong thunderstorms were reported by the NWS. This NWS NEXRAD image from about noon on Saturday shows several strong cells including one area flagged as severe. This composite image from WSI/Intellicast.com at 2:45 pm PDT shows estimated cloud tops, cell movement and hail. Half inch hail was reported in several locations by NWS spotters and there was one report of 1.25 inch hail near Lake Los Angeles. A wide swath of enhanced thunderstorm activity developed in the afternoon to the north and northeast of Los Angeles.

Some of the higher rainfall totals reported by the NWS include Lake Palmdale at 0.98 inch; Palmdale 0.84 inch and Lockwood Valley 0.58 inch. Downtown Los Angeles recorded a trace. Here's a NWS Los Angeles/Oxnard tabulation of some rainfall totals from around the area. Here also is a NWS San Joaquin Valley/Hanford tabulation of some rainfall totals in the southern San Joaquin Valley and Sierra.

In a Southern California Weathernotes post in February I commented that the tough question regarding La Niña was whether warming would continue into the NH summer, with neutral ENSO conditions prevailing as we move into autumn; or if the ENSO cycle would swing back toward a La Nina state. It was noted that since 1949, about half of the first year La Nina episodes have continued into a second or third year.

Climate scientist and Multivariate ENSO Index (MEI) codeveloper Klaus Wolter first mentioned the roughly 50/50 chance that La Nina would return following a summer respite in September 2010. A glance at a plot of the extended MEI index, going back to 1871 clearly shows a propensity for a return of La Nina in the year (or years) following stronger La Nina episodes.

In a September 8, 2011 press release NOAA reported that La Nina is back. More precisely, in their September 8 ENSO Diagnostic Discussion CPC states "La Niña conditions have returned and are expected to gradually strengthen and continue into the Northern Hemisphere winter 2011-12." The Multivariate ENSO Index (MEI) has dropped below the weak La Nina threshold, and several oceanic and atmospheric factors are indicative of a reemerging La Nina. There's still a (historically) slight chance of a return to ENSO neutral conditions this fall, but it doesn't appear likely. We'll see!

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

The wetter ensemble member forecasts prevailed in last Friday's and Saturday's rain and snow event. Downtown Los Angeles recorded 1.20 inches for the storm. This brought the water year total to 15.78 inches, which is about 104% of normal. Although this might seem unusual in a La Nina influenced rain season, during two of the strongest La Ninas in the past 60 years -- 1955-56 and 1973-74 -- Los Angeles recorded 99% and 106% of normal rainfall. Here's an archived copy of a NWS tabulation of some rainfall totals from around the area from early Friday, February 25, to 10:00 p.m. Saturday, February 26.

The NAM and GFS, as well as the SREF and eWRF ensembles, had a tough time forecasting the rainfall produced by our current scenario -- a relatively shallow, moist westerly flow accompanied by modest jet stream dynamics. As this BUFKIT time-height cross section shows, the wettest eWRF ensemble member depicted a 125 kt jet max overhead, suggesting the models had some difficulty in forecasting the structure and strength of the jet. At best, the models forecast a few hundredths of an inch of rain in the Los Angeles area, but the most recent NWS tabulation of rainfall totals for the event lists many stations with between 0.1 and 0.25 inch. As of 4:00 a.m. this morning, Downtown Los Angeles has recorded 0.16 inches for the storm, bumping the water year total to 105% of normal.

As a result of the westerly flow, snow levels are currently high, but one of the interesting facets of last Friday's storm was the possibility of very low elevation snow. A NWS Winter Weather Advisory issued February 25 for the Santa Monica Mountains Recreation Area forecast the snow level to drop overnight from above 3000' to between 1000' and 1500'. Snow accumulations from 1 to 3 inches were expected. Some low elevation post frontal showers of graupel or snow were reported in the east San Fernando Valley and La Crescenta Saturday afternoon, but based on the conditions on Sandstone Peak (el. 3111'), the snow anticipated in the SMMRA did not occur. It was an especially challenging forecast, pitting pre-frontal warm air advection, against post-frontal cold air advection, falling humidity, and fluctuating regions of snow growth and omega.

Developed in part to help forecasters deal with lake effect snow, BUFKIT includes a number of special features for snow forecasting and visualization. For example, following are two time-height cross sections generated by BUFKIT from NAM data for VNY (Van Nuys) from 4:00 p.m. Friday, February 25. The elevation for the Bourgoiun precipitation type analysis for each cross section has been raised to 2711 feet MSL. The first BUFKIT cross section shows snow growth, omega, temperature, and the projected precipitation amount and type. The second cross section shows the same parameters, but with relative humidity instead of omega. The purple and yellow rectilinear contours are snow growth, and the red and blue contours are omega. The green bars are rain amounts, and the blue bars snow. Some of the things to note in these cross sections are the omega/snow growth "cross hair," freezing level, and rapid decrease in humidity with the frontal passage.

What the discrete NAM forecast can't show is the variability of the forecast situation. In its most recent release BUFKIT added support for SREF and eWRF ensembles. When combined with BUFKIT's precipitation type and other tools, ensemble forecasts can provide much additional information and insight. In this case eWRF ensemble members for VNY exhibited widely varying thermal profiles, regions of snow growth, omega, and humidity. BUFKIT's Bourgoiun precipitation type analysis for the eWRF ensemble members at 8:00 a.m. Saturday morning showed freezing levels as high as 2882' and as low as 1176'. The pattern of post-frontal precipitation was also quite variable.

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

The transition from a persistent West Coast ridge to a West Coast trough occurred on Monday more or less as forecast by medium range guidance back on February 5. A series of systems and impulses riding up and over a high amplitude Eastern Pacific ridge are expected to continue to produce cool, blustery weather in Southern California with periods of showers and rain into next week.

Precipitation on Valentine's Day was generally limited to areas north of the Los Angeles basin, but was a little more widespread late Monday night into Tuesday morning, with a number of stations in the Los Angeles area recording a few hundredths of an inch of rain.

Wednesday's system was much more energetic. Because of the convective nature of the precipitation, rainfall amounts were extremely variable, ranging 0.25 inch or less to 1.0 inch or more. Downtown Los Angeles (USC) recorded 0.65 inch of rain for the storm, bringing the water year total for Los Angeles to 13.14 inches. This is well above normal for the date, and about 87% of normal for the water year. Here's an archived copy of a NWS tabulation of some rainfall totals for the period 4:00 p.m. Tuesday to 4:00 a.m. Thursday.

Los Angeles only needs about 2 more inches of rain to top the 100% mark for the water year, and it looks like we could get most of that Friday and Saturday. BUFKIT analysis of 12z NAM data for Van Nuys indicates about 1.6 inches of rain Friday into Saturday, and then another 0.5 inch Saturday night. The HPC 3-day Quantitative Precipitation Forecast for the period ending 4:00 a.m. Sunday also indicates about 2.0 inches of rain in Southern California. This morning's 09z SREF puts the probability of more than 1.0 inch rain for the 24 hour period ending 1:00 p.m. Saturday at about 70%.

Temperature data at the surface and at depth in the equatorial Pacific suggest the La Nina is moderating. The tough question is whether warming will continue into the NH summer, with neutral ENSO conditions prevailing as we move into autumn; or if the ENSO cycle will swing back toward a La Nina state. Since 1949, about half of the first year La Nina episodes continued into a second or third year. At the moment the majority of computer models favor a transition to ENSO neutral or warm conditions by July 2011. Here is a compilation of ENSO Forecasts from the IRI ENSO Quick Look for February 17, 2011. We'll see!

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

Over the next two weeks, the MJO signal currently propagating across the western Pacific is forecast to destructively interfere with the La Nina base state, suppressing convection in the Indian Ocean and Indonesia and enhancing convection near the dateline. This is forecast to result in a transition from upper tropospheric anomalous cyclones to anticyclones near the dateline, and an extension of the East Asian/Pacific jet. These circulation changes may eventually lead to a retrogression of our persistent West Coast ridge, or perhaps a breakthrough of the westerlies underneath the highly amplified ridge.

The key word here is eventually. Over the next week or so, the extending Pacific jet is forecast to reamplify the ridge over the West Coast. But at some point, perhaps near the beginning of February, destructive interference in the Western Pacific will dwindle and convection in the Indian Ocean will strengthen. This should result in retraction of the Pacific jet, and possibly, a West Coast trough. The 240 hr forecast of the 12z ECMWF hinted at this possibility, as did the 18z GFS. We'll see!

After starting out cold and wet, the weather this January in Southern California, and much of the state, has been warm and dry. Monday, Pierce College in Woodland Hills hit a high of 88°F, and Downtown Los Angeles (USC) 84°F. So far this January, only 0.58 of rain has been recorded at Downtown Los Angeles (USC), which is a little less than one-third of normal for the month. However, because of our prodigious December rainfall the water year total at Los Angeles is currently 6.5 inches above the normal for the date of 5.78 inches.

Earlier this month I photographed these mid-level mammatus clouds over the western San Fernando Valley. They were produced by the moisture and dynamics associated with a closed upper level low that was about 390 miles WSW of Los Angeles. To see how the clouds evolved and more about the scenario at the time, see this post on PhotographyontheRun.com.

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

If current precipitation forecasts and outlooks validate, December 2010 could become one of the wettest on record in California, and at the higher elevations of the Sierra, one of the most snowy. This morning's 5-day precipitation forecast from the Hydrometeorological Prediction Center, for the 5-day period ending early Wednesday morning, indicates up to 7.9 inches of precipitation locally, and 14.7 inches (water equivalent) in the Sierra. And additional precipitation is forecast beyond the 5 day period.

A large Pacific low working in concert with a low west of Hawaii has established a tropical connection, and is pumping copious amounts of moisture into California. This tropical moisture conveyor belt scenario is currently forecast to continue into next week. Late Sunday, shortwave impulses dig down into the low and elongate it southwestward, creating a large longwave trough. This northern part of this trough is forecast to evolve into another large upper and surface low in the northeastern Pacific. The southern part of the trough is forecast to develop a surface low and front that moves into Southern California Tuesday into Wednesday.

In the short term, BUFKIT analysis of WRF ensemble data for Van Nuys shows precipitation totals through early Sunday morning ranging from about 1.0 inch up to about 3.5 inches. Analysis of 12z NAM data for Van Nuys produces around 1.8 inches through early Sunday morning and nearly 5 inches through early Monday morning. The 09z SREF pegs the probability of more than 1.0 inch of precipitation in Los Angeles area at around 70%. Precipitable water values of over 1.0 inch, high relative humidity, and a strong south to southwesterly inflow of over 30 knots should produce significantly enhanced precipitation on favored slopes -- generally those with a south to southwest aspect.

It's a little early to put much credence in forecasts for next week, other than more rain appears likely. The 06z GFS produced astronomic precipitation totals mid-week -- several inches more than either yesterday afternoon's 00z run (2.75 inches), or this morning's 12z run (4.6 inches). We'll see!

As of yesterday Downtown Los Angeles (USC) rainfall was 0.91 inch below normal for the water year, and much of Southern California was falling behind in the rainfall department. It looks like by next week we'll be ahead of the La Nina game again, perhaps way ahead.

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

October was an unusually wet month in coastal Southern California. Many areas recorded more than an inch above normal rainfall, and some as much as 3 inches above normal. In a month where around 0.5 inch is considered normal, some areas received several times the normal amount of rainfall.

Most of October's rain resulted from two energetic upper lows, and a relatively strong trough/front that swept through the area October 30th. The front produced a new precipitation record for the date of 1.09 inches at Santa Barbara Airport Here's an archived NWS Public Information Statement with some rainfall totals for that storm.

Last week, an approaching Pacific trough pumped up a thick high pressure ridge over California, sending temperatures soaring and breaking temperature records across the area. November 3rd Long Beach Airport set a new high temperature record for the date of 100°F, and Downtown Los Angeles set a new record of 97°F. November 4th temps were once again in high 90s and several stations broke or tied high temperature records. Here are archived copies of the NWS Record Event reports for November 3 and November 4.

The southern part of the Pacific trough that produced our record high temps cut-off into a small upper low as it approached the coast, and moved onshore north of the Los Angeles basin on Saturday. The remainder of the trough and its associated front moved through the area Sunday evening into Monday. The front held together a little better than expected, and rainfall totals in the Los Angeles basin and valleys generally ranged from about 0.10 inch to 0.30 inch. Here's an archived NWS Public Information Statement with some rainfall totals from around the area..

The latest round of rainfall leaves many areas of Southern California way ahead on rainfall totals for the water year. Downtown Los Angeles (USC) missed out on a couple of the systems, and as of close of business yesterday has recorded 1.10 inch for the water year, which is only 0.06 inch above normal. By comparison, Los Angeles Airport is 0.85 inch above normal, and Camarillo/Oxnard is 0.97 inch above normal. This headstart on the rain season should help keep our rainfall totals a little closer to normal as our La Nina influenced rain season continues.

Speaking of La Nina, the September/October value of the Multivariate ENSO Index (MEI) increased slightly by 0.079 sigma, ending the record plunge from El Nino conditions earlier this year. In the past 30 days there was some decrease in the magnitude of negative SST anomalies in the east-central equatorial Pacific, but subsurface temperatures remain quite cool. Computer models are split on whether Nino 3.4 SST anomalies will continue to decline in magnitude during the Northern Hemisphere winter, but nearly all models indicate decreasing anomalies in early 2011. We'll see!

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