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.

Overall it's been a wet water year in Southern California, but in the fashion of a Mediterranean climate the wet water year has been comprised of a patchwork of wet and dry months.

If it were not for an unusually stormy December in which Downtown Los Angeles recorded 8.32 inches more rain than normal, Los Angeles would be looking at about 76% of normal rainfall this water year instead of 131%. This reduced amount would be more typical of the rainfall recorded in Los Angeles during a La Nina influenced water year.

March was wet, but April and May have been dry. Sunday morning's wet weather added 0.07 inch to May's total for Los Angeles, but the combined April/May rainfall deficit is still about 0.9 inch.

BUFKIT analysis of WRF ensemble data for Van Nuys and LAX suggests tomorrow's storm is likely to produce between 0.1 inch and 0.3 inch of rain in the Los Angeles basin, and perhaps up to 0.4 inch in the San Fernando Valley. SREF Ensemble probabilities and HPC Probabilistic Precipitation Guidance support this estimate.

It won't take a lot of rain to break the record for May 17 at Downtown Los Angeles -- 0.18 inch would do it -- and it would also raise the water year rainfall total for Los Angeles to over 20 inches. The previous precipitation record for May 17 was 0.17", set in 1883. Note: Record for USC Campus (KCQT) is 0.06, set in 1949, but data from Civic Center is used for Downtown Los Angeles records prior to 1921.

There's a chance of more rain Wednesday, and then perhaps again Sunday into Monday. We'll see!

Update Tuesday, May 17, 2011. Rainfall amounts from the storm that affected Los Angeles last night and this morning were generally within the range projected by the WRF ensembles. Downtown Los Angeles (USC) recorded 0.15 inch for the storm, bring the water year total to 19.99 inches. Here's a report compiled by the NWS with some rainfall totals from around the area. Tonight's system is forecast to produce similar rainfall totals. At the moment some drizzle or light rain looks like a possibility in the Los Angeles area Sunday and early Monday.

Update Wednesday, May 18, 2011. Thanks to the system that moved into the area last night, Downtown Los Angeles (USC) recorded additional rainfall Tuesday evening, nudging the rainfall total for May 17 up to 0.18 inch, and breaking the old rainfall record for the date set in 1883. Shower activity appears to be winding down and partly cloudy skies are forecast for Los Angeles this afternoon. Rainfall totals from the second system were generally similar to Tuesday's amounts. Here's a report compiled by the NWS Los Angeles/Oxnard that lists the cumulative rainfall that has resulted from these two storms for a number of stations around the area. The water year total for Downtown Los Angeles (USC) now stands at 20.19 inches.

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

Running in the rain and wind, and splashing through innumerable puddles, Markos Geneti set a new Los Angeles Marathon record in a time of 2:06:35. It was a remarkable performance, in adverse conditions. From 8:00 a.m. to 12:00 p.m. the Mt. Washington RAWS, near the start of the marathon, recorded 0.59 inches of rain; and the Beverly Hills RAWS near mile 17, recorded 1.27 inches. Periods of heavy rain were reported at Santa Monica Airport, near the finish line, through much of the race. According to a report in the Los Angeles Times sports blog, The Fabulous Forum, thousands of runners were evaluated for hypothermia and 25 runners were hospitalized.

Many rainfall records were set around the area Sunday, with some locations recording as much as one-third of a year's normal rainfall in 24 hours! In the early evening rainfall rates of over an inch an hour were recorded in Woodland Hills and Canoga Park. According to a NWS Storm Report, at 6:14 p.m. a flash flood was reported in Woodland Hills with "mud and debris flowing down the street" and "at least four to five vehicles stuck in flowing water."

Downtown Los Angeles (USC) recorded 2.42 inches of rain yesterday, breaking a record set in 1943. Santa Barbara Airport had its wettest day on record, recording 5.23 inches of rain. Here's an archived copy of the NWS Record Report for March 20, listing some of the rainfall records for the day.

Storm totals in excess of 3 inches have been common in Los Angeles, Ventura, Santa Barbara, and San Luis Obispo counties. Higher totals were recorded in many areas. As of 5:00 p.m. the NWS reported that Van Nuys had recorded 6.74 inches of rainfall, Northridge 6.08 inches, Newhall 7.20 inches, Santa Barbara Airport 6.27 inches, San Marcos Pass 10.72 inches, and Gibraltar Dam 11.73 inches. Here's an archived copy of a NWS report with some rainfall and snowfall amounts from around the area.

Downtown Los Angeles made up its rainfall deficit for the month of March and more, increasing its water year rainfall total to 18.55 inches, or about 123% of normal. This makes the 2010-2011 water year the wettest in Los Angeles during a La Nina over the last 60 years, surpassing the totals recorded during the strong La Ninas of 1955-56 (99% of normal) and 1973-74 (106% of normal), and weak La Ninas of 1967-68 (110%) and 2000-01 (118%).

Sunday's heavy rain appears to have had the earmarks of an atmospheric river event. NRL SSM/I IPW imagery (lower right) shows a pre-frontal band of IPW values of 35-40 kg/m2 impinging the coast, NEDIS blended TPW imagery showed a band of ~25 mm TPW extending some 4000 km to the longitude of Hawaii. A BUFKIT cross section based on 12z NAM data for VNY depicted a low level jet (LLJ) structure coinciding with the period of heaviest rainfall. Research by Dr. F. Martin Ralph, et. al., observed that in atmospheric rivers over the eastern Pacific, 75% of the water vapor transport below 500 mb takes place within the lowest 2.25 km and occurs with LLJ wind structure. In addition in cases where a LLJ is present, there was 50% greater precipitation efficiency.

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.

Up until a couple of days ago it looked like there might be some damp weather for the Rose Parade and even a better chance for the Rose Bowl game. An upper level low was the culprit, and they are notoriously difficult to model and forecast. The low is out there, it's just further to the north, and so is the rain.

How likely is rain on New Year's Day in Pasadena? Here's a plot generated on the Western Regional Climate Center web site, that shows the probability of various amounts of rain in a 1-day period at Pasadena. The probability of measurable rain (0.01 inch) in a 24 hour period around January 1 is about 20%, or about 1 in 5. As the rainfall amounts increase, the probability drops. The chance of 1.0 inch of rain is about 4%, or 1 in 25. Keep in mind that the probabilities indicated are for a 24 hour period. The chance of rain for the relatively short two hour duration of the Rose Parade is quite a bit less -- as history demonstrates.

Wednesday's cold front, the latest in our series of surprisingly frequent December weather systems, upped the water year rainfall total at Downtown Los Angeles (USC) to 11.70 inches. As of December 31, L.A.'s rainfall total is about 8 inches above normal, and more than three times the normal amount of rainfall for the water year to date. For the first six months of the 2010-11 water year, the rainfall total for Downtown Los Angeles is the fourth wettest in the 133 years that records have been kept. The water year starts on July 1 and ends June 30.

And we're not done yet. In a normal year, Downtown Los Angeles records about two-thirds of its annual rainfall in the months of January, February and March. This amounts to a little over 10 inches of rain. Depending on how you look at the data, rainfall for Downtown Los Angeles during a La Nina episode is usually about 70%-80% of normal. In addition, the composite plus trend plot for Jan-Feb-Mar precipitation anomaly (from CPC) is particularly dry in coastal Southern California, indicating negative anomalies in excess of 3 inches, with a high frequency of occurrence. Even so, it looks like Los Angeles has a good chance of exceeding the normal amount of annual rainfall of 15.14 inches. All we need before June 30 is another 3.45 inches!

The upper level low postponed on account of the Rose Parade, but due in tomorrow, should help with that. BUFKIT analysis of this morning's WRF ensembles projects anywhere from 0.6 inch to 0.9 inch of rain at Van Nuys through early Monday morning. The 18z NAM isn't as wet, projecting about 0.4 inch. The models are still having difficulties with the forecast, and slight changes in the position and behavior of the upper low could have a significant impact on the amount of rainfall. We'll see!

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

After producing phenomenal rainfall totals in Southern California and snowfall totals in the higher elevations of the Sierra, the Pacific low and trough responsible for days and days of wet weather has finally moved east.

Some stations recorded more rain in the last 7 days that would normally be recorded over an entire year. Precipitation amounts over 10 inches were common, and several stations recorded over 20 inches. Tanbark, in the San Gabriel Mountains recorded 24.7 inches, NF Matilija in the Ventura Mountains recorded 24.09 inches, and Lytle Creek in the San Bernardino Mountains recorded an astonishing 26.35 inches!

Here's are snapshots of provisional 7-day rainfall totals in Ventura County and 7-day rainfall totals in Los Angeles County from the Ventura County Watershed Protection District ALERT Map Viewer; and a snapshot of provisional 7-day rainfall totals from the NWS San Diego's Experimental Rainfall Summary Display. And here are archived copies of NWS precipitation summaries with preliminary rainfall totals from the NWS Los Angeles/Oxnard and NWS San Diego offices.

The remarkable amount of rainfall makes December 2010 one of the wettest on record in Southern California. According to preliminary NWS data Downtown Los Angeles (USC) has recorded 8.52 inches of rain so far this December. This ranks it as the third wettest December in Los Angeles since recordkeeping began in 1877, exceeded only by December 1889 (15.8 inches) and December 2004 (8.77 inches). With additional rain a possibility, December 2010 only needs 0.25 inch to surpass 2004. For more detailed info see "A Look at the Record-Breaking Week of Rainfall" a PDF from the NWS Los Angeles/Oxnard.

The Sierra recorded gargantuan snowfall totals, with some of the largest being in the Southern Sierra. In the last week the snow sensor at Farewell Gap recorded 22.6 inches snow water equivalent, which roughly translates to 18 feet of snow. This morning California Cooperative Snow Surveys reports the Southern Section Sierra snowpack at 287% of normal, and the snowpack overall in the Sierra at 207%! Mammoth Mountain reports 167 inches of snow so far this December, a new record for the period 1968 to date, obliterating the records set in 1971 (139.8 inches) and 2002 (134.4 inches).

As of yesterday Downtown Los Angeles (USC) rainfall was a whopping 6.88 inches above normal for the water year. That puts us way, way ahead of what is typical for a La Nina influenced rain season -- at least for a couple of months.

Update December 24, 2010. Looks like there will be two opportunities before the end of December for Downtown Los Angeles (USC) to increase its rainfall total for the month and become the second wettest December since recordkeeping began in 1877. The first chance is Saturday night. For the 24 hour period ending 10:00 a.m. Sunday, the 09z SREF puts the probability of more than 0.25 inch of precipitation in the Los Angeles area at about 70%. The WRF ensembles precipitation forecast for LAX for Saturday night ranges from a low of about 0.25 inch to a high of about 0.7 inch. The 18z GFS says the second opportunity will be Wednesday, and forecasts a little under an inch of rain at LAX. We'll see!

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.