# Sunday, 09 January 2011

Following is a chart comparing the 2010-11 La Nina to 22 other cold ENSO episodes that have occurred since 1949. With the exception of 1961-62 and 2008-09, the cold episodes are based on the Oceanic Niño Index (ONI) and are those specified in the CPC's tabulation of Cold & Warm Episodes by Season. A description of the parameters follows the chart. A GWO phase space plot is included for those cold episodes for which AAM data is available.

Year Nov-Mar
AAM
Peak MEI4 Peak MEI Season Peak
ONI
Peak ONI Season L.A. Rain GWO
Phase Plot
1949-501,2 -- -1.423 APRMAY -1.7 DJF 9.94 --
1950-511 -- -1.261 NOVDEC -1.0 NDJ, DJF 8.21 --
1954-551 -- -1.578 MAYJUN (54) -1.2 ASO 11.94 --
1955-561 -- -2.276 MAYJUN (55) -2.0 OND 16.00 --
1956-571 -- -1.516 MAYJUN (56) -0.9 SON, OND 9.54 --
1961-623 -0.511 -1.081 DECJAN -0.6 ASO, SON 18.79 Click for Nov-Mar GWO Phase Space Plot
1962-63 -1.254 -0.849 JANFEB -0.7 OND, NDJ 8.38 Click for Nov-Mar GWO Phase Space Plot
1964-65 -1.141 -1.496 JULAUG -1.2 SON, OND 13.69 Click for Nov-Mar GWO Phase Space Plot
1967-68 -0.767 -1.060 APRMAY -0.9 JFM 16.58 Click for Nov-Mar GWO Phase Space Plot
1970-71 -0.973 -1.898 MARAPR -1.3 DJF, JFM 12.32 Click for Nov-Mar GWO Phase Space Plot
1971-72 -0.172 -1.463 AUGSEP -1.0 OND 7.17 Click for Nov-Mar GWO Phase Space Plot
1973-74 -1.326 -1.937 DECJAN -2.1 NDJ 14.92 Click for Nov-Mar GWO Phase Space Plot
1974-75 -0.839 -1.256 OCTNOV -0.9 OND 14.35 Click for Nov-Mar GWO Phase Space Plot
1975-76 -0.711 -2.000 SEPOCT -1.7 OND, NDJ 7.22 Click for Nov-Mar GWO Phase Space Plot
1984-85 -0.595 -0.743 APRMAY -1.1 NDJ 12.82 Click for Nov-Mar GWO Phase Space Plot
1988-89 -1.135 -1.591 AUGSEP -1.9 OND, NDJ 8.08 Click for Nov-Mar GWO Phase Space Plot
1995-96 -0.226 -0.641 DECJAN -0.7 OND to JFM 12.46 Click for Nov-Mar GWO Phase Space Plot
1998-99 -0.539 -1.219 JANFEB -1.4 NDJ, DJF 9.09 Click for Nov-Mar GWO Phase Space Plot
1999-00 -0.778 -1.228 JANFEB -1.6 NDJ, DJF 11.57 Click for Nov-Mar GWO Phase Space Plot
2000-01 -0.795 -.747 OCTNOV -0.7 NDJ 17.94 Click for Nov-Mar GWO Phase Space Plot
2007-08 -1.007 -1.619 FEBMAR -1.4 DJF, JFM 13.53 Click for Nov-Mar GWO Phase Space Plot
2008-093 -0.594 -.783 SEPOCT -0.8 DJF 9.08 Click for Nov-Mar GWO Phase Space Plot
2010-11 -0.595 -2.037 AUGSEP -1.4 SON, OND, NDJ 20.20 Click for Nov-Mar GWO Phase Space Plot
1. AAM and AAM tendency anomaly data not available.
2. Based on ONI values beginning with DJF 1949-50.
3. ONI did not meet threshold of 5 consecutive overlapping seasons.
4. MEI values are normalized and may change as new data is added. Specified values were current as of November 8, 2011.

Nov-Mar AAM: The mean of the global relative atmospheric angular momentum anomaly for the period November 1 to March 31 of the following year. Data is from the GWO phase space data file linked on the Global Synoptic Dynamic Model page of the PSD Map Room Climate Products. Reference Weickmann and Berry, 2008.

Peak MEI: The peak seasonal value of the Multivariate ENSO Index (MEI). Reference Wolter and Timlin, 1993, 1998. MEI values are normalized and may change as new data is added.

Peak MEI Season: The peak bi-monthly season(s) for which the MEI is computed.

Peak ONI: The peak Oceanic Niño Index (ONI) based on SST anomalies in the Niño 3.4 region. Reference Climate Prediction Center Cold & Warm Episodes by Season.

Peak ONI Season: The peak tri-monthly season(s) for which the ONI is computed.

L.A. Rain: The water year precipitation total in inches for Downtown Los Angeles (USC). Reference NWS Los Angeles/Oxnard Downtown Los Angeles Climate Page, 1921-2006 Calendar Year Rainfall.

GWO Phase Space Plot: Plot of global relative atmospheric angular momentum anomaly vs. global relative atmospheric angular momentum tendency anomaly for the period November 1 to March 31 of the following year. Data is from the GWO phase space data file linked on the Global Synoptic Dynamic Model page of the PSD Map Room Climate Products. Reference Weickmann and Berry, 2008.

Sunday, 09 January 2011 15:02:14 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Saturday, 01 January 2011

Click
WRF Ensembles Projected Precipitation
Through Monday, January 3, 2011 at 4:00 a.m.

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.

Saturday, 01 January 2011 15:13:31 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Sunday, 26 December 2010

Click
WSI Composite Radar
December 25, 2010 at 8:00 p.m.

The strong cold front that swept through Southern California Christmas night resulted in 0.90 inch of rain at Downtown Los Angeles (USC), increasing December's rainfall total to 9.67 inches. This makes December 2010 the wettest December in 121 years (since 1889), and the second wettest December since recordkeeping began in 1877.

Rainfall totals from last night's fast moving front generally ranged from about 0.4 inch to 0.9 inch. Here's an archived copy of a Precipitation Summary from the NWS Los Angeles/Oxnard with some rainfall totals from around the area.

This morning's model runs continue to forecast more rain for Wednesday. The 12z NAM projects about 0.5 inch at LAX during the day Wednesday. The 09z SREF puts the probability of more than 0.25 inch of rain in the Los Angeles area at about 50% for the 24 hour period ending 4:00 p.m. Wednesday afternoon.

Depending on how much rain we get Wednesday, for the first six months of the 2010-11 water year (July 1 to December 31), the rainfall total for Downtown Los Angeles will likely rank as the fourth or fifth wettest in the 133 years that records have been kept.

In the medium range outlook, the 12z GFS and 12z ECMWF differ in how they handle an upper level low that they forecast to develop off the California coast New Year's morning. The 12z GFS is quite wet New Year's weekend, but it's too early to put much credence in that forecast. We'll see!

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

Sunday, 26 December 2010 14:01:15 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, 23 December 2010

AHPS Precipitation (Percent of Normal) For 90 Days Ending December 23, 2010 at 4:00 a.m Click
AHPS Precipitation (Percent of Normal)
For 90 Days Ending December 23, 2010 at 4:00 a.m.

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.

Thursday, 23 December 2010 12:30:24 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Friday, 17 December 2010

HPC 5-Day Precipitation Forecast For the 5 Day Period Ending December 22, 2010 at 4:00 a.m. Click
HPC 5-Day Precipitation Forecast
For the 5 Day Period Ending December 22, 2010 at 4:00 a.m.

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.

Friday, 17 December 2010 10:48:15 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Monday, 22 November 2010

Click
AHPS Precipitation (Percent of Normal)
For 60 Days Ending November 22, 2010 at 4:00 a.m.

Saturday's front wasn't as strong as expected, but still produced some resepectable rainfall totals. Sunday's front was better defined and had better dynamics, but moved through the area relatively quickly. The end result is that water year precipitation totals remain near to above normal for many Southern California locales, and about 0.15 inch below normal for Downtown Los Angeles.

Precipitation totals varied widely. In the Los Angeles basin and surrounding valleys amounts generally ranged from about 0.25 inch to 1.0 inch. Foothill and mountain locations in Los Angeles county recorded about 1.0 to 2.0 inches, but several stations recorded over 2.0 inches. Tanbark in the San Gabriel Mountains recorded 3.51 inches, and Ortega Hill in the Ventura Mountains recorded 5.0 inches! Here's an archived copy of a NWS Los Angeles/Oxnard Public Information Statement with some precipitation totals from around the area.

A drier version of last weekend's system -- with a more inland track -- is forecast to produce more rain and snow in the northern two-thirds of California through midweek, but not much precipitation is expected in Southern California.

This morning's 09z SREF pegged the chance of more than 0.1 inch in coastal Southern California for the 24 hours ending 4:00 a.m. Wednesday at between 30% and 50%, and the chance of more than 0.01 inch at around 90%. The 00z 11/23/10 NAM forecast is slightly wetter south of Pt. Conception than the 18z NAM, but is still forecasting under 0.1 inch in most areas.

It was cold in Southern California last night with lows dipping into the 30's in many valley locations. Malibu Canyon recorded a low of 30°F. Cold overnight temperatures are exepected to continue through Thanksgiving.

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

Monday, 22 November 2010 20:12:02 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Friday, 19 November 2010

Click
HPC Forecast Precipitation
For the 2-Day Period Ending 4:00 p.m. Sunday.

Thanks to October's unusual amount of rainfall, precipitation totals for many areas of Southern California remain near or above normal for the 2010-11 water year. But November has been relatively dry, and Downtown Los Angeles (USC) is now about 0.30 inch below normal for the month, and for the water year. That's expected to change over the weekend.

A series of impulses on the downstream side of a high amplitude upper ridge is forecast to amplify a trough and generate an energetic upper low along the coast of the western U.S. tonight.

The 00z NAM/WRF shows a strong front moving down the coast and through Southern California on Saturday, a second impulse on Sunday, and possibly a third on Monday.

BUFKIT analysis of this evening's 00z NAM data for Van Nuys project rainfall totals of about an inch over the 24 hr. period ending Saturday at 4:00 p.m., and then another third of an inch or so Sunday. The 00z GFS isn't as generous with the rainfall amounts, generating about 0.5 inch on Saturday and 0.3 inch on Sunday, but appears too fast when compared to current satellite and radar images.

SREF means & spreads suggest amounts across the area should generally range from about 0.5 inch to 1.25 inches, with more expected in the mountains. We'll see!

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

Friday, 19 November 2010 21:58:38 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Tuesday, 09 November 2010

AHPS Precipitation (Percent of Normal) for 30 Days Ending November 1. Click
AHPS Precipitation (Percent of Normal)
For 30 Days Ending November 1, 2010.

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.

Tuesday, 09 November 2010 10:21:12 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Saturday, 23 October 2010

RAMDIS GOES-11 Water Vapor Image October 19, 2010 at 2:30 p.m. Click
RAMDIS GOES-11 Water Vapor Image
October 19, 2010 at 2:30 p.m.

Sunday evening through Wednesday afternoon, another energetic cutoff upper level low spun showers and thunderstorms into Southern California, increasing water year rainfall totals for many stations to near or above normal. Precipitation amounts varied widely, with some stations recording a few hundredths of an inch or less, and some as much as an inch or more. As was the case with the upper low at the end of September, this system produced a number of strong thunderstorms. A severe thunderstorm warning was issued as a result of a cluster of strong thunderstorms in the Antelope Valley Tuesday evening.

Downtown Los Angeles (USC) was one of those stations that didn't have its rain bucket in the right place. The station recorded only 0.08 inch from Sunday evening to Wednesday afternoon, bringing its water year rainfall total to 0.51 inch, which is about 0.16 below normal. Here is an archived Public Information Statement (PDF) from the NWS with some rainfall totals from around the area.

The Aug/Sep value of the Multivariate ENSO Index (MEI), released October 6, dropped another 0.18 sigma to -1.99. This is the lowest value of the MEI since Jul/Aug of 1955, and ranks the current La Nina as the strongest in the MEI record for the Aug/Sep season. Equatorial SST have continued to cool, and this plot of 5-day TAO/TRITON SST shows anomalies of -2.0°C or greater extending west to nearly the dateline.

Cooling of SSTs in the Central Pacific appears to have been enhanced by strong 850mb easterly winds associated with an MJO which propagated into the Western Pacific over the past 2 weeks and slowly diminished in amplitude. Divergence aloft associated with the MJO appears to have enhanced typhoon and tropical storm development in the Indian Ocean and Western Pacific. CIRA TPW loops show a lot of tropical moisture being injected into an extending Pacific jet. This could amplify rainfall amounts along the west coast of the U.S. the next few days.

There's been some drizzle and light rain in the Los Angeles area this morning. BUFKIT analysis of 12z NAM data for Van Nuys and LAX generates about 0.1 inch of rain late Sunday afternoon and evening. The bulk of the precipitation is forecast to occur in the northern two-thirds of California and into the Pacific Northwest. We'll see!

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

 

Saturday, 23 October 2010 10:26:24 (Pacific Daylight Time, UTC-07:00)  #    Disclaimer  |   | 
# Thursday, 07 October 2010

AHPS Precipitation Totals For the 7-day Period Ending 10/07/10 12z Click
AHPS Precipitation Totals
For the 7-day Period Ending 10/07/10 12z

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

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

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

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

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

Thursday, 07 October 2010 12:10:13 (Pacific Daylight Time, UTC-07:00)  #    Disclaimer  |   | 
# Thursday, 30 September 2010


Composite Precipitation for Eight Moderate to Strong Transitional La Ninas
(Mouse Over Image is Composite Precipitation for 16 La Ninas)

Updated October 6, 2010. Replaced 1975 with 2007 in composite of climate division precipitation for eight transitional moderate to strong La Ninas, and other revisions.

Monday at 12:15 p.m. PDT the temperature at Downtown Los Angeles (USC) reached 113°F (45°C). This is the highest temperature recorded downtown since weather recordkeeping began in 1877. All-time and daily records were broken at several location. Here's an archived NWS Public Information Statement with a list of the records that were broken.

A moist south to southeasterly flow associated with a high over the Great Basin and an upper low off the coast resulted in some thundershowers (and rainbows) in the Los Angeles area yesterday, and there's a chance of more thundershowers today. According to the NWS zone forecasts, a chance of thunderstorms will continue in the mountains through Saturday evening. (See update below.)

Since the peak of last Winter's El Nino, there has been an unprecedented drop in the Multivariate ENSO Index (MEI). MEI bimonthly (e.g. Sep/Oct) seasonal ranks range from 1 to the total number of years for the season in the record, currently 60 or 61. A rank of 1 represents the strongest La Nina for the season, and a rank of 60 or 61 the strongest El Nino. Based on MEI seasonal ranks, in six months ENSO conditions switched from the fifth strongest El Nino for the Feb/Mar season to the second strongest La Nina for the Jul/Aug season. As Klaus Wolter pointed out in his September 3rd MEI discussion, the only stronger La Nina in the MEI record for the Jul/Aug season was in 1955, and the last time the MEI was lower in any season was 1975.

In all 11 cases since 1950 in which the MEI was -1.0 or less in the Jul/Aug season, La Nina conditions persisted through the Nov/Dec season. In all but two cases, 1950 and 1964, the MEI remained below -1.0 from the Jul/Aug season through the Nov/Dec season. Most ENSO assessments project that La Nina conditions will persist through at least the end of 2010.

As they say, past performance does not guarantee future results, but in the cases of well established La Ninas and El Ninos, historical composites can provide some insight into what is likely. CPC's ENSO Temperature & Precipitation Composites page now includes composites which incorporate recent precipitation trends. Frequency of occurrence maps are also included for each composite. These composites indicate coastal Southern California has been trending drier in the rain season during the past 15 years. The composite plus trend plot for Jan-Feb-Mar precipitation anomaly is particularly dry in coastal Southern California, indicating negative anomalies in excess of 3 inches, with a high frequency of occurrence.

To get an idea of how US precipitation anomalies might differ in a strong La Nina versus the more general La Nina case, the ESRL/PSD US Climate Division Dataset Mapping Page was used to construct maps of US composite precipitation anomalies. The two cases can be compared by moving the mouse cursor over the image. The initial image is a composite based on eight moderate to strong La Nina years which were preceded by El Nino or neutral conditions. The "mouse over" image is a composite of 16 cases* in which the MEI was in the lower 30%-tile of MEI ranks.

Another consideration in assessing potential ENSO impacts is the likelihood that a La Nina (or El Nino) will result in either extremely dry or extremely wet conditions. Here are maps showing the risk of precipitation extremes for the periods Nov-Dec-Jan, Dec-Jan-Feb and Jan-Feb-Mar during La Nina. These were generated using the ESRL/PSD page Risk of Seasonal Climate Extremes in the U.S. Related to ENSO. According to these maps, Southern California isn't likely to have an extremely dry 2010-2011 rain season, although CPC ENSO composites suggest there is at least some possibility of a dry rain season, particularly in the period Jan-Feb-Mar.

CPC's current operational precipitation outlook for Nov-Dec-Jan, issued September 16, 2010, shows roughly equal probabilities of above average, near average, or below average precipitation in the Coastal Southern California climate division. As the rain season progresses, these probabilities become more skewed toward below normal precipitation. In the period Jan-Feb-Mar the probability of below normal precipitation is calculated to be about 40%, while the chances of above normal drop to around 25%. The skew become slightly more pronounced in the period Feb-Mar-Apr.

All of this suggests below average precipitation in the Coastal Southern California climate division for the period November 2010 through March 2011. How much below average? Perhaps on the order of 3 to 6 inches. Over the 16 years used in the La Nina climate division composite, rainfall at Downtown Los Angeles (USC) has ranged from 7.17" (1971) to 16.00" (1955), with an average value of 10.95". This is about 72% of the normal 15.14". If only the lowest 8 MEI ranked (strongest La Nina) years are included, the average rainfall for Los Angeles works out to 11.08", or about 73% of normal.

Although it appears unlikely, average to above average seasonal rainfall could occur. Also, damaging storms with heavy or prolonged precipitation are still a possibility.

*The 16 years in the La Nina composite were taken from the 19 lowest MEI ranks for Sep/Oct. This includes 1950, 1954, 1955, 1956, 1961, 1962, 1964, 1967, 1970, 1971, 1973, 1974, 1975, 1988, 1995, 1998, 1999, 2007 and 2008. The marginal La Nina years of 1961, 1967 and 1995 were excluded in the 16 year set. An Oceanic Niño Index (ONI) based selection of cold episodes for SON would include 1984 and 2000, and exclude 1961, 1967 and 2008.

Update October 1, 2010. Intellicast.com composite radar shows some strong thunderstorms in Southern California this afternoon, with hail reported, and tops indicated at 35,000 to 40,000 ft. The moist subtropical flow is forecast to continue through Sunday, and along with it, a chance of showers and thunderstorms.

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

Thursday, 30 September 2010 14:33:23 (Pacific Daylight Time, UTC-07:00)  #    Disclaimer  |   | 
# Wednesday, 25 August 2010

BUFKIT GFS Temperature Forecast August 24, 2010 12z Click
GFS Temperature Forecast for Van Nuys
August 24, 2010 12z

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

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

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

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

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

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

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

 

Wednesday, 25 August 2010 15:38:44 (Pacific Daylight Time, UTC-07:00)  #    Disclaimer  |   |