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# Friday, 07 November 2014

SST NINO Region Anomalies from the CPC ENSO Diagnostic Discussion 11/6/2014. Click
SST NINO Region Anomalies
From the CPC ENSO Diagnostic Discussion 11/6/2014.

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.


Friday, 07 November 2014 14:18:36 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, 03 July 2014

Percentage of Normal Precipitation for July 1, 2012 to June 30, 2014 (WRCC) Click
Two-Year Percentage of Normal Precipitation
July 1, 2012 to June 30, 2014 (WRCC).

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.


Thursday, 03 July 2014 13:48:36 (Pacific Daylight Time, UTC-07:00)  #    Disclaimer  |   | 
# Wednesday, 05 March 2014

NRL Terra-MODIS Composite 02/26/2014 2150 GMT Click
NRL Terra-MODIS Composite 02/26/2014 2150 GMT
02/26-02/27 System Approaching Coast; 02/28-03/02 System West of 140W.

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.


Wednesday, 05 March 2014 12:26:55 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Friday, 20 November 2009

AHPS Observed Precipitation For 30 days Ending 11/20/09 Click
AHPS Observed Precipitation
For 30 days Ending 11/20/09

Drier than average weather in the Southwestern U.S., and wet weather in the Pacific Northwest in recent weeks has some folks wondering if the 2009-2010 El Niño is going to be another one of the quirky, underachieving El Niños we've been seeing this decade.

For much of September and October, the primary area of equatorial convection in the Eastern Hemisphere was in the Central Pacific, just west of the dateline. Convection was suppressed in the Indian Ocean. Such a pattern is consistent with El Niño.

Enhanced convection centered at about 160E in early October may have helped to extend the Pacific jet following an East Asian mountain torque event around October 5. This in turn may have contributed to an atmospheric river precipitation event in California October 13-14, 2009.

At that time it looked like we might finally be off and running with a "real" El Niño event. Particularly because during October equatorial Pacific SSTs and heat content anomalies increased dramatically. Theses increases were the result of of a strong Westerly Wind Burst (WWB), an associated downwelling Kelvin wave, and diminished tradewinds,

However -- and I think I hear Ed Berry's Rottweiler growling -- beginning in mid October, strong convection developed in the Indian Ocean. This essentially neutralized El Niño, and perhaps contributed to the La Nina like precipitation pattern we've been experiencing along the west coast of the U.S.

Indicative of the ocean-atmosphere issues, relative Atmospheric Angular Momentum (AAM) has been negative since early September, and the Global Wind Oscillation (GWO) has remained on the La Nina side of the GWO phase space.

Does this mean we've had an El Niño false alarm?

I wouldn't put away my rain gear just yet. Although November rainfall for Downtown Los Angeles is about 0.6 inch below normal, we usually only get about an inch of rain in November. And don't forget, because of the big storm in October, we are still about 0.6 inch ABOVE normal rainfall for the water year.

Update November 24, 2009. Current equatorial heat content anomalies are about the same as in 2006, not greater. See this composite TAO/TRITON section plot comparing equatorial heat content in 2006 and 2009.

And there are some positive signs. Equatorial Pacific SST and heat content anomalies are much greater than in the short-lived 2006 El Niño, and are more along the lines of those observed in the 2002-2003 El Niño. In addition, a strong MJO has developed. It has remained coherent, and is propagating eastward into the Western Pacific. It already appears to be helping to enhance convection in the Western and Central Pacific, and could kick-start the El Niño engine over the next week or two. We'll see!

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

Friday, 20 November 2009 07:32:56 (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, 06 August 2009

Global Wind Oscillation Phase Space - May 7, 2009 vs August 4, 2009 Click
Global Wind Oscillation Phase Space
May 7, 2009 vs August 4, 2009

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

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

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

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

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

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

Thursday, 06 August 2009 08:14:57 (Pacific Daylight Time, UTC-07:00)  #    Disclaimer  |   |