The frontal band associated with the first Pacific upper level low system and trough of Autumn 2009 is producing some showers in Central California. Some snow showers could occur at the higher elevations of the Sierra. A strong onshore flow has dramatically cooled temperatures throughout the state.

Today's cool temps are a welcome respite from several days of hot weather. Wednesday and Thursday of last week, Downtown Los Angeles (USC) recorded a high of 100°F; and Saturday, Woodland Hills (Pierce College) reported a high of 107°F. Temperatures are expected to rebound Wednesday, Thursday and Friday, but are forecast to cool again over the weekend.

Similar to what occurred in July there has been little change in equatorial Pacific SST anomalies, the Southern Oscillation Index (SOI) has switched from negative to positive, the Aug-Sep Multivariate ENSO Index (MEI) value is not expected to significantly increase, and the Global Wind Oscillation (GWO) has dropped its orbit to a lower relative AAM state.

On the plus side, a large area of enhanced convection has redeveloped in the West Central Pacific from about 150E to the dateline, and another of a series of Westerly Wind Bursts (WWB) is occurring -- the strongest observed so far during this ENSO transition. This WWB will likely be followed by another eastward propagating downwelling Kelvin wave, which could help increase upper-ocean heat content anomalies in the equatorial Pacific.

Moderate El Niños come in many flavors and have varying impacts. Under the guise of such El Niños Los Angeles experienced its second wettest water year on record in 2004-2005, when 37.25 inches of rain was recorded; then in 2006-2007 had its driest water year on record, when only 3.21" was recorded.

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

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.

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Los Angeles Temps Continue Below Normal.

Downtown Los Angeles (USC) ended the water year on June 30th having recorded 9.08 inches of rain, which is 6.06 inches below the 1971-2000 norm of 15.14 inches. Some additional water year rainfall totals:

According to the El Nino/Southern Oscillation (ENSO) Diagnostic Discussion issued July 9 by the Climate Prediction Center/NCEP/NWS, "During June 2009, conditions across the equatorial Pacific Ocean transitioned from ENSO-neutral to El Niño conditions." According to the report, El Nino conditions are expected to continue to develop, and last through the NH Winter.

The May-June Multivariate ENSO Index (MEI) value, reported July 6, has increased by 0.51 to +0.85. As was the case last month, the 3-month rise of the MEI (since February-March) is the 4th highest on record for this time of year, and was last exceeded by the strong El Niño of 1997. According to the MEI's originator, Dr. Klaus Wolter, "the combination of already border-line moderate El Niño conditions along with such a big rise in the MEI at this time of year has always been followed by continued El Niño conditions through the remainder of the calendar year, at least in the modern MEI record (since 1950)."

However, as the truncated 2006-07 El Niño event demonstrates, an El Niño is more than just warm Pacific equatorial SSTs. Through complex forcing and feedback mechanisms, the atmosphere and oceans have to cooperate on a global scale. As climate scientist Ed Berry cautions in his July 3rd Atmospheric Insights post, "Should total AAM departures become comparable to that observed during this past January and February, my concerns of an El-Nino 'false alarm' for the weather-climate dynamical system will be significantly raised."

Generally speaking, the momentum of the atmosphere increases when there is an El Niño, and decreases during a La Niña. Over the past 40 days the Global Wind Oscillation (GWO) has retreated from the higher AAM (El Niño) side of the GWO phase space to the lower AAM (La Niña) side. One concern is that two areas of tropical forcing (Indian Ocean and Pacific) may interact in such a way as to quash the El Niño engine. We'll see!

Closer to home, although temps in Southern California have recently been more seasonable, particularly in the Valleys, the last time the average daily temperature in Downtown Los Angeles was above normal was back on May 21 -- 48 days ago.

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Instability and moisture associated with a very late season upper level low produced more rain in Southern California Friday, setting a record for June rainfall at Santa Barbara Airport.

Downtown Los Angeles (USC) recorded a total of 0.13 inch of rain Friday. The rainfall increased the water year precipitation total for Los Angeles to 9.08 inches, which is 6.05 inches below normal. Santa Barbara Airport recorded 0.51 inch, setting not only a new record for the date, but for any day in June, as well as for the total amount of rain in June. Here's an archived copy of a NWS Public Information Statement with some preliminary rainfall totals from around the area as of 5:00 p.m. Friday.

The April-May Multivariate ENSO Index (MEI) value (released today) has increased by 0.54 to +0.34. As discussed by MEI originator, Klaus Wolter, the 3-month rise of the MEI since January-February is the 4th highest on record for this time of year, exceeded last by the strong Niño of 1997. According to Dr. Wolter, if next month's MEI rank is at least the same as this month (37th), "it would be unprecedented for it to drop below that high-neutral ENSO-phase range by the end of 2009, virtually excluding a return to La Niña, based on the MEI record since 1950." We'll see!

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Unusual weather for June. Not so much that there were thunderstorms, but that the thunderstorms were in part the result of an unseasonably strong jet embedded in the base of an offshore upper level low.

It's a bit of a stretch, but an argument could be made that these storms were a calling card of an increasingly energetic atmosphere, and a developing El Niño.

Downtown Los Angeles (USC) recorded a total of 0.02 inch of rain Wednesday. The rainfall increased the water year precipitation total for Los Angeles to 8.95 inches, which is 6.16 inches below normal.

It's not often that a trace of rainfall sets a record, but that was the case at Camarillo Airport and Palmdale, which had not previously recorded precipitation on June 3. The 0.03 inches recorded at Sandberg was also a record. Here's a CNRFC graphic with some 24 hour rainfall totals in Los Angeles County.

Several factors point to an increased probability of El Niño conditions developing over the next few months. Among them, Equatorial Pacific SSTs have increased, and the subsurface heat content is the highest it's been since the El Niño of 2006-07.

But as the short-lived 2006-07 El Niño event demonstrates, an El Niño is more than just warm Pacific equatorial SSTs. Through complex forcing and feedback mechanisms, the atmosphere and oceans have to cooperate on a global scale. Generally speaking the momentum of the atmosphere increases when there is an El Niño, and decreases during a La Nina.

And it looks like the atmospheric momentum may be increasing. Orbits of the Global Wind Oscillation (GWO), a measure of atmospheric momentum, have been shifting upward, in the direction of more energetic values usually associated with an El Niño.

However, an El Niño is not a done deal. The climate system is just leaning in that direction. As climate scientist Klaus Wolter has previously pointed out, in a similar situation in 1973-1975, the climate fell back into a La Niña for another year. But at this point it appears we may be diverging from that analog case. (See June 6 Weathernotes.) We'll see!

Update 06/05/09. Unseasonably strong 110+ kt jet overhead as upper low center moves onshore in Central California. At 7:15 this morning, Intellicast composite radar showed most of the shower activity occurring to the north of Los Angeles, but there has been scattered showers in the Los Angeles area as well, with measurable rain recorded at a number of stations. Here's an archived copy of a NWS Public Information Statement with some preliminary rainfall totals as of 5:00 p.m.

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An unseasonably strong, late season Pacific storm system brought rain to much of California Friday and Saturday, including some areas of Southern California.

Los Angeles basin and valley areas generally recorded a trace of precipitation, but some stations in the Ventura mountains recorded as much as 0.3 inch. Here is an archived copy of a NWS Public Information Statement, with some precipitation totals from around the area.

Heavier rain occurred in Central California. A new rainfall record for May 1 was set in Sacramento, and higher elevations of the Sierra received about a foot of snow. Enhanced southwesterly, sub-tropical flow, and enhanced convection associated with the phase 7 MJO may have contributed to the strength of the system.

SSTs have warmed in the equatorial Pacific, and at least for the moment, orbits of the GWO have shifted to a more ENSO neutral stance. Some longer term ENSO forecasts are now predicting warm ENSO conditions to develop by next Winter.

A warming trend is expected in Southern California over the next several days, with high temperatures in the valleys near 90 by mid-week.

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Upper low off the coast of Southern California this morning is forecast to track into Northern Baja by Saturday, and then continue east into Texas Easter Sunday. There's a chance of a shower, or possibly a thunderstorm, over much of Southern California today, diminishing tonight. Tomorrow is expected to be partly cloudy, with mostly sunny skies forecast for Easter Sunday.

Downtown Los Angeles (USC) recorded at total of 0.03 inch of rain Tuesday into Wednesday as a weak front associated with an upper level low moved through the basin. The rainfall increased the water year precipitation total for Los Angeles to 8.93 inches, which is about 5.5 inches below normal. Here is an archived copy of a NWS Public Information Statement, with some precipitation totals from around the area.

A warming trend is expected over the next several days, with high temperatures near 90 by the end of the week.

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

Compared to last week, GFS and ECMWF runs this week have been much drier in Southern California, with most of the activity focused on the Pacific Northwest. No significant rain is forecast here next week, and both the 6-10 and 8-14 day forecasts project below average precipitation for Southern California.

Recent runs of the GFS show the East Asian jet pumping up a huge, high amplitude ridge in the central Pacific around the dateline, but the resulting downstream trough, jet energy and storm track are forecast to remain above about 40°N.

Model performance has not been the best in recent weeks, and generally suffers during seasonal transitions. However, if the 12z GFS forecast is on the mark, March rainfall for Downtown Los Angeles (USC) would fall well below the 3.14 inch norm. Since normal April rainfall is 0.83 inches, and May only 0.31 inches, beyond March it becomes increasingly unlikely that a big rain event will significantly boost our rainfall total.

Since November 1, Downtown Los Angeles has recorded 8.80 inches of precipitation. This is consistent with the mean November-March precipitation for coastal Southern California during 9 La Niña events from 1948 to the present. (See the composite precipitation map in Weathernotes for October 31, 2008.) The water year total for Los Angeles is now 3.32 inches below normal.

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The precipitation total for Downtown Los Angeles (USC) for February was a little above normal, but not enough to offset January's dry weather. We started out March with a water year rainfall total about 2.4 inches below normal, and as of today we're about 2.73 inches behind.

Tuesday's frontal passages helped a little. Here's a GOES-11 water vapor image from 2:30 in the afternoon, and an Intellicast.com composite radar image from 2:45 p.m. Downtown Los Angeles (USC) recorded 0.25 inch of rain for the storm, and rainfall totals generally ranged from about 0.25 to 0.50 inch. Here is an archived copy of a NWS Public Information Statement listing some rainfall totals.

It looks like our rain season may not be over. Recent runs of the GFS have been hinting at some rain in the 8-10 day timeframe, with a series of systems following. This is consistent with Ed Berry's Atmospheric Insights post today. In response to increased subtropical westerly wind flow, he suggests the possibility of an extended East Asian jet in the week 2-3 timeframe, with possible western USA impacts. We'll see!

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

As forecast by the computer models, most of the precipitation associated with a stream of sub-tropical moisture drawn into California by a large low off the West Coast occurred north of Pt. Conception. Here is an archived copy of a NWS Public Information Statement listing some rainfall totals for the period Saturday night to Monday afternoon. Downtown Los Angeles (USC) recorded only a trace of precipitation.

Both the 6-10 day and 8-14 day NWS Precipitation Outlooks continue to project above normal precipitation in California. At the moment, it looks like the next chance for some rain south of Pt. Conception may be in the Sunday to Monday timeframe, with another opportunity midweek. We'll see!

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Downtown Los Angeles (USC) recorded 1.40 inch of rain in our latest storm, bringing the water year total to 9.37 inches, which is only 0.8 inches below normal for the date. Rainfall totals generally ranged from 1.0 to 2.0 inches in the basin and valleys, with somewhat higher amounts recorded in favored foothill and mountain locations. There was heavy snow in the mountains. Here is an archived copy of a NWS Public Information Statement listing some rain and snowfall totals for the storm.

A large cut-off low pressure complex currently at about 140°W is forecast to spin tauntingly off the coast through Friday or Saturday. Pinwheeling low and vorticity centers, a building ridge, a 150 kt. jet, tropical moisture, and an upstream shortwave are in the forecast mix. At this point it looks like some form of the system moves onshore on Sunday, with most of the rain occurring north of Pt. Conception. However, the situation is extraordinarily complex, and we'll have to see how the forecast evolves.

Beyond next week it looks like there will be additional opportunities for rain in Southern California. Both the 6-10 day and 8-14 day NWS Precipitation Outlooks project above normal precipitation.

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

Models have been trending somewhat wetter with the front approaching Southern California this morning. BUFKIT analysis of 12z data for Van Nuys generates about 0.4 inch rain, starting around noon and ending this evening. At Santa Barbara the generated total is about 0.5 inch, and at LAX about 0.25 inch. Locally higher amounts are possible in thunderstorms, and on southwest to west facing slopes.

This front is the first of several systems that have been forecast to affect the area over the next week. Model output has been variable regarding the timing and strength of these systems.

According to the models, the system originally forecast for Sunday is still in the works, but has been has been pushed back to Sunday evening or early Monday morning. The !2z NAM and GFS model runs forecast about 1.1 to 1.5 inch at Van Nuys from Sunday evening through Monday. Very strong southerly inflow is forecast with this system, so significantly higher totals are possible on south facing foothill and mountain slopes.

Beyond early next week, the models have been all over the place with some runs forecasting several inches of rain late in the week and other runs almost none at all -- the amount of rain being dependent on the proximity of a strong low to the coast. This morning's GEFS ensembles for next Thursday evening are indicative of the uncertainty. We'll see how the forecast evolves over the next few days.

Update 02/18/09. Here is an archived copy of a NWS Public Information Statement with some rainfall totals for Friday's storm.

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