# Friday, March 26, 2010

Global Wind Oscillation (GWO) November 1, 2009 - March 24, 2010 Click
Global Wind Oscillation (GWO)
November 1, 2009 - March 24, 2010

After peaking at a standardized relative AAM anomaly of 2.21 around February 5, 2010, the Global Wind Oscillation (GWO) has plunged 3 SD in relative AAM to a value of -0.90. This is the lowest value of relative AAM anomaly since mid October, 2009. Much of the loss in AAM was from the mid latitudes of the NH. From the week of February 22-28 to March 19-25, 7-day averaged 250 mb Zonal Mean Zonal Winds decreased from about 43 m/s at 30°N to 33 m/s at 30°N-45°N. The decrease in relative AAM reduced the likelihood of an extended and southward displaced North Pacific Ocean jet, and associated Southern California El Nino impacts.

In fact, there has been no measurable precipitation at Downtown Los Angeles (USC) since March 6, 2010. Los Angeles rainfall is now 2.23 inches below normal for the month of March, and only 1.15 inches above the water year norm for the date.

In the last week there has been an increase in relative AAM from about 40°N to 50°N. The GWO and AAM appears to have bottomed out and cyclical subseasonal processes may be working to revitalize Western Pacific convection and El Nino related forcing. If the GWO continues its current orbit, relative AAM would be expected to increase in the 6-10 day period.

The GFS and ECMWF have been showing a strong Pacific jet and trough affecting the West Coast next week. There are significant differences in the GFS and ECMWF model solutions, and although 5-day model performance has been good recently, performance often suffers during seasonal transitions. This morning's 12z GFS puts the initial focus of the event in the Pacific Northwest early in the week. Precipitation is forecast to spread into Northern and Central California, and finally Southern California, as the week progresses, and the trough deepens and moves onshore.

It's way too early to say how this system will affect Southern California. Many processes are in play that could affect the amount of precipitation here, pro or con. Yesterday afternoon's 00z ECMWF appeared to project a wetter solution for Southern California than this morning's 12z GFS, with the upper low and trough evolving a little more to the west. It does look like there is the potential for heavy snow in the Sierra. We'll see!

Update 04/03/10. The frontal band and upper low associated with the trough that moved through the area overnight Wednesday into Thursday produced some showers and some spectacular clouds in the Los Angeles area. The bulk of the precipitation occurred from Central California north into the Pacific Northwest. The upper low stayed offshore as it moved down the coast and past the Los Angeles basin. Avalon and some mountain locations recorded more than 0.1 inch of precipitation, but generally amounts south of Pt. Conception ranged from a trace to few hundredths. Here's an archived copy of a NWS Public Information Statement with some rainfall totals from around the area. A similar system is forecast to produce some rain in Los Angeles area Sunday night. BUFKIT analysis of 18z NAM data for Van Nuys projects about 0.4 inch of precipitation. The 09z SREF shows a high probability of measurable rain, but a sharp decrease in the probability of more than 0.1 inch/day south of Pt. Conception. We'll see!

There was an interesting lenticular wave train northwest of Los Angeles earlier this month. The situation was peculiar because the wind at nearly all levels at that time was from the northwest, and the wind forming the wave clouds appeared to be from the north-northeast. The tops of the wave clouds were being sheared by winds blowing from the northwest (left to right). An ARL NAM-12 based wind profile for the area on March 10 for 06z, shows a possible source of the winds that produced the waves, as well as the shear. Here's an animated series of NRL satellite photos showing the complex wind and wave pattern at the time.

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

 

Friday, March 26, 2010 9:44:05 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Monday, March 08, 2010

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GOES-11 IR Satellite Image
March 6, 2010 - 3:00 p.m. PST

As forecast, an energetic upper low skirted the Los Angeles area yesterday on its way south into northern Baja, Mexico. As the low moved south, a vorticity lobe spinning around the low arced into the Southern California coast, producing rain, thunderstorms, waterspouts, and blustery winds.

Rain amounts were variable, generally ranging from about 0.25 inch to 0.75 inch around the Los Angeles area. Indicative of the convective nature of the precipitation, Downtown Los Angeles (USC) recorded 0.42 inch, while a few miles away LAX recorded only 0.18 inch for the storm. The water year total for Downtown Los Angeles now stands at 14.66 inches -- only 0.48 inch less than the annual norm of 15.14 inches. Here is an archived copy of NWS Public Information Statement with some more rainfall amounts for this rain event.

Reflecting the enhancement of El Niño by the MJO at the end of January, the Multivariate ENSO Index (MEI) for January-February has increased to 1.5. As discussed by Klaus Wolter, MEI originator, this is the highest value of the Jan-Feb MEI since the strong El Niño of 1997-1998, and the fifth highest MEI value for Jan-Feb since 1950. This places the El Nino of 2009-10 in the top 10% of MEI rankings for the season since 1950, and above the "strong" El Niño threshold, as measured by the MEI. Here is a chart comparing the current El Niño to others since 1950.

The increase in the MEI corresponds to the increase in relative AAM since mid January and underscores the potential role of the Global Wind Oscillation (GWO) in assessing the response of the atmosphere to El Niño. It also reinforces the notion that the El Niño phenomena is more than just warm SSTs in the central equatorial Pacific. As we've seen several times in recent years, warm SST anomalies in the Niño 3.4 region alone are not sufficient for evaluating an El Niño and its potential impacts.

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

 

Monday, March 08, 2010 9:05:26 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, March 04, 2010

AHPS 24 Hour Precipitation for the 24 hours ending March 4, 2010 - 4:00 a.m. PST Click
AHPS 24 Hour Precipitation
Ending March 4, 2010 - 4:00 a.m. PST

Tuesday's trough was even more feeble than expected south of Santa Barbara, barely managing to wet the streets and generate an official "trace" of rain in the Los Angeles area. Wednesday's upper low dug a little further south than expected, resulting in some moderate showers. Rainfall amounts were variable, generally ranging from a few hundredths in the basin to 0.25 inch or so at some mountain locations . Here are archived copies of NWS Public Information Statements with some rainfall amounts for Tuesday's trough and Wednesday's upper low.

Downtown Los Angeles (USC) recorded 0.06 inch of rain Wednesday, bringing the water year total (July 1-June 30) to 14.24 inches, which is 3.05 inches above normal for the date.

Another finicky upper low may produce some rain in Southern California this weekend. Just how much depends on the behavior of the low and it's proximity to the Southern California coast. If the low track is too far offshore, rainfall amounts could be dramatically reduced. This is well illustrated by 09z SREF plot showing the forecast probability that precipitation will exceed 0.1 inch for the 24 hr. period ending Saturday at 7:00 p.m. BUFKIT analyses of today's 18z NAM data for Santa Barbara, Van Nuys and LAX all produce about 0.3 inch of rain, mostly on Saturday.

Update 03/05/10 8:00 a.m. The models continue to have difficulty with the timing and precipitation amounts for this weekend's storm. This isn't surprising since small changes in the track of the upper low, the amount of moisture entrained, and other factors could have a big effect on the amount of rain in a particular region. Yesterday afternoon's run of the NAM generated nearly an inch of rain at Van Nuys, but this morning's 12z run of the NAM is back down to the neighborhood of 0.3 inch. Today's 09z SREF suggests the highest precipitation amounts may occur to the south of Los Angeles. Here's an SREF plot of the probability that precipitation will exceed 0.25 inch for the 24 hr. period ending 10:00 a.m. Sunday. We'll see!

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

 

Thursday, March 04, 2010 2:13:37 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Monday, March 01, 2010

NEXRAD Regional Radar (NCAR) February 27, 2010 - 3:00 a.m. PST Click
NEXRAD Regional Radar (NCAR)
February 27, 2010 - 3:00 a.m. PST

A primary frontal band, secondary frontal band, and upper low associated with a strong Pacific storm system produced periods of heavy rain in Southern California on Saturday. Rainfall totals for the system generally ranged from about 1.0 to 2.0 inches, with somewhat higher amounts recorded at a few mountain locations. Here is an archived copy of a NWS Public Information Statement with some precipitation totals from around the area.

Downtown Los Angeles (USC) recorded 0.80 inch of rain for the storm, bringing the water year total (Jul 1-Jun 30) to 14.18 inches, which is 3.25 inches above normal. Saturday, Burbank set a new rainfall record for the date of 1.6 inches.

The weather forecast for the Los Angeles area this week looks like a near repeat of last week, with a chance of showers mid-week, and then a possibility of a somewhat stronger storm for the weekend.

Just 24 hours before a trough is forecast to produce a chance of showers in the Los Angeles area, the models are still having difficulty with the forecast. The nature of the system is the culprit -- the evolution of the trough and precise track of a following upper low is far from certain. Today's 09z SREF suggest a high probability (70%-90%) of very light rain (>0.01 inch) for the 24 hr. period ending 7:00 a.m. Wednesday morning, but south of Pt. Conception there is a rapid decrease of the probability of more than 0.1 inch of precipitation over the same period.

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

 

Monday, March 01, 2010 3:22:51 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, February 25, 2010

GOES-11 Water Vapor Image (UW-MAD) February 25, 2010 - 7:00 a.m. PST Click
GOES-11 Water Vapor Image (UW-MAD)
February 25, 2010 - 7:00 a.m. PST

A weakening frontal band produced some light rain in Southern California yesterday afternoon and evening. Precipitation amounts in the Los Angeles area ranged from a trace to a few hundredths of an inch. Amounts were higher to the north, closer to the surface low. A few stations in Santa Barbara and San Luis Obispo counties recorded more than 0.5 inch and some recorded more than an inch. Here is an archived copy of a NWS Public Information Statement with some precipitation totals from around the area.

Today, a much stronger Pacific storm system begins to impact the West Coast, with the highest precipitation totals forecast for Northern California and the Sierra Nevada. BUFKIT analysis of this morning's 12z NAM data for Van Nuys generates about 1.3 inch of rain, beginning late Friday night and continuing into Saturday afternoon. The 18z NAM run starts the rain a little earlier Friday night, and extends the period of precipitation into Saturday evening. It produces about 1.8 inch of rain at Van Nuys over the period. Today's 09z SREF indicated a probability of about 50%-70% that precipitation in the Los Angeles area would exceed 0.5 inch for the 24 hr. period ending Saturday afternoon at 4:00 p.m. Precipitable water values approaching an inch and strong southerly low level inflow could produce higher rain rates and precipitation totals on south to southwest facing foothill and mountain slopes.

Since being reinforced by a strong MJO at the end of January, El Niño convection has remained active in the Central Pacific (animation). In terms of relative atmospheric angular momentum (AAM), the atmosphere's response is the strongest since the El Ninos of 1997-1998 and 2004-2005. Average AAM for our rain season (November to date) has increased from -0.046 at the beginning of the year to 0.341 as of February 23. This El Nino Comparison Chart shows how this El Niño compares to others since 1950.

So far this rain season, Downtown Los Angeles (USC) has recorded above average rainfall every month, except for November. And November's rain came early -- in October.

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

 

Thursday, February 25, 2010 8:26:22 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Saturday, February 20, 2010

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Yesterday's Weak Pacific System Moving Onshore
NRL NexSat Day/Night 02/19/10 4:00 p.m.

Last night's weak Pacific system had its moments, and managed to produce precipitation amounts ranging from about 0.1 inch to 0.25 inch in the Los Angeles area. Here is a NWS Public Information Statement with some precipitation totals. Downtown Los Angeles (USC) recorded 0.14 inches for the storm, increasing the water year total to 13.34 inches. The is 3.58 inches above normal for the date.

A moisture-starved trough on the downstream side of a pinched upper level ridge over the West Coast is forecast to dig southward and could produce some additional precipitation tomorrow. Today's 09z SREF indicated a probability of about 30%-50% that precipitation in the Los Angeles area would exceed 0.1 inch for the 24 hr. period ending Monday morning at 4:00 a.m. The forecast probability that it would exceed 0.25 inch over the same period is about 10%-30%.

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

 

Saturday, February 20, 2010 4:13:13 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Wednesday, February 10, 2010

AHPS 90 Day Precipitation (Percent of Normal) as of February 10, 2010 4:00 a.m. PST Click
AHPS 90 Day Precipitation (Percent of Normal)
February 10, 2010
4:00 a.m. PST

Yesterday's cold upper level low behaved about as expected, following a track just off the coast. Because of the convective nature of the system, rainfall amounts varied from under 0.25 inch to over 1.0 inch in some foothill and mountain locations. About a foot of snow was reported at the mountain resorts. Here is a NWS Public Information Statement with precipitation totals from around the area. Downtown Los Angeles (USC) recorded 0.44 inches for the storm, increasing the water year total to 13.2 inches. The is 4.78 inches above normal for the date.

Compared to the AHPS 90 Day Precipitation map from about 40 days ago, a fairly typical El Niño precipitation pattern has emerged in the West. Medium range models are projecting about a 7-10 day break in what has turned out to be a busy rainy season in Southern California. Assuming these forecasts verify, such a break could not come a better time for those threatened by mudslides and debris flows. The recent enhancement of El Niño convection by the MJO, and the ongoing phase 6-7-8 transition of the GWO might have resulted in a wetter pattern, and this has occurred in similar circumstances in past El Ninos.

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

 

Wednesday, February 10, 2010 3:34:58 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Saturday, February 06, 2010

UCAR NEXRAD Regional Composite Image February 6, 2010 - 4:45 a.m. PST Click
UCAR NEXRAD Regional Composite Image
February 6, 2010 - 4:45 a.m. PST

Wet antecedent conditions and heavy rain early this morning combined to amplify the amount of runoff from our latest Winter storm. In addition to localized street flooding, mud and debris flows have occurred in the vicinity of the Station Fire burn area. The trough associated with the system, and a second frontal band are moving onshore this afternoon, and are producing some additional rainfall in the Los Angeles area. Please refer to www.weather.gov/losangeles for the latest warnings and weather information.

Yesterday's runs of the NAM/WRF did a pretty good job of forecasting the area of enhanced precipitation that developed overnight in Southern California. BUFKIT analysis of 12z NAM data from yesterday generated about 2.4 inches of precipitation at KLAX for the 24 hr. period ending early this morning. According to preliminary NWS data, as of 10:00 a.m. LAX had recorded 2.31 inches for the storm, and Downtown Los Angeles (USC) had recorded 2.84 inches. The water year total rainfall for Los Angeles is now about 5 inches above normal. Here is a NWS Public Information Statement with rainfall totals from around the area. (Link will be updated as revised totals become available.)

The recent enhancement of El Nino convection in the equatorial Pacific by the Madden-Julian Oscillation (MJO) has triggered a strong atmospheric response. A Global Wind Oscillation (GWO) phase space plot shows large increases in relative atmospheric angular momentum (AAM) and AAM tendency. As a result of this increase, the average relative AAM anomaly for the rain season to date is now positive. As mentioned in this post from December 2009, relative AAM is correlated with rain season precipitation in Southern California. This suggests an increased likelihood of wet weather in Southern California in the medium range outlook period.

So what happens next? The ECMWF and GFS projections have not been particularly consistent. At the moment, it looks like a shortwave trough could affect Southern California in the Tuesday evening or Wednesday timeframe and then again Friday. We'll see how the week develops.

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

 

Saturday, February 06, 2010 3:09:12 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Friday, January 29, 2010

NRL Pacific Basin Composite Image January 29, 2010 - 10:00 a..m. PST Click
NRL Pacific Basin Composite Image
January 29, 2010 - 10:00 a.m. PST

The active phase of the Madden-Julian Oscillation (MJO) is enhancing El Nino convection in the equatorial Pacific, increasing the probability of above average rainfall in California over the next two weeks. Here are today's 6-10 day outlook and 8-14 day outlook periods from the Climate Prediction Center (CPC). Once again, a number of the CPC's best fitting 6-10 day outlook analogs and 8-14 day outlook analogs are from wet El Nino years.

Today's Global Wind Oscillation (GWO) phase plot reflects the atmosphere's response to the MJO, and enhanced central equatorial Pacific convection. ECMWF-based 250 hPa streamfunction anomalies from 01/29/10 00:00 GMT appear to be consistent with the ERSL/PSD phase 7 MJO streamfunction composite and phase 6-7 GWO streamfunction composite centered on January 27.

Another strong Westerly Wind Burst (WWB) is also occurring, and appears to be one of the mechanisms associated with MJO enhancement of deep El Nino convection. This enhancement and resulting feedback could slow the decline of, or even increase, equatorial Pacific SST anomalies in the weeks ahead.

Both the ECMWF and the GFS are projecting a return to a wet pattern in California next week. The first skirmish occurs in central and northern California over the weekend, and then a transition to a much wetter, and possibly persistent, pattern is forecast to occur during the week. We'll see!

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

 

Friday, January 29, 2010 2:17:37 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Saturday, January 23, 2010

NWS AHPS 7-Day Observed Precipitation For the 7 Days Ending January 23, 2010 - 4:00 a.m. PST Click
NWS AHPS 7-Day Observed Precipitation
For the 7 Days Ending January 23, 2010 - 4:00 a.m. PST

 

Following six consecutive days with rain, skies are partly cloudy today and Southern Californians can finally get out and enjoy a little sunshine. According to NWS data, Downtown Los Angeles (USC) received 4.58 inches of rain from the series of storms, bringing its water year total to 9.71 inches, which is 3.57 inches above normal for the date.

The Los Angeles/Oxnard office of the NWS has released a Public Information Statement with rain and snowfall totals for the 6 day period 10:00 a.m. Sunday morning to 10:00 a.m. today (Saturday).

Computer projections have been inconsistent, but it looks like we'll probably get some more rain early in the week. We'll see!

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

Saturday, January 23, 2010 5:20:12 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Friday, January 22, 2010

NRL AquaMODIS Composite Image January 21, 2010 - 1:30 p.m. PST Click
NRL AquaMODIS Composite Image
January 21, 2010 - 1:30 p.m. PST

The fifth and final system of the series that began Sunday brought thunderstorms, damaging winds, record low pressure, heavy rain and snow to Southern California yesterday. The unusually large trough associated with the system covered most of the northeastern Pacific yesterday, and unsettled weather is expected today as it continues to move onshore.

Very strong wings, possibly a tornado or thunderstorm downburst, felled trees and damaged structures in Ventura and Santa Barbara. Record rainfall for the date was recorded at Santa Maria, Palmdale, and Lancaster, but more extraordinary, new all-time records for lowest barometric pressure were set at several locations in the Los Angeles area. Note: The NWS documented an EF-0 tornado in Ventura (3Mb PDF).

Here are some preliminary rainfall totals from around the area, compiled by the NWS, for the period 10:00 p.m. Tuesday to 10:00 p.m. this evening. Note that this combines rainfall from system #4 on Wednesday, and our current Thursday-Friday system. And here's an NWS Public Information Statement with some impressive snowfall totals for the week. Wow -- Mt. Baldy got 7 feet of snow! (Updated 01/23/10)

As of yesterday, Downtown Los Angeles (USC) has received 4.21 inches of rain from the series of storms, bringing its water year total to 9.34 inches, which is 3.44 inches above normal. Over the week, the Sierra Nevada has received several feet of snow, and the statewide average snowpack is now above normal.

Please refer to www.weather.gov/losangeles for the latest warnings and weather information.

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

 

Friday, January 22, 2010 8:13:14 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Wednesday, January 20, 2010

NRL GOES Day/Night Image January 20, 2010 - 11:30 a.m. PST Click
NRL GOES Day/Night Image
January 20, 2010 - 11:30 a.m. PST

In a classic case of "be careful what you wish for," Southern California has been awash in a series of storms that rival those of the biggest El Nino years. Coming on top of the rain we've already recorded this week, today's and Thursday's systems appear likely to deliver a combination punch with potentially serious consequences.

Yesterday's vigorous system was characterized by scattered thunderstorms, heavy downpours, and damaging winds. Rainfall amounts generally ranged from about 0.5 inch to 1.0 inch, with some isolated higher amounts. Here's an archived copy of NWS Public Information Statement with preliminary rainfall totals for the period 5:00 a.m. to 4:00 p.m. Tuesday.

Today's frontal system, the fourth in the series since Sunday, is producing heavy rain, strong winds, and the possibility of thunderstorms. More rain is expected than resulted from yesterday's storm. BUFKIT analysis of 18z NAM data for Van Nuys (VNY) indicates about 2 inches of rain today and tonight, with generalized rain rates peaking at about 0.4 inch/hour. Higher localized rainfall rates could occur in the vicinity of thunderstorms. Here is an archived copy of a NWS Los Angeles Quantitative Precipitation Statement with more detailed information.

Unlike previous systems this week, Thursday's system is forecast to develop a surface low west of Pt. Conception. According to the WRF forecast a multi-centered surface low will develop and deepen into a 972 mb low west to northwest of Pt. Conception. In addition to the usual dynamics associated with such a low, a very strong jet is forecast to be in a position overhead that would maximize system dynamics. The formation of a surface low could also slow the progress of the system, potentially increasing precipitation totals.

Please refer to www.weather.gov/losangeles for the latest warnings and weather information.

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

 

Wednesday, January 20, 2010 1:44:28 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   |