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# 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 6, 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 6, 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  |   | 
# Wednesday, December 16, 2009

AHPS Observed Precipitation For 7-days Ending 12/15/09 at 4:00 a.m. PST Click
AHPS Observed Precipitation
For 7-days Ending 12/15/09 at 4:00 a.m. PST

Following several weeks of dry weather, a shift to a wetter pattern brought much needed rain and snow to California this past week. The pattern change was apparently induced by MJO enhancement of El Niño convection in the equatorial Pacific.

Downtown Los Angeles recorded 1.78 inches of rain from Thursday to Sunday, bringing the water year total to 4.81 inches. This is about 2.20 inches above normal for the date. Rainfall totals generally ranged from 2 to 4 inches in the basin and valleys, and 2 to 6 inches in the mountains. Here's an archived copy of a NWS Public Information Statement with rainfall totals from around the area.

Will the current El Niño produce the expected seasonal impacts in Southern California? A new tool that can help gain some insight into the linkage of climate and weather is the Global Wind Oscillation (GWO) phase space plot. Ed Berry repeatedly demonstrated the usefulness of this tool in his blog Atmospheric Insights. Although the blog has been discontinued, its content remains a valuable resource.

The GWO is a recurring subseasonal phenomenon that involves the transport and interchange of momentum in the earth-atmosphere system. It encompasses the MJO and occurs on a similar timescale. Analogous to the MJO phase space plot, but based on a framework of atmospheric angular momentum (AAM), the GWO phase space plot is a measure global relative atmospheric angular momentum and it's tendency. For details, see Weickmann & Berry, 2008.

Relative AAM is generally positive during an El Niño and negative during a La Nina. Relative AAM is correlated with rain season precipitation in Southern California*, and can be helpful in assessing potential El Niño impacts. The following November to March GWO phase plots show the distinctly different behavior of the GWO during the strong El Niño of 1997-98, and the strong La Nina of 1973-74:

GWO El Niño 1997-1998 GWO La Niña 1973-1974
GWO El Nino 1997-1998
Click for larger image
GWO La Nina 1973-1974
Click for larger image

In a decade characterized by quirky El Niños, the El Niño of 2009-10 has been acting like another odd one. From a Southern California perspective, the concern has been that it might be like the El Niño of 2006-07 when Downtown Los Angeles recorded only 3.21" of rain over the water year. In the early stages of the El Niño of 2006 strong convection developed in the Indian Ocean during November, but an MJO did not develop until a second round of Indian Ocean convection occurred in mid December. Relative AAM remained negative, and in terms of the atmosphere, the 2006 El Niño didn't make it to 2007.

This year, Indian Ocean convection did spawn an MJO which eventually enhanced El Niño convection near the dateline. Significant momentum was added to the mid-latitudes of the Northern Hemisphere, energizing the westerlies and contributing to the pattern change that resulted in our recent wet weather. However, the increase in mid-latitude AAM has been mostly offset by negative anomalies at higher latitudes. Following are the GWO phase space plots for the current rain season to date, and the quirky El Niño of 2006-07.

 

GWO El Niño 2009-2010 GWO El Niño 2006-2007
GWO El Nino 2009-2010
Click for larger image
GWO El Nino 2006-2007
Click for larger image

So where do we go from here?. For at least a few days the storm track has shifted north. It appears the GWO might be starting an orbit that will take it farther into the El Niño side of the phase space. Several MJO forecasts show the next MJO cycle shortcutting it's orbit back into the Western Pacific. In the 6-10 day period, the GEFS ensemble MJO forecast indicates enhancement of equatorial convection around 90E, and suppression (by the MJO contribution) of El Niño convection near the dateline. This could modify any short term response of the El Niño signal and the GWO, once again reducing AAM.

Below are the GWO phase plots for two El Niños with ONI and MEI values comparable to the current El Niño. In the 1986-87 El Niño, November to March AAM averaged 0.02, and it wasn't until February that the GWO climbed out of the AAM doldrums. That water year Los Angeles recorded 7.66 inches of rain. On the other hand, in the 1994-95 El Niño, November to March AAM averaged 0.76. In mid December the GWO began to reflect an ocean-atmosphere coupled El Niño response, and Los Angeles went on to record 24.35 inches of rain for the water year. At the moment it looks like Southern California might not be as dry as 1986-87, but probably not as wet as 1994-95. We'll see!

GWO El Niño 1986-1987 GWO El Niño 1994-1995
GWO El Nino 1986-1987
Click for larger image
GWO El Nino 1994-1995
Click for larger image

*1958-2008 Precipitation/AAM correlation plot generated using US Climate Division Dataset Seasonal Correlation Page.

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

Wednesday, December 16, 2009 1:40:43 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Saturday, December 5, 2009

NAM 500 mb Heights 12/05/09 18z Click
NAM 500 mb Heights 12/05/09 18z

Induced by MJO enhanced convection in the West Pacific, a change in pattern is underway which is expected to produce significant rainfall in California over the next week.

The first shortwave is forecast to ride down the east side of a very high amplitude blocking ridge along the West Coast, dig offshore on Sunday evening, and merge with another shortwave as the westerlies try to undercut the ridge on Monday.

It's an extraordinarily complex scenario, and just how it will play out is hard to say. A BUFKIT analysis of 18z NAM data generates about 1.2" at LAX and about 1.5" at VNY. Relatively strong south to southwest inflow is forecast and would be expected to produce higher precipitation amounts in favored foothill and mountain locations.

Mid-week the westerlies are forecast to break through underneath the ridge, opening the door to more wet weather.

Update 12/08/09 5:00 p.m. The best dynamics worked out to be south and east of Los Angeles county, and as this AHPS graphic shows that's where the most precipitation was recorded. But the Los Angeles area still received it's fair share of precipitation and managed to do so without inundating the areas burned in the San Gabriel Mountains by the Station Fire. This was due in part to the generally moderate rainfall rates and the low snow level. This Intellicast.com composite radar image from 2:30 Monday afternoon shows the jet enhanced precipitation to the south of the Los Angeles basin, and the frontal band north of the basin.

Over the course of the storm Downtown Los Angeles (USC) recorded 0.95 inches, bringing the water year total to 2.28 inches, which is about 0.75 inches above normal for the date. Here's an archived copy of a NWS Public Information Statement with rainfall -- and snowfall -- totals from around the area.

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

Saturday, December 5, 2009 3:43:09 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Monday, May 4, 2009

Intellicast.com Composite Radar - May 1, 2009 6:15 p.m. PDT Click
Intellicast.com Composite Radar
May 1, 2009 6:15 p.m. PDT

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.

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

Monday, May 4, 2009 8:32:27 AM (Pacific Daylight Time, UTC-07:00)  #    Disclaimer  |   | 
# Monday, January 26, 2009

GEFS Ensemble Forecasts - For February 4, 2009 4:00 a.m. PST Click
GEFS Ensemble Forecasts
For February 4, 2009 4:00 a.m. PST

Weather is by nature chaotic, and not perfectly predictable. From a given starting point, small variations in initial temperature, pressure, moisture, and other factors in the weather system can lead to dramatically different outcomes. In addition, interactions of earth's oceans and terrain with the atmosphere further complicate predictability.

Two periods of rain were recently forecast in Southern California -- one from Wednesday into Saturday, and another from Sunday evening into Tuesday. Model projections varied from day to day and run to run. At one time it looked like the Los Angeles area might get about an inch or rain out of both systems. So what happened?

Considering the first period, Wednesday into Saturday, here are the preliminary rainfall totals from the NWS. Note how much the rainfall varies. In the Los Angeles basin, Downtown Los Angeles (USC) recorded 0.34 inch, LAX recorded 0.51 inch and Santa Monica recorded 0.76 inch. Locations that benefited from orographic enhancement recorded higher totals. For example. Pacioma Dam recorded 1.11 inch, Warm Springs 1.31 inch, and OPIDS Camp 1.54 inch. Some stations in Santa Barbara and San Luis Opisbo counties received as much as 2-3 inches of rain. A slight change in the position of low to the south on Wednesday and Thursday, or the low to the west on Friday and Saturday could have easily resulted in much more rain.

We're in the middle of the second period now. Instead of an over the ocean path forecast a couple of days ago, the cold trough and upper low that is over the southwest took a drier overland course down the West Coast. Nonetheless, there has been scattered showers in Southern California, and even isolated thunderstorms, lightning and hail. It's not a huge step to speculate the rainfall would have been more widespread if the system had taken a more westward path, and picked up additional moisture.

So what's next? Globally, some interesting things are occurring that could impact our weather over the next few weeks. The MJO and GWO are expected to continue their circuit into phase 3-4. This might open the door to more West Coast troughs, or MJO enhanced rainfall. A Sudden Stratospheric Warming (SSW) event appears to be underway and may result in polar air outbreaks that would affect the mid-latitude circulation.

However, in the short term Los Angeles rainfall remains below normal. As of January 25, Downtown Los Angeles (USC) has recorded 5.00 inches of rain this water year, which is 1.38 inches below normal. Last year, the water year total on January 25 was 9.35 inches. The ECMWF and GEFS ensembles suggest the possibility of a strong trough affecting California around February 4th or 5th, but that is a long way out, and we'll have to see!

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

Monday, January 26, 2009 8:55:48 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Tuesday, January 20, 2009

HPC QPF Forecast - 48 hrs. Ending 4:00 pm 01/22/09 Click
HPC QPF Forecast
48 hrs. Ending 4:00 pm 01/22/09

Including today, Los Angeles has enjoyed an unprecedented ten straight January days with highs in the eighties. In the yin yang of weather, the western half of the U.S. has been enjoying unusually warm temps, while the eastern half of the country has shivered.

Since the start of the water year on July 1, Downtown Los Angeles (USC) has recorded 4.66 inches of rain. Just 10 days ago, 4.66 inches was almost exactly normal rainfall for the date. However, as is so often the case in Southern California, this apparent normality was the sum of offsetting wet and dry periods. December's precipitation was generally well above normal, but January has been dry, dry, dry. Today, the Los Angeles rainfall total is about 1.10 inch below normal, and every day it doesn't rain, our deficit increases by about 0.10 inch.

Our warm temperatures and dry weather have been the result of a high amplitude ridge, pushed up over the West Coast by a very strong and extended Pacific jet stream. Big upper level ridges such as this are consistent with La Nina, and have been a recurring theme this Fall and Winter. Much of our rainfall and cold weather this season has occurred when an extended Pacific jet collapses or contracts -- as is occurring now -- and the blocking ridge shifts westward, opening the door to cold storms plunging down the backside of the ridge from the north.

This time there is a wildcard in the mix. One of the reasons the Pacific jet has been extended is the active phase of the Madden-Julian Oscillation (MJO) has recently propagated from west to east, across the Pacific. The system that is forecast to affect Southern California Wednesday night into Thursday looks like it might be enhanced by an inflow of moisture from an area of tropical convection that may be associated with the MJO.

How much might it rain? A BUFKIT analysis of 12z NAM data generates 1.3 inches of rain at Van Nuys for the period Wednesday evening through Thursday afternoon. The HPC 2 Day forecast is not so bullish, indicating about 0.50 inch to 0.75 inch over the area. We'll see!

Medium range models suggest the weather in the West could remain unsettled into next week and beyond. Of particular interest is this morning's 12z ECMWF forecast for mid-week next week, which projects the retrogression of the West Coast ridge to 150-160°W, with a cold, wet pattern similar to what we saw in December. That would be quite a change!

Update 01/22/09. It seems the NAM and GFS models can't get a handle on the system currently affecting our area. The 12z NAM generates 1.6 inches (!) of rain at Van Nuys and the 12z GFS 1.2 inches at LAX from this morning into Saturday afternoon. On the other hand the 09z SREF Ensembles puts the probability of more than 0.25 inch of rain for the 24 hr. period ending mid-morning Friday at about 50%, and then only about 10-30% for the following 24 hr. period. Upslope enhancement may produce higher totals on south facing foothill and mountain slopes. The GFS continues to advertise the possibility of a significant rain event Monday afternoon into Tuesday or Wednesday, but given recent model performance, we'll wait and see!

Update 01/21/09. The 12z models now extend the rainy period for the first system into Saturday. The 12z NAM generates about 1.0 inch of rain at Van Nuys, beginning Thursday morning and ending Saturday midday. The 12z GFS also produces about 1.0 inch, beginning Wednesday evening and ending Saturday afternoon. The GFS adds another 0.9 inch from Sunday morning to Monday night. Somewhat higher amounts would be expected in some foothill and mountain locations. As always, we'll see!

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

Tuesday, January 20, 2009 8:57:46 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Tuesday, February 5, 2008

Drought Monitor Classification Change for Water Year Ending January 29, 2008 Click!
Drought Monitor Classification Change
Water Year Ending January 29, 2008

Sunday's storm added more rain to Southern California's unexpectedly wet water year totals, and more snow to the Sierra snowpack. This rain season western storms have reduced drought classifications by as much as three steps in some areas of California and four in Arizona.

As of February 4, Downtown Los Angeles (USC- KCQT) has recorded 12.13 inches of rain since July 1-- about 4.5 inches above normal for the date. Here is an archived PDF of a NWS Public Information Statement with some preliminary rainfall totals for Sunday's storm.

Sierra snow course measurements for February 1 are coming in and manual measurements are confirming what automatic snow sensors have already reported -- that the Sierra snowpack is well above normal for the date.

Will the wet western weather continue? The GFS and ECMWF medium range models, and NCEP and PSD ensembles are forecasting a generally quiescent, rain free period for Southern California over the next several days, and into the extended period. Today's NWS 6-10 Day and 8-14 Day Precipitation Outlooks for Southern California are indicating Below Normal precipitation.

However, there are suggestions (see Atmospheric Insights) that sometime around February 15-20, the currently active MJO may more or less phase with the GWO, and amplify La Nina to produce a energetic, extended Pacific jet and west coast trough, similar to what occurred around January 3, 2008. We'll see!

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

Tuesday, February 5, 2008 9:30:08 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   |