# Friday, March 18, 2011

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HPC 3-Day Precipitation Forecast
For the Period Ending 11:00 a.m. Monday, March 21

Along with January and February, March is one of the big three precipitation months in Southern California in which the average rainfall for Downtown Los Angeles exceeds 3.0 inches. In January we were about 2.5 inches below normal rainfall, and in February about 0.4 inch below normal. So far this March, we're nearly two inches below normal for the month. If it were not for an unusually wet December in which Los Angeles recorded 8.32 inches above normal rainfall, we'd be looking at 50% water year right now instead of over 100%.

If today's 12z NAM and GFS forecasts verify, we could make up that March deficit, and push our water year total up to nearly 120%! According to these numerical forecasts, we could see two to three inches of rain in the Los Angeles area by Monday afternoon, and significantly more in orographically favored foothill and mountain locations. The NAM forecasts moist southerly winds approaching 60 kts at the 5000 ft. level Sunday evening.

BUFKIT analysis of 12z NAM/GFS data generates 2.16/4.11 inches of rain for LAX, 3.56/3.57 inches for Van Nuys, and 4.01/4.20 inches for Santa Barbara. The GFS brings in the second front faster than the NAM, forecasting the onset of heavier rain during the day Sunday, rather than Sunday evening. The 09z SREF puts the probability of more than 1.0 inch of rain in the Los Angeles area for the 24 hour period ending Monday morning at about 90%.

And yes, it does look like there's a chance of rain for the Los Angeles Marathon. Although the heavies rain is forecast to hold off until Sunday afternoon or evening, the 09z SREF pegs the probability of 0.1 inch of rain or more in the Los Angeles area over the 6 hours ending at 11:00 a.m. Sunday at about 70%. BUFKIT analysis of 12z NAM data generates about 0.20 inch at LAX from 8:00 a.m. to noon, with temps in the mid 50's. The GFS says about 0.50 inch over the same period. We'll see!

Be sure to check the NWS Los Angeles/Oxnard web site for the latest forecasts, advisories and warnings. More information about Southern California weather and climate can be found using our WEATHER LINKS page.

Friday, March 18, 2011 11:41:41 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, March 03, 2011

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BUFKIT Bourgoiun Precipitation Type Analysis
For eWRF Ensembles for VNY at 8:00 am 02/26/11

The wetter ensemble member forecasts prevailed in last Friday's and Saturday's rain and snow event. Downtown Los Angeles recorded 1.20 inches for the storm. This brought the water year total to 15.78 inches, which is about 104% of normal. Although this might seem unusual in a La Nina influenced rain season, during two of the strongest La Ninas in the past 60 years -- 1955-56 and 1973-74 -- Los Angeles recorded 99% and 106% of normal rainfall. Here's an archived copy of a NWS tabulation of some rainfall totals from around the area from early Friday, February 25, to 10:00 p.m. Saturday, February 26.

The NAM and GFS, as well as the SREF and eWRF ensembles, had a tough time forecasting the rainfall produced by our current scenario -- a relatively shallow, moist westerly flow accompanied by modest jet stream dynamics. As this BUFKIT time-height cross section shows, the wettest eWRF ensemble member depicted a 125 kt jet max overhead, suggesting the models had some difficulty in forecasting the structure and strength of the jet. At best, the models forecast a few hundredths of an inch of rain in the Los Angeles area, but the most recent NWS tabulation of rainfall totals for the event lists many stations with between 0.1 and 0.25 inch. As of 4:00 a.m. this morning, Downtown Los Angeles has recorded 0.16 inches for the storm, bumping the water year total to 105% of normal.

As a result of the westerly flow, snow levels are currently high, but one of the interesting facets of last Friday's storm was the possibility of very low elevation snow. A NWS Winter Weather Advisory issued February 25 for the Santa Monica Mountains Recreation Area forecast the snow level to drop overnight from above 3000' to between 1000' and 1500'. Snow accumulations from 1 to 3 inches were expected. Some low elevation post frontal showers of graupel or snow were reported in the east San Fernando Valley and La Crescenta Saturday afternoon, but based on the conditions on Sandstone Peak (el. 3111'), the snow anticipated in the SMMRA did not occur. It was an especially challenging forecast, pitting pre-frontal warm air advection, against post-frontal cold air advection, falling humidity, and fluctuating regions of snow growth and omega.

Developed in part to help forecasters deal with lake effect snow, BUFKIT includes a number of special features for snow forecasting and visualization. For example, following are two time-height cross sections generated by BUFKIT from NAM data for VNY (Van Nuys) from 4:00 p.m. Friday, February 25. The elevation for the Bourgoiun precipitation type analysis for each cross section has been raised to 2711 feet MSL. The first BUFKIT cross section shows snow growth, omega, temperature, and the projected precipitation amount and type. The second cross section shows the same parameters, but with relative humidity instead of omega. The purple and yellow rectilinear contours are snow growth, and the red and blue contours are omega. The green bars are rain amounts, and the blue bars snow. Some of the things to note in these cross sections are the omega/snow growth "cross hair," freezing level, and rapid decrease in humidity with the frontal passage.

What the discrete NAM forecast can't show is the variability of the forecast situation. In its most recent release BUFKIT added support for SREF and eWRF ensembles. When combined with BUFKIT's precipitation type and other tools, ensemble forecasts can provide much additional information and insight. In this case eWRF ensemble members for VNY exhibited widely varying thermal profiles, regions of snow growth, omega, and humidity. BUFKIT's Bourgoiun precipitation type analysis for the eWRF ensemble members at 8:00 a.m. Saturday morning showed freezing levels as high as 2882' and as low as 1176'. The pattern of post-frontal precipitation was also quite variable.

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

Thursday, March 03, 2011 8:58:48 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Monday, February 21, 2011

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Experimental HPC 6-Hour Probabilistic Precipitation Guidance
For the 6 Hour Period Ending 4:00 p.m. Tuesday

The rainfall totals from Friday's and Saturday's systems were generally a little below the 2 inches forecast, but it was still enough to boost the water year rainfall total for Downtown Los Angeles to 14.58 inches. This is about 96% of the normal water year total of 15.14 inches. The water year extends from July 1 to June 30, so we still have four months to receive the 0.56 inch of rain that would put us at 100%. Here's an archived copy of a NWS tabulation of some rainfall totals from around the area from early Thursday morning to 10:00 a.m. Sunday.

Southern California's wet weather pattern didn't extend into this week as it looked like it might. The upper low that might have produced the rainfall is now forecast to stay offshore. At the moment it looks like it will mainly result in some clouds as it moves down the coast today through Wednesday. Rain chances increase near the end of the week and into the weekend as an upper low and trough work down the West Coast. Ensemble members vary quite a bit on the track of the upper low, with some solutions wetter in Southern California than others. We'll see!

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

Monday, February 21, 2011 2:28:03 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, February 17, 2011

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HPC 3-Day Precipitation Forecast
For the Period Ending 4:00 a.m. Sunday Morning

The transition from a persistent West Coast ridge to a West Coast trough occurred on Monday more or less as forecast by medium range guidance back on February 5. A series of systems and impulses riding up and over a high amplitude Eastern Pacific ridge are expected to continue to produce cool, blustery weather in Southern California with periods of showers and rain into next week.

Precipitation on Valentine's Day was generally limited to areas north of the Los Angeles basin, but was a little more widespread late Monday night into Tuesday morning, with a number of stations in the Los Angeles area recording a few hundredths of an inch of rain.

Wednesday's system was much more energetic. Because of the convective nature of the precipitation, rainfall amounts were extremely variable, ranging 0.25 inch or less to 1.0 inch or more. Downtown Los Angeles (USC) recorded 0.65 inch of rain for the storm, bringing the water year total for Los Angeles to 13.14 inches. This is well above normal for the date, and about 87% of normal for the water year. Here's an archived copy of a NWS tabulation of some rainfall totals for the period 4:00 p.m. Tuesday to 4:00 a.m. Thursday.

Los Angeles only needs about 2 more inches of rain to top the 100% mark for the water year, and it looks like we could get most of that Friday and Saturday. BUFKIT analysis of 12z NAM data for Van Nuys indicates about 1.6 inches of rain Friday into Saturday, and then another 0.5 inch Saturday night. The HPC 3-day Quantitative Precipitation Forecast for the period ending 4:00 a.m. Sunday also indicates about 2.0 inches of rain in Southern California. This morning's 09z SREF puts the probability of more than 1.0 inch rain for the 24 hour period ending 1:00 p.m. Saturday at about 70%.

Temperature data at the surface and at depth in the equatorial Pacific suggest the La Nina is moderating. The tough question is whether warming will continue into the NH summer, with neutral ENSO conditions prevailing as we move into autumn; or if the ENSO cycle will swing back toward a La Nina state. Since 1949, about half of the first year La Nina episodes continued into a second or third year. At the moment the majority of computer models favor a transition to ENSO neutral or warm conditions by July 2011. Here is a compilation of ENSO Forecasts from the IRI ENSO Quick Look for February 17, 2011. We'll see!

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

Thursday, February 17, 2011 1:31:48 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Saturday, February 05, 2011

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NCEP Ensemble Forecast
240 hr. Forecast for February 14 at 4:00 p.m. PST

Although the Pacific jet did expand as anticipated through about January 27, and then retract around January 29, leading to a upper low off the California coast, it wasn't the discontinuous ridge regression which had been hoped for -- or the change in pattern that would open the door to Pacific storms wishing to visit sunny Southern California.

The area did manage to get a bit of rainfall out of the system. Downtown Los Angeles (USC) recorded 0.21 inch on Sunday, bringing the rainfall total for January up to a paltry 0.79 inches, which is more than 2.5 inches below normal for the month. At this point Downtown Los Angeles remains almost 5 inches above normal for the water year (July 1 to June 30), but with each dry day that cushion grows a little smaller. Here's an archived copy of a NWS tabulation of some rainfall totals from last Sunday's storm.

Well, what the MJO couldn't do maybe random influences will. Both the GFS and ECMWF are forecasting a change to a wetter pattern for California around Valentine's Day. This is supported by the NCEP Ensembles, and ESRL/PSD Analog Probabilistic Precipitation Forecasts for the 8-14 day period. As this CPC 8-14 day Precipitation Outlook indicates, the chances look the best from Central California, north into Oregon. We'll see!

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

Saturday, February 05, 2011 4:48:56 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, January 20, 2011

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Extended Pacific Jet and Resulting Mega-Ridge
GFS Forecast for Sunday Afternoon

Over the next two weeks, the MJO signal currently propagating across the western Pacific is forecast to destructively interfere with the La Nina base state, suppressing convection in the Indian Ocean and Indonesia and enhancing convection near the dateline. This is forecast to result in a transition from upper tropospheric anomalous cyclones to anticyclones near the dateline, and an extension of the East Asian/Pacific jet. These circulation changes may eventually lead to a retrogression of our persistent West Coast ridge, or perhaps a breakthrough of the westerlies underneath the highly amplified ridge.

The key word here is eventually. Over the next week or so, the extending Pacific jet is forecast to reamplify the ridge over the West Coast. But at some point, perhaps near the beginning of February, destructive interference in the Western Pacific will dwindle and convection in the Indian Ocean will strengthen. This should result in retraction of the Pacific jet, and possibly, a West Coast trough. The 240 hr forecast of the 12z ECMWF hinted at this possibility, as did the 18z GFS. We'll see!

After starting out cold and wet, the weather this January in Southern California, and much of the state, has been warm and dry. Monday, Pierce College in Woodland Hills hit a high of 88°F, and Downtown Los Angeles (USC) 84°F. So far this January, only 0.58 of rain has been recorded at Downtown Los Angeles (USC), which is a little less than one-third of normal for the month. However, because of our prodigious December rainfall the water year total at Los Angeles is currently 6.5 inches above the normal for the date of 5.78 inches.

Earlier this month I photographed these mid-level mammatus clouds over the western San Fernando Valley. They were produced by the moisture and dynamics associated with a closed upper level low that was about 390 miles WSW of Los Angeles. To see how the clouds evolved and more about the scenario at the time, see this post on PhotographyontheRun.com.

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

Thursday, January 20, 2011 7:25:42 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Sunday, January 09, 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, January 09, 2011 3:02:14 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Saturday, January 01, 2011

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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, January 01, 2011 3:13:31 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Sunday, December 26, 2010

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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, December 26, 2010 2:01:15 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Thursday, December 23, 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, December 23, 2010 12:30:24 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Friday, December 17, 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, December 17, 2010 10:48:15 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Monday, November 22, 2010

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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, November 22, 2010 8:12:02 PM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   |