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.

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.

A Pacific storm system -- which at one point was expected to be the wettest of the rain season thus far -- took a more westward track and skirted the coast of Southern California on Christmas Eve, generally producing only modest rainfall across the area.

Today, the trough digging down the coast is further west that forecast. (In retrospect this might have been foreseen taking into account the GWO phase 1-2 transition.) In any case, the flow into California has been moist and cyclonic, and rain has continued in many areas, with snow at the higher elevations.

A front associated with this trough is working its way down the coast, and early this afternoon is just north of Los Angeles. This could add a little more to our rainfall totals as it moves through later today. We'll see!

Update 12/26/08 10:30 am. Here are some preliminary storm totals from the NWS Los Angeles/Oxnard and storm totals from the NWS San Diego.

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

A potent appearing Pacific storm system is developing off the coast of Oregon and Northern California and headed our way. Significant rain, with snow at the higher elevations, is expected over much of the state.

Rain associated with this system is forecast to begin in Northern California this evening. At the moment, it looks like prefrontal rain could develop in the Los Angeles area tomorrow morning. Rainfall rates are forecast to increase Christmas Eve, and then taper off during the day on Christmas.

Although the models appear to be converging on similar solutions, there has been a lot of variability from run to run. This is due in part to the difficulty of the forecast. The intensity of the low, the path of the low center, the amount of moisture available to the system, the upper and lower level jet dynamics and other factors could change the amount of rainfall.

The HPC QPF forecast for the period Tuesday afternoon to Friday afternoon shows about 1.25 to 1.50 inches along the coast with up to about 3 inches in the mountains. These amounts could be somewhat understated. We'll see!

The system that moved through the Los Angeles area Sunday night into Monday was a little stronger than expected, with rainfall amounts generally ranging from about 0.1 inch to 0.25 inch. Here are some preliminary rainfall totals from the NWS.

Update 12/24/08 11:00 am. 12z NAM and GFS forecast a more westward track of the low and have backed off precipitation amounts forecast for the Los Angeles area. The 12z NAM generates about 0.6 inch at Van Nuys and the 12z GFS only about 0.26 inch for the storm. HPC's latest 2-day Quantitative Precipitation Forecast indicates precipitation amounts ranging from about 0.75 inch to 1.50 inch in coastal Southern California. On the other hand, the 06z high resolution WRF-NMM was quite wet, with precipitation amounts in our area ranging from 1.0 to 4.0 inches.

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

A cold upper level low produced widespread rain and snow in Southern California, closing highways, fouling traffic, and chilling Southlanders. As much as a foot of snow was reported in the Antelope Valley and the snow level dropped to near 2000 ft in the foothills and mountains of Ventura and Los Angeles counties.

As forecast by the NAM/WRF, the precipitation totals were higher to the south of the Los Angeles basin. This Intellicast.com animated loop shows the pattern of rain and snowfall from Tuesday evening to Wednesday evening. Here are some preliminary storm totals from the NWS Los Angeles/Oxnard and storm totals from the NWS San Diego.

Downtown Los Angeles (USC) recorded 0.51 inch for the storm, bringing the water year rainfall total at Los Angeles to 2.79 inches. As of December 17, this is 1.41 inches above normal, and 0.34 inch more rain than we had last year on this date.

What's next? RAMDIS 4km water vapor animation shows a weak disturbance northwest of Pt. Conception, moving east toward the Central Coast. Although today's 18z NAM/WRF is dry overnight south of about Monterey, this area of vorticity could produce a shower north of Pt. Conception. After that, things should stay dry in Southern California until around Monday, when a fast moving front sweeps through the state. Later in the week, sometime around Christmas, the models are suggesting the possibility of a major system impacting California. We'll see!

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

An intense upper level low, fed in part by moisture that originated in the subtropical mid-Pacific, produced widespread rain in Southern California, with snow at the higher elevations. Precipitation totals generally ranged from about 1 to 3 inches, and new rainfall records for December 15 were set at LAX, UCLA, Long Beach, San Gabriel and Santa Barbara. Here are some preliminary rainfall totals from the NWS.

A very cold core upper level cutoff low is currently spinning off the California coast. A GOES sounding from this afternoon indicated a temperature of -33°C at the 500 mb level and significant instability. The low is forecast to move onshore over the next 24 hours, and slight changes in its trajectory could have a big impact on precipitation amounts and intensity.

The NAM/WRF has been quite a bit drier in the Los Angeles area than the GFS, with the heaviest precipitation occurring south of the basin. The latest (00z) run of the NAM appears to be wetter in the Los Angeles area than runs earlier today. With such cold temperatures aloft, heavy rain and strong thunderstorms are a possibility, particularly wherever the low moves onshore. BUFKIT analysis of PMD and VNY data suggests the snow level could drop to 2000-2500 ft., perhaps lower in heavy showers. We'll see!

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

It's been a couple of weeks since an upper level low set rainfall records for the date at several Southern California locations, and pushed water year rainfall totals above normal at Los Angeles. Since then we've had almost no rain, and as of yesterday, Downtown Los Angeles (USC) rainfall has dropped to about six-tenths of an inch below normal. If a storm later this weekend works out as expected, we could be back above normal by Monday night.

Several factors could affect the forecast. As we've seen earlier this season, the computer models don't do a particularly good job with high amplitude flows, retrograding ridges and digging troughs. Throw into the mix a strong 150-170 kt. jet stream, sub-tropical moisture being advected from Hawaii (nearly) to the Aleutians, and cold air being drawn down from Canada and Alaska, and there is the potential for high impact weather.

A chance of showers is forecast Saturday into Sunday, but at the moment, the main event is forecast for Sunday evening through Monday. HPC's QPF for that period indicates rainfall amounts ranging from about 0.75 inch to 2 inches or more in the mountains. We'll see!

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

Happy Thanksgiving! Our record-setting upper low system is slowly moving east out of the area, but this morning there's still some shower activity occurring south and east of the Los Angeles basin. The best chance of showers today is in the mountains, and areas south and east of the basin, but a shower could pop up just about anywhere in Southern California.

Rainfall totals for the storm have generally ranged from 1.0 to 2.0 inches in the coastal and valley areas, with some higher amounts recorded in foothill and mountain locations. Rainfall records for November 26th were broken at several locations, including Lancaster, Palmdale, Sandberg, UCLA, Burbank, Woodland Hills, and San Gabriel. OPIDS Camp, in the San Gabriel Mountains has recorded 3.29 inches. Here are some preliminary rainfall totals from the NWS.

Through yesterday, Downtown Los Angeles (USC) has recorded a storm total of 1.60 inches, bringing the water year total to 1.87 inches, which is 0.17 inches above normal for the date.

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

In the past week the weather models have been all over the map -- and sometimes off of it -- trying to come up with a forecast for this week and the Thanksgiving holidays. Now that a potent looking upper level low system is bearing down on the Southland, the 18z NAM and 18z GFS both agree we are going to get drenched.

The Hydrometeorological Prediction Center (HPC) is projecting from one to two inches of rain over a large area of Southern California. BUFKIT analysis of 18z data for Van Nuys generates rainfall totals of in excess of 2 inches for the storm. Depending on the strength of the southerly inflow, rainfall totals could be higher on south facing foothill and mountain slopes.

Radar is already showing some echoes aloft, and some showers could occur overnight and into tomorrow. Rain is expected to begin in earnest during the day tomorrow, perhaps as late as tomorrow afternoon or evening, and taper off late Wednesday night into Thursday morning.

NOAA has just released their 2008-2009 Winter Outlook. The precipitation outlook for December through February indicates an equal chance of below normal, near normal, or above normal precipitation for California. The IRI Multi Model Seasonal Forecast for Dec-Jan-Feb indicates below average precipitation is slightly more likely in southern third of California.

Update 11/25/08. BUFKIT analysis of 12z NAM and GFS data for Van Nuys isn't quite as wet as yesterday's 18z run. Over the next 36 hours, the 12z GFS generates about 0.8 inch, and the 12z NAM about 1.7 inch of rain, with maximum rain rates of about 0.4 inch/hour. Rain is expected to move into the Los Angeles area later this afternoon or evening.

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

Downtown Los Angeles (USC) happened to be under the right cloud yesterday, and recorded 0.02 inch of rain. There's a continuing chance of showers in Southern California through the weekend. The area could use some rain. The last storm to produce more than a couple tenths of inch of rain at Downtown Los Angeles was back on February 24, 2008 -- more than eight months ago.

Last year, California's rain season might have been scripted by Dr. Jekyll and Mr. Hyde. Lowland rain totals and the Sierra snowpack were well above normal through the end of February, and then the spigot was turned off. The storm track shifted north, and California had its driest March to May on record. (The March 29, 2008 post in Ed Berry's Atmospheric Insights blog describes the events that may have led to and maintained this shift.)

Which rain season personality will emerge this Winter? Earlier this year the equatorial Pacific tried to transition to El Nino, but the atmosphere didn't cooperate. As measured by the Multivariate ENSO Index (MEI), the ENSO state has returned to the weak La Nina threshold. And as measured by the Global Wind Oscillation (GWO), an uncooperative atmosphere has continued to cycle through a La Nina base state.

This suggests a winter precipitation outlook with a La Nina flavor -- similar to last year. This precipitation map, generated by the ESRL-PSD Composite ENSO plots page, shows the mean November-March precipitation for the U.S. during 9 La Niña events from 1948 to the present. The average La Niña rainfall indicated for coastal Southern California is in the 7.0-10.5 inch range. Last season's November-March rainfall in Downtown Los Angeles was about 12 inches.

Here's the current Nov-Dec-Jan precipitation outlook issued October 16 by the Climate Prediction Center. The "EC" means an equal chance of below normal, near normal, or above normal precipitation. The IRI Multi Model Seasonal Forecast for Nov-Dec-Jan is similar. We'll see!

Update 11/4/08. Here are the NWS Public Information Statements with preliminary rainfall totals for the period from Friday evening to Sunday morning, and from overnight last night to 10:00 a.m. this morning. The current Climate Prediction Center 6-10 day and 8-14 Precipitation Outlooks project below normal rainfall for Southern California. In Atmospheric Insights, Ed Berry paints a scenario with an extended North Pacific Jet collapsing into a strong western U.S. trough in the week 2-3 timeframe. We'll see!

Update 11/1/08. Did a trail run this morning in the Pt. Mugu State Park Area. Over the course of 3+ hours watched 2-3 waves of convection march northeast from the Santa Barbara Channel across the Oxnard Plain. We were showered on a few times, but were very happy to be east of these training T-storms and heavy rain. Here is a NWS Public Information Statement with some rainfall totals as of 11:00 a.m.

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

Downtown Los Angeles recorded a trace of rain, and a few other stations in Southern California recorded meager, but measurable, precipitation Monday. Where it did rain, amounts of a few hundredths of an inch or less were the norm. A good cell must have developed over the southwestern San Gabriel Mountains -- a NWS technical discussion mentioned that 0.5 inch was recorded at Big Tujunga Dam.

Following our "welcome to Summer" heatwave back in June, summertime temperatures have generally been seasonable. Yesterday, Los Angeles Pierce College recorded a high of 102°F, and inland temperatures today appear to be running a little warmer.

This weekend temperatures are expected to drop dramatically as a Pacific storm system and it's unseasonably strong 165+ kt jet affect the west coast this Friday into Saturday. If this morning's 12z GFS verifies, as much as 3-4 inches of precipitation could occur in the mountains of Northern California, Oregon and Washington. At high elevation some of this precip could be in the form of snow.

The front associated with the trough is currently forecast to hold together south of Pt. Conception and could produce some showers in the Los Angeles area on Saturday, particularly in the mountains. A BUFKIT analysis of 12z GFS data generates about 0.3 inch of rain at Van Nuys, but given the time of year, and the ample time remaining between now and Saturday, we'll have to see!

Ed Berry first mentioned the possibility of an "anomalous extended North Pacific Ocean jet collapsing into a western USA trough" in a September 6, 2008 post on Atmospheric Insights, and refined the projection, including a timeframe, in subsequent posts.

Update 10/06/08. Saturday's trough and front behaved about as expected with scattered light rainfall around the area. Precipitation amounts generally ranged from a trace or less to about 0.1 inch or so in some foothill and mountain locations.

Update 10/03/08. Focus of the incoming storm system continues to be Northern California and the Pacific Northwest. A BUFKIT analysis of 12z NAM data shows the expected north to south gradient in projected precipitation. About 0.5 inch is projected at Monterey, 0.2 inch at San Luis Obisbo, 0.1 inch at Santa Barbara, and 0.06 inch at Van Nuys. The 09z SREF Ensembles suggest a high probability, low precipitation event in the Los Angeles area with about a 90% probability of more than 0.01 inch of rain, but less than a 10% probability of more than 0.1 inch. We'll see!

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

Monsoonal flow that began last Thursday has resulted in numerous thunderstorms in the mountains and deserts of Southern California, with flash flooding and debris flows reported in several locations.

Areas in or near recently burned terrain are particularly susceptible, and flash floods and debris flows appear to have originated in the 2007 Oak Fire burn area in the eastern Sierra, and the still burning Piute Fire burn area near Lake Isabella.

This sequence of three GOES-WEST VIS/IR satellite images from NRL's NEXSAT web site shows the explosive development of a convective cell north of Mt. Whitney on Saturday afternoon. This appears to be the thunderstorm that produced the heavy rain in the area burned by the Oak Fire, and resulted in the Oak Creek debris flow near Independence. Here's an AVI loop, generated on the NEXSAT web site, for the period 3:00 p.m to 6:00 p.m. PDT on Saturday.

A particularly large thunderstorm cell/complex developed in the Sierra Nevada south of Mt. Whitney midday Saturday and drifted south down the Kern River drainage over the course of the afternoon. At about 2:45 p.m. the cell/complex was centered near the confluence of the Little Kern River and N.F. Kern River and measured about 44 miles in diameter. Here's a Google Earth image of the cell/complex with a 250m resolution True Color Aqua cloud layer imported from the MODIS Rapid Response System.

This GOES-East/West Precipitable Water composite from 9:00 this morning indicates precipitable water values as high as 1.6" in some areas of Southern California, and the possibility of more mountain and desert thunderstorms continues today.

Update July 25, 2008. The large cell/complex that developed south of Mt. Whitney on Saturday, July 12, and drifted south down the Kern River drainage appears to have produced the flash flooding and debris flows on Erskine Creek and other creeks in the Lake Isabella area late Saturday afternoon. This cell/complex can also be seen in sequence of three GOES-WEST VIS/IR satellite images and the AVI loop previously referenced.

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