CalClim: California Climate Data Archive

CALIFORNIA CLIMATE WATCH
CURRENT ISSUE: April 2005

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A Winter To Remember

By Laura Edwards
Western Regional Climate Center

In water supply circles in California, we often refer to the water year, as opposed to the calendar year, to measure precipitation. Because little (if any) precipitation falls in the summer season, and snowfall and snowpack are vitally important to water supply, the water year begins on October 1. At the end of April we reached the midway point for the 2005 Water Year. The last 6 months have been exceptional weather- and climate-wise, with records being broken nearly as fast as these words can be typed.

There are many parts to the complicated story of Water Year 2005. Precipitation, snowfall, snowdepth, temperature, streamflow, runoff, large-scale atmospheric patterns, landslides and injuries and deaths due to the weather are all chapters to this story. Each has an important role in creating the plot of interesting weather behavior.

Figure 1. Percent of normal precipitation in California as of May 8, 2005.

Numerous precipitation records have been set throughout south and central California. Los Angeles has received an exceptionally large amount of precipitation, near double their normal annual average of 15.14 inches. As of press time, downtown Los Angeles had 30.71 inches on the books since October 1, and 37.11 since July 1, the beginning of the rain year. This surpasses the annual average of 37.07 inches for Seattle-Tacoma airport in Washington. Other stations in the area, such as Opids Camp in the San Gabriel Mountains have not been excluded from the extreme precipitation. It is not abnormal for Opids Camp to receive about three times the precipitation as Los Angeles, and as early as February 25th 107 inches of precipitation had been recorded since July 1. Figure 1 shows the percent of normal precipitation so far this water year in California.

In the southwest desert region, this abnormal amount of rain led to flooding in a number of rivers, including the Amargosa River. Two men, one from the Bay Area and his friend from southern California, ran the river in a kayak and a canoe for miles. Under normal circumstances, this is a dry riverbed. The seemingly constant pitter-patter of raindrops also produced one of the best spring wildflower displays in Death Valley in nearly a century.

Some of the same weather systems that brought rain to central and southern California produced large snowfall totals in the Sierra Nevada mountains as well. The storms that rolled through in late December into mid-January dropped 13 feet of snow at the Central Sierra Snow Lab and 10 feet at Tahoe City (1). This season's snowfall in Reno is currently measured in feet, not inches, with over 7 feet falling in the December-January period alone.

Despite these large precipitation amounts around most of the state, northern California has experienced drier than normal conditions for much of the water year, particularly in the Klamath River basin on the Oregon border. Due to anomalously low precipitation and snowfall, the California Department of Water Resources' Bulletin 120 has consistently predicted below average runoff and streamflow in this region for the April-July period. This is an extension of the drought that has prevailed the Pacific Northwest due, at least in part, to a more southerly jet stream track that has brought California its fortunes in water. Although the Klamath River basin has been drier than normal, and the Colorado Basin has had a multi-year drought, overall water storage in the system is above normal this water year. Warm days and cool nights in the spring have helped keep the snowpack in place and melt slowly, which is also beneficial for the reservoirs, groundwater supply and water storage.

Figure 2. October 2004- March 2005 anomalous wind speed at 200 mb, the jet stream level. From Climate Diagnostics Center.

WATER YEAR WEATHERMAKERS:
Major contributing factors to the weather California has experienced include a blocking pattern on the West Coast, Madden Julian Oscillation activity, and corresponding changes in the jet stream and other atmospheric variables.

A weak warm ENSO event, or weak El Nino event, was also declared in October. With this announcement came some expectations for the winter season's climate. The weak El Nino event of this past winter differed from a "normal" event, however. The warm sea surface temperatures that migrate eastward in the tropical Pacific did not move much past the international dateline. A second characteristic of the sea surface temperatures was the magnitude of the anomalously warm temperatures, much higher than a usual El Nino event in this dateline region. It is interesting to note that the north dry/south wet pattern is a traditional El Nino year result, but the oceanic properties did not align in a traditional way.

This is where the Madden-Julian Oscillation (MJO) makes an appearance. This oceanic phenomenon carries warm water from the Indian Ocean. Figure 3 illustrates the sea surface temperatures since November. The orange colors represent warmer waters. From late December to mid-January this warm pool moved across the Pacific, which is a sign of increased storminess. In combination with MJO at this time, the subtropical jet was fairly south of its usual winter track, and thus carried warm, moist air from the tropics. This feature is also called the Pineapple Express. While MJO does not tell the entire saga of the 2005 water year, it is a force to be reckoned with. The effect was Los Angeles set records for its largest one-day rain total for December since records began in 1877 with 5.55 inches on December 28th. This is also the third wettest single day in its history.

Figure 3. Pineapple Express illustration, from http://www.cpc.ncep.noaa.gov/products/winter_outlook/pineapple.gif

In mid-January, southern California was impacted again with heavy rainfall. The large-scale weather pattern was a little different, with cold air coming from a low pressure system off the Pacific Northwest coast and moist air from the subtropical jet stream. Combine orographic influences with the blocking pattern that was setting up in the Pacific, the state was headed for a prolonged period of precipitation. (2) Compared to December's storms, this event brought about 50% more precipitation to southern California. One example is Opids Camp, which received 19.83 inches December 26-January 5, and then 31.94 inches January 7-11. This event also caused a number of landslides, most notably at La Conchita where 10 people died.

February brought another phenomenon to the West Coast, the Rex block. The Rex block is a stationary weather pattern where high pressure off the Canadian coast is north of low pressure off the California coast. This pattern brings shortwave energy and subtropical moisture from the ocean around this low pressure in a counter-clockwise fashion. The strongest of these shortwaves were headed for the southern California coast, and this condition persisted for nearly a week, producing 8-10 feet of snow in the mountains above Los Angeles, and 4-8 inches of rain in the coastal locations. By this time, Los Angeles and San Diego had both reached their third wettest years since their records began.

Currently, Los Angeles has 1.12 inches to go before breaking its rain year precipitation record by June 30. The normal precipitation for this station for May and June do not quite total this amount, but then again, this hasn't been a normal year. There is no doubt that if no more records are broken this year, it will continue to remain in our memories as one of the wettest winters in over a century.

Page last updated 5/16/05.

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Climate Watch Archive All data in the Climate Watch is provisional and subject to change. Jan 2005 [PDF] [Word] Feb 2005 [PDF] [Word] FMar 2005 [PDF] [Word] For 2004 stories, click here Monthly Summaries by Bill Mork 2005 2004	CALIFORNIA CLIMATE WATCH CURRENT ISSUE: April 2005 Read the full newsletter here: Word Format or PDF format A Winter To Remember By Laura Edwards Western Regional Climate Center In water supply circles in California, we often refer to the water year, as opposed to the calendar year, to measure precipitation. Because little (if any) precipitation falls in the summer season, and snowfall and snowpack are vitally important to water supply, the water year begins on October 1. At the end of April we reached the midway point for the 2005 Water Year. The last 6 months have been exceptional weather- and climate-wise, with records being broken nearly as fast as these words can be typed. There are many parts to the complicated story of Water Year 2005. Precipitation, snowfall, snowdepth, temperature, streamflow, runoff, large-scale atmospheric patterns, landslides and injuries and deaths due to the weather are all chapters to this story. Each has an important role in creating the plot of interesting weather behavior. Figure 1. Percent of normal precipitation in California as of May 8, 2005. Numerous precipitation records have been set throughout south and central California. Los Angeles has received an exceptionally large amount of precipitation, near double their normal annual average of 15.14 inches. As of press time, downtown Los Angeles had 30.71 inches on the books since October 1, and 37.11 since July 1, the beginning of the rain year. This surpasses the annual average of 37.07 inches for Seattle-Tacoma airport in Washington. Other stations in the area, such as Opids Camp in the San Gabriel Mountains have not been excluded from the extreme precipitation. It is not abnormal for Opids Camp to receive about three times the precipitation as Los Angeles, and as early as February 25th 107 inches of precipitation had been recorded since July 1. Figure 1 shows the percent of normal precipitation so far this water year in California. In the southwest desert region, this abnormal amount of rain led to flooding in a number of rivers, including the Amargosa River. Two men, one from the Bay Area and his friend from southern California, ran the river in a kayak and a canoe for miles. Under normal circumstances, this is a dry riverbed. The seemingly constant pitter-patter of raindrops also produced one of the best spring wildflower displays in Death Valley in nearly a century. Some of the same weather systems that brought rain to central and southern California produced large snowfall totals in the Sierra Nevada mountains as well. The storms that rolled through in late December into mid-January dropped 13 feet of snow at the Central Sierra Snow Lab and 10 feet at Tahoe City (1). This season's snowfall in Reno is currently measured in feet, not inches, with over 7 feet falling in the December-January period alone. Despite these large precipitation amounts around most of the state, northern California has experienced drier than normal conditions for much of the water year, particularly in the Klamath River basin on the Oregon border. Due to anomalously low precipitation and snowfall, the California Department of Water Resources' Bulletin 120 has consistently predicted below average runoff and streamflow in this region for the April-July period. This is an extension of the drought that has prevailed the Pacific Northwest due, at least in part, to a more southerly jet stream track that has brought California its fortunes in water. Although the Klamath River basin has been drier than normal, and the Colorado Basin has had a multi-year drought, overall water storage in the system is above normal this water year. Warm days and cool nights in the spring have helped keep the snowpack in place and melt slowly, which is also beneficial for the reservoirs, groundwater supply and water storage. Figure 2. October 2004- March 2005 anomalous wind speed at 200 mb, the jet stream level. From Climate Diagnostics Center. WATER YEAR WEATHERMAKERS: Major contributing factors to the weather California has experienced include a blocking pattern on the West Coast, Madden Julian Oscillation activity, and corresponding changes in the jet stream and other atmospheric variables. A weak warm ENSO event, or weak El Nino event, was also declared in October. With this announcement came some expectations for the winter season's climate. The weak El Nino event of this past winter differed from a "normal" event, however. The warm sea surface temperatures that migrate eastward in the tropical Pacific did not move much past the international dateline. A second characteristic of the sea surface temperatures was the magnitude of the anomalously warm temperatures, much higher than a usual El Nino event in this dateline region. It is interesting to note that the north dry/south wet pattern is a traditional El Nino year result, but the oceanic properties did not align in a traditional way. This is where the Madden-Julian Oscillation (MJO) makes an appearance. This oceanic phenomenon carries warm water from the Indian Ocean. Figure 3 illustrates the sea surface temperatures since November. The orange colors represent warmer waters. From late December to mid-January this warm pool moved across the Pacific, which is a sign of increased storminess. In combination with MJO at this time, the subtropical jet was fairly south of its usual winter track, and thus carried warm, moist air from the tropics. This feature is also called the Pineapple Express. While MJO does not tell the entire saga of the 2005 water year, it is a force to be reckoned with. The effect was Los Angeles set records for its largest one-day rain total for December since records began in 1877 with 5.55 inches on December 28th. This is also the third wettest single day in its history. Figure 3. Pineapple Express illustration, from http://www.cpc.ncep.noaa.gov/products/winter_outlook/pineapple.gif In mid-January, southern California was impacted again with heavy rainfall. The large-scale weather pattern was a little different, with cold air coming from a low pressure system off the Pacific Northwest coast and moist air from the subtropical jet stream. Combine orographic influences with the blocking pattern that was setting up in the Pacific, the state was headed for a prolonged period of precipitation. (2) Compared to December's storms, this event brought about 50% more precipitation to southern California. One example is Opids Camp, which received 19.83 inches December 26-January 5, and then 31.94 inches January 7-11. This event also caused a number of landslides, most notably at La Conchita where 10 people died. February brought another phenomenon to the West Coast, the Rex block. The Rex block is a stationary weather pattern where high pressure off the Canadian coast is north of low pressure off the California coast. This pattern brings shortwave energy and subtropical moisture from the ocean around this low pressure in a counter-clockwise fashion. The strongest of these shortwaves were headed for the southern California coast, and this condition persisted for nearly a week, producing 8-10 feet of snow in the mountains above Los Angeles, and 4-8 inches of rain in the coastal locations. By this time, Los Angeles and San Diego had both reached their third wettest years since their records began. Currently, Los Angeles has 1.12 inches to go before breaking its rain year precipitation record by June 30. The normal precipitation for this station for May and June do not quite total this amount, but then again, this hasn't been a normal year. There is no doubt that if no more records are broken this year, it will continue to remain in our memories as one of the wettest winters in over a century. Page last updated 5/16/05.