Friday Map Discussion: 26 October 2007

Lance was out of town on Friday, but map discussion went on as usual, with the topic of discussion the meteorological conditions leading to the recent devastating southern California wildfires. As of Sunday evening 10/28, the fires had killed 7 people, burned more than a half-million acres, and destroyed about 2,300 homes. A New York Times interactive graphic nicely shows the daily extent of the largest of the 24 fires during the period 21–27 October.

The favorable meteorological and climate conditions for the California fires that were identified on Friday were (i) the severe drought in southern California (2006–present), (ii) western U.S. ridge amplification in association with the passage of a Rossby wave train (19–23 October 2007), and (iii) the onset and persistence of Santa Ana conditions (21–25 October 2007).

1. Severe southern California drought (2006–present)

The driest water year on record for downtown Los Angeles occurred between 1 July 2006 and 30 June 2007 (see water year precipitation time series obtained from http://home.att.net/~station_climo/). Only 3.21” inches of precipitation were recorded during the period (see 2006–2007 daily precipitation time series) – about one-fifth of the mean water year precipitation total of 15.08”. This precipitation total was more than an inch lower than the previous precipitation record of 4.42” set during the 2001–2002 water year. Several other stations in southwestern California also recorded their driest season on record, including Burbank (2.83”), Lancaster (1.40”), and Palmdale (0.65”) (source: NWS Los Angeles/Oxnard WFO). In more recent months (July–October 2007), dry conditions have persisted in southern California. A time series of 2007 Los Angeles daily precipitation shows that, with the exception of a two-day precipitation event in September, no precipitation has been recorded there since last April, at the end of the last rainy season (see graph of Los Angeles seasonal precipitation frequency distribution).

In addition to the dearth of rain over the past 1.5 years, southern California has experienced periods of unusually warm weather. A time series of 2007 Los Angeles daily average temperature indicates that average temperatures during June 2007 and the period from early August through early September 2007 were above normal, and a 2006 daily average temperature time series shows that exceptionally hot weather occurred from May through early September 2006. Plots of 850-hPa temperature and temperature anomaly for May–August 2006 show that 850-hPa average temperatures of 21–23°C over southern California during the period corresponded to anomalies of greater than +2°C.

The combination of much below-normal precipitation and periods of above-normal temperatures has led to extreme drought conditions in southern California, according to the National Drought Mitigation Center (see 23 October 2007 U.S. Drought Monitor map). Archives of the weekly Drought Monitor maps allow for an investigation of the evolution of drought spatial coverage and severity. Comparison of the 23 October 2007 Drought Monitor map with the Drought Monitor map from approximately 1 year ago (24 October 2006) shows a dramatic displacement of the U.S. drought region from the High Plains, southern Plains, and Arizona, to California and the interior West. In addition, the areal coverage of severe drought conditions has increased substantially from one year ago. (Also of note is the incredible expansion and increase in severity of the southeastern U.S. drought in the past year). According to the Drought Monitor text summaries (e.g., the 13 March 2007 summary), snow pack in the southern California mountains during the 2006–2007 winter was below normal and melted earlier than usual, exacerbating drought conditions.

2. Western U.S. ridge amplification connection to Rossby wave train passage (19–23 October 2007)

Of interest is the role of a Rossby wave train (downstream development) in the amplification of a ridge over the western U.S. that led to the establishment of an interior surface high and strong downsloping flow west of the San Jacinto, San Bernadino, San Gabriel, and Santa Ana mountains (topographic map from UCLA Department of Atmospheric and Oceanic Sciences). Plots of 1000–500-hPa thickness, 300-hPa wind speed, and sea level pressure obtained from Ron McTaggart-Cowen’s GFS analysis animation builder for 00Z on 18, 19, 20, 21, 22, and 23 October 2007 show the rapid transition of the Pacific jet from zonal to highly amplified, with a clear signal of a Rossby wave train evident in the successive pattern of troughs and ridges that propagates across the Pacific Ocean and North America. The development of a strong surface high over the interior West was associated with the amplification and eastward surge of an upper-level ridge over the eastern Pacific/western U.S. (see above 1000–500-hPa thickness, 300-hPa wind speed, and sea level pressure plots for 00Z 21–23 October). A signature of Rossby wave train propagation is also clearly evident in a Hovmöller diagram of 300-hPa meridional wind averaged between 30° and 50° N for October 2007 (generated at the ESRL/PSD website).

3. Santa Ana conditions (21–25 October 2007)

Santa Ana conditions developed rapidly on 21 October across southern California as high pressure became established over the interior West. A composite plot of visible satellite imagery and NAM sea level pressure analysis (obtained from San Francisco State University) for 00Z 22 October shows a complete absence of a marine layer near the California coast. (Also of interest is the composite plot for 18Z 23 October, which features smoke plumes from the fires that extend several hundred kilometers offshore.) 10-day meteograms from Ramona (northeast of San Diego) and Valley Center (north-northeast of San Diego) obtained from the MesoWest website at the University of Utah show that strong offshore winds and very dry conditions developed shortly before 12Z on 21 October. At Ramona, the dew point dropped from 40°F (4°C) to around 5°F (−15°C) in less than 4 hrs. At Valley Center, the changes were more dramatic, with a dew point drop of more than 75°F (44°C), from near 50°F (10°C) to near −30°F (−34°C). At both stations, the wind direction changed from variable to easterly and increased in speed from under 5 mph to 20–35 mph (17–30 kt), with gusts to 50 mph (44 kt). The dew points at both stations begin on 23 October to rebound slowly, though the wind direction remains predominantly from the east (offshore) until 25 October.

Soundings from San Diego (NKX) taken at 00Z 21 October, 12Z 21 October, and 00Z 22 October (obtained from University of Wyoming website) show the period of transition from marine layer conditions (characterized by onshore flow and a shallow, moist boundary layer) to Santa Ana conditions (characterized by offshore flow and a deep, well-mixed, dry boundary layer).

To finish, I’ll offer up a few unaddressed/unresolved science issues:

1. The extent that the drought in southern California over the past 1+ year can be attributed to La Nina conditions (an issue brought to light by Lynn Gribble).

2. The impact of local terrain such as flow channeling in the behavior of the fires in this case and others.

3. The trigger(s) (of tropical or extratropical origin) of the Rossby wave train associated with the western U.S. ridge amplification.

4. The frequency that Rossby wave trains play a role in western U.S. fire outbreaks.

Cheers,

Heather