Santa Ana Winds and Wildfires
February 21, 2018
Over a several week period lasting into December 2017, a prolonged period of Santa Ana winds over southwest California provided conditions favorable for large-scale wildfires, which caused widespread damage to vegetation and structures and a deterioration in air quality. Santa Ana wind conditions over southern California typically occur in fall, when high pressure over the Rocky Mountains/Desert southwest region produces an east or northeast flow of sinking air on the west side of topographic barriers. As the air flows towards the southwest, warming and drying occurs, and surface winds can be significantly enhanced when channeled through gaps in the topography. The dry, hot, and windy environment associated with Santa Ana events facilitates the growth and spread of wildfires.

In December 2017, surface high pressure over the northern Rockies drove the downslope northeasterly Santa Ana winds over southern California (Figure 1). The large-scale upper level flow over North America shows the jet stream bending northward to very high latitudes over the west coast, supporting the surface high pressure area in the west. Further east, the jet stream dove southward around low pressure over the eastern United States. This jet stream pattern is associated with warm air extending northward to high latitudes and cold air extending well into the central and eastern United States.

Santa Ana Figure

Figure 1. NOAA surface analysis at 1200 UTC December 6, 2017. The solid brown lines are the sea level pressure contours. The bright red lines with semi-circles depict warm fronts, and the blue lines with triangles depict cold fronts. The red arrows across southern California show the direction of the surface wind flow.

Santa Ana Figure

Figure 2. NOAA 500 mb constant pressure analysis (at about 18,000 feet) for December 6, 2017. The red arrows show the location of the jet stream. The blue contours show heights of the 500mb pressure value. Dashed lines are temperature contours.

This large-scale weather pattern supported prolonged Santa Ana wind conditions and wildfires over southern California during December 2017. The MODIS Polar orbiting satellite showed the transport of wildfire-generated smoke plumes southwestward over the ocean within the northeast low-level wind flow (Figure 3).

Santa Ana Figure

Figure 3. MODIS polar orbiting satellite image at 250 meter resolution on December 6, 2017. Image courtesy of the University of Wisconsin.

By January 9, 2018 the large-scale flow pattern had changed dramatically and was dominated by a westerly flow extending from the Pacific across the United States and a strengthening east Pacific low pressure area approaching Southern California. Figure 4 shows low pressure and a cold front with a southwest wind flow across southern California. Figure 5 shows the jet stream flow with an upper low pressure area near the southern California coast. These weather conditions resulted in several inches of heavy rainfall across southern California.

Santa Ana Figure

Figure 4. NOAA surface weather analysis at 1200 UTC on January 9, 2018. The solid brown lines are the sea level pressure contours. The bright red lines with semi-circles depict warm fronts, and the blue lines with triangles depict cold fronts. The red arrows show the direction of the surface wind flow over southern California.

Santa Ana Figure


Figure 5. NOAA 500 mb constant pressure analysis (at about 18,000 feet) for January 9, 2018. The red arrows show the location of the jet stream. The blue contours show heights of the 500mb pressure value. Dashed lines are temperature contours.

How Exponent Can Help?

Exponent’s services include:

  • Meteorological Analysis – Exponent has strong capabilities and deep experience in meteorological analysis including state of the science numerical meteorological modeling with the Weather Research and Forecast model (WRF), computational Fluid Dynamics modeling (CFD), weather radar analysis, satellite imagery interpretation, and use of NOAA three-dimensional numerical analysis data such as the High-Resolution Rapid Refresh (HRRR) data. Exponent scientists can use the WRF model Fire module to perform numerical simulations of wildfires. Radar data coupled with available rain gauge data can be used to develop rainfall estimates at high spatial resolution over areas of interest. 
  • Health Effects Analysis – Exponent’s industrial hygienists and health consultants provide support related to health impacts from moisture and water intrusion, environmental mold, odors, and other related issues from furniture, consumer products; food and other materials.
  • Investigation of Structural and Architectural Damage – Exponent’s engineers work with adjusters to determine the nature and extent of damage, evaluate the contributions of perhaps several causal factors, and identify the most appropriate repairs.
  • Environmental Analysis – Exponent’s environmental scientists have evaluated post-fire wildland and vegetation recovery, fire-related impacts to storm water and drinking water quality, releases of fuels and hazardous materials during fire and flood events, crop loss and agricultural damage, and ecological impacts following fires. 
  • Geotechnical Analysis – Exponent has analyzed mudslides and debris flows as well as erosion and soil loss following fires and rain events. 
  • Landslide Investigation – Exponent geologists and engineers investigate pertinent aspects of landslides and erosion. Key elements in an investigation depend on site characteristics and the needs of the client and can include emergency response and evaluation of imminent hazards to persons and structures, as well as assessment of damage to structures and infrastructure.
  • Flood Hazard Analysis – Exponent staff have considerable experience using proven approaches for investigating flood hazards. We integrate elements of geologic mapping, mapping of geomorphic features, detailed topographic data using Light Detection and Ranging (LiDAR) technology, hydrologic analysis to determine flood frequencies and peak discharges, hydraulic analysis of offsite and onsite drainage channels and flood control pathways, retention/detention basin analysis, sediment transport, and numerical simulations of flood propagation.
  • Wildfire Impacts – Exponent is skilled in the evaluation of wildfires and their impacts, including geologic and hydrologic hazards, as well as the ecological and water quality impacts that a fire has within and downstream of the burned areas. We can evaluate the hazards resulting from wildfires and provide remedial solutions tailored to the local site conditions.

For more information, please contact the following:

Meteorology

Alfred M. Klausmann, CCM
Managing Scientist, Health Sciences
(978) 461-4628 | aklausmann@exponent.com

Environmental Analysis

Susan C. Paulsen, Ph.D., P.E.
Principal Scientist & Practice Director, Environmental & Earth Sciences
(626) 204-4089 | spaulsen@exponent.com

Flood Studies, Landslides, etc.

Macan Doroudian, Ph.D., P.E., G.E.
Managing Engineer, Civil Engineering
(949) 242-6025 | mdoroudian@exponent.com

Philip J. Shaller, Ph.D., P.G., C.E.G.
Senior Managing Scientist, Civil Engineering
(949) 242-6006 | pshaller@exponent.com

Buildings & Structures

Brian McDonald, Ph.D. S.E
Principal Engineer and Practice Director, Buildings & Structures
(650) 688-6946 |  mcdonald@exponent.com


Wildfires

Erik Christiansen, Ph.D, P.E., C.F.I.
Principal Engineer, Thermal Sciences
(310) 754-2723 |  echristiansen@exponent.com

Abid Kemal, Ph.D.
Principal, Thermal Sciences

(650) 688-7181 | akemal@exponent.com

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