Across the contiguous United States, extreme weather conditions dominated the third quarter of 2017 and brought departures from long-term average global horizontal irradiance, a proxy for PV plant production.

GTM has partnered with Vaisala, a company that regularly monitorssolarperformance across the U.S., to give the solar industry a better understanding of how these weather trends are driving solar performance around the country. This allows the industry to reconcile project performance by comparing it with recent solar conditions and putting this in context with long-term average conditions. The maps published below and the data behind them serve as tools for better understanding how weather impacts solar energy. 

The most recent set of U.S. maps evaluate departures from long-term average solar irradiance conditions in the third quarter of 2017. 

While the industry was focused on the potentially disastrous International Trade Commission ruling this summer, there were actual weather disasters that plant operators dealing with. Since July 2017, six dramatic weather and climate events impacted the solar industry, including the western U.S. wildfires, drought in the Northern Rockies and Plains regions, and major Hurricanes Harvey, Irma and Maria.

Nationally, 2017 tied the record year of 2011 for the number of billion-dollar weather-related disasters during the January-September period.

For July and August in particular, some states with significant solar deployed such as California, Nevada and Texas saw reductions in solar resource of 5 percent or greater due to weather events such as wildfires and hurricanes. The timing of these events during the high-production summer months is particularly significant, since the second half of the year may not be able to offset these losses.

For example, Kern County boasts an installed capacity of 4,881 megawatts of solar power. Vaisala estimates that a 1 percent loss in production in Kern County during a typical single month of peak summer production would equate to over $940,000 in lost revenues for operators across the region.  Of course, the impact increases when multiple months are affected or if the loss in production is significant, both of which were the case in Q3 2017.

A question we often get when reviewing these scenarios is: How well do solar resource anomalies correlate to production anomalies? To a first order, power generation is proportional to irradiance in the plane of the array. In the U.S., in the absence of plant performance problems, correlations of global horizontal irradiance (GHI) anomalies and power production anomalies are typically on the order of 0.9 or greater, based on tests run by Vaisala at sample locations.  

In fact, if your plant performance is not tracking with the GHI anomaly maps presented here, that can be a way to identify either a production problem or a poor-quality long-term baseline production assessment. 

For more details, read our month-by-month summaries below to learn how specific weather conditions and anomalous patterns throughout July, August and September may have influenced solar production across your portfolio.

July was the best month of the summer for solar production almost uniformly across the contiguous U.S. As noted by the NOAA National Centers for Environmental Information, an upper-level ridge dominated the West, bringing warmer-than-normal and drier-than-normal weather to the region and contributing to a prolonged and intense wildfire season. Above-average irradiance and below-average precipitation were also observed for parts of the central and southern Great Plains, Southeast and northern New England.

On the other end of the scale, below-average irradiance was observed in parts of the Southwest, due to an active monsoon season that kicked into high gear mid-month. As a precursor to the active summer hurricane season, Tropical Storm Emily brought cloud cover and heavy precipitation, soaking Florida at the end of the month.

Above-average precipitation was also observed in parts of the Northeast and Mid-Atlantic, with several intense one-day precipitation events causing significant flooding at numerous locations. The storms left lingering cloud cover, which negatively impacted solar performance, in addition to the impacts of flooding.

Similar to July, the pattern of an upper-level ridge over the western half of the contiguous U.S. and an upper-level trough over the eastern half persisted during August.

The most striking feature on the August anomaly map is the negative irradiance anomaly affecting most of the South after Hurricane Harvey made landfall. Texas received a record-breaking amount of rain, 1,016 millimeters, resulting in extreme flooding in Texas and the Lower Mississippi Valley.

Nearly 68,000 megawatts of generation capacity (almost 25 percent of that capacity being solar), was within Harvey’s path. Most of the solar installations fared as well as could be expected in terms of physical damage. However, the August anomaly map shows how these conditions affected systems in this region, with some areas experiencing resource levels of up to 20 percent lower than normal. 

Areas of the West also had lower-than-normal insolation. Except for isolated locations, this was not due to the local monsoon season but rather increased aerosol emissions from wildfires. Lower-than-normal irradiance values, 5 to 10 percent below average in some locations, are directly related to proximity to the wildfire areas, or to being downwind of the smoke plumes.

NASA's Goddard Space Flight Center has posted an excellent simulation of the global aerosol circulation pattern for August 2017. In the NASA animation, you can clearly see that the areas affected by the smoke plumes from the Western wildfires are co-located with the areas of negative irradiance anomalies shown on the map.

 

The dramatic east-west split down the country was not related to politics, but rather an upper-level circulation pattern that underwent a significant shift over the contiguous U.S. at mid-month. This upper-level shift also affected the temperature anomaly pattern across the U.S. during September. The first half of the month saw higher-than-normal temperatures affecting the western contiguous U.S. and cooler-than-normal temperature affecting the eastern portion. As the upper-level circulation switched, so too did the temperature anomaly.  

Numerous low-pressure systems moving in the western trough brought precipitation to much of the West and Plains regions during the last half of the month. The increased rainfall helped to suppress wildfires, but the increased cloud cover continued the low-solar-resource trend across the region, resulting in the negative irradiance anomalies shown on the September map.

September was an extremely active hurricane month for the North Atlantic Basin, with five hurricanes -- four of which were major hurricanes. Hurricanes Irma and Maria brought rain and devastation to parts of the U.S. but the cloudy conditions did not linger in the same way they did with Hurricane Harvey, at least in the contiguous U.S. Hurricane Irma devastated parts of Puerto Rico, the U.S. Virgin Islands and Florida. Then Hurricane Maria delivered a second punch, causing unprecedented impacts on Puerto Rico and the U.S. Virgin Islands.

The Solar Foundation is currently accepting donations for solar equipment and expertise to help rebuild the Puerto Rico grid and bring power back to critical services. If you’re interested, please consider donating.