by Gwendalyn Bender, Energy Assessment Product Manager, Vaisala

Did your plant underperform expectations last year? If so, you may currently be under pressure to evaluate and fix any of a mirade of possible equipment or operational issues. While it’s likely some instances of low production are due to equipment, others may simply be caused by a lack of solar resource due to cloudy weather. If your project came online last year in the U.S., it is much more likely that the low output was caused by the latter.

 2015 was a wetter than normal year for much of the United States, which was good for states struggling with drought but not so good for the solar industry. For example, Texas and the surrounding area was hit particularly hard in three of the last four quarters, having significantly cloudier conditions than normal. Last year 300MW of new solar projects were expected to come online in Texas. Without factoring in the affect of this year’s weather and its deviation from long-term conditions, many of those projects may have been flagged as “underperforming expectations.” A difficult start for an industry trying to get established in a new region.

 “There was more storm activity than normal this past year,” said Dr. Louise Leahy, Solar Resource Analyst at Vaisala. “Disturbances in the normal upper-level flow patterns in the first half of the year and the onset of a strong El Niño in the second half created particularly wet conditions in the central and western United States.” The Northeast did see a nice break from last year’s storms with higher than average irradiance for much of the year and the Northwest experienced exceptionally high irradiance conditions.

 The onset of a major El Niño event caught significant media attention due to its impact on the wind industry. Reduced power production at many wind farms in the central United States was blamed on lower than average wind speeds caused by the same storm patterns that affected irradiance in the region. While the full effect of El Niño will not be experienced until the first half of 2016, it is clearly already having an effect on the renewable energy industry.

When it comes to solar plants, these weather patterns have a direct impact on power production. Vaisala’s solar performance maps are a good reminder of why it is critical to weather-adjust power performance at solar plants with on-site measurements or a regular weather data feed. Accounting for the weather should be the first step in assessing whether a solar plant is performing as expected. This is particularly important for plants coming online without a benchmark of long-term irradiance variability. In these cases, knowing when solar conditions were well above or below normal can save a lot of effort in chasing down equipment problems that simply don’t exist.

Understanding how these climate patterns impact performance at your solar plant is critical for reconciling recent performance and identifying the true cause of over or underperformance. From a fleet perspective across a group of power generation assets, building a climate resilient portfolio means paying attention to how weather patterns can vary in different geographies and building a portfolio to balance their effects on power production. This might mean building projects in different areas of the country or building some solar and some wind projects in the same geography.

2015 U.S. Solar Performance: Quarter-by-Quarter Summary


Similar to the first quarter of 2014, higher than normal insolation in the western and central U.S. was associated with an exceptionally strong long wave ridge which dominated atmospheric circulation patterns for most of the quarter. This ridging pattern was characterized by clear skies leading to above average insolation in these regions. In the southern and southeastern regions of the U.S., higher than normal precipitation levels were the result of short wave disturbances in the upper-level flow pattern. As the quarter progressed, above normal precipitation levels affected most of the southern and Gulf Coast states. This pattern was a result of the close proximity of the North Atlantic High to the southeastern coast. The flanking low pressure area to the north of the North Atlantic High brought vigorous storm activity to the Southeast and along the coast as far north as the mid-Atlantic states, producing above normal precipitation in this region and negative solar irradiance anomalies.


Strong negative anomalies over the Central and Southern Plains, and the Gulf Coast states, were a direct result of the numerous storm systems traveling though the jet-stream. The southerly flow associated with these low-pressure systems fueled the storms with moisture from the Gulf of Mexico. Many states received 20 inches or more of rain, producing severe flooding in Texas and Oklahoma. Texas had the wettest May on record. With the storm tracks located in the southern U.S., below-normal precipitation levels were experienced in the Northwest, producing positive solar anomalies in that region. Washington state experienced the second driest April to May period on record.


High pressure ridges in the Pacific Northwest and in the southern U.S. brought warmer and drier than normal conditions to these regions, resulting in positive anomalies in solar irradiance. Many El Niño-enhanced storms formed in the Equatorial Pacific, but they did not directly impact the West Coast because of the presence of the ridge. During September, many short wave disturbances traveled along the upper level flow, with some troughs lingering across the western U.S. This resulted in ridges dominating downstream of the lows, over the central and northeast U.S., corresponding to the positive solar anomalies observed in these regions.


Throughout this quarter, atmospheric circulations over the U.S. were strongly influenced by El Niño. Energy from El Niño created a highly active jet-stream, which in turn produced shortwave troughing and ridging activity along its path. In the southern U.S., the fourth quarter began with several low-pressure weather systems joining paths with the remnants of the slow-moving Hurricane Patricia. This resulted, once again, in severe flooding in Texas, and in Louisiana. In fact, from January through December 2015 Texas had the wettest 12 months on record. As the quarter progressed, weather systems continued to bring moisture from the Gulf of Mexico into the Gulf States, the Carolinas, and up to the mid-Atlantic states, generating above-normal precipitation levels which produced strong negative insolation anomalies in these regions. The positive solar irradiance anomaly over the Northeast is attributed to a ridging pattern which resulted in drier than normal conditions.