Atmospheric river cuts irradiance across Western North America

Irradiance patterns across North America in November showed a stark east-west divide, according to analysis using the Solcast API. The southeastern United States and Mexico recorded strong positive anomalies, while the western regions endured persistent cloud cover and heavy rainfall, driving irradiances well below average. Seasonal forecasts suggest this pattern will persist through December, with only minor shifts in the southwest.

The month was particularly sunny for the South East United States and Mexico. Areas adjacent to the Caribbean Sea saw irradiances up to 30% above climatology, extending inland across most of Mexico and the southern U.S., reaching portions of the Midwest. Eastern states from Florida through Virginia and up to New Jersey also benefited from clear skies. In contrast, the far northeast experienced negative anomalies, which further reduced irradiance during a season with already characteristically high solar zeniths. These conditions were linked to a negative North Atlantic Oscillation, which established a blocking pattern that not only favored clear skies in the southeast but also brought colder than-average weather to eastern Canada and parts of the U.S..

Meanwhile, the western half of the continent faced a very different scenario. November was dominated by an atmospheric river that delivered heavy rain, dense cloud cover, and widespread fog. Irradiances fell 10–20% below climatology across much of the western and northern U.S., Baja California, and parts of western Canada. Central California’s Central Valley saw the signature of “Tule fog” in the data, with anomalies plunging as much as 40% below average. This persistent instability also drove record or near-record November rainfall at several locations, reinforcing the suppression of solar resources.

Current anomalies align closely with seasonal forecasts issued earlier in the month, which anticipated strong irradiance in the southeast and subdued conditions in the west. This pattern is expected to hold through December, with the exception of a flip in the southwest U.S..

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