May irradiance patterns across North America showed a familiar east–west split, with strong positive anomalies across the western half of the continent and widespread deficits in the south and east, according to analysis using the Solcast API. A high pressure system supported clear skies and positive anomalies across much of the western half of the continent, while a more active, convectively unstable regime dominated the eastern and southern regions, limiting solar resource through frequent cloud and rainfall events.
Across the western interior, high pressure system remained anchored through much of May, promoting large-scale subsidence and suppressing cloud development. This setup produced broadly positive irradiance anomalies across the western United States. The strongest departures from the long-term average were observed over the northern Plains, Wyoming, Montana, and the western Great Lakes, where anomalies reached around +20%. In these regions, the sustained clear-sky conditions allowed irradiance totals to accumulate well above average for the month.
Further south, the arid interior of the Southwest, including inland California, Nevada, Utah, Arizona, and New Mexico also recorded positive anomalies generally ranging between 10% and 20%. Exceptionally dry surface conditions, reinforced by low snowpack and worsening drought, limited local moisture availability and helped sustain clearer-than-average conditions. Large portions of the Colorado River Basin received less than 3 mm of rainfall during May, while Utah declared a statewide drought emergency on 21 May.
The Pacific Northwest interior registered only modest positive anomalies of around 5% despite experiencing record-breaking surface temperatures, while coastal areas remained close to long-term average levels as marine influences continued to support near-normal cloud cover.
In contrast, below-average irradiance was concentrated across the southern and eastern flanks of the continent. Texas, the Gulf Coast, and the Lower Mississippi Valley experienced the deepest negative anomalies, reaching up to 20%. These reductions aligned with an active convective pattern, delivering 100 200 mm of rainfall from coastal Texas into the central Gulf region. Repeated thunderstorm outbreaks, including dailyrecord rainfall totals such as more than 60 mm in St. Louis on 18 May, and severe wind events producing gusts near 70 knots across parts of the Plains and Midwest, kept cloud cover frequent and persistent.
The Northeast, Mid-Atlantic, and eastern Canada also finished the month below average overall. A late-May heat burst briefly drove record high temperatures into the upper 30°C along the Eastern Seaboard, but this short-lived event was insufficient to offset an otherwise cloudier-than-normal month. Negative anomalies were strongest across the Northeast and eastern Canada, reaching up to 20%, while values were milder, and in some cases near neutral, from New York south toward North Carolina.
Further south, much of Mexico experienced below-average irradiance with negative anomalies, reaching up to -15% in some areas. The spatial pattern, concentrated over mountainous and highland regions, suggests early-season terrain-driven convection, with rising air over higher ground supporting cloud development and reducing irradiance. Northern Mexican desert regions blended into the positive anomaly signal observed across the U.S. Southwest.
Solcast produces these figures by tracking clouds and aerosols at 1-2km resolution globally, using satellite data and proprietary AI/ML algorithms. This data is used to drive irradiance models, enabling Solcast to calculate irradiance at high resolution, with typical bias of less than 2%, and also cloud-tracking forecasts. This data is used by more than 350 companies managing over 350 GW of solar assets globally.
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