While the West hangs onto summer, the Great Lakes may get an early taste of fall as Canadian air slips south during the first half of September

The West is experiencing warmer days while the Great Lakes see a more variable mix of temperatures to start the month.

During this active period, shifting weather patterns begin their journey eastward as they adjust to the next phase of the seasons.

These broader continental transitions set the stage for evolving weather patterns across the United States and Canada. In response, the National Weather Service and other centers have announced upcoming weather variations.

Evaluating key macro-climate drivers and El Niño

Macro-climate drivers of these seasonal changes must be evaluated in relation to each other. These processes occur thousands of miles apart from one another.

Oceans primarily control how our atmosphere functions.

The Climate Prediction Center shows that recent climate trends underscore this relationship.

A number of global climate indices are currently influencing long-term forecasts. The Climate Prediction Center maintains an active El Niño advisory.

Equatorial atmospheric circulation anomalies continue to align directly with active El Niño conditions.

Meanwhile, sea surface temperatures continue to trend significantly warmer than average across both the Central and Eastern Pacific Ocean basin.

Models show there is a greater than 90% chance of a very strong El Niño event persisting through the fall and winter. Projections raise the odds of Northern Hemisphere climate anomalies, though local impacts remain probabilistic.

Teleconnections and global pressure patterns

As warmer-than-average sea surface temperatures provide energy to the atmosphere, they initiate teleconnection responses around the world.

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Consequently, these elevated temperatures modify jet stream pathways, cause new highs and lows, and redirect global moisture tracks.

The Madden-Julian Oscillation (MJO) is also contributing to this scenario. This active wave has now interacted with the developing strong El Niño.

Forecasts provided by the Climate Prediction Center suggest that the MJO will move eastward across the Western Hemisphere by September.

Following that path, it will loop back towards the Indian Ocean.

Through this process, the MJO modulates tropical convection and subsequent downstream atmospheric ridging.

Collectively, these phenomena contribute to large-scale pressure anomalies around the globe. These anomalies also alter atmospheric circulation over North America, creating unique regional climatic zones as summer transitions to fall.

Continental anomalies and seasonal temperature trends

A large anomalous ridge is evident within mid-troposphere pressure patterns (500 hPa geopotential heights) near Canada’s west coast and the north-central U.S. border.

Model output indicates that widespread warm conditions are expected. Most of the Contiguous U.S. and Alaska will experience generally above-average temperatures by mid-September.

Several areas exhibit conflicting signals within these regional forecasts. Some model outputs predict less prominent trends or “equal chance” for coastal regions of the West Coast and certain northern regions.

Where forecast confidence lowers, models show lower predictability across these transition zones.

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Hawaii will also experience warm temperatures consistent with those experienced by the bulk of the Continental U.S.

Regional sector impacts and precipitation forecasts

Seasonal forecasting relies heavily on tracking these broad atmospheric signals. Data assimilation models help analysts map upcoming temperature and moisture trends.

Persistent surface pressure systems continue to influence regional climate norms.

Meteorologists continue tracking these complex interactions to maintain accuracy across multi-week outlooks.

Alongside these thermal shifts, experimental CPC outlooks indicate wetter conditions across the Southwest, Northern Rockies, and parts of the West Coast. Conversely, drier conditions are expected in the Northeast and Middle Atlantic seaboard.

Meanwhile, Hawaii anticipates above-normal moisture totals for the period.

Extended-range forecasting involves balancing tropical ocean telemetry against shifting upper-atmospheric jet streams.

From September 5 through September 18, these atmospheric processes will dictate local shifts. They will determine whether regions retain lingering warmth or transition rapidly into true fall weather.

The Climate Prediction Center will publish weekly updates detailing the unfolding seasonal transition.

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