La Niña Arrives, Blob Strengthens

Global sea-surface-temperature (SST) anomalies as of 8/14/2020. Note the massive Blob in the A Northeast Pacific and a developing La Nina in the Tropical Pacific. The strongest parts of the Blob have sea-surface-temperature anomalies exceeding 5oC (8oF). Credit: NOAA

Seasonal forecasting is the Achilles Heel of weather forecasters. Despite the plethora of seasonal forecasts offered by various government, educational, and commercial institutions, these forecasts have relatively little skill, and they have made minimal improvements over the last decade.

However, there are a few key features that can make seasonal forecasting a bit easier, at least for the Pacific Northwest. One is the status of the tropical Pacific and whether it is in an El Nino, Neutral, or La Nina state. The other is the “Blob” – the area of warm, sterile water that occasionally forms in the Northeast Pacific. We currently have a burgeoning La Nina in the tropical Pacific that is predicted to strengthen into a weak-to-moderate La Nina this winter, and we have an extraordinarily strong Blob of warm water in the Northeast Pacific well offshore.

La Nina and the Blob effect our weather in vastly different ways. La Nina events have well-documented effects on global atmospheric circulation and affect the location and strength of storms that impact the West Coast in the winter. More specifically, La Ninas tend to create an anomalous ridge of high pressure in the Northern Pacific, which then results in the jet stream sliding down into the Pacific Northwest from the WNW and giving us cooler and wetter-than-average weather with well-above-average mountain snowfall.

The Blob, on the other hand, is a result of persistent ridging over the Northeast Pacific and has relatively little effect on the location and strength of the jet stream. But at a more local scale, the Blob increases temperatures for the West Coast by warming and moistening the air that passes over it. The 2014-2015 winter was the warmest on record for many spots in the Pacific NW due to an extraordinarily strong Blob just off the coast, and the 2019 summer saw much warmer-than-average overnight lows due the onshore flow from the Pacific passing over the blob and picking up more water vapor in the process, as water vapor is a strong greenhouse gas and prevents heat from escaping at night.

La Nina and Blob Forecast

The Climate Prediction Center currently has a “La Nina watch” and forecasts a 60% chance of La Nina conditions developing by autumn and persisting through the winter. If a La Nina does develop, it is expected to be weak. However, even weak La Ninas tend to bring wetter, cooler, and snowier-than-average weather to the Pacific Northwest. There are notable exceptions – for example, the 2000-2001 weak La N