Parts of the Midwest and East Coast have been blanketed by the first major snow of 2025, with temperatures dropping as fast as my New Year’s resolutions. Do we have the stratosphere, the polar vortex in particular [footnote #1],  to blame (or thank) for this recent cold air outbreak? We don’t think so. This seems like a case of the polar vortex getting blamed for being a trouble maker, when it was actually minding its own business. We’ll tell you why below. Our usual suspect has an alibi: the polar vortex has been strong  A common way in which the stratospheric polar vortex contributes to  cold air outbreaks over the eastern U.S. usually involves two key ingredients.…   Read article

When we launched this blog last winter, the atmosphere came through for us with not one, but two breakdowns of the stratospheric polar vortex. That was very considerate of it, given that many winters pass without even a single one. With all the excitement that occurs when the stratospheric polar vortex is disrupted during a sudden stratospheric warming, our readers might wonder whether there is any way to predict weeks or even months ahead of time the likelihood that one (or more!) will occur this upcoming winter. As we discussed last year, there are certain ingredients that need to come together to drive a sudden stratospheric warming. These can be boiled down to two main factors:  …   Read article

There’s a 59% chance that weak La Niña conditions will develop shortly. This is very similar to last month’s estimate, just applied to November–January. It’s true; if you read last month’s post, you can pretty much carry that information over to this month. However, we have lots of fun sciency details to talk about this month, so stick around! The office holiday party La Niña, the cool phase of the El Niño/Southern Oscillation (ENSO), is a coupled ocean-atmosphere pattern in the tropical Pacific Ocean. To qualify as La Niña conditions, we need to see (1) surface water in the tropical Pacific that is at least 0.5 °C (just shy of 1˚F) cooler than the long-term average (long-term=1991–202…   Read article

Although it’s taking its sweet old time to get going, a weak La Niña, the cool phase of the El Niño/Southern Oscillation (ENSO), is expected to develop this winter. Much like I did last November, I thought it would be a good time to delve into how these expected tropical Pacific conditions might influence the pattern of precipitation over North America this winter. After all, ENSO—the entire El Niño and La Niña system—is the single most predictable influence on year-to-year differences in average winter precipitation across the United States. Without further ado, let’s get to it! Break out the models!  Last November we were anticipating a big El Niño to do…   Read article

Though many aspects of nature in the Northern Hemisphere tend to quiet down during the winter, the Arctic stratospheric polar vortex (we’ll just call it the polar vortex from here on) is just getting started. Read on to get a recap of what the polar vortex is, what happened over the past summer, and what antics the polar vortex has been getting into recently. The polar vortex: a lonely bear or party animal?  For those of you who are new to the polar vortex blog or are reminiscing over those summer polar vortex-less days, let’s start with a brief recap [footnote 1]. The polar vortex is a band of strong west-to-east winds that forms in the stratosphere between about 10 and 30 miles …   Read article