Washington just experienced its second-hottest August on record, and even in the second week of September we’re pushing 100 degrees. So obviously this is the best time to talk about winter.
We’ve got three ways to get an exceptionally cold one. Just removing last winter’s super strong Godzilla El Niño already increases our chances for a colder season ahead, but let’s talk about some interesting scenarios in which we could nail down another polar vortex winter. And, to be clear — we’re not making any predictions here, just walking through the ways in which it could happen! We won’t publish any winter season forecasts until late October.
1. A weak La Niña
In the wake of Godzilla El Niño’s demise, a gentle cooling of the tropical central Pacific waters is allowing a weak La Niña to emerge. For over two months, water temperatures have been slightly cooler than normal. A cool patch is emerging in the tropical Pacific, and climate models project it to remain in the weak La Niña range into next summer.
Weak La Niña winters increase the odds of a cold winter for the Washington area thanks to weaker (warm) tropical forcing influences and improved chances for cold air patterns (due to atmospheric blocking jet stream situations). They increase the odds of cold, but not necessarily the snow part:
Washington sees almost a 66 percent chance of a cold winter when a weak La Niña occurs. Snow results vary wildly though, with nearly a 50-50 chance of an above or below normal season. The “average” value is somewhat misleading because it is skewed by the very snowy 1995-1996 winter.
Interestingly, the way we’re headed into this La Niña is very similar to 1995-1996, including the very hot preceding summer. La Niña winters tend to not have as many moisture connections to the East Coast (we tend to lean drier), so while the cold increases our chances of snow, the lack of a stronger moisture connection means there might not be enough to generate snow.
Side note: Different ways to measure the Tropical Pacific and different viewpoints on what classifies as a weak La Niña exist; these are the best matches in our view looking at multiple sea surface temperature data sets. The general theme is that weaker tropical forcing generally increases odds (against typical climatology) for a colder winter.
Side note 2: NOAA announced today that they are removing the La Niña watch for the winter, but they keep a weak La Niña as the second most likely outcome (I still favor it). Even if just a negative neutral (slightly not cool enough for La Niña), it would be the first one since the cold 2013-2014 winter, which is part of our second way to a cold winter next:
2. A warm North Pacific
It seems odd to talk about warm waters giving us cold winters, but if it’s warm in just the right it place can make a big difference.
Water temperatures in the North Pacific currently match a pattern very similar to a small subset of years since 1950 that went on to become very cold winters in the United States.
The last time this happened was in 2013-2014, which featured an impressively cold winter with above-normal snowfall. Remember that polar vortex? Every single month from November to March was colder than normal in D.C. — not an easy feat.
One can have longer arguments about what drives this pattern, but the typical reflection for that winter that follows is above normal high pressure ridging around the Alaska area that seesaws big chunks of cold air into the Lower 48.
3. A quieter sun
The third potential winter influence is literally out of this world.
Research has shown that a quieter sun (fewer sunspots) is correlated to colder winters in eastern North America and Europe. The sun rolls on an 11-year cycle of active (maximum) and inactive (minimum) periods. In the most recent solar cycle, the sun’s quiet periods have lasted longer than the active ones compared to previous cycles. The active periods also appear to be weaker.
The solar minimum from 2007 to early 2011 registered the quietest solar period seen in at least a century. And sure enough, we experienced a series of cold U.S. winters. Now the sun is getting quiet again — even faster than NOAA can project:
For serious weather geeks, the previous quiet solar period offers some interesting insight. During that time, the Arctic Oscillation and the North Atlantic Oscillation were lowest on record in modern times. It was also the only year (2010) in the record in which every single month reported a negative North Atlantic Oscillation.
For the rest of us, this period led to crazy snowy winters that included our snowiest on record, Snowmaggedon and the following winter’s Commutageddon mess.
One solar caveat: The solar activity decline may slow down, and thus it may require another year or two to get to the levels of the previous minimum; however, we are also not sure about the cumulative effects of a quieting sun that could increase the colder pattern impacts faster this time around. The early 1800s featured a series of quieter solar cycles like this (known as the Dalton Minimum) that resulted in much more frequent cold winters.
So there you have it. At the most simple level, the weak La Niña should increase our odds of at least cold weather, but significant multiplier factors such as a warmer North Pacific and a quieting sun could pile on some additional wildness to our winter weather.