|Above: Snow covers a farm field on Thursday, January 17, 2019, near Ottawa, Illinois, about 50 miles southwest of Chicago. Northern Illinois could get 4" to 8" of snow from Friday into Saturday as a major winter storm sweeps across the Midwest and toward the Northeast U.S. Image credit: Scott Olson/Getty Images.|
As far back as November, seasonal forecasters at The Weather Company (TWC) warned that the upcoming U.S. winter would bristle with late-season cold risk across the East, especially in February. It now looks more and more like that bold call may turn out to be just right, for a variety of reasons.
The imminent chilldown comes on the heels of a remarkable month-long stretch of consistent mildness that’s spanned nearly all of the contiguous U.S. At many locations, temperatures were among the warmest on record for the 30-day period straddling New Year’s Day (Dec. 16 – Jan. 15). The mildness—a welcome break for many, but a curse for snow lovers—kept weather-related holiday travel trouble to a minimum while holding Boston to its slowest snow-season start in 83 years of records.
|Figure 1. U.S. temperatures averaged 4°F to 10°F above normal east of the Rockies for the period from December 15, 2018, through January 14, 2019. Image credit: NOAA/NWS/WPC.|
All this is about to change, and the relative mildness we’ve seen so far this winter will make the transition all the more wrenching. First off, a winter storm dubbed Harper will drop major snowfall from the Central Plains on Friday across the Ohio Valley into the interior Northeast, perhaps dumping 1 to 2 feet over the weekend on parts of New England and upstate New York. See weather.com for frequent updates on Harper.
In Harper’s wake, a large pool of cold air will slide into the Midwest and East, perhaps setting at least a few daily record lows. Long-range forecast models are consistently calling for one or more shots of even colder air in the next 1 to 2 weeks. If the latest TWC outlook is correct, wintry jabs will continue pummeling the central and eastern U.S. straight through February and into early March.
|Figure 2. U.S. temperature outlooks issued by The Weather Company for February and March. These outlooks show the strength of warm and cold departures from average that are deemed most likely by Weather Company forecasters. NOAA’s seasonal forecasts, which cover three-month overlapping periods rather than single months, are keyed to the probability of above- or below-normal temperatures rather than their intensity, although the two variables are often closely linked. Image credit: The Weather Company.|
Splits in the stratosphere
The last few days of 2018 did more than bring mildness to the eastern United States: they also produced a heat spike about 15-20 miles above the North Pole that far outpaced anything seen at ground level. It was a vivid example of a sudden stratospheric warming (SSW), a wintertime circulation readjustment typically triggered by lower-altitude weather features such as blocking highs whose effects percolate upward. In turn, the SSW impacts can filter downward weeks later. This two-step process has long been explored by Judah Cohen (Atmospheric and Environmental Research) and others for its influence on Northern Hemisphere winter weather; see Cohen’s weekly analyses.
Every couple of years, on average, a major SSW sends temperatures in the polar stratosphere soaring by 90°F (50°C) or more in only a day or two. Last month’s SSW was a doozy, as shown in Figure 3.
|Figure 3. Temperatures above the North Pole at the 10-mb level (about 18 miles or 30 kilometers high) jumped more than 60°C (108°F) near the end of December 2018. Temperatures in the magenta trace were calculated for recent weeks by NASA's GEOS FP forecast and analysis system. Image credit: Paul Newman (NASA), Eric Nash (SSAI), and Steven Pawson (NASA), courtesy Judah Cohen (AER).|
SSWs can have a profound impact on the pole-centered circulation known as the stratospheric polar vortex. Often an SSW will disrupt the vortex, morphing it into multiple lobes, pushing it toward lower latitudes, and sometimes even splitting it in two. True to form, the latest SSW broke the polar vortex into two main centers, as discussed this week by Capital Weather Gang, and these “daughter” vortices have been unable to recombine.
The question facing us now is how long it might take before these vortex disruptions work their way downward and influence the weather at ground level. Typically this happens when easterly winds at lower altitudes becomes juxtaposed with stratospheric easterlies produced by the distorted polar vortex, a process that can take up to several weeks.
Once the winds are vertically aligned, it becomes easier for the stratospheric warm air to descend to lower levels, a process nicknamed “dripping paint” for how it appears on plots of atmospheric pressure surfaces over time.
Classic downward propagation of anomalous warm air following the SSWE pic.twitter.com/IXpCet48LG— Michael Ventrice (@MJVentrice) January 11, 2019
As it descends, the warming can induce a blocking pattern near the poles, which in turn supports the build-up of cold air masses that can push well southward into U.S. latitudes. Sometimes this evolution is expressed as a negative phase of the Atlantic Oscillation (AO) and/or the North Atlantic Oscillation, the latter being an index of the strength of flow across the North Atlantic. The strongest nor’easters tend to occur with a negative NAO, which allows storms to intensify near the East Coast rather than scooting quickly offshore. A negative NAO also favors intrusions of Siberian air westward into northern Europe.
The longest-range day-to-day weather forecast models, including multiple runs of the GFS and European models, increasingly agree that such a scenario may unfold around the last week of January. Other, more general dynamical and statistical models that extend out a month or more, such as the NMME, strongly support the idea of cold blasts extending through February across the Midwest and East. (Keep in mind that in midwinter, even intense cold isn’t always record cold—especially in our warming climate, where it’s become increasingly difficult to set large numbers of record lows.)
About as classic as it gets post SSW. Massive cold air outbreak will overtake the Eastern half of North America and spread all across Europe/northern Asia to end January and begin February. Look for this pattern to wax and wane throughout February. pic.twitter.com/xAq3zvXIP0— Jason Furtado (@wxjay) January 17, 2019
El Niño: from go-go to no-show?
When forging into the wilderness of seasonal weather prediction, the forecaster’s most reliable guides are El Niño and La Niña. Both of these quasi-cyclic Pacific phenomena tend to be at their strongest during northern winter, when they serve as the single greatest influence on year-to-year variability over the United States. What’s more, the most powerful El Niño and La Niña events are often ramping up by autumn, giving forecasters months of lead time for winter impacts.
A strong El Niño event tends to “smoosh” U.S. temperature contrasts, often producing a mild, dry north and a cool, damp south. La Niña tends to accentuate the contrasts instead of muting them, favoring a even-colder-than-usual northern tier, an even-warmer-than-usual Sun Belt, and an active jet stream in between.
Although an El Niño event seemed to be building this past autumn, the ocean and atmosphere haven’t quite managed to coalesce over the tropical Pacific. NOAA reduced the odds of El Niño development from around 90% to around 65% with its latest monthly outlook, issued January 10.
Nevertheless, there is plenty of warm water across the tropical Pacific, and this winter’s U.S. temperatures have largely followed the playbook of a weak to moderate El Niño event based on analogs from the last 60-plus years. February most often turns cold in the eastern U.S. during such an event, which lends support to the TWC seasonal forecast.
There’s another player in the seasonal prediction game, and it’s one that many forecasters have long derided or dismissed. We’ll have the details in a follow-up post on Friday afternoon.