This plot shows the 500-hPa geopotential height anomalies for most of December, and January to date. Mid-late December had all of the classic features of an anomalously mild North American pattern – negative NAO with persistent troughing across Greenland, working in tandem with Alaskan and northeast Pacific troughing to result in a mild Pacific airmass spreading throughout much of North America. The greatest warm anomalies were concentrated over Canada, as mean southwesterly flow aloft meant near negligible pathways for frigid Arctic air to advect southward into Canada and the US. The result was a series of cutoff lows and cyclones undercutting the Canadian ridging, but with a near total lack of any substantial cold air, led to many mild rain events in the East Coast event with cyclone tracks that normally would produce snow in their cold sector during winter.

Now let's look at the height anomalies for January to date. This is a very different picture from December, most notably with persistent high-latitude blocking. This prolonged blocking episode was in part a combination of constructive interaction between a weakened lower stratospheric polar vortex, following multiple stratospheric warming episodes with one marginally qualifying as a sudden stratospheric warming (SSW) event, and multiple cyclonic wavebreaking episodes in the North Atlantic allowing ridging to build north of the UK, split off from the main waveguide and becoming a block, and retrograding west towards North America, which I discussed briefly in my last blog post.

In addition to Alaskan ridging, this allowed a lobe of the lower stratospheric/upper tropospheric vortex to cut off over Canada and become displaced towards the Hudson Bay. Such a configuration is commonly associated with major central US cold air outbreaks, and indeed this is what we saw. Relative to the last major cold outbreak, a historic one in January 2021, the southern extent was not as notable, the core of the cold was farther west as indicated by the mean trough axis leaning towards the Rockies, and the duration was shorter as well.

This configuration initially favored the Plains for major snowstorms, as a deep trough axis was centered over the southwest Rockies in addition to a Greenland ridge linking with a broad eastern US ridge. Following a transient shift to western North American ridging, the mean trough axis shifted farther east, but the waveguide became more progressive as well, resulting in only minor to locally moderate snow events in the Northeast and Mid Atlantic. January 19-20 specifically came close to being a much larger snow event, having many of the ingredients typically associated with major snowstorms, but the upstream trough was too flat, too progressive and not deep enough. From a research perspective, this may be an opportunity to run feature perturbation experiments to test just how close this came to a major snowstorm.

As a snow enthusiast that lives near New York City, it is also only appropriate I add a section about the ongoing NYC snow hole. "Snow holes" are an interesting phenomenon – sometimes they are merely a subjective perception, other times they are actually supported by evidence. Furthermore, snow holes are relatively short and merely statistical artifacts in decadal and multi-decadal timescales. Such snow holes have happened in most winters in the region, and in some cases over populated areas too, such as 2016-17 and 2017-18 in Washington DC, and 2019-20 in Philadelphia. It now appears to be New York City's turn to experience the "snow hole". There are numerous metrics to quantify this snow hole in NYC:

• Days since the last 2" snowstorm: It has been 722 days and counting since the last time Central Park recorded both a one-day and two-day cumulative 2" or greater snow total. This is a record for both metrics.
• Spatial maps: Nearly all locations north, west, south and east of NYC have had greater cumulative snowfall this winter to date. Specifically, NYC is the only major I-95 city north of Richmond to have received less than 3" of snow this winter, and among Mid Atlantic cities, both Washington DC, Baltimore and Philadelphia have received at least one 4-6 inch snow event.

From a subseasonal perspective, this appears to be the consequence of random chance. While two snow events affected the Mid Atlantic region, both had heavier snow bands south of NYC, with a stubborn subsidence axis over the NYC metro leading to poor snow growth, less precipitation and lower snow-to-liquid ratios than models predicted. This was my reasoning for anticipating less than an inch of snow in Central Park with the January 19-20 event (verification was 0.4 inch), though while this was a short-term forecast, such details cannot be predicted with precision even at several days lead time.

As the lower stratospheric polar vortex recovers from its most recent warming episode, and high-latitude blocking over North America rapidly collapses, it is increasingly evident that the spell of cold and snow in mid-January was the exception rather than the norm this winter thus far. The background state continues to favor troughing over western North America and Greenland, and accordingly we see a rapid transition back to high-latitude troughing over northern North America in the coming days. In addition to a cutoff low over the southwest US, this leads to above-normal temperatures across the eastern US this coming week. In fact, those arriving early for the 104th annual American Meteorological Society meeting in Baltimore may be treated to some unusually mild weather!

A cold air outbreak is currently ongoing over eastern Asia. While temperature anomalies aren't as large as the December cold outbreak, the southward extent is quite unusual with snow and ice down to southern China and even higher elevations in Taiwan. The result of this cold outbreak, similarly to December, is once again a strengthened and extended Pacific jet towards the last few days of the month.

Initially, cyclogenesis in the left-exit region of the jet looks to aid in ridge amplification over western North America, and this plot shows a classic split flow regime with a subtropical jet to the south and polar jet to the north, while northwesterly flow over eastern North America allows for some equatorward cold air advection from high latitudes over the eastern side of the continent.

However, this is not a warm look for the Northeast US, with the Mid Atlantic eventually cooling down as well. Given that the mean ridge axis extends east of the Canadian rockies, this pattern configuration favors NNW flow over eastern Canada and a pathway for a cold Arctic airmass to be advected southward along the eastern side of the continent. As this limited cold air intrusion occurs with the SW Rockies cutoff low being ejected northeastward, this opens up a pathway for a Northeast US minor-moderate impact snowstorm, primarily favored north of I-80 and north of NYC.

Given limited cold air over Canada and lack of an antecedent cold airmass, NYC and locations southward appear to primarily end up with rain or only minor wet snow accumulation at most regardless of low track, not unlike the early January snowstorm, continuing the record-breaking NYC snow drought. Towards the interior Northeast into much of New England, precipitation initially looks to start as wet snow especially in higher elevations, but as the cold airmass intersects the cyclone as it begins to depart the region, colder thermal profiles look to increasingly support colder, higher ratio snow. Depending on the low track, this may lead to accumulating snow – perhaps several inches – into Boston and perhaps even southern New England.

As the Pacific jet extension continues into the end of January, we see another reconfiguration of the North American waveguide, but one even more conducive to anomalously warm temperatures into Canada and the northern Midwest. A largely zonal and southward displaced flow across the North Pacific with negative height anomalies extending into the western US, associated with an extended & equatorward shifted Pacific jet, is a classic indicator of a pattern favorable for Atmospheric Rivers (ARs) across the West Coast, as subtropical moisture plumes are transported towards California.

Indeed, both GEFS and EPS ensembles favor well above average precipitation across California to start out February. Alongside the previously noted western-central Canadian ridging, broad warm air advection and time-mean southwesterly flow across North America favors widespread above-normal heights across North America east of the Rockies and north of the Southeast US.

Temperature anomalies during the beginning of February are forecast to be well above average across much of Canada, while below average over Alaska and Greenland. This signal, however, is not a predominantly warm one for the US East Coast, as the greatest positive temperature anomalies are over the Midwest US into southern and central Canada; in fact, the continued NNW flow over eastern Canada as previously noted may allow for limited cold air intrusions towards New England and temperatures remaining near or perhaps even below average.

Going back to the 500-hPa height anomaly map, the eastern US is located downstream of the time and ensemble mean ridge axis. This region of northwesterly flow downstream of the ridge axis is dynamically associated with synoptic-scale subsidence, favoring a high pressure, and can occasionally be an indicator of back-door cold fronts or sneaky cold air masses to the east of the ridge axis. While this time frame does not appear to be particularly cold for the eastern US, it doesn't appear to be anywhere near record or even anomalously warm either. This becomes important for the next section in this post.

Heading beyond the first week of February, there is quite a bit of uncertainty regarding the evolution of the broad region of anomalously high heights over the northern US and central Canada. But one thing that jumps at me looking at this from a pattern recognition viewpoint is the potential for a "sneaky" low-predictability northern Mid Atlantic and Northeast snow event as early as the beginning of the second week of February, in the event that the continental-scale waveguide in early February evolves similarly to current forecasts.

The primary uncertainty appears to be whether the higher height anomalies propagate poleward towards Greenland/eastern Canada (as some extended GEFS members depict), Alaska (as the extended 101-member EPS mean depicts), or both (as another subset of extended GEFS members depict). But in any case, high-latitude blocking is favored once again as we approach mid-February, with a growing signal for Alaskan ridging favoring increasing cold transport into the eastern half of the US, while a continued active subtropical jet favors near to above normal precipitation.

Altogether, these signals indicate that, not only is early February not looking like a particularly warm time frame in the East US, but another round of potential cold and snow, including the Mid Atlantic, may be on track from the 2nd week of February onwards. Depending on the high latitude block evolution, the current modeled configuration of Alaskan ridging indicates there may be potential for another CONUS cold outbreak centered towards the Plains in the second half of February. And while it's too early to know if NYC breaks its snow drought, as the Mid Atlantic may be especially favored for snow events, at least parts of the I-95 corridor may break their prolonged major snowstorm drought before this winter ends.