Since late December we have been warning that the relatively warm, wet conditions that have dominated the eastern half of the nation would erode by the middle of January, making way for a colder, snowier second half of winter. Temperatures are poised to warm back to seasonal temperatures this week, but the long range outlook still highlights more frequent cold shots and chances for snow for the Eastern US. The Northeast’s first significant winter storm could impact the region as soon as next weekend. It seems this weekend’s cross-country snowstorm was merely a precursor of things to come.
January 20th appears to be the critical date at which the gates to the arctic open to the Eastern US. Readers may recall from last week’s Sunday Storm that this date is the first to feature a completely split polar vortex pattern in the Northeast. The polar vortex is the semi-permanent region of low-pressure situated near the north pole. When some external factor causes it to split, daughter polar vortexes appear south of the north pole and blast the region beneath them with polar air. This forecast split will be partially provoked by squashing from the stratosphere above, which recently underwent sudden warming.
Naturally, a split polar vortex is a weak polar vortex. This trait is quantitatively described by the plummeting Arctic Oscillation (AO) later this week. The AO is related to the pressure difference between the arctic and the mid-latitudes. When it is in its negative phase, the pressure difference is weaker than average. The weaker pressure gradient between the poles and the mid-latitudes results in a weaker jet stream more susceptible to incursions of warm and cold air.
Visually, the upcoming pattern first fully appears in the most recent 12 UTC GEFS cycle on January 20 as demonstrated below, courtesy of Tropical Tidbits. The plot features three deeply amplified troughs (blue anomalies) extending from the arctic southward toward the northern sub-tropical latitudes of the Gulf of Mexico, northern Africa, and the central Pacific. Each trough contains a piece of the polar vortex.
Of note are two branches of the jet stream in eastern North America that are fully aligned–or phased–with one another. Each branch has an upper-level region of low pressure system associated with it. The northern branch has a closed low centered over the Hudson Bay and the southern branch has an open wave. In tandem they mutually amplify the other, resulting in one highly amplified trough dragging arctic air toward the poles. It is by no coincidence then that a winter storm is expected to ride the forefront of the arctic airmass and intensify on its journey eastward from the West Coast Thursday toward the Northeast US by Sunday.
The dynamic setup for the storm is different than most storms so far this winter. Most systems that have impacted the Eastern US since roughly Thanksgiving have originated as upper-level waves of low pressure in the stronger, southern branch of the jet stream off the California coast. The weaker, northern branch of the jet stream has generally been confined to Canada. The dominance of the southern branch has precluded phasing (merging) of the two branches. These upper-level low pressure systems developed before reaching North America, where the jet stream was decelerating over the relatively cool waters near the continental shelf. Eventually, they spawned surface low pressure systems near the Gulf of Mexico in their progression eastward. The main results of this pattern were northward surging subtropical air from the Gulf accompanied by heavy downpours. Occasionally, upper-level features of the northern branch would reach the East Coast too early and suppress the southern branch. This would enable cold air to produce snow, but only south of the northern branch of the jets stream in the Midwest and the lower Mid-Atlantic.
Like previous storms, the southern branch of the jet stream will continue to be the dynamic power-house. That’s where all commonalities end, however. Unlike previous storms, the upper-level feature driving the cross-country winter storm will actually originate Thursday in the Sierra-Nevada mountains of northern California. This is where the jet stream will decelerate, as opposed to the eastern Pacific near southern California like previous storms. As explained last week, air rises and forms low pressure to the north of where air decelerates.
Since the southern jet stream will be aligned further north than its position for most storms this winter, the upper-level wave of low pressure will not trek toward the Gulf of Mexico. Instead it will take a route across the heartlands of the US, crossing the Rocky Mountains en route. Compression and stretching of the atmosphere in traversing the Rockies will intensify the wave of low pressure and help draw arctic air north of the nearby northern jet stream branch. It will simultaneously spawn a surface low pressure system just east of the Rockies Friday morning.
The newly-formed surface-low will continue eastward, spreading snow across the northern and central Plains before interacting with southerly flow from the Gulf of Mexico upon reaching the southern Plains. The precipitation-shield associated with the storm will gradually begin to expand at this point. But how far north the snow spreads in the low’s journey to the coastal Northeast depends on one big dynamical factor.
The abundance of cold air will be well established for this system. Previous storms were either suppressed by the early arrival of cold air, or lacked cold air entirely. A large region of high pressure will dominate from the northern Plains to southern Québec, a feature earlier patterns lacked. This cold-core high pressure system will limit the northern extent of the path of the cross-country winter storm. Its strength and positioning will impact how soon the storm curves northeast and how far south cold air can penetrate. Too strong and positioned too far south, the storm would be kicked out-to-sea and snow would never reach the Northeast. Too weak and positioned too far northwest, the storm would curve northeast too early and allow warm air to prevent rain from changing over to snow.
The phasing of the northern and southern branches of the jet stream will dictate the positioning of the aforementioned high pressure system. When the jet stream branches phase, the resulting confluence results in sinking motion that forms and intensifies high pressure. As is typical for one week of lead time, there is no model consensus on this critical parameter. The two dominant global models paint two completely different scenarios for how this phasing occurs.
The GFS before this afternoon cycles (12 UTC and earlier) depicted a faster progression of a trough in the northern jet stream. The phasing of the two branches of the jet streams occurs earlier and results in rapid intensification of the storm, even before reaching the Atlantic Coast Saturday. That the phasing happens earlier means high pressure prevents the storm from cutting north of Virginia before recurving northeast. But the rapid intensification also means that once outside the influence of high pressure the storm will quickly recurve to the northeast, passing just east of the Northeast coast. For the I-95 corridor from Washington to Boston, Precipitation would initially fall as rain before changing over to snow on the backside of the storm. This scenario is portrayed below in the model graphic from PivotalWeather and would be an all-snow event for the Ohio Valley and inland parts of the Mid-Atlantic and New England.
The ECMWF paints a completely different picture. The northern-stream feature is depicted to arrive slower. The original upper-level system driving the winter storm in the southern branch would intensify without help from the northern branch. The storm would take a route slightly farther to the south, but would recurve between North Carolina and Virginia Sunday morning toward the Northeast, undergoing rapid intensification en route. The southerly position of the storm before turning northeast would allow precipitation to start as snow everywhere in the northeast. As the low passes close to the shore, coastal locations from Delaware to Massachusetts may undergo a transition to rain or mixed precipitation before changing back to wind-swept snow. Inland locations from Virginia to Maine would experience heavy snow throughout the storm’s duration. The most recent 18 UTC GFS also follows such a scenario.
There is a lot of uncertainty regarding the evolution of the pattern-changing storm. It is still too early to tell where exactly the heaviest snow will fall, but heavy snow is more likely to impact the Northeast than the most recent cross-country winter storm. Regardless of impacts from the weekend storm, cold air will invade the Eastern US on or about January 20, and it appears like it will stick around for a while.