Winter catapulted to a record-setting start with on the East Coast with an unexpected 3-6″ snowfall November 15. But since then, the frequent warm downpours have made it obvious that winter has largely been absent. Over the last two weeks we discussed winter’s whereabouts and why it has been so warm and wet. We alluded in the most recent Sunday Storm that over the next few weeks this warm, wet pattern will gradually become more wintry. In accordance with our expectations, the first signal for a widespread East Coast snow event has finally appeared.
It was noted in the most recent Sunday Storm that another storm could be brewing in the Gulf of Mexico this weekend, but that signals pointed toward a suppressed storm steering clear of the Northeast. Since that writing however, a slew of more possible scenarios have emerged. The most notable trend in numerical guidance has been toward an intensifying coastal storm capable of delivering a heavy snowfall across the coastal Northeast between Sunday, January 13 and Monday, January 14.
Hopeful snow lovers ought to proceed with caution. Characteristics of the overall pattern alongside the poor skill of numerical weather prediction at six days of lead time yield additional, less significant scenarios that should equally be considered.
Before mention of specific model solutions, it would be wise to consider the overall pattern. Winter has been dominated by the Pacific Ocean. A vigorous Pacific jet stream has been responsible for isolating winter’s cold in Canada while encouraging the development of weekly or semi-weekly low pressure systems in the eastern Pacific Ocean. The lasting momentum of the Pacific jet stream has carried many of these systems across the Southwestern US toward the Gulf of Mexico. Upon interacting with the Gulf of Mexico, the Pacific-systems have become revitalized and directed toward Ontario or New England by means of the Mississippi and Ohio River Valleys. But with no viable source of cold air and a dampened polar jet stream, these Gulf-reborn storms have generally been unable to produce snow, instead socking the East with days of heavy rain.
The pattern is slowly starting to change. Stratospheric warming has helped weaken the Polar Vortex (PV), the vast region of low pressure situated near the North Pole. Though the effects of stratospheric warming have yet to directly impact the position of the PV in the troposphere, it has contributed to weaker than average pressure differences between Greenland and the Arctic, a phenomenon characterized by the negative phase by the Arctic Oscillation. In it’s negative phase, the Eastern US and Western Europe are more susceptible to cold-air outbreaks, generally irrespective of the Pacific Ocean.
As has been the theme this winter, a shortwave trough of low pressure is expected to develop later this week off the coast of southern California and the Baja peninsula. The low pressure will form Wednesday evening in response to Pacific jet momentum decelerating towards North America while a larger feature escapes toward western Canada. The upper-level area of low pressure will intensify as it crosses the Rocky Mountains Friday and generate cold air, largely without enlisting aid from the cold reservoirs locked away in Canada.
The upper-level low pressure system will be in reach of the Gulf of Mexico by Friday night. Its newly created cold air-mass will perturb the temperature gradient near the warm Gulf of Mexico and trigger the development of yet another Gulf of Mexico set to propagate up the lower Mississippi River Saturday.
Overall the setup for this weekend’s storm is quite generic. Previous storms this season have delivered heavy rain to the Southeast, Midwest and Northeast under an identical upper-air pattern. But there will be one unique factor in the Northeast that has been absent nearly all winter: cold.
With the AO in its negative phase, cold is much more flexible to spread across eastern North America and excite the northern branch of the jet stream as it does so. This will occur in two shots across the eastern half of the US this week. The first, most potent arctic shot, is expected behind a cold front that will pass through the Northeast Tuesday evening. The cold will stick around through the end of the week, with a second shot of arctic air arriving Sunday. This second shot will arrive just in time to reinforce the cold air and invigorate the northern jet stream ahead of the Atlantic-bound Gulf of Mexico storm.
The timing and intensity of the second arctic air mass will dictate the quantity of snow that falls on the East Coast, if any falls at all. If the cold air infiltrates the East before the Gulf storm reaches the Southeast Coast, the storm will be deflected out to sea, bringing heavy rain to much of the Deep South and moderate snow to inland portions of the Carolinas and Virginia Saturday night and Sunday. Sunday in the Mid-Atlantic and New England would be a typical blistery but dry winter day if this scenario were to pan out. The UKMET, ECMWF and its ensemble mean, plus the GFS ensemble mean all currently support this scenario.
Alternatively, the cold could arrive with the storm’s northeastward progression across the Southeast. The storm would intensify at a faster pace and it would take a route closer to the coast if this were to occur.
Parts of the lower Mid-Atlantic and inland Southeast would experience a wintry mix given this storm track, with torrential downpours likely in the eastern Carolinas. Meanwhile, cities along the I-95 corridor from Richmond, VA to Portland, ME will all have a chance to receive snow between Sunday and Monday of next week if this scenario were to occur. This scenario is only supported by the deterministic GFS and the GDPS, Canada’s equivalent to the GFS.
Finally, the cold could be weaker than currently modeled, and arrive too late to keep the storm track east enough to allow cold air to reach the coast. If late enough the storm could even trek toward the Great Lakes. This scenario would favor rain up and down the I-95 corridor, with the heaviest snow further inland. A rainy scenario represented by either track is only represented by some members of the ECMWF ensemble.
Whichever track the storm takes, there will likely be a swath of moderate snow somewhere between the southern Plains and the Ohio Valley Saturday. Where the secondary corridor of heavy snow develops will depend on the storm track. Specific model solutions ought not to be taken seriously so early as six days is plenty of time for the model forecast to drastically change.
It is still far too soon to say where–and if- heavy snow falls this weekend according to the above scenarios. But the pattern would favor the storm being in proximity to the coast at least partially up the coast. How far up the coast precipitation develops is yet to determined. WeatherOptics will continue to provide updates on this potential snowstorm throughout the week.