The long-advertised change to a cold and snowy wintry pattern for the eastern half of the country is finally poised to arrive this weekend. Inaugurating the change will be a significant cross-country winter storm expected to impact a corridor from northern California to the Northeast. But before this storm takes shape, a weaker system will clear a path for it across the Midwest and Northeast between Thursday and Friday. The path and intensity of this weaker system will ultimately decide the fate of the potentially more significant weekend storm.
The behavior of the jet stream will set the storm path and intensity. Split into a northern and southern branch, each branch will contribute in some way to the weekend storm. The northern branch will supply the cold while the southern branch will supply the initial disturbance. The disturbance will develop from enhanced energy from the Pacific decelerating over California’s Sierra-Nevada mountains Wednesday.
The new disturbance will proceed to traverse the country en route to the Northeast by Sunday. The surface component to the storm will emerge east of the Rocky Mountains Thursday morning and intensify as it undergoes a strong thermal gradient between the Gulf of Mexico and southern Canada. The Sierra-Nevada Mountains, the Great Basin, the Rocky Mountains and parts of the northern and central Plains all will undergo heavy snow accumulations before the surface low emerges from the other side of the Rockies on Friday. Forecast uncertainty plummets thereafter.
We discussed in last night’s Sunday Storm the importance of the northern and southern branches of the jet stream merging together near the Northeastern US this weekend. Where the jet stream branches merge, sinking air develops and intensifies high pressure. If this were to happen in southern Ontario, cold high pressure would simultaneously act to lock cold air into the eastern US while keeping the weekend storm from tracking too far inland. Coastal locations would consequentially undergo periods of heavy snow. An inland track would occur if the merging were to occur further northeast in Québec and would allow warm air to surge northward. The warmth would hold precipitation as rain along and east of the Northeast’s heavily populated I-95 corridor while delivering a heavy snowfall to inland areas. So where will the confluence (merging) of the two branches of the jet stream occur?
The weaker storm Thursday through Friday will dictate where this confluence occurs. The warmer scenario for the coast would evolve if the first system intensifies slower and progresses faster. The developing weekend storm would take longer to reach the cold northern jet stream under this scenario. A rainier weekend storm, perhaps preceded by and concluding with a brief period of snow, would ensue from Washington, DC to Boston. By contrast, inland cities like Pittsburgh, Scranton, Syracuse, and Manchester would be pummeled by heavy snow.
The colder scenario for the coast would evolve if the first system intensifies faster and progresses slower. This would draw cold air and the northern jet stream branch southward, closer to the developing weekend storm. The proximity of the northern branch would permit the two branches to combine earlier and further south and west. Heavy snow, perhaps with a period of mixed precipitation or heavy rain during the height of the storm, would engulf the Washington, DC to Boston corridor.
Regardless of the fate of the potentially significant storm this weekend, light precipitation is expected to overspread the eastern two-thirds of the US Thursday and Friday as the weaker system propagates northeastward. Light rain is expected to progress northeastward Thursday across all of the central and southern US but the eastern Gulf Coast and southern Georgia. Thursday night and Friday the precipitation will spread into the Mid-Atlantic and southern New England. Rain may be preceded by a brief thumping of snow along the I-95 corridor but only a quick coating may accumulate before the changeover. Most coastal areas are expected to remain light rain.
The first storm will only produce minor rainfall of a few tenths of an inch across most of the eastern US, with higher amounts possible along the central Appalachians and the Ohio River valley. But for northern and western Pennsylvania, upstate New York, and interior New England, the late week system will be the first of a back-to-back sequence of snowstorms. These areas will likely undergo light to modest snow accumulations Thursday night through Friday afternoon with only 24 – 36 hours’ reprieve before what could be a much more significant snowfall.
Over the past 24 hours the first system has trended to be slower and slightly more intense. That would favor a stronger, colder weekend storm. Of course this outlook may change over the next few days. The jet stream energy that will produce the late-week precursor storm is still in the open waters of the eastern Pacific Ocean and is not expected to arrive over North America until Wednesday. Until then numerical guidance will continue to shift in track and intensity of the first storm, shifts which become even larger in the depiction of the second storm. Stick with WeatherOptics throughout the week, as we will continue to provide updates as we know more.