Tis’ the season to be rainy. The weekly cross-continental train of moisture that has been in service since the end of November will continue this week. Its route will be nearly identical to that of the last several weeks, making stops in the Pacific Northwest and Rocky Mountains early in the week before picking up more moisture in the Gulf of Mexico. From there, the moisture will disembark over the Ohio Valley and the East Coast.
The locomotive already arrived on the West Coast in the form of an upper-level shortwave trough of low-pressure. The dynamics of this upper-level system are responsible for forcing the propagation of moisture across North America. Its arrival was met with the deluge of heavy rain and snow across the Pacific Northwest. The rain and snow will persist until Wednesday morning, long after the departure of the driving locomotive. That’s because the upper-level low pressure system will leave behind a robust northern jet stream flowing from the ocean to the Cascade mountains. The jet stream’s dynamics can unload precipitation by itself without the help of the wave of low pressure.
Like its predecessors, the wave is oriented along a northwest to southeast axis. As the base of the wave is traveling faster than the average jet stream speed, the wave will take a southern track toward the East Coast. The wave locomotive will refuel as it crosses the Rocky Mountains, which will amplify the wave so that its base reaches the Gulf of Mexico.
With open access to the open waters of the Gulf, the upper-level wave will quickly draw moisture and begin to produce heavy rainfall over the Southeast. The passage of a low pressure system thousands of miles to the north in Ontario will draw cold air toward the intensifying low pressure system. The resulting temperature gradient will accelerate the growth of the open wave, again increasing its amplitude and its capacity to hold moisture.
Some of the added moisture from the Gulf of Mexico will disembark early. The contrasting airmasses between the Gulf and the mainland matched with the invigorated upper-level shortwave will accelerate rising air to such a degree that strong thunderstorms could result along and near the Gulf Coast Wednesday afternoon and evening.
While heavy downpours and thunderstorms deluge the South Wednesday and Thursday, a related system will intensify near the Montana/Canada border. Some of the momentum that will have produced several days of rain and snow in the Pacific Northwest will be energized as it crosses the northern Rockies. Eventually this shorter northern wave will overlap to the north of the much stronger wave over the South. As the two upper level waves of low pressure overlap, the northern and southern branches of the jet stream will combine, or more formally, they will phase.
The phased jet stream will stretch roughly from south to north, from the Gulf of Mexico to Ontario, by Thursday night. There will be no means of redirecting the new combined low pressure systems from traveling directly northeastward up the east coast as there was the most recent system. Just like conventional locomotives require a lot of force to slow down, so too does this atmospheric locomotive. Typically a high pressure system directing cold air into the Northeast or southern Canada is one way to achieve redirection. Despite early week cold in the Midwest and Northeast, the pattern will be too progressive for the cold to last until the arrival of the moisture train.
The consequence of the two upper-level waves phasing so early is that their combined force will seamlessly eject the high pressure system bringing early week chills to the Northeast eastward toward the Canadian maritimes. With no high pressure system around to suppress the northward trajectory of the low pressure system and its Gulf of Mexico warmth, rain will likely be the dominant form of precipitation, even in Northern New England. Only the highest mountain peaks could experience any other form of precipitation.
Behind the rain, a cold front will usher in seasonably cold air throughout the Eastern Seaboard. Enough straggling precipitation appears to remain such that Lake Effect Snow could deliver locally heavy snowfall, or enough to ensure a White Christmas for the typical snowbelt regions of the Lower Great Lakes. Elsewhere in the east, prospects of a White Christmas appear bleak at this time.
It is too early to tell how much rain will fall from this next system, although amounts of at least 0.5″-1.5″ are possible. Check back with WeatherOptics throughout the week as the forecast becomes clearer.