Last evening, as Tropical Storm Arthur churned off the east coast of Florida, a broad dip in the jet stream, known as an atmospheric trough, stretched from the Southern Rockies to the Lower Mississippi Valley and thence northeastward across the Appalachian Plateau. Along its leading edge, cold air was knifing beneath warm, humid air to its south and east, igniting a long chain of thunderstorms.
The cold front and thunderstorms will push into the Mid-Atlantic and Northeastern States today and, ahead of the trough, southerly winds will begin to nudge Arthur to the north. The advance of the cold front will eventually determine the path of Arthur, which is forecast to become a hurricane by tomorrow afternoon; should the front stall east of the Appalachians, the storm might rake the Atlantic Seaboard while, if it continues to push eastward, Arthur will stay well offshore.
The current forecast indicates that Hurricane Arthur will likely cross the Outer Banks of North Carolina and, farther north, lash Cape Cod and the Canadian Maritimes. However, depending on its exact path, the storm may impact most of the Atlantic Coast, sending in high waves, producing rip currents and eroding beaches. On the positive side, Arthur is expected to move rapidly to the northeast and, if so, its effects will be short-lived for any given location.