The official start of Meteorological Fall is September 1. Meteorological Fall is the transition between the quarter of the year with the highest average temperatures and the quarter with the lowest average temperatures in the northern hemisphere. Hints of Autumn have already arrived behind Wednesday night’s cold frontal passage over the interior Northeast, where high temperatures will remain in the 60s and low 70s through Saturday. Thursday will mark the last day of summer heat for the I-95 corridor before cooler air invades from the Atlantic Friday and Saturday, just in time to mark the beginning of fall. But don’t close the pool just yet.
The calendar may mark fall but the overall weather pattern is still the epitome of summer. A subtropical ridge of high pressure will remain centered just off the Carolina coast through this weekend. The ridge and the heat beneath it were suppressed by the passage of a broad upper-level trough of low pressure Wednesday. Surface high pressure centered just west of the trough axis will expand and draw cool air from the Canadian Maritimes southwestward into the Northeast and Lower Mid-Atlantic. This Canadian Maritime air will flow against the rim of another surface high associated with the subtropical ridge. A game of tug-of-war will ensue between the two highs, and there can only be one victor.
Autumn’s weapon against summer’s subtropical dome of heat is a series of cold shots from Canada. In general these are small but relatively intense shortwave troughs. Their collision against the ridge generally result in heavy thunderstorms in the Midwest. Without any means to continue the flow of cold air beneath them, the collision of these shortwaves will actually enhance the already strong ridge over the East. Condensation to form the thunderstorms releases immense quantities of heat that only act to reinforce the boundaries of the ridge. As heat builds underneath the ridge, high pressure will continue to build, further intensifying the ridge. This isn’t good news for the cool surface high pressure system over the Canadian Maritimes funneling fall relief to the Northeast.
The ridge will first begin to rebound northward Saturday but the tug-of-war will continue. As the ridge expands, the surface high over the Canadian maritimes will be squished southward by much stronger high pressure to the north associated with a strong trough in northern Québec. With surface high pressure beneath the subtropical ridge intensifying, the high responsible for the Holiday Weekend relief will yield, merging with the subtropical surface high on Sunday. The center of the surface high will drift southward, gradually changing the wind direction from the cool northeast to the hot southeast Sunday and Monday.
Temperatures will rise to the upper 80s to about 90 in urban areas Monday. Temperatures will be on the rise through Wednesday as the ridge reaches its maximum extent into southern Canada. Scathing heat for September will bake most of the East, with high temperatures climbing to the low to mid 90s along the I-95 Corridor from Florida to Massachusetts. Upper 80s and Low 90s will reach as far north as the Champlain Valley. Maine could be excluded from this high heat. Alongside the heat, humidity will also be high, with dew points rebounding into the low 70s, making it feel like the mid 90s to near 100 degrees.
It is uncertain how long the suptropical ridge will funnel heat into the Northeast, but it is expected that heat will continue through much of next week. The best chance for a shakeup is for a tropical system in the Atlantic or the Pacific. If a tropical cyclone forms next week over the Atlantic, heat may be initially intensified or later be suppressed depending on its track. Alternatively, northward traveling typhoons over the Pacific tend to prompt cool air to invade the Eastern US within roughly a week. We will continue to provide updates as we know more about the intensity and extent of the resurgence of summer heat.