Tuesday marks Day 5 of the heatwave for many cities in the Northeast. A cold front will bring a few passing storms Tuesday afternoon and evening but extended relief will not arrive until the approach of a strong cold front Thursday night through Friday evening. Behind the front, temperatures and humidity will nosedive for a comfortable and sunny weekend. But this relief comes at a steep price.
Throughout the week, the dome of high pressure responsible for the heat in the central and eastern US will expand westward, enveloping the entire eastern two-thirds of thirds of the country under a steamy air mass. Accordingly, the heatwave will continue through Thursday for the majority of the Northeast except for the coast and elevated areas of Pennsylvania, New York and northern New England.
The heat will not be as intense as it has been since the weekend, however. Temperatures are only expected to reach about 90 where the heatwave officially continues. This is because the westward expansion of the high pressure system will shift the transport of the hottest air towards the Southern Plains and the lower Mississippi River Valley. But combined with the high humidity, it will continue to feel as if were in the upper-90s in the Northeast as far north as Burlington and Montreal both Wednesday and Thursday afternoons.
Extended relief will finally grace the Northeast between Thursday night and Friday evening. A strong upper-level low and its associated cold front will drape southward Thursday night into Friday. Tropical Atlantic moisture from a westward-building Bermuda high will flow directly into the southeastward moving cold front, resulting in a wide swath of heavy downpours. In the Mid-Atlantic, where the air modest instability will accompany the flow of moisture, strong thunderstorms will be embedded in the heavy showers. These storms will be slow moving, resulting in a moderate flash-flooding potential wherever the heaviest downpours develop.
Showers and thunderstorms will develop over the Ohio River Valley, the Appalachians, western New York and western Pennsylvania late Thursday evening. These storms will spread into most of Upstate New York and northern New England overnight Thursday. Low levels of instability and wind shear will prevent these storms from becoming severe. That does not necessarily mean they will be harmless, however. Since they will be slow moving, localized flash flooding in valleys and near banks of rivers and streams will be a threat with these overnight storms.
The downpours will move into southeastern New England and the I-95 Corridor by midday Friday. Modest instability and marginal wind shear present in this area will be sufficient to generate a few strong storms, some of which may be capable of producing wind damage and extended periods of torrential downpours. Localized flash flooding will be possible in areas of poor drainage and along streams where downpours form and redevelop.
The rain will clear in most of Pennsylvania, New York and northern New England by early Friday evening. Storms will continue along and southeast of the I-95 corridor until the overnight hours. In the wake of the rain, a beautiful weekend awaits. Sunny skies, low humidity, and temperatures in the mid 70s to low 80s will provide the opportunity to enjoy the outdoors without oppressive environmental conditions.