The mercury will climb above 90 degrees Friday afternoon for many cities in the Northeast, marking the beginning of the first heat wave of the summer. An expanding and intensifying dome of high pressure over the eastern US will pump heat and humidity into the Northeast, resulting in at least 4 days at or above 90 degrees into next week. For many cities, the string of consecutive 90 degree days could last for more than a week.
A strong ridge of high pressure centered over the middle Mississippi River Valley Friday morning will intensify as it is pushed eastward by an upper-level low pressure system. This is the same system that is expected to bring severe storms to parts of the Midwest Friday evening.
Before centering itself over the East, the ridge will deliver a hot and very humid airmass to the Midwest. Temperatures will climb into the low to mid 90s Friday and Saturday afternoon. Alongside dew points in the upper 70s, it will feel like 105-110 across much of the region, with some localized urban areas feeling even hotter. The heat will end for the upper Midwest Saturday and the Ohio River Valley Sunday as the ridge moves further east.
As the ridge intensifies though, so will the heat. The height of the 500 hPa surface over the eastern half of the US from Saturday through at least the first week of July will be between 594 dm and 600 dm, or up to 6 km. The thicker the layer between the surface and any layer of the atmosphere, the warmer the average temperature must be. The 500 hPa surface is significant because roughly half of the atmosphere’s mass is contained beneath this layer.
The average height of the 500 hPa surface in the summer is roughly 579 dm over the northeast. Small changes in this height surface usually result in significant changes in weather at the surface. For example, surface temperatures are generally around freezing when this height surface falls to 540 dm. A 500 hPa surface sustaining for so long means that unseasonably warm conditions will continue until these heights lower.
In the Northeast, temperatures will warm into the 90s throughout the urban coastal plains Friday afternoon. The heatwave will begin in most other areas of the Northeast Saturday, with northwestern Maine not getting it until Monday. The south fork of Long Island, Cape Cod, and coastal Maine will miss out on the heat entirely. Temperatures are not expected to reach 90 degrees in these coastal areas.
While Saturday will be hot, it will likely be the most comfortable day until at least the July 4th holiday. Humidity levels will be comfortable Saturday, with oppressive dew point in the 70s not arriving until Sunday. The high dew points will generally stay north and west of I-95 in the northern Mid-Atlantic and New England, so that Boston, New York, and Philadelphia will generally escape the most oppressive conditions.
Although the big cities along I-95 will escape the worst humidity, they are not going to get off easy. The most intense heat is expected Sunday through Tuesday, when temperatures will skyrocket into the upper-90s in eastern Virginia northward into the lower Champlain Valley. The mercury may even crack the formidable 100 degree mark in the lowest elevations of New York’s Mohawk and Hudson Valleys as well as along the I-95 corridor between New York City and Richmond, Va.
Unfortunately the humidity will make it feel even hotter. Between Sunday and Tuesday, the heat index in northeastern New Jersey, southwestern Connecticut, and the Hudson, Mohawk, and southern Champlain River Valleys will escalate up to 110 degrees. The heat index in surroundings areas away from the coast and higher terrain could reach 105 degrees, including cities as far north as Burlington, VT and Watertown, NY. The heat index incorporates relative humidity and gives a measure of what it actually feels like.
The heat will pose health, environmental, and electrical threats. The high heat and humidity may cause heat exhaustion. The first signs of heat exhaustion are excessive thirst, muscle weakness, and headache. Get inside and drink plenty of water before symptoms exasperate into heat stroke. If it is necessary to be outside, drink plenty of fluids, seek shade, and avoid strenuous labor to prevent heat exhaustion.
Many areas of the northeast will face poor air quality as pollution builds in the lower atmosphere under high pressure, especially inside and to the northeast of urban areas. The elderly, young children, and those with asthma should limit their exposure outside. Further, cooling systems will strain the electrical grid. To prevent an overload, electric companies may instigate “rolling blackouts” in which electric service is temporarily cut or reduced. An overload in the grid can result in a fire risk and can damage essential parts.
The worst of the heat may end Tuesday, but most of the Northeast will see little relief. The only appreciable source of relief will come from sea breezes at the coast and isolated thunderstorms Tuesday afternoon in Upstate New York and northern New England ahead of a very weak cold front. The front will stall northwest of I-95 Wednesday, but being closer to the center of high pressure, the chance of thunderstorms is negligible.
The reason the heat will continue is because the eastern ridge providing this weekend’s heat and humidity will expand until it merges with a ridge over the desert southwest, forming one strong dome of high pressure encapsulating the eastern two-thirds of the country.
New England and Upstate New York will be on the northeastern periphery of this new enormous ridge. Dry air may work its way aloft. If any weak impulses ride along this periphery, these areas may see a few thunderstorms. Some storms could reach the New England coast. Otherwise, the Northeast will be dry throughout the heatwave.