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The line on the National Weather Service map stops short of the coast. The heat advisory the weather service issued for Massachusetts runs from noon Thursday to 8 p.m. Friday and covers western Essex County, where heat indices could reach 98 — but not eastern Essex, the coastal zone that includes Marblehead, where the ocean is expected to take the worst edge off the warmest air mass of the season.
That does not mean the town gets a pass. Forecasters at the weather service office in Norton expect temperatures across eastern Massachusetts to hold in the mid 80s to low 90s both days, with dewpoints surging to near 70 pushing heat index values into the low 90s, especially away from the immediate coastline. A heat index of 90 or above falls into the federal extreme caution range, where heat cramps and heat exhaustion become possible with long exposure or exercise.
The geography of the advisory tells the story of who suffers most. The worst conditions are expected in the Connecticut River Valley, where heat index values approach 100 Thursday and likely top 100 Friday, while advisories stretch across Franklin, Hampshire and Hampden counties, much of Middlesex and Worcester counties and western portions of Norfolk and Plymouth counties. Marblehead, on a peninsula in Massachusetts Bay, sits in the narrow strip the sea breeze protects.
Heat is most dangerous when it arrives early. Hospital visits for heat illness tend to climb in the first few days of hot weather, before bodies have adjusted — a pattern that prompted weather service offices across New England to lower their heat advisory thresholds after research showed deaths and emergency department visits rise significantly on days when the heat index hits 95, and that Northeast residents are uniquely vulnerable because they are less physiologically adapted to extreme heat and fewer homes have air conditioning.
Relief reaches the coast first. Coastal temperatures are expected to drift downward Saturday as the sea breeze kicks in, while inland towns could see a four-day stretch above 90 — though humidity drops for the weekend.
Until then, the weather service’s advice is the same on both sides of the advisory line: drink plenty of fluids, stay in air-conditioned rooms, keep out of the sun and check on relatives and neighbors, especially older people and those who live alone.
