New England

FIRST ALERT: Heat Index to Reach Rare 105 Degrees

Heat and humidity make a dangerous combination for those without cooling

NBC Universal, Inc.

A First Alert continues through Friday from our NBC10 Boston, NECN and Telemundo New England weather team for intense heat and humidity.

Wednesday's heat index or "feels like" forecast is expected to reach 95 to 100 degrees during the afternoon, with Thursday and Friday topping out at 100 to 105 degrees – the hottest feeling this season.

Heat index is something our weather team fields questions about from time to time: is it real?  What does it mean?  The heat index is a complex mathematical formula that takes both the temperature and amount of moisture in the air into account.

The basis for heat index and the impact of humidity on the body comes from the understanding that our bodies cool by the evaporation of sweat from our skin – if it's more humid, sweat evaporates slower, meaning our bodies can't cool as effectively. This reduced ability for the body to cool means the impact on the body is as if the temperature were even hotter under normal humidity.

With the thermometer topping out around 90 Wednesday and in the mid-90s Thursday and Friday, it's the oppressive humidity with dew point temperatures in the 70s that generates the highest impact on the body so far this summer in this stretch.

Furthermore, multiple studies have shown when the heat index fails to drop below 80 degrees overnight, those without air conditioning don't have a chance for the body to rest and recuperate effectively, increasing the stress from heat in the following day(s), creating the dangerous aspect of these hot stretches.

Of course, lots of heat and humidity in New England also can lead to strong thunderstorms if there's a strong enough "trigger" to touch off the storms – that appears to be the case Wednesday, Friday and Saturday, all with a chance of thunder in the afternoon to evening from west to east, respectively.

Thursday actually may have fewer storms in New England – the most likely ones in the mountains – as any warm/cool clash in the sky is mitigated, frankly, by purely hot air, decreasing the clash needed for storm development.

A cold front passes through New England from northwest to southeast, producing a number of showers and thunderstorms, but slow enough that temperatures will still reach 90 degrees in much of southern New England, extending the heat wave to a fourth day before a dramatically different air arrives from Sunday through next week, dropping daytime high temperatures to around 80 with coastal sea breezes and vastly reduced humidity.

Toward the end of next week, it's possible we see some showers or rain from whatever will be left of the newly formed Tropical Storm Fred, located south of Puerto Rico as of Wednesday morning, but it's early to say and quite possible our dome of cooler, fair weather next week may hold that moisture at bay.

Contact Us