Temperatures rose into the 80s and 90s during the first part of the work week with Tuesday’s temperatures approaching record levels — including an astonishing 97° F in Provincetown. Those temperatures, combined with dewpoint values above 70° F, made it feel like the heart of summer on the Cape.
Our heat index value — or “feels-like” temperature — passed 100° F in spots, prompting the issuance of a heat advisory from the National Weather Service for our neck of the woods. Inland was worse, with dangerous heat index values approaching 110° F throughout southern New England. And we weren’t alone. The NWS estimated that nearly 160 million Americans were under some kind of heat-related weather alert during this early season heat wave.
Thankfully, the anomalous upper-level ridge responsible for the extreme heat has weakened and retreated to the south. This has allowed a surface cold front to make its way into New England. On the backside of that boundary, cooler and less humid air has oozed in from Canada, putting an end to the sultry weather and returning us to more average levels for this time of year.
Speaking of “average” — what exactly is average for this time of year? And what do we mean when we say “the average high” for such and such a date? After some quality control and data analysis, the process is rather straightforward: take the daily maximum temperatures for a specific calendar date over a 30-year span, average them, and that becomes the “average” high temperature for that date. (Here on the Outer Cape, that value for June 27 is about 75° F.) It’s a useful benchmark for forecasters and the public.
The interesting part? The values come from NOAA’s 30-year climate normals — and that 30-year window shifts every 10 years. Our current average temperatures are based on the period 1991 to 2020. Due to human-driven climate change, both high and low temperatures are trending warmer (among many other weather variables that are changing, which we can talk about another time). The updated averages released in 2021 showed noticeable warming compared to the previous 1981-to-2010 set. For example, the average highest maximum temperature on the Cape in late July climbed from around 78° F to about 81° F.
Anyhow, as we look ahead to the next several days, that cold front looks to waffle north and south, bringing opportunities for wet weather and yielding temperatures at more seasonable levels — with the cooler temperatures focused on Thursday and Friday. Humidity levels will be down dramatically to start the period, before winds come back around to the south and values climb over the course of the weekend.
