Cool, Canadian high pressure built into the region early in the week, yielding a couple of days of very nice midsummer weather. Temperatures held in the 70s with plenty of sunshine and comfortable humidity levels — something we don’t get to enjoy much at this time of year. It even felt a bit cool Monday and Tuesday nights as light winds and clear skies allowed temperatures to fall after sundown. Unfortunately — assuming you enjoyed those weather conditions — the cool high-pressure cell responsible for the September-like weather has slipped offshore, and, in response, winds have swung back around to the south and west. That southwest wind is transporting much warmer and more humid air north and eastward into the area — taking temperatures back to midsummer heat over the course of 24 hours.
Like last week, a frontal zone is approaching the region from the north just in time for the weekend. This time, however, the front’s boundary doesn’t have much “oomph” and won’t deliver a nice air-mass change to the area. Instead, the boundary looks to wash out nearby, leaving us on the warm and humid side for several days. As of this writing, there are no big signals for widespread precipitation, but with that frontal zone lurking nearby, some wet weather can’t be ruled out during the weekend and into the first half of next week. Right now, it appears the best opportunity for some shower or thunderstorm activity would be Tuesday or Wednesday.
Circling back to the early week weather for a moment: the rather “chilly” nights we enjoyed Monday and Tuesday came courtesy of ideal “radiational cooling” conditions — clear skies and calm winds with relatively dry air. While we might normally associate these kinds of nights with the winter season, they can happen any time of year and can produce some dramatic short-term temperature changes. During the day, the sun rapidly heats the land surface, and the air temperature rises accordingly. With a high-pressure center directly overhead, however, clear skies and light winds are almost always the result. Consequently, at night, all that daytime heat can easily and rapidly escape the atmosphere. Picture sleeping on a bed with no pajamas, no sheets, and no blankets — nothing to trap the warmth against your body. Brrr.
With radiational cooling, temperatures can easily fall 20 to 30 degrees overnight, taking a daytime high of near 80º F down to the 50s. In the winter, these conditions often result in one of those perfectly still but frigidly cold nights. In the summer, it just means we get to turn off the A/C.
