Heat Wave & Water Temp, June 2020

[CanisLupusArctos]

On Sunday, 21 June the air temperature at Black Cat Island reached 90 degrees for the third consecutive day, making an official heat wave. While 90 degrees is not unusual on the island between May and September, official heat waves are much less common.

Surrounded by much cooler lake water, the island requires a great deal more heat energy (and 100 percent cooperation from the Big Lake's quirky microclimate) for that to happen. Typically, when the lakes region towns get an official heat wave, the lake islands miss 90-degrees by just a couple of notches on at least one of the days, like a meteorological game of tic-tac-toe.

The last official heat wave on the island was Independence Day weekend 2018.

The max temperatures of this month's heat wave were:

19th: 92 F
20th: 90 F
21st: 91 F

A greater-than expected onshore airflow (North Atlantic Ocean influence with fog) held today's morning temperatures down until midday, and as a result the sun had only enough time to produce a max of 87 F, thus ending the waterfront heat wave of Father's Day Weekend 2020.

There is some potential for a return to these temperatures later this month (but forecast confidence in that is not high) after a cool-down (75-80) later this week. A round of showers and thunderstorms will mark the transition on Wednesday.


...Water Temp...

After triggering cold water safety messages from the National Weather Service on Memorial Day Weekend, the water temperature spiked in response to the heat and humidity, reaching 79 F today. Several swimmers near the site exuberantly confirmed validity of that reading.

That is the warmest June water temperature ever recorded at Black Cat Island since records began in 2004. The average summer seasonal max is 75 at that location -- a boat-stirred, wind-exposed part of the lake. An 80-degree water temperature occurs about once every five years and last occurred in August-September 2017.


...Air Mass Thunderstorms...

There have been pop-up thunderstorms in this heat. Staying safe from those can require a great deal more vigilance than is necessary for a well-predicted squall line ahead of a front (which is more typical here.) There is no knowing which town, or which part of the lake, will have enough of an updraft to create a thunderhead over it. They generally remain stationary until the updraft cools and collapses. That has proven destructive because the collapsing air column hits the ground with a "whump" (if you can imagine) and flows out in all directions like when you point a leaf blower directly down.

Sudden outflow from collapsing thunderheads this past weekend put a tree across the road/wires in Sanbornton and caught a few lake paddlers off-guard in the otherwise-calm conditions. First, a sudden outflow wind from a collapsing storm over the Ossipees, and an hour later it became an equally-strong outflow wind from the collapsing storm over Sanbornton in the opposite direction.

Tonight some are seeing lightning over the southwestern shores, but any boater checking the radar can see there is nothing in the area. No, the radar's not broken. Thunderheads are tall, often reaching the cruising altitudes of commercial flights, and lightning in the upper portions of those clouds can be visible 100 miles away in otherwise-clear air in a wide-open area with no visual obstructions like trees. Regional radar and lightning detection indicate that thunderstorm-in-question to be over southern Vermont.


Weather quirks like this are what make this area the water equivalent of the Presidential Range in the White Mountains.