Enjoy Tuesday. Compared to the increasingly humid weather ahead of the Independence Day holiday weekend, Tuesday will feel like an early summer gift.
Much less humid air holds on Tuesday night and early Wednesday, with temperatures just a shade below normal. Highs will still be in the 80s, but Tuesday night will be pleasantly cool, as low temperatures by daybreak on Wednesday finish in the lower 60s.
But it is the end of June, and that type of weather cannot last. Those who have followed weather in Virginia a long time know about the Bermuda High, and what it means for summers in the Southeast. It is a broad area of high pressure that meanders in the western Atlantic for much of the summer, driving Virginia's fundamental summer weather pattern.
Because the Earth is round and spins on its axis, the wind flow around a high pressure area in the northern hemisphere is clockwise. In the broadest sense, the position of the high means that wind in Virginia usually comes from the south or southwest during the summer months. Those are winds originating from the very warm Gulf of Mexico, Caribbean, or the Bahamas. And that means high humidity gets sent into the Old Dominion.
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The Bermuda High is not static, it does ebb and flow during the summer, both in its strength and precise location. Those smaller fluctuations during the summer will help drive the frequency of high humidity air into Virginia and impact the track of individual storms coming out of the tropics.
Summer is the dominant time for the Bermuda High due to the temperature difference between the continent of North America and the Atlantic Ocean. Over the course of a summer, the land warms more than the ocean. As a result, air over the western Atlantic Ocean tends to be cooler than the air immediately over the North American landmass.
Air pressure is directly related to temperature, with cool surfaces having relatively higher pressure. Consequently, a broad area of high pressure will set up for much of the summer in the western Atlantic.
By later in the fall, as the continent cools, the regularity of the Bermuda High gradually fades, allowing cooler and drier air to start making its way into Virginia more often.
But as this weekend nears, the Bermuda High will flex its might, and send some especially humid air into Virginia. Friday, Saturday, and Sunday will be muggy with afternoon temperatures in the lower to middle 90s, with the nights staying in the 70s. Not coincidentally, the strength of the high will keep the winds from the east in the deeper tropics, keeping the current topical system (to be called Bonnie) on track to cross the Caribbean and avoid turning north into the Gulf of Mexico.
Humidity will be high enough in Richmond to generate scattered afternoon thunderstorms on Saturday and Sunday, so keep those in the back of your mind as you start putting together your weekend plans. Independence Day brings an even better chance of showers and thunderstorms, but that is several days away, so it’s probably too early to be canceling any specific plans for the Fourth.