Warm and Humid Weather on the Way

THU 6/5: 86, 86, 84. Those are computer generated highs for Falmouth for Saturday Sunday and Monday. That's toasty for Cape Cod in early June. 66, 68, 63. Those are computer generated dew points for the same period. That's flat out muggy. Put the two together, and we've got the makings for a good stretch of summertime heat and humidity on the way, especially away from the water's edge.

Unfortunately, before we get to the summer weekend weather, we have to shake the low clouds, fog and chilly temperatures that are blanketing the Northeast Thursday. The cool and damp weather comes courtesy of a chilly north to northeast wind blowing in behind Wednesday's storm system. Normally behind a departing storm we'd expect to see drying conditions and developing sunshine. This go around, while the air is drying out aloft, a lot of moisture is stuck in the lowest few thousand feet of the atmosphere thanks to an onshore wind. The northeast wind is keeping the lowest 4000 feet of the atmosphere nearly saturated, giving us the thick low overcast and areas of fog and patchy mist. This set up will hold strong through the day on Thursday, meaning highs will be well below normal, probably struggling to reach 60F. It's possible that we see some brightening skies later in the day, but overall the theme is gray, damp and cool.

In order to turn this around and get back to some sunshine and milder air, we need to shift the wind direction and mix in some drier air. The first part of that equation will come early Friday, as high pressure shifts south of the region and allows for a more south to southwest wind to take hold during the course of the day. The second part, the drier air, will come Friday Night and Saturday, as a southwest flow of air takes hold at all levels of the atmosphere, mixing out the thick, stable cold marine layer. With that said, Friday will be a transition day. A frontal zone dividing the cool marine air over the Northeast from some ridiculous summer heat over the Ohio Valley and Southern Mid-Atlantic will shift back to the north, signaling the beginning of our summer time weather pattern. In doing so, this front will leave us with a mix of clouds and sunshine and perhaps even a spotty shower or thunderstorm. Temperatures will be in the upper 60s - milder if we can manage more sun, cooler if clouds hold strong.

By Saturday, the warm front will have lifted north of the region and southwest winds will be taking hold. As we head through the weekend, high pressure will be set up shop east of the Carolinas, right near the island of Bermuda. This so-called "Bermuda High" is a traditional summer feature for the Eastern United States and is responsible for most of the heat waves the Northeast sees and this will be no exception. At the surface, the clockwise flow around the big warm high pressure center pulls heat and humidity northward from the south and southwest. Aloft, a large upper level ridge blocks any incoming frontal zones and helps to maintain the surface high, allowing the heat to persist for more than just a day or two.

Looking out through the next five to seven days, there are questions as to just how long this Bermuda High will persist. Some computer guidance is quicker to break it down, allowing cool air to sneak southward early next week. However, other guidance maintains the strength of the ridge and effectively blocks any southward progression of cooler air, causing a frontal zone or two to wash out over the Northeast and never really sag south of the region. In fact, some of the guidance used in the medium to extended range really emphasizes the heat Monday and Tuesday of next week, with the weekend just being a primer.

Either way it goes, for us here on Cape Cod, there are a couple of factors to look at with regard to the weekend forecast and just how warm it gets. Most important is the wind direction. Terrible heat waves in the Northeast can often just be a period of warm and muggy weather on Cape Cod if the wind is coming in from the southwest. For the Cape to really bake, we need a more westerly component to the wind, which promotes downsloping, alleviates the marine influence and carries the heat right to the coast. It remains to be seen what exact direction dominates, but the current trend is more southwest than west, meaning temperature-wise, Cape Cod may be the place to be this weekend. Low 80s is far better than mid to upper 90s! The other two issues relate indirectly to the wind. The first is fog. With a warm, humid air mass advecting northward toward the region, fog and low clouds will develop over the cooler waters surrounding the region. These should be most common at night, yielding to the strong June sunshine by day. However, that low cloud/fog deck can be tough to shake at times with a southwest wind, particularly right along Nantucket Sound and out near Chatham. So we will have to watch that each day. That cloud deck can be a spoiler if the atmosphere doesn't dry out enough. The last issue is any thunderstorm activity that might want to pop during the afternoon(s). Overall, the probabilities are low given the expected wind direction, but storms could crank over interior southern New England and drift to the coast before fizzling out during the evenings. So don't be shocked to see increasing afternoon clouds and to hear a rumble of thunder - though I would say the chances are only around 1 or 2 in 10.