A Hot Start to the Week

MON 6/9: It's warm. There's not much else to say. You know it's going to be a hot day when it's 75F at 6AM - which is what it was this morning at Hyannis. In fact, the unofficial overnight low was 74 at Hyannis! That's actually above our average daily high for the day.

So with the incredibly warm start, where will the temperature end up this afternoon? Chances are, readings will rocket through the 70s and 80s this morning before settling out in the upper 80s to around 90F for a high - cooler right along the water's edge and perhaps out across some of the Outer Cape where the ocean water is still in the 50s and any sea breeze does a good job of holding things in check. (Looking for some relief? Try an Outer Cape ocean beach or a south-facing shore along Nantucket Sound - both locations should have a wind off the water). Anyway, the only thing stopping us from the mid 90s today is the weak pressure gradient, which will allow the light northwest winds this morning (which will actually be a big part of the fast morning warm-up) to become a southerly or easterly sea breeze in most spots - capping readings. Still, add a dew point of 68 to 71-ish and it will be very uncomfortable.

While all is hot and calm this morning, the combination of extreme heat and humidity over interior Southern New England should provide enough fuel for thunderstorm development later today. Surface-based "CAPE" values, a measure of available energy in the atmosphere, over interior sections will spike to several thousand joules per kg, levels only seen a few times a season in the Northeast. Given temperature profiles aloft are also favorable, storms should be able to fire as they did Sunday - but perhaps further northeast. However, similar to yesterday, those storms that do pop are likely to weaken as they drift eastward toward the coast. Still, one or two isolated storms could survive the journey and a late day spotty thunderstorm is in today's forecast.

As for Tuesday, similar conditions are expected. However, with a more pronounced west wind, even hotter temperatures are possible. Most of the area should make a run at 90F to 93F, with the cool spots holding in the upper 80s. (If you check back to last week, you might recall that there was some long term computer guidance hinting that Tuesday would be the worst of the weather.) The combination of heat and dew point could push heat index values to the mid to upper 90s for a time - not as bad as inland locations, but still quite awful. In terms of the bigger picture, the main difference in Monday and Tuesday's weather will be the approach of a cold front Tuesday afternoon and evening. Out ahead of this surface boundary, severe thunderstorms are likely over interior parts of the Northeast. If the timing is right, this has the makings of a pretty good severe weather outbreak from central New England southwest to the Appalachians of the Mid-Atlantic and eastward to the coastline. These situations don't often yield severe weather here on Cape Cod, but it's certainly not out of the question that a strong storm could approach the region during the evening hours. Keep an eye to the sky and the local radar.

The front responsible for Tuesday evening's storms will be delivering a shot of cooler and drier air to the region for the second half of the work week. Before that refreshing air arrives, however, we will deal with one more warm and somewhat muggy day on Wednesday. It's not uncommon for the refreshing air to be delayed here on the Cape for a solid 12 to 18 hours. In fact, I wouldn't rule out a shower or thunderstorm during the first half of Wednesday as the last of the warm and humid air is forced offshore. By Thursday, dew points will be back in the 50s with temperatures in the 70s.