This is one of those weekend forecasts that is going to be somewhat frustrating both for those trying to enjoy the weather and for those of us trying to predict it.
The basic setup is a very slow moving frontal system will transition across New England between now and Sunday. If this front were moving at a typical speed, it would pass offshore during today and set up a nice weekend with sunshine and low humidity. Unfortunately the glacial pace of this front means that the clouds and chance of rainfall are going to linger. It won’t rain all the time, but Saturday and at least Sunday morning have a chance of showers and even downpours.
But first, there’s Friday morning, when we find a line of heavier showers concentrated across Western New England. This will slowly move Eastward during the day. Even though the main area of rain is well west of greater Boston as I write, the strong southerly flow does have showers and even the chance for thunder before the main band arrives. If you look closely at the loop above, you can see some small showers moving north ahead of that band. It’s within the scattered storms that we could see a stronger and even severe thunderstorm midday. Even a small tornado isn’t out of the question in this flow of tropical air. It’s equally likely that you’ll see the sun pop out before that main concentration of showers arrive this afternoon.
The showers will be most numerous in greater Boston between about 3 pm and 8 pm this evening, although here again it won’t rain all the time. Outdoor activities will be impacted in some areas, but once this line passes offshore, the dry hours will outweigh the wet ones.
Saturday morning looks to be relatively rain free. It may still be very humid across Cape Cod, but across the Worcester area and points West the dew points will be low and it won’t be uncomfortable anymore.
Saturday afternoon and Saturday evening a new wave of low pressure is going to develop along the frontal system and move northward. Because the boundary between the humidity and the dry air will be so close to eastern Massachusetts, many areas will see another round of rainfall.
As long as things keep moving, Sunday will improve. After the showers or even steady downpours in the morning, I do expect some clearing to develop by afternoon. This will lead to sunshine and temperatures in the 70s along with lower dew points. That will mean the air will no longer feel tropical.
Early next week it looks dry and seasonable and by the middle of the week we may see even cooler air move in. Boston still has not had a day under 70 degrees since way back on July 4th and this is the latest in the season that we’ve ever gone without a sub 70 degree day since records were first kept in the 1870s.