Dry, sunny spell with record heat to end with showers and t-storms on Sunday and especially Monday with some severe t-storms possible; much cooler afterwards but not too chilly

Plain-language summary:

Today marks the 10th day in a row without rain and 3rd day in a row with record high temperatures in many locations. The dryness and record heat is about to end as the weather pattern blocking the normal progression of weather systems is breaking down. After some afternoon cumulus clouds and higher-elevation scattered showers today, clouds and humidity will increase for everyone tonight into tomorrow, with intermittent showers and perhaps a thunderstorm expected for most tomorrow with somewhat cooler temperatures. Monday will be quite warm, humid, and breezy for the 1st half of the day before a cold front brings showers and thunderstorms in the late afternoon and evening, a few of which could be severe. It turns much cooler afterwards with brisk westerly to northwesterly winds and lingering clouds and showers for Tuesday into Tuesday night, especially along and northwest of the Appalachians. On Wednesday into Wednesday night, winds weaken and clouds decrease yielding modest daytime warming but a cool night, though only cold mountain hollows are expected to experience frosts or freezes.

Meteorological discussion:

For the 3rd day in a row, temperatures are soaring under plentiful sunshine and reaching or exceeding 86F (30C) in lower elevations, breaking daily record highs in many locations. In fact, this is one of the earliest, if not the earliest, on record that lower elevations reached 86F (30C) for 3 days in a row. In addition, today is the 10th day in a row without rain for the vast majority of locations.

Source: PivotalWeather and NOAA

However, the big Rex block is breaking down and heights are falling, unlike the past couple of days, the humidity is noticeably higher, leading to afternoon cumulus popping up, especially in the higher elevations. Even some scattered showers have formed, but most of us will stay dry today.

Some moisture left over from the slow-moving and weakening upper-level low that was located to the south all week will finally reach our region tonight into tomorrow, helping to trigger more widespread showers and a few thunderstorms. With the clouds and showers, it will be noticeably cooler but also more humid tomorrow. It will temporarily dry out as that patch of moisture moves off to the east Sunday night into Monday morning.

Source: PivotalWeather

Monday will be an interesting day weather-wise. A strengthening storm will pass to the northwest and cause strong surface southerly winds across the region, especially in the Champlain Valley where they will be enhanced by terrain channeling. The southerly winds will help maintain the warmth and moisture, and the sun will shine for at least the first half of the day, except in eastern Ontario where the storm will arrive first. Meanwhile, winds will be strengthening but blowing from the southwest aloft. The resulting instability and changing in wind direction with height will lead to some strong to severe thunderstorms to develop, for the first time this year, along the cold front. This will occur first in northern New York in the late afternoon and then more likely in southeastern Quebec and Vermont in the evening before weakening due to loss of daytime heating as they move farther east. The models have not been handling this storm well, and only in very recent runs have shown a strengthening storm passing this close, so more changes to the forecast can be expected. Surface winds could turn more easterly in southwestern Quebec, leading to enhanced directional wind shear and the threat of an isolated tornado or two. The U.S. Storm Prediction Center shows a slight risk of severe weather in northern New York and Vermont, which can be reasonably extended into southern Quebec.

Source: PivotalWeather

Behind the cold front, it will turn much cooler with breezy westerly to northwesterly winds bringing colder air from central Canada. With the storm wound up in central Quebec on Tuesday, cloudy conditions with lingering showers will occur from the Appalachians on northwestward, with warmer and sunnier conditions to the east due to downsloping. The highest elevations may even see snow showers, but accumulations are not expected. The breezy conditions will prevent frosts and freezes Tuesday night despite the cold air mass. 

Source: PivotalWeather

The storm pulls away by Wednesday, leading to decreasing clouds though it will still be breezy. High pressure moves closer Wednesday night, leading to a chilly night especially over the Adirondacks, eastern Ontario, and southern Quebec, which will be closer to the high pressure center and therefore will have better radiational cooling. But despite the better radiational cooling, the air mass will have modified enough by this time to limit the frost and freeze threat to the cold mountain hollows.

Source: PivotalWeather