Sunny today, then mix of sun and clouds with a few thunderstorms Monday through Wednesday; hot and humid through Tuesday or Wednesday, then cooler for a couple of days before turning warmer but staying mostly dry unlike most of this summer so far

Plain-language summary:

 

Hot and humid weather will prevail through Tuesday for everyone and Wednesday for the Champlain Valley on southeastward, though the extreme heat will stay to our south like it always has for the past few weeks. It will be sunny and rain-free today, an uncommon occurrence this summer, though there will be some more clouds and scattered showers and thunderstorms for some on Monday and Tuesday. After Thursday, it turns cooler and drier, then warms up, but it is uncertain whether it gets truly hot or if we get more cooldowns. In either case, it appears to be a drier pattern than we have had over most of the past month.

Meteorological discussion:

Today will be sunny and rain-free (not common for this summer!), as well as rather hot and humid with weak deep-layer westerly flow and upper-level ridging. It will approach or reach 90F (32C) in some lower elevation locations. A disturbance moving in from the west will bring clouds and maybe a few showers tonight, but the unfavorable nighttime timing will preclude much thunderstorm activity. The disturbance will move out in time for clearing again on Monday. There will still be some cloudiness, with another disturbance producing more clouds and scattered thunderstorms in eastern Ontario and northern New York late Monday afternoon, but there will be enough sunshine for it to be another similarly hot and humid day. Tuesday will be similar, though it is a bit uncertain just how hot due to uncertainties in timing of subtle disturbances that would bring showers and thunderstorms. Just as in for the past few weeks, the real extreme heat will stay to the south of our region.

 

Source: TropicalTidbits

 

A potent upper-level trough will into northern Ontario on Wednesday, with an associated surface low-pressure over central Quebec and its cold front draping into southern Ontario and moving southeastward, sparking some showers and thunderstorms, perhaps more widespread than on Monday and Tuesday, but the positive tilt of the trough and pieces of energy being a bit spread out may preclude widespread showers and thunderstorms. It will likely still be hot and humid from the Champlain Valley on southeastward. Depending on the speed of the storm, there could still be lingering showers on Thursday, but the cooler and drier air from the northwest will be moving in by then with the surface high-pressure over the Great Lakes by then. 

 

Source: TropicalTidbits

The humidity and rain will be pushed southeastward by Thursday night, likely starting a rather dry weather pattern for quite a while, in stark contrast to the frequent and sometimes torrential rains of the past month. The synoptic-scale weather pattern over North America will change; a strong upper-level ridge will build in western and central Canada, associated with a positive PNA, sometimes poking eastward into eastern Canada and New England, while there are upper-level troughs over Labrador and the south central U.S. This will provide a deep-layer westerly to northwesterly upper-level flow out of central Canada or the north central U.S., providing lower humidity and mostly dry weather and clear skies and pushing the tropical humidity and rain to the south. It will initially be cool Thursday and Friday with the northwesterly flow advecting in an arctic or subarctic air mass, but eventually, as the surface high pressure moves over our region, the cool air mass will quickly modify and warm with the July sun angle. With the upper-level ridging and heat wave building in western Canada, this type of weather pattern could eventually allow for some of the heat to spread east into our region overtop slightly cooler air to the south from time to time, the opposite of the unusually strong meridional temperature gradient recently. However, if the upper-level trough over Labrador is too close, it could also prevent most of the heat from reaching our region, getting cut off by cold fronts from the north and surface high-pressure systems over Quebec that would provide northerly low-level flow. Either way, our region will likely escape the extreme heat for the foreseeable future. It does appear that the slightly positive NAO and AO associated with the lack of high-latitude blocking will prevent troughs from digging too deep into our region bringing cool and rainy weather, despite the +PNA.

Source: TropicalTidbits

Source: TropicalTidbits

Source: Climate Prediction Center