Skies are likely to be mostly cloudy, from an astronomy observing perspective, as a result of an extensive layer of jetstream-level moisture which is forecast to be over most of southern Ontario during most of the night (illustrated below for late evening). The high cloud might be thin enough to cause the impression of clear skies over some regions where starlight is visible but the high cirrus cloud is not obviously visible.

Will skies be clear over southern Ontario Monday night?

Mostly clear skies are expected, with the past weekend's high cloud and haze moving east during Monday, except for some mid-level clouds that may appear over southern Ontario, caused by a region of mid-level moisture and uplift which is forecast to move eastward across southern Ontario overnight (illustrated below for 2 am EDT).

Will skies be clear over southern Ontario Friday night?
Mostly cloudy skies are likely, except for some periods of less cloudiness, due to regions of mid-level moisture moving across southern Ontario overnight (illustrated below for late evening). However, the periods of partly clear skies will have some high jetstream-level moisture over southern Ontario (illustrated below at late evening, but present all night) that will be visible as some degree of high cloudiness.

Will skies be clear over southern Ontario Thursday night?
Skies are likely to be mostly cloudy during the evening and then partial clearing is likely after midnight. Lots of moisture is forecast to be present during the evening and a region of upward motion is likely to move across southern Ontario and to cause cloudiness. After midnight, lots of moisture will also be present, but no significant regions of uplift are expected, and partly clearing skies may develop over many regions.

Will skies be clear over southern Ontario Wednesday night?

Mostly cloudy skies are likely, except for partly clear skies over some regions during the evening before increasing mid-level cloudiness (illustrated below for 700 hPa level for late evening) moves northeastward over southern Ontario ahead of Thursday's warm front and causes cloudy skies after midnight. The regions that remain partly clear will have extensive jetstream-level moisture over southern Ontario (illustrated below for late evening) which will most likely appear as high thin cloudiness.

Will skies be clear over southern Ontario Tuesday night?
Mostly clear skies are likely during the evening, except for some high clouds that might be visible. After midnight, increasing cloudiness is expected ahead of a weak cold front which is forecast to move southeastward over southern Ontario during Wednesday morning. Moisture ahead of the front (illustrated below for early morning) is forecast to cause increasing cloudiness after midnight.

Monday night: clear skies are expected in all regions.

Will skies be clear over southern Ontario Thursday night?

Mostly clear skies are expected for most regions, except for regions near and west of western Lake Ontario, where low and mid-level clouds may persist during the evening. The region of mid-moisture causing the cloudiness (illustrated below for late evening) is forecast to diminish after midnight.

Will skies be clear over southern Ontario Wednesday night?

Skies will most likely be mostly clear, except for some low and mid-level cloudiness over some regions especially during the evening, caused by a region of moisture which is forecast to move southward across southern Ontario overnight (illustrated below for late evening).

Will skies be clear over southern Ontario Tuesday night?

Mostly clear skies are expected, except for some high-level cirrus clouds that may appear, due to jetstream-related cirrus cloud near the jetstream winds over southern Ontario (illustrated below for late evening) and except for some mid-level cloudiness that may appear north of Lake Erie due to mid-level moisture remaining over southwestern Ontario (illustrated below for late evening).

Will skies be clear over southern Ontario Monday night?

Cloudy skies are expected by late afternoon and during the evening, caused by a low pressure trough and cold front moving eastward over southern Ontario. Following passage of the front, clearing skies are likely before dawn over northern regions of southern Ontario and near and east of the GTA as mid-level moisture moves south-eastward (illustrated below for early Tuesday) but southern regions seem likely to remain cloudy all night.

Will skies be clear over southern Ontario Saturday night?
Skies are likely to become mostly clear by sunset over most regions, except for cloudiness near Lake Erie during the evening.Later, by dawn, more moisture is forecast to move southeastward over southern Ontario and cause cloudy skies.

Will skies be clear over southern Ontario Friday night?

Skies are expected to be cloudy by midnight and partly or mostly clear during the early evening until mid-level moisture spreads eastward (illustrated below for late evening) over all regions by about midnight. The most clear skies will likely be in easternmost regions of southern Ontario, mainly near the GTHA and Niagara Peninsula (as well as eastern Ontario), but high thin cloud may become visible and cause partly cloudy skies well before midnight.

Will skies be clear over southern Ontario Wednesday night?

Skies will most likely appear mostly clear, since the extensive high-level moisture and cloudiness that has been over southern Ontario during the past several days is forecast to move south of the province by evening. Some moisture at mid and high levels will still be over southern Ontario (illustrated below for late evening) but with considerably less moisture than during the past couple of nights, only thin occasional clouds may be present and so skies will most likely appear to be mostly clear.

Will skies be clear over southern Ontario Monday night?
Mostly clear skies are likely, except for high-level clouds which may be visible from time to time over most or all regions (especially southwestern regions), caused by a region of jetstream-level moisture over southern Ontario (illustrated below for late evening).

Will skies be clear over southern Ontario Sunday night?
Clear skies are likely over most regions, except for some mid-level cloudiness that is likely over the regions north of eastern Lake Erie during the evening, and scattered low clouds over other regions of southern Ontario. After midnight, the region of mid-level moisture (illustrated below for late evening), that is expected to cause some evening cloudiness, is forecast to be south of Ontario and mostly clear skies are expected.