Plain-language summary:
After an unseasonably chilly air mass for the past 1-2 days with frosts and freezes in normally colder locations, it will quickly warm up through Monday before turning cloudier with light rain late Monday into Tuesday. It will be mostly dry on Wednesday, but with some remaining cloud cover, with rain likely on Thursday and Friday, especially south of the U.S./Canada border, though this is still a bit uncertain. It will turn dry, clear, and cool again on Sunday before warming back up, but not turning hot, for the following week, with the dry weather leading to the drought intensification resuming.
Meteorological discussion:
Unseasonably cool air mass to quickly moderate through Monday, then light rain late Monday into Tuesday
An unseasonably cool and dry air mass directly from the north pushed into our region yesterday associated with a cool and very dry Canadian surface high-pressure system, resulting in frost and freezes for many locations outside the broader valleys like the St. Lawrence and Champlain Valleys as well as away from large bodies of water. The Maniwaki sounding yesterday evening showed that the air mass was actually the driest on record for this date, with a precipitable water of only 0.25" (6.4 mm). The drought is also helping nights turn chilly with the ground not adding any moisture into the air to slow the cool down. However, the high-pressure system is quickly moving off to the east, with strengthening southerly flow on the back side leading to a quick warmup through Monday. It will also turn cloudier and breezier (in contrast to the mostly calm winds recently) as a disturbance approaches our region from the Great Lakes, and the disturbance will actually bring at least scattered light rain late Monday into Tuesday, the first rain in more than 2 weeks for many locations. As has been the case for the past 2 months though, the extremely dry antecedent air and lack of time to sufficiently advect Gulf of Mexico or western Atlantic moisture into the region mean that rainfall amounts will be relatively light.
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| Source: Storm Prediction Center |
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| Source: PivotalWeather |
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| Source: TropicalTidbits |
Complicated weather pattern with more clouds this week and rain likely Thursday and Friday especially south of U.S./Canada border
It is a somewhat complicated or uncertain weather setup next week, with actually several days of potential rainfall, especially south of the U.S./Canada border. Although the disturbance mentioned earlier will push off to our east by Wednesday, another disturbance lagging behind to the west over the Midwest U.S. will actually be slightly cut-off from the main flow and meander through the central and eastern U.S., with upper-level and surface ridging building overtop of it to the north. As is always the case with cut-off lows, the timing and track of the system is still rather uncertain. If it is a little farther north, feels more of the mid-latitude jet stream to the north, and the jet stream is configured to have west-southwest flow over our region, the cut-off low could push northeast into our region and bring widespread light to moderate rain Thursday into Friday. On the other hand, if it is a little farther south, or the mid-latitude jet stream to the north is configured to bring a west-northwest flow suppressing the system farther south, then the system could move more slowly and miss our region to the south entirely with its closest approach Friday into Saturday, which would be a missed opportunity to dent the intensifying drought, though the systems would almost undoubtedly lead to a cloudier week overall than for most of the past 2 months. Today, models are in better agreement on the first, rainier scenario, but it is still too early to have high confidence on this.
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| Source: TropicalTidbits |
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| Source: TropicalTidbits |
Dry, clear, and cool again next Sunday before warming up but not turning hot, with drought intensification likely resuming
Regardless of what happens with the cut-off low, there is good model agreement on another dry, chilly Canadian surface high-pressure system sneaking in next Sunday, though the cold air will not penetrate very far south, and the bulk of it could stay to our north. Like with this weekend's cool shot, nighttime frosts and freezes are possible for many locations before the high-pressure system pushes to the east, allowing for a warming trend for the following week. However, with the rapidly lowering sun angle and average temperatures, these future warmups will become less and less likely to yield summerlike temperatures and especially humidity. Still, with the mean upper-level trough axis continuing to be near the North American West Coast leading to a downstream broad upper-level ridge over central and eastern Canada, temperatures will likely be above average most of the time, though this pattern also allows for occasional dry, chilly Canadian surface high-pressure systems to favor frost and freezes for normally colder locations, like this weekend and next weekend and possibly later the following week, especially with the ongoing drought. The teleconnections do not yield any strong signals, though the slightly positive NAO and AO favors a rather zonal jet stream to the north blocking cold arctic air from reaching the entire contiguous U.S. and southern Canada, which matches ensemble projections of widespread warmth from West Coast to East Coast. However, this is a bit uncertain in the long-range, as models have occasionally shown bigger cooldowns in eastern North America perhaps associated with a slightly more positive PNA promoting more upper-level troughing in eastern North America. Also, the models, especially the ECMWF model, have underestimated the intensity of the cooldowns over the past 2 months in the long-range. Still, our location generally to the east of the upper-level ridge axis and west of the upper-level trough axis is also likely to promote sinking air and dry weather most of the time through early October, intensifying the drought further, even with the seasonal cooldown leading to lower evaporation rates.
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| Source: TropicalTidbits |
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| Source: Climate Prediction Center |
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