Plain-language summary:
After an incredible cool spell, some unsettled weather with a stalled backdoor cold front and a narrow zone of heavy rain will occur for mid-week. Like during last week's heat wave to the west, it will be hotter to the west and cooler to the east and where there is heavy rain. Summer heat will then arrive this weekend. It will cool down again early next week, but the core of the cool air appears to be aimed to the west for a change and does not appear to be anywhere close to the record chill last weekend.
Meteorological discussion:
After record chill last weekend and a warmer early this week, some unsettled and weird weather will arrive for mid-week. The deep upper-level trough that brought the record chill over the weekend has slowly moved east into Atlantic Canada, but the southern part of it will cut off and retrograde back to the west as ridging builds to the north, approaching eastern New England by Wednesday night. This evolution has been difficult for models to pin down, but it appears that they finally have a good handle on it now.
 |
| Source: TropicalTidBits |
 |
| Source: TropicalTidBits |
This rather uncommon setup will cause a backdoor cold front (containing both the leftover, modified cold air mass and cool Atlantic air) to move west into upstate New York and eventually stall as it clashes with and blocks a very hot and humid air mass coming from the west. On Wednesday, highs will approach 86F (30C) in eastern Ontario but only around 68-75F (20-24C) in eastern New England. This clash will create a narrow area of showers and thunderstorms, potentially heavy at times. Due to the front stalling, this area of heavier rain could remain almost stationary, leading to isolated amounts of >3" (75 mm) and flooding. This appears most likely in eastern New York and southwestern Quebec. Like last week, there will be a pronounced west to east temperature gradient during this time, with and likely below 68F (20C) in the heavy rain area.
Eventually, another subtle upper-level trough over Hudson Bay will finally pick up the cut-off low and push it off to the east by Friday, allowing a broad ridge from the central U.S. to spread eastward. Winds will turn nearly calm for Friday and Saturday as this occurs. The extreme heat in the central and midwestern U.S. will have collapsed by this time, similar to last week. It is amazing how all the heat from the southwest has simply failed to reach our region this month, with the upper Midwest, northern New England, and parts of upstate New York being the only places in the eastern U.S. to be cooler than average this month so far.
 |
| Source: Northeast Regional Climate Center |
However, this time there will not be a strong trough from the north to provide more cold air for the end of the week. As such, it will dry out and warm up for Friday, except in Maine which could still be cloudy due to the upper-level low near there. Today is the summer solstice, and while it has not felt like summer lately, it will by this weekend everywhere. Westerly to southwesterly flow aloft will push any cold air out and bring by far the hottest temperatures of the month, with many lower-elevation locations likely approaching or reaching 90F (32C) on Sunday, especially immediately to the north and east of the Adirondacks which will receive a boost from downsloping. It will turn breezy from the Champlain Valley on northward with the increasing pressure gradient. The southerly component to the flow could bring a slight moderation from the Atlantic Ocean from southeastern Vermont to southeastern Maine, though it currently appears that the flow will be weak enough there for moderation to be confined to close to the immediate coast.
 |
| Source: TropicalTidBits |
A cold front will pass through between Sunday evening and Monday afternoon, with models disagreeing with each other by up to 18 hours on timing. Given the heat and humidity ahead of it, it will likely be accompanied by showers and thunderstorms, but how strong or severe they become depend on the front's timing. They will be stronger if the front arrives in the afternoon or evening when the low-level air is the hottest and most unstable. The cold front will move relatively slowly but will likely not stall until reaching the mid-Atlantic U.S. The coolest air will be aimed to the west and will not be comparable to last weekend's record chill.
 |
| Source: TropicalTidBits |