Incredible warm, dry spell about to end with rain Monday, with much cooler and showery weather Tuesday and Wednesday; a shorter warmup likely follows Friday and Saturday, but cooler and at times unsettled weather likely to prevail after

Plain-language summary:

Today will be the last very warm day for areas west of the Appalachians. Tomorrow will still be warm for parts of southern Quebec, but a cold front will bring rain and cooler temperatures to everyone by late afternoon. Much cooler temperatures with scattered showers, mixing with snow in higher elevations with minimal accumulation, will occur on Tuesday and Wednesday. It will likely warm up on Friday or Saturday again, but it will not be as warm and long-lasting as this past warm spell and likely will be preceded and followed by modest rain. Cool but not cold and at times unsettled weather will prevail starting about a week from now.

Meteorological discussion:

Today will be the last incredibly, possibly record-breaking warm day for many, especially west of the Appalachians with light southerly flow, downsloping, and mostly sunny skies, with lower elevations reaching 77-86F (25-30C) again. When it is sunny at this time of year, before all the leaves are up to block the sun from reaching the ground, it gets warmer than nearly all model guidance, as evidenced by the record temperatures experienced on Thursday and Friday, especially south of the U.S./Canada border. East of the Appalachians, a maritime air mass chilled by the still cool Atlantic water, combined with some low clouds, will lead to much cooler temperatures, with high temperatures as low as 50F (10C) along the Maine coast.

 

Source: TropicalTidbits

 

The unseasonably warm air mass will be pinched from the east by the maritime air mass and from the west by an approaching cold front as the associated storm tracks well to the northwest, a classic scenario given the persistent trough in western North America and ridge in eastern North America. The cold air mass is targeting the U.S. Midwest first before spreading eastward, which will lead to modification. Monday will still be warm in areas that get any sunshine before the cold front, again especially west of the Appalachians. Due to the NNW-SSE orientation of the cold front, the warmest areas will actually be in St. Lawrence Valley in southern Quebec. However, with the arrival of clouds and rain with the cold front, everyone will cool down by Monday evening. The rain will finally dampen the ground that has now become extremely dry from the prolonged warm and dry spell and end any fire threat for the time being. The amount of rain will not be that impressive as the rain will only last 6 hours or less at any given location. 

 

Source: TropicalTidbits

 

It will actually be cooler than average on Tuesday and Wednesday, but not as much as farther west due to the cold air modification. Although it will be mostly cloudy, breaks of sun with the April sun angle will still try to heat up the ground while cold air moves in aloft, leading to some instability and showers, especially west of the Appalachians, closer to the upper-level low. Higher elevation spots could even mix with snow and graupel at times, though any accumulation will likely be limited to the highest elevations, as usual for mid-April and given the showery nature of the precipitation. It will be cooler and cloudier to the west, especially over eastern Ontario, and warmer and sunnier east of the Appalachians due to downsloping with the westerly flow.

 

Source: TropicalTidbits

 

Source: PivotalWeather

 

Once again, the cold will be short-lived, as another trough moves into the western U.S. and ejects into the Plains by Wednesday, a familiar pattern driven by the feedback loop I wrote about a couple of weeks ago: cold ocean temperature anomalies off the West Coast and warm ocean temperatures in the Gulf of Mexico and western Atlantic leads to the trough in the West and ridge in the East, which support the continuation of the ocean temperature anomalies, etc. However, this time, a cool high pressure over northern Quebec will provide more resistance to the surge of warmth from the southwest. The warmth attempting to come in could instead initially get lifted above the colder low-level air on Thursday or Thursday night, producing clouds and rain, especially in northern areas. The warmth likely will arrive for a day, either Friday or Saturday, especially south of the U.S./Canada border due to two low pressures tracking to the northwest, but it would likely be cloudier with wetter soil due to the antecedent rain, preventing the type of record heat seen this week. There would also be some rain with the cold front later Saturday or Saturday night. However, there is still substantial uncertainty in how the systems evolve.

 

Source: TropicalTidbits

 

 

Cooler air will likely quickly follow as a Canadian high pressure noses in. However, it is unlikely to be an arctic cold air mass, which means that if there is sunshine, daytime temperatures will not be too far below average, but nighttime frosts and freezes will occur, which is not unusual for late April anyway. The chill is unlikely to be intense enough to erase the large warm anomaly induced by this week’s record heat. If there is upper-level troughing over the area with cold air aloft, this pattern could lead to daily pop-up showers, especially in the afternoon and evening when instability is the greatest. There would also be snow showers at higher elevations. There is also the chance of a slow-moving cut-off low producing unsettled weather for days at a particular location, which is not unusual in late spring, but it is too early to speculate on that. It does appear that we will enter a pattern of general upper-level troughing with cold air aloft, but with no accompanying arctic cold low-level air mass. This would mean a showery and possibly cloudy pattern, with no sustained cold or warmth.

 

Source: TropicalTidbits

 

Source: Northeast Regional Climate Center