Warmth ends with rain and snow today before mostly dry upcoming week; hard freeze Monday night before seasonably cool with stable temperatures later in week; seasonable temperatures likely to continue into following week, perhaps mostly dry early on before turning wetter and cooler

Plain-language summary:

 

After being unseasonably warm for most yesterday, especially west of the Appalachians, it is abruptly and dramatically colder today with a cold rain at lower elevations and even wet snow at higher elevations with minor accumulations. After a few scattered snow showers tonight, it will be a mostly dry upcoming week. It first turns colder and drier with a hard freeze expected Monday night before moderating with sunshine on Tuesday. After a few scattered rain or snow showers Wednesday into Wednesday night in Maine and Eastern Townships of Quebec, seasonably cool and dry weather with partly to mostly sunny skies is then expected through next weekend with relatively little day-to-day temperature variations. Temperatures will likely remain relatively seasonable for the following week, perhaps mostly dry to start before turning wetter and cooler later in the week, though that is a bit more uncertain.

 

Meteorological discussion: 

 

Warmth ends abruptly and dramatically with cold rain in lower elevations and wet snow for higher elevations today, then colder and drier with hard freeze Monday night before moderating Tuesday 

 

Yesterday was the last really warm day (especially west of the Appalachians) for our region for 10 days or likely longer. A sharp cold front passing through today with much colder air from the subarctic or arctic is leading to temperatures being 20-30F (11-17C) cooler than yesterday for most, with rain for many as a disturbance riding along the cold front leads to lingering precipitation behind the front for a few hours before drier air moves in. In higher elevations, the rain will change to snow, with minor accumulations possible though limited by marginal temperatures and daytime occurrence (which matters a lot now given the high late April sun angle). Wet snow fell briefly even in lower elevations of the St. Lawrence and Champlain Valleys this morning with isolated areas of minor accumulations. A weak secondary front will bring scattered snow showers tonight, mostly areas along and northwest of the Appalachians, before clearing by Monday. Again, given marginal temperatures, accumulations will be mostly at higher elevations, though the nighttime occurrence will remove the inhibition by the April sun angle. The front will bring an even colder shot of air directly from the north, with a hard freeze expected Monday night as winds lighten with the cold surface high-pressure system directly over our region and skies remain clear. The cold shot will be relatively brief, with the surface high-pressure system already sliding southeast of our region by later Tuesday, allowing the northerly flow to shift to a southerly flow and the April sun angle to quickly moderate the cold air mass.

 

Source: TropicalTidbits

 

Seasonably cool and mostly dry with relatively small day-to-day temperature fluctuations late week into weekend 

 

A rapidly dropping NAO and AO into negative territory signals high-latitude blocking building over the next week or two, the opposite of most of March and April thus far. It is impressive with a strong Greenland cut-off upper-level ridge overtop a strong cut-off upper-level low over Atlantic Canada. This pushes the storm track and jet stream south of our region, as well as slows down all the weather systems, while also allowing cool air from the higher latitudes to dominate and preventing any of the persistent summerlike air in the southern and central U.S. from reaching our region. I alluded to this in my blog post a week ago, but at that time, models indicated one more warmup later this upcoming week beforehand, which no longer is likely. The PNA turning positive is also aiding this shift. However, ironically, at the same time, the cold air at high latitudes is rapidly weakening compared to the earlier incredibly cold strong air masses (including the coldest April temperature on record in Quebec of -40.4C on April 1). This means that after the cold shot through Tuesday, even the cooler air masses from the northwest will not be far below average, especially on sunnier days given the strong late April sun angle. The push of storm track to our south will also lead to a drier weather pattern, and the lack of strong temperature contrasts will lead to relatively stable temperatures from one day to the next, with no particularly cold or hot days.

 

Source: Climate Prediction Center

 

There is a weak backdoor mostly dry cold front likely Wednesday that will temporarily bring increased clouds and maybe a few scattered showers (possibly mixing with snow with little or no accumulation) in Maine and the Eastern Townships of Quebec. Then, a surface high-pressure system parks over far northern Quebec through next weekend, whose cooler air will battle with warm, moist air over the central U.S. along a stationary diffuse front or baroclinic zone likely near or just southwest of our region, a symptom of the developing blocking pattern. Given that the core of the cold air will remain to the northeast of our region, temperatures will just be seasonably cool with continued dry weather with partly to mostly sunny skies (very much unlike last week's daily rains). Increased clouds and periods of rain are likely along this baroclinic zone, though most models show this staying mostly southwest of our region at least through Sunday.

 

Source: TropicalTidbits

 

Source: TropicalTidbits

 

Seasonable temperatures perhaps with relatively dry weather likely for early following week, with a possibly cooler and rainier weather later in week

 

The blocking pattern is likely to continue into the following week but likely without any true cold air to tap into, leading to relatively stable and seasonable temperatures with a rather weak temperature gradient typical of blocked patterns in spring. It could also continue to be dry if the baroclinic zone with unsettled weather just stays to the southwest of our region. But it is also possible that the baroclinic zone slowly pushes into our region, leading to increased clouds and showers the following Monday, though it will likely be weakening as it approaches and eventually fizzling out by Tuesday or Wednesday. Then, an upper-level trough moves into the western U.S. and could be strong enough for the associated storm over the Plains to push northeastward into our region later in the following week despite the blocking pattern, bringing an initial surge of modestly warmer air before it turns rainier and cooler as the storm moves slowly across the region. Days with increased clouds and rain are likely to be cooler just because of the lack of sun, not due to any cold air from higher latitudes, though somewhat chillier air could be brought in behind the storm with a possible upper-level ridge/trough in western/eastern North America setting up again leading to predominantly below average temperatures, though the core of it is likely to be to the southwest of our region (again, the opposite of March and the first half of April).

 

Source: TropicalTidbits