The expression April Showers is founded from days like this. The strong springtime sunshine heats up the land and along with low levels of the Atmosphere, while left over winter cold swirls in cold pools aloft. Because warm air is less dense than cold, we get updrafts. The warm updrafts lift air from near the ground up into the cold aloft. Rising air then cools, with the latent moisture condensing into cloud droplets. The condensation process causes latent heat release creating positive feedback for further updraft and bigger cloud formation.The upper low (cold pool aloft) that caused the worst April Tornado Outbreak since, perhaps, 1974 is now crossing Ontario. The front with that Ontario low raced ahead through New England Saturday Night with 3" of rain, and 70 mph wind. But the low has one more trick up it's sleeve. it will cross New England this evening, generating a powerful squall line. This squall line is sort of a combination Windex Event / April Shower. Most April showers dissipate at sunset, not this one. In fact the air aloft is so cold, the squall that starts as rain will also contain hail, and snow. Thunderstorms may even accompany the line. Timing looks to be about 7 PM at the New York Boarder, racing East to the Massachusetts and Maine Coast by 11 PM. After that we get colder, windy dry weather into Monday Morning.
But more changes are in store for Marathon Monday. The front that pushed offshore last night will lift back north as a warm front in the afternoon, bringing lighter wind, clouds and shower by sunset Patriot's Day.
The next outbreak severe starts tomorrow in Colorado, arriving in New England Wednesday.