That day, maybe a week ago, it really came down.
I know nothing about late spring snow. Nothing. When I was a child, in New York, it snowed during our Easter break -it was early April. The day prior was warm, even the day it snowed it was warm, so much so that I was out riding my bike in the street with my brother. Although it was cloudy, the big, wet flake snow came without warning.
This snowfall is different, intermittent pellets and flakes. It was windy too, driving the pellets hard. As is often the case, the snow did not stick. The snow was not the trouble at all. It was the cold that presented itself the following night.
I woke to find a frost on the little wetland.
Crystals coated all the leafed out, saturated-looking plants in the early sun.
The Virginia Wetleaf succumbed (but recovered) to the eight or so hours well below freezing.
The last frost date for our location is roughly May twenty. I do not think anyone would suggest that the last two months have had ordinary temperatures, we haven't. Since March, we have had days that topped out at 10ºF and 82ºF, although most have been in the forties through the sixties. Our March monthly average high temperature was nearly 46ºF and the April average so far is 59ºF. Daytime temperatures have long suggested I should be growing things that California is having trouble providing. Think twice. I watch the trees and the vegetable gardens. Only this week are the oaks beginning to show the chartreuse of spring and there has been zero garden activity.
Warm air masses, heated by their descent from the Rockies and Great Plains, move in from the south and west, and locally there is sunlight warming the thermal mass of land without the cooling influence of great bodies of water. The day warms nicely. At night, however, without the moderating influence of clouds, radiational cooling is strong. I recall a typical temperature differential in NYC to be about 15 degrees. Here, in Minnesota, I have seen 20+ as the norm. Beyond nightly cooling, there is always the threat of a cold airmass coming down from the north whenever the jet stream decides to do something funky. Minnesota is the common entry point for cold air, it is the reason people think this state is cold.
Which brings me to another weather detail. I noticed the window box of just planted pansies was bone dry. What? I had watered it in, deeply, just the day before. Hmm. Something unusual had happened -dry air, exceptionally dry air. Two days after the snowfall, and the day of the overnight freeze, our relative humidity had dropped to 12%, twelve percent! Our dewpoint was nearly 1ºF by the late afternoon. Meanwhile, our high temperature was 55ºF and the winds were up. The water simply evaporated. Despite this, the pansies toughed out the freeze and drought, as those in the pot above attest.