There’s no denying Thursday’s forecast was way off. We [meteorologists] freely admitted on-air and through social media “we got it wrong” and were fine with calling it a “surprise storm.” That’s something we don’t like to say very often.
Thursday’s event doesn’t compare to the famous “dusting” forecast from the 90s that ended up with a foot of snow. However, it’s safe to say the April storm came as a surprise to most of you.
What should have been an inch of rain, came down as a heavy, wet snow. In Roanoke and Lynchburg, the snow amounts were modest, reaching 1” or less. However, where the snow came down hard, places like Montgomery, Pulaski and Carroll counties ended up with five or more inches of snow.
While some yell at the meteorologists, others laugh it off as another one of winter’s last gasps before spring truly settles in.
SO WHAT HAPPENED?
When a forecast goes this wrong, you don’t just shrug your shoulders and say, “better luck next time.” A responsible forecaster will look into what happened.
The most obvious explanation in this case was a process called “evaporational cooling.”
The crystal clear blue skies the past few days were a sure sign the atmosphere was very dry.
High pressure delivering the tranquil weather eventually weakened and moved off the northeast coast. Northeasterly winds wedged cold air against the mountains. At the same time, moisture was advancing from the south.
We said earlier in the week, if this were January or February, we’d be looking at a major winter storm. Little did we know just how close we would come.
As the snow dropped from the clouds, it fell into that very dry air. Since evaporation is a cooling process, the air a few thousand feet above us started to get colder and colder. Another way to put evaporational cooling into perspective is to think about the last time you got out of the pool, or shower. As the water evaporates from your skin, you feel chilly.
Thursday, this evaporation continued until the air was cold enough to keep the snowflakes frozen all the way to the ground.
DIDN’T YOU CONSIDER THIS?
We consider lots of scenarios when coming up with the forecast. In this event, we were expecting the wedge of cool air to be taken over by the warmer air coming with the storm. Since the storm was approaching from the south it was transporting some of that warm air from the Gulf of Mexico and southern states. That finally happened, but only after several inches of snow had fallen.
HOW DOES THIS COMPARE?
We’ve seen April snows, but it has been a while. The last time Roanoke topped 1” of snow in April was in 1992.
*The record for most April snow in Roanoke was set on April 6, 1971, when 4.2” fell. The next day, 3.1” more fell, putting the total for that April 1971 at 7.3” of snow.
The latest snow ever recorded in Roanoke was May 29, 1940 when 3” fell.