Crozet Weather Almanac: November 2013


Freezing Rain… Curse of the Mountain Valleys

November 2013 Weather Data (Chart courtesy Heidi and Roscoe)
November 2013 Weather Data (Chart courtesy Heidi and Roscoe)

By Roscoe Shaw and Heidi Sonen

Some people love snow. Some even like cold. A rare soul loves wind. But I don’t know anybody who likes freezing rain or its cousin, sleet. Unfortunately, we get a lot of the slippery slime around here and the mountains are partially to blame. So, here are some FAQs (frequently asked questions) about freezing rain.

What is freezing rain?
Freezing rain is just plain rain that freezes when it hits the ground.

How is that different than sleet?
Sleet is frozen when it reaches the ground. It could be rain that froze or snow that partially melted and refroze or some combination, but it is frozen.

Which is worse?
Almost always freezing rain. It can be unbelievably slippery. Sometimes sleet is coarse almost like sand and not at all slick. But in Virginia, we often get both mixed in the same storm with snow or just plain rain and the resulting mixture is always different.

How is this even possible? Why isn’t it snow below 32 degrees?
Snowflakes only form in certain environments where supercooled droplets in a supersaturated environment grow into crystals. Often, a snowflake will fall through a warm layer of air and melt. When it hits colder air again, it won’t turn back into a snowflake. Now it is just rain or might freeze into sleet but the miracle of a snowflake is gone.

What is a typical freezing rain storm here in Crozet?
Cold air comes down from Canada and gets stuck here against the mountains. Warm, moist air invades from the Gulf of Mexico and is forced upward over the cold Canadian air. Often, snow falls at first but then warmer air aloft wins and the snow turns into sleet and then freezing rain.

Why do the mountains make it worse?
“Cold Air Damming” happens when the cold dense air gets stuck in the low valleys and hollows. If the ground was flat, the approaching warm, wet air would force the cold air out. But here, the cold air can hide and last for a long time, turning a rain storm into a very unpleasant freezing rain storm.

When a cold air damming pattern emerges, Heidi always says “Damn that cold air!”

What is the worst you’ve ever seen?
Once, in Ohio, Heidi and I awoke to thunder and lightning. Temperatures were in the teens and rain poured all night. The next day, a pack of kids, off from school, ice skated right through our yard and all over the neighborhood. Everything was covered in two inches of solid, clear ice.

November Recap
The month started warm but ended on a cold wave and finished 3.6 degrees colder than normal. The first day was warmest with 77 degrees but 17 days dropped below freezing. The month was mostly dry except for 2.18” of rain the two days before Thanksgiving. Snow fell briefly on the 12th and most of the afternoon on the 27th, enough to bring a white Thanksgiving to the hills above town.

Rainfall totals

  • Crozet 2.68”
  • Greenwood 2.71”
  • Old Trail 2.83”
  • Wintergreen 3.79”
  • Nellysford 3.17”
  • Univ. of  VA 4.60”
  • Waynesboro 2.04”
  • CHO Airport 2.87”