Norm Knows: Snow, sleet, freezing rain all in a day? What gives?
Thu, 28 Feb 2013 22:33:23 GMT —
In the last couple of days, parts of the WNWO viewing area have gone from one type of precipitation to the next. And in a few cases...back again for a second trip!
So what gives? How can one location go from rain, to freezing rain...then to sleet and snow all within the span of a few hours?
It all has to do with temperature...not only at the surface where we all dwell...but in the lower levels of the atmosphere.
Cold air is more heavy and dense that warm air. (This is why hot air balloons float). As two differing air masses bump up against each other, the warmer air is often forced up and above the colder air below.
And when that colder air is below the freezing mark...some interesting things cam happen.
Most all precipitation starts high up in the clouds where temperatures are well below freezing. Ice crystals, and eventually larger flakes, are formed and begin to fall.
If Mr. Snowflake can make it all the way down to the ground in air that is below freezing...then we get snow.
But, should he travel through a layer of air that is above freezing...he melts into a raindrop, and the temperature of the air the rest of the way down makes him behave differently.
If that warm air layer isn't very thick, and he falls back into air that's below freezing, he then re-freezes into an ice pellet. That's what we know as sleet.
However, if that warm air layer is deeper, then that raindrop can stay liquid all they way down to the ground. If that ground and the surrounding air are below freezing, Mr. Raindrop freezes on contact and we end up with freezing rain...by far one of the most dangerous types of winter weather.
Lastly, if Mr. Snowflake falls into a column of air that is entirely above freezing...all the way down to the ground...then we get plain old rain.
Part of the difficulty with forecasting winter weather in changing temperature profiles is nailing down exactly where those changing levels of warm and cold air will be...both aloft and at the surface.
As forecasters, we generally do a good job detailing where each type of precipitation will fall. But even slight changes in temperatures at the ground up through about 4000 feet...sometimes as much as a degree or two...can greatly change the mode of precipitation that reaches the ground.
Factor in changes in moisture content of the air through that depth and you're adding even more room for error.
And to be honest...as good as our computer modeling is, we still don't have the resolution needed to really see all of the little nuances that exist in our atmosphere. We'd need to double our weather balloon network, have more and better satellites and have more surface observations taken to really improve the product.
You know that old saying about computers..."garbage in, garbage out"? It's desperately true with computer forecast modeling.
Suffice it to say...forecasting winter weather is the most difficult part of our jobs. Mother Nature holds all the cards.
And even though we're pretty good at guessing what she's going to play...she remains a good bluffer. So you'll have to forgive us when our "stack" gets a little short once in a while.