Monday, December 13, 2010

Ice This Week?

We've got the potential for an icing event by the middle of the week, but don't get too worried just yet. It's not going to be a BIG ICE STORM like we had two years ago. Get that out of your mind first!

The above image is BUFKIT showing the latest GFS model run. The timebar goes from right to left (you guys should know that by now). Anywho, it shows some icing through the day on Wednesday, as indicated by the red bars. The red line is the temperature and I have made a "freezing line" in light blue.

The GFS does have us at or below freezing as the precipitation moves in. This would lead to some light icing if this happened. Two things to note. First, models have a hard time handling shallow cold air. (I'll explain shallow cold air shortly) Second, notice the big gap between the green and red line. That is the difference between the temperature and dewpoint, which indicates dry air ahead of this system. This should lead to some "evaporative cooling". Lick the back of you hand. Go ahead... it's cleaner than the palm of your hand! Now, blow on it. Colder?? That's evaporative cooling.
NOW, the above image is the NAM model. It has us warming above freezing before the moisture arrives! Remember though, models have a tough time handling temperatures in this setup. We could be colder. Regardless, there is not a TON of moisture with this, but enough to cause some slick spots if it happens.

Now, what do I mean when I say "shallow cold air"? Below is a forecast Skew T of the GFS model...
This is a single point in time (1PM Wed) and it shows the atmosphere from bottom to top. I have colored in some areas to help show what's happening. First, the blue area is the freezing area. The red area is above 32. The "shallow" area of cold air is the freezing air at the bottom. Water falls through that air, and may not have time to freeze before it hits the grounds, but it freezes on contact as a "glaze"... if it DOES freeze before it hits the ground, it is sleet.

For it to be snow, the entire column of air would need to be at or close to 32.


Jason H said...

You thinking any Sleet or all rain/fzr ?

Ryan said...

I think sleet is possible. Especially at the onset, but warm air should change it to freezing rain... and eventually (hopefully) to all rain.

Terri said...

Thanks for keeping us posted Ryan, you are the best. What would we do without you?lol

Wintermute said...

This blog post appears very distractingly cluttered; but maybe it's just the wine kicking in. What do I know about blogging, anyway?