Monday, January 14, 2008

Weather update: Getting very interesting

The 0z run of the NAM is out and it's a major hit -- 1-3 inches of snow followed by a serious ice storm. Is it right? Could be. As you will see from the picture I'm posting, the temps are as close as you can get. Above is what is called a meteogram. In layman's terms, this shows precise temperature forecasts at a given weather station, in this case, Shelby's airport.

As you can see, the NAM shows 29.2 mm of precip fall during this event -- that's just under 1 inch.

The two lines that are key to observe are the 850mb temperature line and the 2m temperature line.

For there to be snow, very generally speaking, both of those lines need to below 0C. As you can see, that's the case for the first 6 hours of the event during which about 8 mm of precip falls -- that's about 1/4 of the total precip which would be a 1-3 inch snow.

Again, generally speaking, when 850 temps are above freezing and 2m temps are below, you have ice. So, this depicts icing for the last .75 inches of precip -- which would not be good.

Now, a couple of things: First, the temps here are very close -- normally to get a real killer ice storm, you need temperatures as low as 30 or even the upper 20s -- otherwise what is called "latent heat release" (subtle, slight warmth generated when water freezes) can cause the temperature to rise above freezing.

On the other hand, even in this short range, models struggle to pick up on the strength of CAD.

In any event, this bears very close watching -- the NAM keeps trending stronger with precip and the GFS is trending stronger with the high pressure (up to 1032 mb last run). If both of those are right ..... could get REAL interesting.
Not sure if I'll stay up for the 0z GFS, but if so, I'll make a quick post.

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