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Jack’s

Watching Next Week’s Wintry Potential

Posted on: 6:36 pm, January 11, 2013, by

Before we get to far into this, I’ll say what I’ve said about every chance we have had so far: most winter weather events sneak up on us. They are not usually very visible in the distance because while our forecast models are pretty good, they are not perfect. There is some skew toward climatology in most model guidance, and models have some known issues (biases) that hurt their long range solutions. So, while the model may be showing us the pattern for a winter weather episode, the devil is in the details that cannot be resolved at a long distance due to a lot of factors.

Since Wednesday night’s run of the ECMWF (European model), interest has been up about the potential for some snow or ice next week.

CAUTION: Before you continue reading, understand this is a discussion about a possibility; it is not a part of the forecast we are talking about on TV. If all you want is the forecast (highest confidence weather information), check out WHNT.com/Weather. There is a lot of information there that will help guide you do making decisions about the coming change in the weather!

Here’s the beginning – a huge temperature change that takes us from 20º above normal to 10º below normal next week:

1

While there will be some variability in the numbers, the message isn’t going to change for next week: it gets a LOT colder!

The real uncertainty in the forecast comes due to big disagreements in the long-range guidance about what happens with an upper-air disturbance called a shortwave between Wednesday and Friday of next week.

Here is a comparison between the morning run of the ECMWF midday run of the GFS. (There is no midday ECMWF to compare).

Screen Shot 2013-01-11 at 6.06.58 PM

The orange highlight shows the position of the short wave at the exact same time in the forecast. Is it an instance of forecast model bias?  It very well could be.  The ECMWF historically sends those shortwaves out of the southwestern US too quickly.  That’s a BIG difference! In fact, it could mean the difference between a dry, cool day with a high near 50º and a wet, cold day with at least a chance of some wintry precipitation if you take the model’s solution as a “forecast:”

Wet/possibly wintry vs. sunny & dry.  A computer model conundrum.

Wet/possibly wintry vs. sunny & dry. A computer model conundrum.

Just for added confusion, the Canadian GEMS model splits the difference bringing the rain and possibility of wintry weather into the region on Wednesday; not Thursday.

Canadian Model forecast for the same weather feature coming in at least 24 hours before the other two.

Canadian Model forecast for the same weather feature coming in at least 24 hours before the other two.

So what does one do with this kind of chaos? Unfortunately, you have to envision a scenario that takes into account model biases like how the GEMS seems to always run too high on precipitation, the ECMWF tends to send these shortwaves east too quickly, and the GFS’ biases are generally related to precipitation being too light.

That’s why at the top of this post I said the devil is in the details, and those details will not clear up for a while. It’s going to be an interesting pattern to watch, but if you’re looking for a guarantee of snow or ice, it’s not clear enough yet.  We’ll watch the trends over the weekend and keep you posted!

-Jason
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Winter Weather