Saturday, December 5, 2009

Here comes the cold (dadadada) + What's up with Probcast?

December 5, 2009
9:46 P.M.
Greetings. Looking at the latest radar image, it does not seem like we are going to get much in the way of snow tonight, as most of the snow appears to be heading east of the area. Most of it is wrapped around the northeastern corner of the Olympics by Sequim. Here, northeasterly winds flowing out from the Fraser River Valley pick up moisture as they come across the strait, and as they slam into the mountains by Sequim, clouds and precipitation (usually in the form of snow) are created. The ironic thing is that the Olympics usually keep Sequim dry; most of our weather comes from the southwest, leaving Sequim on the drier, lee side of most storms. However, when winds come from the northeast, orographic (topographically enhanced) precipitation occurs around Sequim and other areas around there. There is also some snow extending southward from there, but it is quite light and accumulations should not amount to much. Sequim should see several inches of snow, and as long as we are getting these arctic outflow winds from the Fraser River Valley, I can't think of any reason why this "Strait Effect" snow would stop.
Let's take a look at one of my favorite charts of them all, the UW mm5 1000-500 mb thickness chart. Remember, this is the thickness in linear units of the atmosphere, and is good for giving an approximation of how warm or cold the air is. Since cold air is more dense, it will have a lower thickness within the same millibar levels. The inverse is true for hot air. So the tropics have much higher thickness levels than the polar regions.

As you can see with this guy, very cold arctic air is seeping into our region from Canada. However, the highs here during this period are expected to be much higher than places further east with similar thicknesses. We should see a high of around 30 Monday, but Butte, Montana, will be lucky to get a positive value for a high. Why is this? You see, when we have arctic outbreaks, the air flows out of the Fraser River valley, but due to the many decreases in elevation it undergoes as it makes its way southward, it warms up and is significantly modified. If we didn't have the Cascades or Rockies blocking us from this cold air, we would get quite a bit more snow days.

I am expecting highs in the low 30s tomorrow and around 30 Monday. Tuesday and Wednesday will be warmer, with highs in the mid 30s and around 40 respectively. It will be sunny and dry all of these days. Sorry guys. :(

I should note that it is possible that the cold weather will stick around longer though, because sometimes the cold air at the surface is tough to scour out even as more moderating Pacific air overrides it.

Speaking of Pacific air, we will start to see some more moisture work its way into our region later in the week, but by then, highs will be in the low 40s, so I'm not expecting much in the ways of snow. That said, details can change, and it is very possible that we may see a fringe snow event. But I am expecting nothing like last year.

One thing that I noticed people talking on Cliff Mass' blog about and that Garfield sophomore Nicholas Efthimiadis (a self-proclaimed weather nut in his own right) were the completely wacky predictions made by Probcast, a forecast model that works using probability and statistics to convey to probabilities of different outcomes occuring as opposed to just having one model run and trying to make a forecast after that. It is Cliff Mass' project, and is extremely accurate in most cases.

Right now, though, it is not accurate at all. The image above is a mapped prediction of low temperatures for Monday night in the region. It is expecting Seattle to have a low of 7 degrees. That's soooooo cold! It says the lowest we could possibly get is 3 degrees (which would be one of the coldest nights ever) and the highest we could get is 12 degrees (which would still be among some of the colder nights ever). I don't have to overthink this scenario. Probcast is wrong. Although I wish it wasn't, it is. I wonder what exactly is up with it.

I'll keep you guys posted. Keep reading, and let me know what you like! This blog exists because of you - otherwise I'd just keep a weather journal or something. Have a good weekend and bundle up!



  1. hm... maybe it means celsius? i want snow!!!!

  2. I think one of the big advantages of a system like Probcast is that it's putting model data out there. While the folk who run the models have a natural tendency to shove massive screwups like these 'way too cold' forecasts during the cold-and-clear period, Probcast make the mistakes obvious - and then the people running the models are forced to acknowledge those errors and hopefully fix them. As a result, the models get better!