It has been 735 days since we saw snow in Sea-Tac Airport. Since our last snow, same-sex marriage was legalized nationwide, Donald Trump became president, CO2 levels permanently passed the 400 ppm threshold, and Michael Phelps won five more gold medals. There is a chance our snow-less streak could finally come to an end this week. But it will be close.
Snow is one of, if not the, hardest thing for meteorologists to forecast here in the Pacific Northwest. It’s not necessarily that our temperature/precipitation forecasts are more inaccurate during snow events; it’s that small changes in temperature and precipitation can play a humongous role in the forecast outcome. For example, nobody would notice/care about the high only reaching 70 in August when the forecast was 73, or if you got a half of an inch of rain at your house instead of the quarter of an inch your local weatherman forecast. But when you are close to freezing, the smallest changes can make the biggest differences in your forecast.
Let’s zoom out and get a look at what’s happening over the North Pacific right now.
Credit: University of Washington Atmospheric Sciences
We currently have a HUGE ridge of high pressure off the coast of California as well as a decent-sized trough in the Gulf of Alaska. Between these two, we have a large difference in pressure, which results in strong upper-level winds. At the 300 mb level of the atmosphere (around 30,000 feet), the jet stream is blowing at an amazing 185 knots!
Not surprisingly, air traffic from Tokyo to Seattle is taking the longer (distance-wise) route over the Pacific because they have a massive tailwind. Air traffic from Seattle to Tokyo is taking the “great circle route,” which is the shortest distance between two points on a sphere.
Over the next couple days, the ridge will move south and the trough will slip southeast, directing that powerful jet straight into our area. Below is the forecast for 500 mb heights (~18,000 feet) at the 1 pm Tuesday.
But what is really fascinating is how cold the air is behind this trough. Here is the chart for temperatures at 925 mb, pressure at sea level, and the wind at 10 m at the same time. Absurdly cold air over the Yukon and Northern Canada, with cool air over the Northern Pacific.
Our first chance at snow comes Sunday night/Monday morning as a very weak low pressure system arrives from the NW.
The placement and timing of this low is critical. Too far north, and we get warmer southerly winds. Too far south, and we don’t get enough moisture. We want precipitation to start around 4 am when the atmosphere is cool, and we want Sunday night to be clear and calm so that we can have more radiational cooling: i.e. that heat can more effectively escape the surface to space.
Because we are coming off a record-warm November and the air will not be that cold, any snow that may accumulate will likely not stick to the roads.
During these high-uncertainty situations, I like to refer to ensembles, which are a suite of forecasts run from a single model with minor perturbations in the initial conditions (well within the range of uncertainty) and model physics. The model below shows a suite of 21 model members from the GEFS (Global Ensemble Forecast System). Amazingly, all the models show snow at Sea-Tac.
Unfortunately, I am still pessimistic that this event will give us snow. First off, the ensembles are much lower resolution than the singular GFS… .5 degrees (35 km over our area) vs 13 km. The lower the resolution, the more terrain features are “smoothed out.” Because Puget Sound is surrounded by two mountain ranges, the models often think it is higher than it actually is. At this point, it looks like the hilltops and foothills from 500-1000 feet have the best chance of snow, while downtown Seattle will likely just see a rain-snow mix or non-sticking snow.
After Sunday night/Monday morning’s system, even colder air pours into the region. Look at those temperatures Wednesday morning! Shiver me timbers.
Our next chance at snow occurs Wednesday evening into Thursday morning as a warm Pacific system heads into the area. For this system, the snow will change over to rain by Thursday, but we may get a quick inch before that. And because roads will be cold, this snow, if it occurred, would likely stick to the roads at first. This would cause quite a hassle for the Thursday morning commute… you know how Seattle drivers fare in the snow.
The model above warms us up too quickly and shows the precipitation arriving midday, giving us essentially no snow for most of the Puget Sound lowlands. Meanwhile, the normally superior ECMWF (European) model gives us several inches. Needless to say, there is a lot of uncertainty with this feature.
In conclusion, I am pessimistic that we will see as much snow as modeled here in the lowlands because of (a) model resolution, (b) marginally cold temperatures, and (c) the fact that we’ve gotten gipped “snow” many times with these snows. Our best chances for snow are Sunday night/Monday and Thursday. There is significant model disagreement, and hopefully details will become more clear over the weekend, but we may not know if we will see snow until flakes start falling.
Two things are certain: we will get pretty darn cold Tuesday and Wednesday night, and the mountains will see heaps of the white stuff. Unfortunately for winter weather lovers, moisture will be lacking Tuesday and Wednesday, so no snow is expected. Cover your plants and take precautions to avoid bursting pipes. On the positive side, look at the predicted snowfall for the next 72 hours. Feet of snow in the Cascades and Olympics. Lower-elevation resorts like Snoqualmie will have enough to open next week.
Finally, a Calvin and Hobbes strip, one of my favorites.
Retrieved from Pinterest
Have a nice night!
Charlie
1 Comment
Excellent post! Thanks for the update. We’re keeping our fingers crossed for snow, too.
I want to gently call your attention to the etymology of the word “gipped”. I assume that you, like most people these days, are totally unaware that it is, in fact, a racial slur directed at the Roma people (aka “gypsies.”) While they are often portrayed as some mythic fantasy culture, they are a real people and they are present in our country and community. I hope that now that you know this you will choose another word when you mean cheated or ripped off.
Best,
Renee