Snow Much Less Likely For Portland, More Likely For Seattle

Tonight’s model runs are in, and every model has shifted the plume of moisture I was talking about in last night’s blog slightly further north. Now, it appears as though Portland will be too warm for snow, while Seattle will see several inches of extremely wet, gloppy snow that may have trouble sticking at the lowest elevations.

Let’s start by looking at current conditions around the Pacific Northwest. As you can see, temperatures are still quite mild around the Portland area, but are much cooler by the Canadian Border.

Looking at the profilers from Sand Point, Washington and Troutdale, Oregon, things are relatively warm aloft as well. The most recent sounding from Sandpoint has the temperature at 5 degrees C (41 degrees F) at just over 600 meters above the surface (~2000 feet), and Troutdale’s sounding is even warmer.

And yet, with such warm temperatures aloft and another system quickly approaching from the SW, all models are showing somewhere between NW Oregon and Western Washington getting hit with a ton of snow. It sounds counterintuitive, but if we look at the details, things start to make sense.

We currently have an upper-level trough with chilly, but not overly cold, air off the coast of Central Vancouver Island. This upper-level trough is sliding around the east side of a massive ridge of high pressure extending northward into Alaska. This feature has been directing showers into the lowlands and snow into the mountains all day… the Summit at Snoqualmie has had nearly a foot since 5 AM this morning and snow is continuing to pile up.

Behind this trough is much colder air in Southern Canada. This air is nowhere near as cold as some of the arctic blasts they saw in January, but it is still quite chilly. Williams Lake in south-central British Columbia is currently reporting 7 degrees, which is chilly weather for even our neighbors to the north.

A look at the most recent 00Z WRF-GFS run from the University of Washington shows this cold air hanging over Southern British Columbia. There is an associated area of high pressure at the surface there, since cold air is dense and weighs more than warm air. Meanwhile, there is a significant temperature gradient over the West Coast – temperatures at the 925 mb level of the atmosphere (approximately 2,500 feet) are in the 50s over Central California but are only in the upper 20s  off the Washington Coast. For our relatively mild climate, that’s a pretty big temperature change over a short distance.

But if you think that’s a big temperature change, take a look at what happens next.

An area of low pressure forms along the coast of Northern California early tomorrow morning, creating one of the most drastic warm fronts I have ever seen on this model over the Pacific Northwest. A plume of moisture is pointed directly towards Oregon at this time, bringing heavy rain to the area.

This low continues to track NE along this temperature gradient, and the moisture plume remains stationary over the same area as it does so. Temperatures over Oregon are too warm for snow, and we will be right on the fringe in for snow down to sea level in Western Washington. A northerly gradient over Western Washington will help usher in a reinforcing shot of cold air from Southern B.C., but will it be enough to keep us in the form of snow? It’s hard to tell.

As I stated in my previous blog post, the track of this low is critical. The track shown would keep the Portland Metro Area too warm for much snow and would instead give an epic rain-snow mix later Sunday into Monday. Seattle is further away from the moisture but would be cooler, with 6+ inches of snow possible. Olympia would be in the bullseye because they would still have heavy precipitation but would be just cold enough for heavy snow.

Finally, the low swings into BC, bringing another band of showers in its wake. Snow looks like a pretty good bet with these in NW Washington, and if these showers are heavy enough, they could bring snow into NW Oregon as well.

Another huge determining factor in whether Western Washington will see snow of not is how heavy precipitation is. If you have heavy snow falling into an area slightly above freezing, that snow will melt, taking heat energy out of the atmosphere in the process and cooling temperatures. If the precipitation is not heavy enough, less melting will occur and the atmosphere will be too warm for snow.

Most of our snows occur when moisture runs into cold air and begins as snow. The setup above, with heavy precipitation falling into a cool air mass and lowering the snow level via melting, is generally not a situation that gives us snow. However, it has happened before, and models are saying it will happen again on Sunday night/Monday.

And now the map you’ve all been waiting for: the 48-hour snowfall totals from this storm.

This is certain to change, but it our current thoughts are that Portland will begin to see heavy rain around 10 AM Sunday and will remain rainy throughout the event. There is the possibility Portland could see some snow showers Monday morning as cooler air filters in behind this system, but given the northward trend in the storm and the corresponding increase in temperatures, this is looking less and less likely.

Seattle, on the other hand, is trending snowier. Rain will begin Sunday afternoon, and the big question here is if it will change to snow in the lowlands due to melting snow sucking heat out of the atmosphere. At this point, it looks like the higher hills will see snow, while downtown Seattle will see a rain-snow mix. But as we go into early Monday morning, things could change to snow for everyone. Finally, the aformentioned band of showers Monday afternoon could drop another couple inches of snow throughout the lowlands.

This is still an extremely difficult forecast, with small changes in the track and intensity of the low having huge effects. Pay attention to your local NWS office for updates, and I’ll do my best to keep you updated as well.

Portland NWS: http://www.wrh.noaa.gov/pqr/
Portland NWS Twitter: https://twitter.com/NWSPortland

Seattle NWS: http://www.wrh.noaa.gov/sew/
Seattle NWS Twitter: https://twitter.com/NWSSeattle