More Precipitation Today

Lets examine what happened yesterday and then move to today's situation. Although the forecasts were not perfect, they did get a great deal correct. Looking at a range of reports (and I very much thank those of you who provided snowfall amounts), there was roughly 3.5 to 8 inches for the communities adjacent to central Puget Sound, while 6-15 inches occurred over Kitsap and neighboring westwide communities. Less snow--2 to 6 occurred over the east side. So the amounts in Seattle and immediately north and south were right on and the predicted east-west snow gradient was apparent, but was less than forecast (the Kitsap had less and the eastside had more than predicted). This is consistent with wind evolution as we will see.

Now lets talk about the winds. First, this was never going to be a general windstorm...from the first it a downslope wind event on the western slopes of the Cascades and a gap flow event over the Strait. The strong winds verified in the Strait and there were strong winds near the western foothills...sustained 30-40 mph, with gusts to 70. But the strong winds were not as extensive as our best local models suggested...even when we had a historic pressure difference across the mountains. That difference...17.3 millibars between Seattle and Yakima...was the highest I have seen in 30 years. The winds in Snoqualmie pass were fierce and closed down the pass. Anyway, the weaker easterly winds contributed to higher snowfall on the east side of Puget Sound and less snow on the western portion. I could go on about this...but my colleagues and I will try to understand this forecast error.

One issue I didn't mention was the freezing rain. During the event, warmer air was moving in aloft, while cold air was maintained at the surface. The temperatures several thousand feet up got above freezing, resulting in rain falling into the cold air below--particularly south of Snohomish County. The result was freezing rain as the rain was cooled as it fell into colder air below. Thus, there is a crust on the beautiful fluffy snow we had last night.

Several of you commented about the nature of the snow last night. Most of you are used to the large, dendritic crystals that fall when temperatures are near freezing...our usual situation. Last night you got to enjoy the type of snow they get in colder climates.

What about today? More precipitation is coming. Another system is now moving into our area (see satellite picture) and the radar shows the precipitation moving in from the north and northwest (see radar). Rain, occasionally heavy, has moved into the coast...and Shelton has reported snow and ice pellets.

I REALLY wish we had a coastal weather radar to see the details of this approaching system...it would very much help (see my link on the right about this issue). This afternoon and early evening, the precipitation will spread over us...and the big question is: will it be snow, freezing rain, or rain? The problem is that temperatures are now marginal for snow..in fact there are above freezing temperature above Puget Sound now. Temperatures will begin to cool aloft this afternoon and temperatures can be cooled by the melting of heavy precipitation. If the moisture falls as snow, we could have 3-4 more inches. The NWS is now angling towards snow and the computer models suggest a good chance of snow over Kitsap (which there will upslope on the Olympics) and light snow north of Sea Tac. But we are on the edge and there is considerable uncertainty. At this point, I would lean towards wet snow this afternoon north of Seattle-Tacoma, with a "wintry" mix to the south. But there is considerable uncertainty about this.