Wednesday, December 17, 2008
Snow Situation Today
noon update: The forecast needs an update. Some parts have worked out well...the rainshadow appeared as predicted and snow--heavy in places--occurred to the north and south of it (mainly north). So what about the CZ snow? It is occurring...but is further north the predicted...but probably will move southward later. Look at the latest radar...you see the heavy shower from roughly Port Townsend that extend to the NE...thats the cz showers...and some areas are getting substantial snowfalls in it. There is heavy snow north of Everett...with lots of places getting 3-6 inches. There appears to have been more troughing (lowering of pressure) offshore and that has resulted in the winds aloft being more southwesterly than expected..the result...the cz being further north. The latest computer models suggest that later this afternoon and evening the winds will turn more westerly and the CZ will move southward and finally give some snow over central PS. I know some people are disappointed and some school districts cancelled classes...
The weather system from the north started bringing light precipitation (generally snow away from the coast) to the region after midnight--with the precipitation extending over the area by 5 AM (see radar). As predicted by the high resolution models there is a profound snow shadow extending from the Olympics to Seattle....the radar image is really quite amazing. The borders of the rainshadow have moved a bit over the past few hours...but this feature is shielding central Puget Sound. Away from the shadow, light to occasionally moderate snow is fall...from Snohomish County up to Bellingham and the far eastern suburbs of Seattle where upslope on the Cascades brings back the snow. There is also snow south of the rainshadow...particularly south of Tacoma.
With the system, the winds aloft have switched to southerly or southwesterly and temps have increased aloft...even at the surface many temperatures are near or just below freezing. Along the coast it is warm enough for rain.
Now this is a snow event with two acts. We are in the first one now, and light snow will continue in the non-snow shadowed areas through commute time, with several inches possible.
Act II will start between commute time and 1 PM: convergence zone snow. The latest computer models suggest a band of enhanced CZ snow will form north of Seattle --probably centered in Snohomish County-and then move slowly southward during the afternoon and early evening before dissipating later tonight. In the CZ far heavier snow amounts are possible...4-8 inches are not unreasonable. See the attached model snow forecast for the 24-h ending 4 AM tomorrow. And the mountains are getting a very good dump, with at least of foot.
There is a long history of convergence zone snows in the area...one of the most memorable being the event of Dec 18 1990 when Seattle had a foot and virtually nothing fell at Everett and Sea-Tac airport (my book has a section on this storm). That cz snow was completely unforecast...we didn't have the high resolution models we use today.
Tomorrow things should dry out..but then ANOTHER surge of cold air and strong NE winds out the Fraser River valley should develop. And yes, the weekend snow storm is still out there.
Posted by Cliff Mass at 4:59 AM