The coming system

Update: 8 PM....looks like the central Puget Sound region will be snowfree for the morning commute due to Olympic rainshadowing through perhaps 9am...at least according to the latest forecast model output. The most serious snow in the urban corridor should be north of Seattle during the afternoon..in the Puget Sound convergence zone--lowest amounts 1-3 inches near the sound, increasing to 6-8 inches to the east. The map you see gives the output from the latest computer model run...with the 24h snow amounts ending 4 AM Thursday and clearly shows the convergence zone snow north of Seattle. Healthy amounts of snow...12-16 inches... will fall in the mountains. Heaven for skiers. And also note the moderate amounts from Bellingham southward to Everett. And keep in mind this is a forecast with inherent uncertainties.


This morning, temperatures across the region varied from the low teens in the cold spots away from the water (e.g, Arlington, North Bend) to mid 20s near major water bodies. Some super cold spots in valleys mights have dropped below 10F.

The big concern is tonight and tomorrow. A trough of low pressure will move southward tonight and will bring precipitation to the area. Temperatures will also warm a bit as more onshore flow develops. I think it will start around 3 AM in the general area. The low center at the surface will be inland and thus this will not be a large regional snow event. With northwesterly flow, the Puget Sound region will be initially rainshadowed...certainly from lower PS basin to the west. As the trough and associated low moves south, a Puget Sound convergence zone will form north of Seattle and then move southward. The snow will mainly be between 4 am and 4 PM. With Convergence Zone snow events the greatest amounts are generally east of the Sound....and 4-6 inches is possible there, with amount trailing off to 1-2 inches near the water. NW Washington, which is less rainshadowed could get 3-6 inches away from the water. Near the coast the temps should be high enough to have precip as rain. The mountains will get snow of course...a substantial accumulation (roughly a foot) that may allow the ski areas to open for the holidays.
Colder air moves in again on Thursday....and we all have to watch the situation on late Saturday and Sunday....when a stronger system will approach from off the Pacific.