Friday, December 26, 2008
Plant Damage, Snow, Rain, Melt Out, and Avalanche Danger
Walking to the bus stop this morning I saw increasing evidence of damage to plants and trees. Large branches broken off by the weight of snow, bushes bent over to the ground or broken. At the UW, the main road was blocked because of heavily leaning fir tree...snow loading had done the trick. Anyway, I suspect many of you will find quite a bit of broken or bent vegetation around your homes and in our parks.
A major weather system is now approaching rapidly...with a warm front tonight and cold front tomorrow morning) and it should be precipitating over the western interior around lunchtime (see satellite and radar images). It is raining on the coast and Quillayute turned from rain to snow.
So what will happen here as it begins to precipitate? Temperatures are generally above freezing at the surface and the Sand Point profiler indicates the freezing level is at 1000 ft. The snow level (the level where all the snow is melted) is usually about 1000 ft below that....sea level in this case. So we may initially see some wet snow at higher elevations (above 300-500ft), and very, very wet snow and rain near sea level. You all know whether you are in the high or low elevation category.
During the afternoon the cold air will hold in place at low levels, but this evening it will be replaced by warm, southwesterly flow and strong winds. You WILL be able to tell when the warm Pacific air hits tonight. Attached is a time-height cross section showing temperatures (C), winds, and humidity forecast over Seattle. You can see the 0C freezing line move nearly straight up in the evening. The heights are in pressure...850 mb is about 5000 ft, 700 mb about 10,000 ft.
This system will be wet, but not a pineapple express. Seattle, partially rain shadowed, will have less than .5 inches....minimizing the threat of urban flooding from the combination of melting snow and rain (but please clean out your street drains to make sure!). Temperatures tomorrow will get back to normal...into the mid-40s and most streets should become passable by late tomorrow.
The mountains will get several feet of snow...and then we come to another issue....avalanches. The snowpack is not large right now, but it has a number of weak embedded layers. With massive snowfalls on top of it, a large avalanche threat will exist by tomorrow...so be very careful if you are skiing out of controlled areas. Even better is to avoid such locations.
And finally...there will probably be strong southeasterly winds over northwest Washington waters tonight as the front approaches.
Posted by Cliff Mass at 9:57 AM