Thursday, December 18, 2008
Seattle's turn and this weekend
Right now there is a band of snow over central Puget Sound...including Seattle...produced by convergence over the area...but with a twist. Take a look at the surface map. Instead of converging airstreams from off the Pacific, we are getting convergence between southwest flow from off the Pacific and northerly flow from the Strait of Georgia and the Fraser River Valley, with the northerly airstream being rather cold. The convergence is producing a band of snow around Seattle--which is not good for the morning commute. (see radar image) The northerly flow is occurring because the low pressure trough has moved south enough to draw the BC flow southward. The convergence zone band should move southward and weaken later this morning and the Puget Sound region will be left in scattered snow showers. But an inch or two is possible.
The latest infrared satellite picture shows the swirling clouds from the low. You can see instability clouds (cumulus type) develop offshore around the Queen Charlottes as cold continental air goes over warmer water. See the clouds over Las Vegas? They had record SNOW there yesterday.
There have been all kinds of comments about the accuracy about yesterday's forecasts. But even the most critical must admit that the models were absolutely correct about the rainshadowing over Seattle....take a look at the output graphics of my previous posts. Seattle should never have cancelled school. The model underplayed the Whidbey snow...but was correct in showing greater amounts north of Lynnwood and over the south Sound. The timing of the convergence zone snow in the north Sound was off (the models were too early by roughly 3-5 hrs). Twenty years ago we could not have provided any of the regional insights into this event...the models didn't even have the local terrain right. A major problem with weather prediction now is communication....and yesterday shows that my profession needs to do better at that. And communicating uncertainy has to be big part of it.
Now a really threatening even is possible for Saturday night and Sunday. Not a wimpy convergence zone...but a major pacific weather features coming in while cold air is over the region. The models's indicate a threat of serious snow over the region....but this is a difficult forecast. Will the cold air hold as strong, moist SW flow invades aloft? This is what we will have to determine in the next few days.
PS: Book availability. A number of local books stores and online locations have run short of my weather book. During the next few days there should be a major restocking as the 5000 printed last week become available. Some local stores like Barnes and Noble and Edmonds Book Store still have copies.
Posted by Cliff Mass at 5:00 AM