Sunday, January 9, 2011
Chances of Major Snowstorm Fading
One of the most difficult things about being a forecaster is maintaining objectivity.
Many of us got into the field because of a major storm event or a love of extreme weather. Like big snowstorms. And there is a tendency to forecast what you yearn for. Yes, we have our fancy models that provide objective guidance. But many times they disagree or have known biases, leaving some room for "professional judgment" and experience to make alterations. In some sense, meteorologists are the last people on earth you want to forecast the weather because we enjoy the major weather events so much. But eventually reality has to rule and this is such a case.
It is clear now that the trend of several of the forecast models is to bring the low and the associated warm air farther north and to move things in earlier. Yes, we might get some light initial snow but right now it looks like it will rapidly turn to rain near sea level.
Before discussing the mid-week event, I should note that our forecasts were pretty good for today, a mixture of rain showers near sea level over Puget Sound and snow showers in the convergence zone and over NW Washington is now occurring. Bellingham and environs has had 1-2 inches (see image at top). Bellingham and NW Washington is being helped along by northeasterly flow coming out of the Fraser River Valley (see surface plot below). Some snow has also fallen over the San Juans and northeast of Olympics such as near Sequim (no jokes about Sequim golfers and retirees in this post). And moderate snow is falling in the Cascades. The freezing level is around 700 ft right now and snow can push to the surface where the intensities are moderate.
OK, lets talk about the potential snowstorm. The type of event is different than the recent snowstorms of the past few years, which have been associated with convergence zones or disturbances coming out of the north. The midweek event was going to be an over-running snowstorm, in which cold air was in place and an approaching major Pacific system pushes moist over over the low level cold air, resulting in snow...and potentially lots of it. A great example of such an over-running snow event occurred during the last week of December 1996. In fact, there were two of them that week. At my house there was nearly 20 inches of snow on the ground by the end.
The issue with overrunning snow events is that many are associated with low centers and associated fronts. If the low goes too far south, you stay cold and dry. (meteorologists hate that!). If it goes north of us, we are warm, wet and often windy. To get massive snow, the low needs to go south of us, close enough to give us the precipitation but far enough to keep the warmest air at bay. And the low south of us helps pull nice cold BC air into the region. When this sets up right you can get HUGE snowstorms, but everything has to be perfect.
Several days ago the model solution looked very good for big snow...the low crossing the coast and staying near the Oregon/WA border. Major snows to the north over WA. And the fun did not stop there--more disturbances followed along a similar route. Big time snow. It might have been the end of KING-TV SnowMeister Jim Forman. At least he would need a second parka. As I noted in my previous post--there was considerable uncertainty in the solution and some major modeling systems (like the Canadian GEM model) were taking the low north of us.
During the last two days the main U.S. global model (GFS) has altered its solution . Precipitation comes in much earlier (early Wed morning) and the low moves farther north.
Let me show you. Here are several forecasts, starting at different times but all valid at the same time.
First a forecast of sea level pressure and temperature (shading,yellow is warm and blue is cold) for Wednesday at 4 PM for a forecasting starting Wednesday at 4 PM. Nice and cold, with the low well to the south.
Here is the latest forecast valid at the same time. See the difference? The low and associate warm front is way farther north.
In the latest runs, precipitation starts coming in late Tuesday...and yes, it could start as snow, but there would be a rapid switch to rain at low levels.
Here is the latest 24-h period snow forecasts. First the 24-h ending Wednesday at 4 AM. Up to six inches along the coast, over SW Washington and the mountains, but only a few inches at most near Puget Sound.
And here is the subsequent 24h ending Thursday at 4 AM. The influence of warmer air is obvious--only in the far northern portion of western WA is snow maintained. But the Cascades gets hammered and snow is king over eastern WA.
Again, there is uncertainty of the forecasts, but that uncertainty is dropping as we get closer to the event. Bottom line: most of you will see snow, but the huge event suggested few days ago appears much more unlikely. Also, be VERY careful when looking at forecasts more than 3-4 days out, particularly events where the exact location of weather features are critical.
Posted by Cliff Mass at 9:08 AM