We've had nearly a week of sunny skies, below normal temperatures, and lots of frost. In fact, I had so much frost around my neighborhood it looked like a light snowfall had occurred.
And now it is all going to change, first with a turn towards wet conditions, then cold, and then a huge ridge will develop and we will turn dry and sunny again.
The changes were obvious today with considerable clouds moving in prior to an approaching Pacific front. The latest infrared satellite pictures shows a complex series of disturbances moving our way and the latest radar (not shown here) indicates the first showers reaching Puget Sound. These disturbances are associated with substantial warming of the lower atmosphere--something you will notice tomorrow.
An interesting issue is the scouring out of the cold air in eastern Washington. Not unlike a bowl, cold air settles in to the depression of eastern Washington and it takes some time to remove that air as the weather changes. Here is a close-in view of the temperatures and pressure over the region at 4 PM--the cold air is shown by the blue colors. With that cold air in place for a while, the precipitation tonight will certainly fall as snow east of the crest for an extended period. What about the western half of the State? The air right now is warming, but still fairly cold. There could be some light snow tonight in some locations of western Washington before it turns to rain, particularly places above 200-300 ft. It should all be history by commute time tomorrow, so the DOT folks can rest easy.
After the transition we will be in a moist, warm flow through Friday morning. Sort of like a junior pineapple express. Here is the predicted precipitation over the next 72h:2-5 inches in the Washington Cascades and nearly twice that in the mountains of BC. California stays dry for a change.
On Friday morning a strong cold front will move in and behind it the air will cool rapidly...in fact by mid-day Saturday it will be cold enough to snow here. Here is a predicted surface chart at 10 AM Friday...this is a beautiful front...far stronger than we normally get. Huge wind shift and a big drop in temperatures behind it. Normally our fronts are really wimpy around here...why? Because of the homogenizing effects of passage over thousands of miles of ocean.
The question I know you want answered next: will it snow over the lowlands this weekend? Well for snow lovers we have the classic problem...right now precipitation is mainly over over the lowlands by the time it is cold enough for snow. But there is still substantial uncertainty about the distribution of precipitation on Saturday, so we will have to watch things. At this point the models are showing some light snow showers on Sunday. In any case, there will be plenty of snow in the mountains before Saturday...probably 1-2 feet before all the action is over the next few days.
Keep in mind the following: our ability to forecast the temperature of the air is far better than our ability to predict precipitation intensity and distribution. The upcoming cold you can bank on, the distribution of precipitation of precipitation and snow you can't. At least until we get closer in time.