The mountains of Washington State will get an early blast of heavy snowfall during the next few days, with a particularly heavy snow dump (1-2 feet) over the eastern slopes of the Cascades.
If this was a few months later, I would be warning about snowfall over the lowlands of western Washington, but this IS mid-October.
Let me start by showing you the snowfall prediction from the NOAA/NWS National Blend of Models, the guidance heavily used by local National Weather Service forecasters.
The snowfall total through Wednesday at 5 PM is impressive and crazy high over and to the east of the North Cascades—as high as 1.5-2 feet on some of the highest terrain. Virtually all of Cascades and Olympics above 2500 ft will get significant snow. Stevens Pass would be treacherous.
What about the UW WRF weather prediction model? The high-resolution (4-km grid spacing) domain also produces heavy snow, but shifts the snow epicenter down to the central Cascades. (see below). Snoqualmie and Stevens passes would both get significant snow. Leavenworth as well.
So why such heavy early-season snow? First, much cooler air will be moving into the region as Arctic air shifts southward on Tuesday and Wednesday.
But there is more: a potent low-pressure system will be moving to the Washington coast as well. To understand the complex geometry of the predicted weather, below is a forecast of sea level pressure, surface winds, and low-level temperatures (color shading) for 2 AM Wednesday morning.
The low center is centered near Hoquiam. Very cold air has moved into southern BC, Alberta, and western Montana. And with the cold air is high pressure (cold air is more dense/heavy than warm air).
But is the snowfall so large on the eastern side of the Cascades?
Because of strong winds from the east that will be forced to rise by the eastern slopes of the Cascades. And rising air is associated with precipitation.
To illustrate the situation, consider the winds and heights (like pressure) at around 5000 ft (850 hPa pressure) at 2 AM Wednesday (see below). Strong easterly flow is predicted… flow that is headed straight up the Cascades.
The result? Lots of snow.
Both the Seattle and Spokane offices have put out special snow warnings (check below).
Seattle
Spokane
Be prepared for winter driving if you are crossing the Cascades on Wednesday!
