The Lowland Snow Threat

I would not run to the food store yet.   But there is a significant chance for some light lowland snow over portions of western Washington during the week.

Uncertainty is still high, but sufficiently cold air will be in place to allow snow to reach sea level for substantial portions of time.  The issue is getting precipitation during those cold periods.

The large-scale flow pattern that will dominate during the next week is a classic for cold and snow in the Northwest (see upper level map below for 1 PM on Saturday---500 hPa level, about 18,000 ft). A big ridge/high pressure area will be positioned over the central Pacific, with northwest flow over the West Coast--which brings in colder air from the north.

Snowstorms are generally associated with a trough moving southward on the eastern side of the ridge/high.   Such a trough causes upward motion, resulting in clouds and precipitation, and helps produce a low pressure area near the SW WA coast, a feature associated with many Puget Sound snow events.

The first potential for lowland snow will occur late Thursday and early Friday, but it is only a minor threat (see sea level pressure and low-level temperature map for 4 AM Friday).  Marginally cold enough air will be in place and an approaching front will bring the precipitation.

The latest UW model run showing the snowfall (not snow accumulation) for the 24-h ending 4 AM Friday is presented below. Some light snow north of Seattle (.5 to1.5 inches), particularly at higher elevations (above 400 ft).  More in the mountains.  Some of you will see some flakes.  Fortunately, temperatures hav been much warmer than normal and light snow will melt on most roadways.

The next, major snow threat is on Monday.  On Sunday, much colder air will push south into WA state, with a low center developing just offshore of the Columbia bars (see below).  The question is how much precipitation will be associated with the low center and the associated upper level trough.  And how far it will push northward.

The UW model forecast for the 24-h snowfall ending 4 PM Monday shows significant (2-4 inch) snow spreading across Portland and southwest Washington, with Seattle on the edge.  A lot of uncertainty where the edge will be.

As described in the blog many times, for situations like this, with lots of uncertainty, one must look at ensembles of many different forecasts to determine the probabilities of snow.  So let's look at the best...the European Center ensemble forecast of total snowfall (not accumulation on the ground, which would be less) at Seattle-Tacoma Airport.

Each line is an individual snowfall forecast, color coded as noted on the right.  The average of the ensemble is shown by the green bars on the bottom and the solid blue line on the bottom is their high-resolution forecast.

Although many forecasts have a dusting here on late Thursday/Friday AM, significant snow waits until late Sunday/Monday/Tuesday (several inches).  But considerable differences are apparent between the runs in amount and timing of the snow.  This is important.

So don't run to the market right away (particularly since I need to go shopping for groceries), but keep your eye on the forecasts, which could change as the details of the approach systems clarify.  But there is a real threat of lowland snow in western Washington and the probability that cold air will move in is very high.