Potential for Light Snow at Lower Elevations

It is getting very late season for significant lowland snow west of the Cascade crest, although light snow accumulations have occurred infrequently into mid-April.   But an upper level low approaching from the west has the potential to produce some light, non-accumulating snow to near sea level, with perhaps some light accumulations about 500 ft.

The difficulty, as I have discussed many times in this blog, are temperatures that are on-the-edge for snow to reach the ground.

On the other hand, the mountains will get significant snow, as will sections of eastern WA.

As I write this, the shield of clouds has now spread across Oregon into Washington State (see infrared satellite image at 6 AM) as a low center approaches from the southwest.   Our blue skies are now history.

The current freezing level is way high--around 2250 ft according to the graphic from the City of Seattle's SnowWatch website (see below).   The means a snow level around 1250 ft.  Thus, as this system spreads precipitation northward, there will be rain over the lowlands of western WA, while snow will occur over the Cascades and eastern WA.

The accumulated snow forecast through 7 PM tonight from the UW WRF high-resolution modeling system shows this distribution.  Portland, exposed to the cold air exiting the Columbia Gorge, may get a few flakes, with the more serious snow over the lower eastern slopes of the Cascades into the Columbia Basin.

But then there is the second act of our meteorological play.  As the weather system exits to the northeast, cooler air comes in behind.  Not the primo arctic cold air that we experienced last month, jetting into our region through the Fraser River Gap, but cold air from off the Pacific, warmed at low levels by its passage over the relatively warm eastern Pacific (see forecast map of sea level pressure and low-level temperatures at 1 AM Thursday).

A trough of surface low pressure develops off the northwest coast (see above), associated with an approaching upper level low (see forecast upper level map below at 4 AM Thursday), which brings some upward motion and thus some precipitation.

Our latest model forecast suggest the potential for some light snow that could whiten some higher hills and produce some flakes down to near sea level.

Going back to the snow accumulation maps, by 7AM Thursday, the UW WRF has some very light snowfall over central Puget Sound (this is snowfall, NOT accumulation)...perhaps a few tenths of an inch, with a bit more over Portland.

Snow showers could occur into Thursday, with the total snowfall getting to perhaps a half-inch over areas away from the water. The biggest initial threat is in the north Sound, where a Puget Sound Convergence Zone may form tonight.  Accumulations on the ground will be very light, particularly over warmer roadway surfaces.

The very latest NOAA HRRR (High Resolution Rapid Refresh) model run, accentuates this "snow threat" with less than an inch over Snohomish County through 10 PM tonight, but much more at high elevations.  This is no Snowmageddon. 

And more general snow showers on Thursday could spread some non-accumulating snow over the region on Thursday (see totals through 4 PM Thursday below).

So don't rush to the supermarket or buy another bag of salt.   Near sea level, you may see some occasional flurries, while they could be a light accumulation on the grass for the higher hills.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/03/potential-for-light-snow-at-lower.html