The Olympic Mountain Snow Shadow and Prospects for Snow this Week

Some light snow is falling over Seattle now, as a weak upper level disturbance moves though.  Perhaps a half-inch on the ground in North Seattle.

But in general, Seattle has been left out of the bounty, even though we are cold enough to snow.  More snow north, south, east and west of us.  But why are we being short changed?

There IS a reason.

Let's start by showing the distribution of snow depth tonight at 10 PM from the NOAA National Snow Analysis.   On the left, is last night and on the right, 10 days ago.

Quite a bit more over the west lowlands, particular south of Olympia, and east of the Cascade crest.  Very light amounts over Seattle and south King County.

A year ago? More snow than this year over the Oregon Cascades, less over the western lowlands and slightly more over the WA Cascades.

So why has snow been generally light or absent over central Puget Sound?

A major reason:  winds in the lower atmosphere have been too persistently out of the Northwest, leaving Seattle in the rain shadow of the Olympics.  To illustrate, here is the National Weather Service precipitation analysis for the last week....lots of precipitation in the mountains, but major rain shadowing over Puget Sound. And with cold air over us, that means snow shadowing as well.

Another issue is that the atmosphere is relatively dry west of an upper level ridge--and that is the pattern we have had for days (see map).

Only when a strong trough works its way around the ridge and drives southward can we get real snow in Seattle in such a situation.

Getting back to the current snow situation....snow should end during the next few hours as the upper trough moves by.  Thursday should be dry.

But we have another chance of snow on Friday, as two weak disturbances make their way south from SE Alaska to our region on Friday/Friday evening, and another on Sunday (see upper level maps for these times).

There is a good chance for light snow over the lowlands on Friday...and much more in the mountains.   Here is the European Center  high-resolution 24-h snow total forecast between 10 AM Friday and 10 AM Saturday. Over a foot in the mountains and about an inch in the lowlands.

In their ensemble (many model runs), the Seattle accumulated snow forecast starting 4 AM this (Wed) morning shows the light snow today (good prediction of about a a half inch), but more (about an inch) on Friday.  I should note that there is some uncertainty in the exact amounts, but some snow looks likely.

So at this point, no big snowstorm in view (at least through Saturday), but enough to think about getting that hot chocolate and enjoying the flakes.

Record Cold and the Northwest Stays in the Freezer for a While

The air over us now is unusually cold for this time of the year, in fact, record breaking cold in some ways.  For example, let's start with the temperature around 5000 ft (850 hPa pressure) at the radiosonde station at Quillayute, on the Washington Coast (see below).   In this plot, the blue line shows the daily record low 850 hPa temperatures.  Today's temperature (-12.5C, shown by a circle) was not only a record for the date, but the lowest for the surrounding dates as well.  Really unusually cold.

The surface air temperatures last night (see below) were held up by the windy conditions (which mix down warmer air above the surface) and some clouds, but still temps dropped into the mid 20s F in much of western Washington and single digits in the mountains  Some valley locations dropped below 0F.  Teens dominated in eastern WA.

Tonight is going to be much colder, with colder air aloft, clear skies, and less wind. The temperatures predicted for tomorrow morning suggest mid-20s in the west and single digits east of the Cascades, with some valleys in eastern WA dropping below -8F.

The latest forecast model output predicts another four days of the really cold stuff.  Here is the NWS GEFS ensemble forecast for Seattle.   Several more days of lows in the mid-20s ahead.

The large scale atmospheric pattern is really locked into a super La-Nina configuration with high pressure offshore and cool northerly flow over the NW.   To show you this, here are the upper level (500 hPa) weather maps for Wednesday and next Tuesday. Quite similar really.

There are occasional disturbances moving southward in to the flow east of the upper level ridge of high pressure that will bring some occasional precipitation, and in some places, snow.

To illustrate, here is the total snow fall forecast for the next 72 h.  Most of the snow is heading for Oregon and SW Washington.  None over Seattle or the WA Cascades.

The next 72 hour is quite different, with massive snows in the WA and Oregon Cascades. Skiers will be pleased.

Keep warm....