An Unusually Sunny Midwinter

One of the minor negatives of living in the Pacific Northwest is the lack of sun during midwinter, the result of our northern latitude and considerable cloudiness.

Some folks get depressed from the darkness, developing  Seasonal Affective Disorder (SAD).   A problem that can be addressed by securing a light box, getting out during midday, or heading to a southern clime during the holiday season.

But this winter has been much better than most, with more sun than usual.

Noon Today

Let me show you the proof.    Aaron Donohoe of my department has examined the solar radiation data collected on our roof since the year 2000.   Here is his plot of cumulative solar radiation during the month of December.  The black-red line is this year.  For much of the month, this year was the sunniest since 2000, but fell into second place at the end.  No complaints.

And the first few days of January have been plenty bright, as illustrated by Space Needle PanoCam above and the solar radiation measured on the Atmospheric Sciences Dept roof (see below).

The sunny December is a huge boon to Seattle's psyche, since this month is generally the most depressing, with the shortest days and weakest sun.   By the end of January, things are starting to brighten and by the end of February, Northwest spring is upon us.

Why such sunny skies?  Because we have had persistent high pressure over the West Coast, and high pressure is the enemy of clouds.  To demonstrate this, here is the anomaly (difference from normal) of sea level pressure during December.  A big positive (high pressure) anomaly over our region.

And the high pressure also brought extensive dry periods.  Take a look at the cumulative precipitation at Sea Tac during the past 4 weeks.   We had less than normal, and nearly all the rain occurred over a few days when weather systems broke through the ridge.  Most of the month was dry, particularly the first two weeks.

The unexpected sunshine has attracted some Northwest residents out of hibernation.  Here is someone you might know....

Northwest Mountains Protect Us From the Cold

Much of the U.S. is in the deep freeze, with temperatures in the single digits along the NE U.S. coast and temperatures dropping to -30F or less in Montana, a one day car ride for most of the readers of this blog.

But today, temperatures in western Washington will be relatively balmy, with highs reaching into the lower to mid 40s over western WA and Oregon, while east of the Rockies, the HIGHS will get perhaps to -15F in some locations (see National Weather Service Forecasts for today, Monday):

Why doesn't the cold dense air within the interior of the continent slosh over us, like it is doing over the east coast? 

The reason:  our mountains, which provide  multiple lines of defense that our beloved Seahawks would be jealous of.

Take a look at the minimum temperatures this morning.  In central and eastern Montana, the lows dropped to -30F and colder.   Big Sky Country is frozen solid.  Cross the Rockies into eastern Washington and the situation is relatively balmy, with lows dropping into the teens and low 20s.  Cross another barrier (the Cascades) and drop into western Washington/Oregon and the lows reach only the mid-20s to around 30F.   Cross another barrier (the coastal mountains including the Olympics) and temperatures only dropped into 30s and mid-40sF in some location. 

So we are talking about an 80F difference in low temperatures between the WA coast and central Montana, a distance of roughly 600 miles.

The key to understanding our temperatures situation is the blocking effect of the major regional terrain barriers, coupled with the relatively warm Pacific Ocean.

Currently, cold high pressure dominates the central portions of the U.S. (see NWS sea level pressure map below).  Over our region, there is a weak offshore pressure gradient (high pressure inland), which results in offshore (easterly flow) over the Northwest.   This pattern reduces the impacts of the warm Pacific for the NW interior (click on image to enlarge, the solid lines are lines of constant sea level pressure)

Our terrain is well known to most of you (see map).  The Rockies are a huge and high barrier...it does the heavy lifting of cold-air protection, blocking the coldest and densest air near the surface.   The Cascades are lower and narrower, but do block the coldest air in the Columbia Basin from reaching the wset, with one exception:  in the sea level Columbia Gorge.

But there is a bit more to the blocking effects, which is best understood by a vertical cross section through our regional barriers (see below).   This figure is from my book on Northwest Weather.

As noted earlier, very cold/dense air at low levels over the continental interior is blocked by the Rockies.   Some air (at the crest level of the Rockies) does make it into eastern Washington, but that air sinks down the western slopes of the Rockies and thus is warmed by compression as it loses elevation.  (The air is going from lower pressure aloft to higher pressure near the surface).   The same process occurs with the Cascades and the coastal mountains....each barrier results in warming of the air reaching the surface.

Near the coast, the relatively warm (roughly 50F) waters of the Pacific come into play.  Any onshore flow will flood the coast with mild marine air.

Another way to see the impacts of terrain is from a forecast of sea level pressure (solid lines) and 925 hPa (around 3000 ft) temperatures (white and purple are the coldest temps) for 10 AM toda (see below).  You can see the impacts of the Cascades, keeping the colder air over eastern Washington away from the west.   Look closely and you will notice tongues of cold air and high pressure trying to push across the Cascades...those are the passes.   And look closer (click on image to expand) and you will see a substantial pressure difference across the Cascades, which can drive easterly winds in the passes and the Columbia Gorge.

At Crown Point in the Columbia Gorge, the winds are now gusting to 75 mph, with SUSTAINED winds just under 50 mph. 

With high pressure over and east of our region, there will be plenty of sun on New Years day.