Dry "Storm" Hits Washington State, Snow/Freezing Rain over Northwest Oregon

Today is a day of amazing weather contrast across the region, with unusual extremes around every corner.  

On one hand, the low-level air over Washington State is extraordinarily dry, because frigid Arctic air has very little moisture content.

Consider dew point, an absolute measure of the water vapor content of the air.

Dew point temperature, the temperature to which air must be cooled to get saturation (100% relative humidity), is in the single digits to below zero F over much of the state (see plot below for 2 PM values).

In western Washington, wintertime dew point temperature is typically in the 30s or low 40s.  To illustrate how unusual things are, below is a plot of the dew point temperature at the UW for the last twelve weeks.

The bottom has plummeted during the last few days, with a dew point near zero. Very dry air.  In contrast, dew point is 36F right now at Khartoum, in the Sahara Desert.

Or perhaps you prefer relative humidity (RH), which tells you how close to saturation the air is  (saturation is 100% RH).   Below is the RH forecast for 4 PM today for the surface air.    Quite low, with values in much of the state below 40%.

In contrast, relative humidity is far higher over Oregon, due to a moist weather system moving to our south (see the 2 PM visible satellite image below)

So the outside air over much of Washington is very dry.  

But now the shocker....it is even DRIER inside our heated buildings.

Below is the indoor relative humidity around Seattle.

OMG.  Many values are below 25% and some are in the teens.   No wonder your mouth is dry and your skin is cracking!

Why such low relative humidities inside?    

Relative humidity is 100 times the ratio of the amount of water vapor in air divided by the maximum amount it can hold.  

Outside air is constantly invading our homes and buildings so the moisture content is pretty much the same inside and out.   But the amount of water vapor air can hold depends on temperature and it is much warmer inside.  Thus, relative humidity is much lower inside during the winter.

Blizzard over the Western Gorge

Finally, while a dry storm is hitting much of Washington State, strong easterly winds are found in the Columbia Gorge, hitting places such as Troutdale and Portland.  At Crown Point, a scenic location in the western Gorge, winds are now gusting above 80 mph.  These winds are being driven by a historic difference between eastern Washington (high pressure) and the coastal areas (with low pressure approaching from offshore).

Snow has now spread across northern Oregon, with areas of localized freezing rain.    This was the wintry weather that was forecast to move into our region by the errant American weather model, the GFS.

Portland at 4 PM has a cover of snow

A Dramatic Arctic Front Pushes Southward into Washington State

This afternoon a powerful Arctic Front pushed southward into Washington State, on both sides of the Cascades.

As a reminder, an Arctic Front is the boundary of cold, dry air from northern Canada and the previous moist, warm air over the region.  Winds behind the Arctic Front are often strong and gusty, particularly when it exits low-level gaps such as the Fraser River Valley.

And some light snow often accompanies this front as well.

The Arctic Front today was one of the strongest in years..... let me show you the details.

This Arctic Front hit like a freight train.   Here is a video from Greg Johnson at Skunk Bay Weather showing what conditions were like over North Kitsap County after passage.  Strong enough for white caps!

Conditions at Bellingham, downstream from the Fraser River gap, were stunning.  Temperatures today dropped from near 40F this morning to 14F a 9 PM.  At the same time, winds gusted to about 55 mph! (see below)

As a result, the wind chill at Bellingham is now well below zero (see below). And it will go lower.

The winds were so strong around Bellingham that a substantial number of power outages occurred:

I plotted the location of the Arctic Front at 5 PM today (red line, see below).  At that time is was located just south of Seattle and further south in Eastern Washington.  Note that single digit temperatures have already entered the northern part of the state.

As the northeasterly (from the NE) Arctic Air hit the Olympics it was forced to rise, producing several inches of snow.    The weather radar around 5:30 PM shows the upslope precipitation very clearly.

Here is a picture from SR 101 west of Port Townsend.....looks quite wintry.

What about snow?  

It is pretty much over north of Seattle, with some snow showers remaining to the south.  The latest UW snow total forecast from 4 PM today through 4 Saturday (below) suggests most of the snow will be over Oregon and the south Cascades.

The latest NOAA/NWS National Blend of Models forecast suggests a few more cold days in Seattle-Tacoma Airport followed by a warm-up (see below).  The predicted low for Saturday morning is extreme  (11F), with temperatures remaining below freezing most of the weekend.  11F would be the lowest temperature in many years at the airport.

Eastern Washington will stay colder longer, with the Tri Cities (shown below) dropping into the single digits through Tuesday.

Sweaters and thermal underwear might be good choices for the next few days.   This going to be a hard freeze, which may damage some plants.