Sunglasses and Caution

If you lost your summer sunglasses, you better find them.  But if you are driving, some caution is advised, since the downside of clear, cold conditions is roadway icing.

The freezing level is now hovering around 1000-1200 ft, which means the snow level is around sea level (generally it takes about 1000 ft for snow to melt in a layer above freezing).  So if you are near sea level and precipitation is very light, you can get rain.  But get above a few hundred feet or get hit by harder precipitation, and wet snow will be falling.   A few remnant showers might be around early Friday, but those will be history very quickly as the skies clear.

The models are building a huge ridge (high pressure) over the eastern Pacific and we will generally be dry and cold into the middle of next week.  Here are some samples of the upper level flow for the next few days.  First, Friday morning we see a deep trough moving east and south of us and ridging building over the eastern Pacific.

Then on Saturday, a week trough moves southward over us, bringing some clouds and perhaps some light precipitation (even snow) to the coast.  Maybe the interior could get a light dusting in places...nothing to be worried about.

And then the ridging goes into overdrive...we are completely high and dry.  Here is the map for next Tuesday to illustrate.  Huge ridge. And it doesn't go away.

But we make a Faustian bargain with the weather devil when we sell our meteorological souls for a big ridge and dry weather.  

The devil's response:  highway icing and sometimes persistent fog and low clouds.   And damaged plants (less of risk this time since the air won't be extremely cold). 

The icing threat begins this (Friday) morning.   The dew points are still fairly high (low 30s in many places) and if the temperature falls to the dew point (and it will in many locations) and the temperatures are below freezing, you will see frost.  We also could see localized fog, and if such fog drifts over a cold road...icing could be substantial and rapid.

An important thing to keep in mind is that on clear, cold nights without strong winds the temperature near the ground can be quite a bit colder from what is reported at weather stations (which generally measure at around 6 ft above the ground).  So don't assume you are safe if local weather stations are reporting 33-35F.   And also keep in mind that the eastern suburbs can easily be 10-15F colder than the central city.

Many of you have cars with air temperature sensors.  Those sensors are mounted a few feet above the roadway surface and thus can be warmer than the ground by several degrees.  If your car sensor says 33-36F you should worry.  And drive slower.   Guess what weather phenomenon injuries and kills more Northwest residents than any other?  Roadway icing.

As I mentioned before, we now have road temperature sensors here in Seattle (thanks to Mayor McGinn and the City Council).  You can get a real-time view of the temperatures at the SNOWWATCH web site.  As shown in this example, during the day and after a period of relative warmth, the roadway temperatures can be warmer than the air temperatures, but that situation will reverse on cold, clear nights as the earth radiates heat to space.  WSDOT also has some road temperature sensors and an excellent roadway weather site.