Some rain into the weekend ... but watch for the most dangerous Northwest weather

Oct 28, 2011

A strong storm front is moving in and will make today and Sunday mostly wet with a break from the rain on Saturday, KPLU's weather expert Cliff Mass says. And trick-or-treaters on Monday should have decent weather. 

After Friday's rain and windstorm subsides, you'll have a chance to ponder what Mass calls Washington's "most dangerous weather phenomenon."

It's not storms, and it's not floods. It's ice on the roads.

Here’s what Mass had to say on his blog about roadway icing:

“I got sensitized to the threat of roadway ice in the first years after I started at the UW. On occasion I would do legal (forensic) consulting and I was surprised that most of the cases were associated with lawsuits dealing with roadway icing. I was stunned by how many people were getting killed and injured. … Many icing deaths and injuries occur each year on our roads (17 deaths in 2009 in Washington alone, according to Federal online stats).  So let me explain how you can protect yourself and your family.

“Roadway icing has two major causes: frost and freezing fog, and the fog is the worse threat. Frost occurs generally on cold, clear nights – the earth radiates heat to space and the earth cools to the dewpoint, and if the dewpoint is at 32F or less, you get frost. Frost can make the road slippery, but it produces a relatively thin layer, which allows the roughness of the road to still supply some traction. …

“The big threat is freezing fog. Classic situation around here:  clear, cold night. ... The roadway temperature drops below freezing – perhaps a little frost – but nothing bad. Near the road, there is a boggy or wet area over which fog forms. The fog then drifts over the roadway and lots of the fog droplets freeze on the roadway, leaving a thick ice deposit. Very dangerous.”

On the Web:

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The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and repeats twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU’s Health and Science reporter Keith Seinfeld. Cliff Mass is a University of Washington Professor of Atmospheric Sciences and renowned Seattle weather prognosticator.Check out the podcast of the show. You can also listen to a podcast of this and previous "Weather with Cliff Mass" shows.