Winter Begins…On The Roads

December 5, 2011

I was asleep, but I hear it was quite a mess on area roads this morning.  Freezing fog and frost on roadways led to numerous accidents all across the metro area.  The 27 recorded at PDX was the coldest so far this season.

Lots of folks were probably going too fast through the dense fog and slid across the black ice on curves or hills.  It was the first time we’ve seen it this winter, but it’ll be quite common from now through early March.  I don’t have stats to back it up, but I wouldn’t be surprised if there are more injuries and accidents from these mornings than from one or two snow events.  There are just a lot of foggy and frosty mornings in our climate in wintertime.  That’s mainly due to abundant water sources (ground, ponds, rivers, ocean) and temps often dropping below freezing.  It’s much drier in Central/Eastern Oregon, so light frost on roads is far more common with fog being somewhat unusual.

I made this graphic   explaining the two different ways we get icy roads on clear and calm nights:  With freezing fog the droplets freeze right onto the road surface…that’s LIQUID to SOLID.  This tends to be clear ice or maybe a scratchy, shiny surface.  It’s not actually black, but since you can see right through it, it’s also called black ice.  When it’s already below freezing and the dew point is lower, the water vapor in the air goes straight into ice crystals on surfaces if they are colder than the dew-point.  That’s GAS TO SOLID (skipping the liquid phase), and it’s called deposition.  Since it’s ice crystals, it has a very white appearance.  This makes for “frosty” roads.  Generally it’s not quite as slippery as the shiny black ice. 

Last night and this morning we probably had both.  For example, frost could form on roads under the clear sky, then fog moves in later coating the frost with plain ice.  A messy morning commute followed!

More of the same on the way for Tuesday morning’s commute under the stagnant weather pattern.  I only see two very weak weather systems moving through the Pacific Northwest for the next 7-10 days.  One late Wednesday; this is the wave that was shown to be the big arctic blast on our models about 5 days ago.    All it’ll do is give us more cloud cover with onshore flow Wednesday.  The next one on Saturday may or may not be a rain maker.  If we get something; it’ll be light.  Beyond that it’s still ridging or split flow through at least the middle of next week (10 days).  If so, we’ll be all dry or mainly dry for the entire first half of December.   This isn’t unheard of, but unusual. 

If you’re looking for an arctic blast or snow setup, I don’t see that either.  Here are the two ensemble charts from the 12z GFS and ECMWF.  Neither one even has any ensemble members showing anything other than near-average 850mb temps for the next week or two.

 Chief Meteorologist Mark Nelsen