2:00 PM UPDATE:
Some of you have inquired more about deformation zones… one of which produced the unexpected rain and snow last night. Here is more information for you:
A deformation zone results when two competing areas of low pressure in the middle and upper levels of the atmosphere "pull the air apart". In between the two lows, there is a line dividing where the air is sucked towards one low and where the air is sucked towards the other low. In the area of the line, there is an absence of air created by the diverging winds. Air rises from below to replace that vacancy of air aloft… and that rising air cools and condenses, resulting in a skinny swath of clouds and precipitation. Specific to last night’s situation, there was an upper low over Canada and an upper low over the southwest U.S…. and the middle (deformation) zone was right over us. Deformation zones are notoriously hard to predict and are often slow moving, dropping rain or snow over the same localized area for hours.
Just when I thought it was safe to say "it’s over"… it’s not.
A "deformation zone" of showers is moving east to west right across the Portland metro area as I write this (1:00 am Sunday morning). Temperatures are too high for the precipitation to fall as snow at the lowest elevations, but above 500 feet I expect snow to be falling through the night in the band. The sticking snow level should be somewhere between 500 and 1,000 feet. Portland’s higher West Hills should have a fresh blanket of snow in the morning, as should the northern Oregon Coast Range (Vernonia has already picked up a few inches) and higher elevations of Clark and Multnomah counties.
With the cloud cover, freezing temperatures are unlikely below 1,000 feet tonight… so if you heard a forecast for "clearing sky and below-freezing temperatures" in the Portland area, that forecaster wasn’t paying attention.
If you wake up to snow in the morning, send your photos to firstname.lastname@example.org.
I’ll update things, if necessary, tomorrow morning sometime.
– Drew J