Freezing Rain Hits Portland with a Threat to WA Passes

Temperatures are rapidly warming aloft as a Pacific weather system approaches.    Sounds good for cold-weary NW residents right?  But those located in or downstream of gaps in the Cascades or in particularly cold areas are experiencing something else:  freezing rain that produces surface icing, a real threat for aircraft and those driving.

As a result, Portland International Airport is essentially closed, with nearly all flights cancelled (see below)

A look at the flightware tracker website shows a lot of overflights but virtually none taking off or landing.

Aircraft are taking off and landing at Seattle Tacoma Airport and the time-height cross section of weather observations from aircraft indicate tremendous warming aloft, with temperatures above 8C in the lower atmosphere (heights in pressure, 850 is around 5000 ft, red is temperature in C)

Right now there is an absolutely HUGE difference in pressure across the Cascades, with high pressure to the east.   The value is nearly 16 hPa (mb).... believe me, that is unusual.  Such a pressure difference drives easterly flow... cold, dry easterly flow... at low levels through gaps and passes.  To show this to you, here is a short forecast (for 10 AM today) of sea level pressure, surface winds, and low-level temperatures (blue is cold).  You can see the tongues of colder temperatures and higher pressure extending into gaps and the large pressure gradient across the Cascades.

With cold, below-freezing air, at low levels and warmer air aloft, one gets a set up for freezing rain.   Precipitation starts as snow higher up and then falls into above-freezing air and melts into rain.  But then the rain falls into a below-freezing layer near the surface and cools below freezing, but remains liquid.  This is called supercooling of water.  Then when the supercooled rain hits the cold surface or an aircraft it freezes into ice....freezing rain.  Portland and the western Gorge is particularly susceptible to such freezing rain events because it is downstream of the Columbia Gorge...a near sea level conduit of cold air from eastern WA.

The latest observations at Portland Airport (KPDX) shows several hours of freezing rain.

My colleagues at the National Weather Service in Portland put out an ice storm warming last night (see below)....a good call.

The ice storm/freezing rain should weaken during the afternoon around Portland as the warm air reaches the surface and pressure differences relax,  Will hold on longer in the Columbia Gorge.

Here is a picture of an iced window looking out from the NWS office in Portland.

Further north, the big freezing rain threats are in the Cascade passes, which also supply a conduit of cold air from the east.  At this point, the NWS is mainly going for heavy snow in the Cascades, but I suspect that there is also a major freezing rain threat.

And did I mention winds?   With a huge pressure difference across the Cascades, winds in the Gorge are gusting to 30-50 mph, even higher in exposed location.   For example, Crown Point in the western Gorge, where gusts are exceeding 70 mph! (see below)

And to make things interesting, the Langley Hill weather radar is broken.  Not good.  Anyway, be careful if you around Portland, the Gorge, or the WA Cascade passes.