Cold Continues, Messy Changeover Expected

As the arctic air continues to impact the Pacific Northwest, more cold temperatures are ahead for the region. Additionally, a messy changeover to rain and warmer temperatures is likely from late Tuesday through Wednesday morning. Let's take a look at the forecast.

We'll start with the forecast for lows on Monday morning, from the European model.

This forecast shows lows in Western Washington in the upper teens to low 20s, with the Portland area dropping to the low to mid teens. Eastern Washington will drop to the low single digits to low teens, with outlying areas (and areas north of Spokane and around Pullman) dropping to -5°.

Let's compare this to the NAM high-resolution forecast, also showing Monday morning lows.

This forecast is very similar, with lows in the lowlands in the upper teens to mid 20s, with areas from Lewis County to Eugene dropping to the low to mid teens. Eastern Washington drops to the low single digits to low teens.

Now, let's take a look at the highs on Monday. These temperatures could indirectly determine some of Tuesday's weather, as overnight lows from Monday into Tuesday could impact how precipitation starts later on Tuesday.

First, the European model's forecast for Monday's highs.

The European model's forecast shows lowland highs reaching the mid 30s, but highs in the Willamette Valley will only reach the mid to upper 20s due to cold air spilling out through the Columbia River Gorge. Highs in Eastern Washington will only reach the mid teens to low 20s.

Let's compare this to the NAM high-resolution forecast for highs on Monday.

There is a notable difference here, as the NAM shows lowland highs in the low 30s, with isolated areas reaching 33-34°. Areas from Kelso to Eugene will only reach the mid to upper 20s, and Eastern Washington will remain in the mid teens to low 20s.

Additionally, expect another round of gap winds on Monday, with gusts in the foothills (especially around North Bend and Enumclaw) reaching 35-45 mph. This will bring more frigid wind chills (single digits to low teens), and potential tree damage.

Now, for the messy changeover from snow and freezing rain to rain. Precipitation will begin on Tuesday afternoon, continuing into Wednesday. It will likely start as brief snow, but more likely freezing rain, transitioning to rain overnight for all areas except Whatcom County. Let's take a look at the NAM model forecast for precipitation type on Tuesday evening.

Notice the pink areas, which show forecasted freezing rain. Freezing rain is caused when falling snow melts into rain in a warm layer aloft, then re-freezes upon contact with the still freezing ground. This forecast shows freezing rain from Seattle to Eugene, extending into the Gorge, with snow extending into Eastern Washington. 

Let's compare to the forecast for precipitation onset in the lowlands on the European model. This forecast is for Tuesday night.

The European model also shows freezing rain on Tuesday night, especially around Portland, from Olympia to Skagit County, and in the Gorge. Snow will extend into Eastern Washington.

Additionally, as precipitation moves north, colder air in the Northwest Interior and BC's Lower Mainland will support snow. Let's take a look at the forecasts for snow through Wednesday night, starting with the European model forecast.

The European model calls for 2-6" of snow for Whatcom County, and up to 6-8" for the Vancouver and Victoria metro areas. Additionally, expect 2-8" of snow across Eastern Washington, with up to 7-9" around Spokane. The passes will likely receive 15-24" of snow through Wednesday night, creating hazardous travel from the Cascades eastward.

Let's compare this to the GFS forecast, seen below.

This forecast is similar, although it shows a trace of snow for the Portland and Seattle metro areas. This shows 3-6" for Whatcom County and SW BC. Eastern Washington will get 2-5" of snow, except 7-12" for Spokane, Pullman, and most of the far eastern part of the state. This forecast also shows 12-18" for the passes.

Now, for one more forecast, let's take a look at the European model's forecast for freezing rain through Wednesday morning.

This forecast shows 0.25-0.35" of ice accretion for the Willamette Valley and 0.1-0.3" for the lowlands from Chehalis to Seattle, with 0.05-0.15" from Seattle northward. The Columbia River Gorge could see up to 0.5". 

This forecast is still uncertain and may change further. Check back for an update by Monday night.