Winter storm watch issued for Portland metro region

PORTLAND, Ore. (KOIN) -- An arctic blast is aiming for much of the country this weekend, with some northern states dropping as cold as 40 below zero by Saturday.

Although the National Weather Service has issued a winter storm watch for the Portland metro area starting Friday, with some forecasts calling between 1-6 inches of snow, there are still some uncertainties for winter weather in the region as of Wednesday late afternoon.

All of the uncertainty will become clear once the timing of moisture and cold air arrival becomes stable. Then it comes down to what weather forecasters call microscale meteorology.

Microscale meteorology is looking at how the local topography plays into the movement of air in a refined area. It's also a closer look at the temperature profile throughout the atmosphere. All of this helps determine what type of weather is possible in and around Portland this weekend.

Weather models are consistently showing east winds over the region as early as Friday afternoon. It's those east winds that help pull the cold air into the Willamette Valley. The Cascades act like a dam, stopping the cold easterly air from moving west. Remember, cold air sinks. So, the subfreezing temperatures struggle to make it over the mountains. The lower elevations of the Columbia River Gorge help give that cold air an easy route into the Portland metro area.

East winds begin to increase as the cold air sinks into the lower elevations of the gorge. That only continues to push the cold eastern air into the Willamette Valley. The snow and ice potential then increases as available moisture moves above the cold air near the surface.

Depending on how thick or tall the cold air is will determine what type of precipitation Portland could see. If the cold air stretches high into the atmosphere, Portland will see snow. If the cold air is extremely shallow and stays near the surface, then ice is possible.

This winter precipitation scenario all depends on whether the moisture remains over western Oregon and Washington on Saturday. Some weather models are pushing the moisture farther south, just as the cold arrives. That's where Portland could see no form of winter precipitation, but will still experience the cold. Other weather models are showing the moisture slowly moving south. That's where snow is more likely for the Portland area Friday night with drying skies by Saturday.

This comes as a winter storm watch has been put into place for the Portland metro area. The chance to see anywhere from 1-6 inches of snow is possible, according to the National Weather Service out of Portland. These snow accumulation predictions will continue to change as the system continues to develop.

So, Portland is guaranteed to see some of the coldest temperatures of the season so far this weekend. As for the chance to see snow or ice on Saturday? That will become more clear to the KOIN 6 Weather Team later this week.