Leftover moisture and cold air aloft will be enough for scattered showers and isolated t-storms throughout the Pacific Northwest on Tuesday. These storms will be more widespread along and west of the Cascades and some may produce small hail and/or graupel. The Columbia Gorge, Hermiston, and the Tri-Cities will be breezy Tuesday into Wednesday as high pressure at the surface filters through the Cascades. Some shower activity may linger in the mountains Wednesday but the Columbia Basin will be mostly sunny. Skies clear by Thursday for Seattle and Portland.
This change is pattern is the result of an upper level ridge moving in off the Pacific Ocean. The ridge will generate mostly sunny skies amd warm temperatures into the beginning of next week. Portland is likely to have their first 70°F (21°C) high and may reach up to around 75°F (24°C) during the weekend. Seattle will be a few degrees cooler.
The wildcard during this event is an upper level low that is likely to dig into the Desert Southwest creating what is called an omega block. The term comes from the jet stream pattern, which when plotted on a map can look like the Greek letter omega (Ω) with the ridge in the middle and lows on the lower left and right sides.
The position of the low in the Desert Southwest will play a significant role in high temperatures both for the Tri-Cities and Portland. The general consensus among models is that the low will be located in a way that produces northeasterly flow aloft. Under this pattern, cool air from the Canadian Rockies will influence temperatures in the Columbia Basin to moderate highs somewhat. Right now looking at upper 60s (20°C) through the weekend, possibly warming early next week as the ridge axis shifts to change the upper level wind direction.
The combination of northeasterly flow aloft and cooler temperatures in the Columbia Basin mean offshore flow is likely in the Portland metro. Wind coming downslope off the Cascades will warm, amplifying temperatures in the city by a few degrees so that while this pattern has a cooling influence east of the Cascades, it is likely to have a warming effect to the west. There is a small chance (maybe 15%) that the low in the Desert Southwest ends up closer to Albuquerque over the weekend instead of Las Vegas. This would produce the same effect but the influence would be weakened.
This period of warm and dry weather will bring a decrease to Pacific Northwest snowpack, which was at 87% of the median for end of day March 10. There is a good chance that we have reached peak snowpack for the season. Omega blocks are fairly resiliant to being broken down by incoming systems. Models do favor the block breaking down with a new ridge building over Alaska toward March 21. I feel reasonably confident in that solution (with the caviat that it is a week and a half out), but it could also just be the models trending toward climatology in the longer range.
The dry season hasn’t started yet, but we are transitioning that way. We will likely see more stormy periods including mountain snow over the remainder of spring but these likely won’t produce enough to compensate for snowpack losses during the omega block pattern.
The featured image is ECMWF modeled 500 mb heights and wind for 00:00 PDT Sunday via Windy.
