Some drizzle was observed in the Tri-Cities area Thursday and Friday as a low pressure system worked its way through the region. That low is on its way east to bring some snow to the Plains and Canadian Prairies (with sharp temperature drops in places like Winnipeg).
The Columbia Basin will be in between regional storms Saturday and Sunday for dry conditions. A cool air mass remains in place above the Pacific Northwest so that mostly cloudy skies are still forecast generally. That said, a few long-lasting sunbreaks are possible – especially during the morning hours before temperatures warm up. Look for highs in the mid- to upper-40s (7-10°C).
The cool air mass is the product of an upper level trough parked on the western half of North America. The jet stream is oriented well to the south producing a remarkable blizzard in California’s Sierra Nevada mountains.
Over the Columbia Basin the upper level trough and associated cool air aloft will produce strong lapse rates – meaning the temperature will decrease quickly with altitude (remember that heat rises, and it rises faster the colder the air is above). Strong lapse rates are what will produce a lot of afternoon cloud cover. In addition, some showers are possible in and near mountains (including locations like Walla Walla).
Most in the basin will remain dry, however, because there isn’t enough moisture making it across the Cascades in between regional storms. A modest breeze is likely to continue for the Tri-Cities and Hermiston thanks to deepening low pressure east of the Rockies. Neighborhoods with a lot of mature trees may not feel this so much but gusts to 30 mph (50 km/h) are possible in exposed location like large fields, along the Columbia, and hilltops.
Some thunder, small hail, and maybe even an isolated tornado are possible west of the Cascades Sunday thanks to strong lapse rates, wind shear, and ample moisture there. As of end of day Friday the Pacific Northwest snowpack is 85% of the median for this time of year and will continue to increase this weekend.
The next event rolls in Monday produce lowland rain and mid- to high-elevation snow. Rain totals in the lowest elevations like the Tri-Cities and Moses Lake will be light. Some minor snow accumulations are possible in the highest parts of the Horse Heaven Hills, Saddle Mountains, and the ridges around Yakima.
Precipitation chances continue Tuesday with another low making landfall in Oregon and quickly passing through. The storm track shifts toward Southeast Alaska beginning Wednesday for mostly clear skies forecast in the Tri-Cities and Yakima during the second half of the week. Those with budding tree fruit should be prepared for hard freeze conditions during that period.
