On both Sunday and Monday highs in the Tri-Cities and surrounding areas came in below the forecast (though still quite hot) because of cloud cover. On Sunday these clouds were remnants of thunderstorm activity from the night before in the Blue Mountains and Central Oregon. On Monday, radar data showed some light shower activity trying to form over the Tri-Cities but with the cloud deck well over 10,000 feet up and relative humidity values around 30% not much made it to the ground.
This activity is thanks to monsoon moisture being pulled into the Pacific Northwest from the Southwestern United States thanks in part to an upper level low centered off the coast of Northern California (see the featured image at the top of this post). This low is driving some moisture up from Arizona, Nevada, and Southern California as well as moist air from the Pacific Ocean off the Southern California coast. This synergy of moisture is generating higher dew points to foster thunderstorm growth in the region.
Despite this, the clouds cleared out for the 5 o’clock hour allowing the Tri-Cities Airport to squeak out a high of 100°, extending the streak of consecutive days reaching that threshold. Cloud cover from overnight thunderstorm activity in Oregon may drift over the Tri-Cities again Tuesday, but it is hard to say how much of a difference this will make for afternoon temperatures. As a shortwave trough transits British Columbia, upper level winds are likely to turn more westerly.
This turn may keep the clouds further south, but at the same time the trough is helping to weaken the ridge that brought the regional heat wave. The National Weather Service has a high for the Tri-Cities on Tuesday right around 100°. If high clouds linger in the Columbia Basin, the high will likely be somewhere around 94°-98° whereas if they stick to the south highs will be closer to 97°-101°.
Monsoon driven thunderstorms are likely to continue in Oregon and Northern California through at least Wednesday. Most will stick close to area mountain ranges as the terrain helps lift the moisture up to create the storms, but some may stray into lowland areas especially west of the Cascades where there’s a bit more water in the air.
Thunderstorms have started several fires over the last couple of days throughout the Pacific Northwest. Smoke from these fires has been carried over the Columbia Basin by the upper level wind pattern. With things shifting more westerly, smoke should be minimal for at least the next couple of days (assuming no new fires start directly west of the area).
