Possible inversion buster this weekend

Areas of fog and freezing fog are expected through Friday and with it little change in daily temperature for those who are stuck in it. Those out of the fog and low clouds can expect mostly sunny conditions Thursday and increasing clouds Friday, with warmer conditions. This is similar to the past couple days and can be pretty clearly demonstrated when we take a look at high temperatures so far this week:

Meacham, which is located in the Blue Mountains between Pendleton and La Grande, has consistently seen temperatures higher than those seen in basin locations like the Tri-Cities and Hermiston. This is despite Meacham’s elevation, which is about 3,000 feet above the Tri-Cities.

This inversion setup is threatened with removal Friday afternoon as a low-pressure system approaches the Oregon Coast. The center of this low is expected to move near where last month’s hurricane-force low made landfall. Despite being reasonably strong now, this upcoming low is expected to be quite a bit weaker than last month’s intense storm when it nears the Pacific Northwest.

Also unlike last month’s storm, there is reasonable model consensus in the path of this low-pressure system. It is anticipated that it will move generally eastward until it nears the coast, at which point it is expected to curve toward a more northeast trajectory taking the center of low pressure across Oregon and toward the Idaho Panhandle. The center tracking closer to the Tri-Cities gives us a better chance of precipitation, but it being in a weakened state makes it so that chance still isn’t exceptionally high for locations in the Columbia Basin and Yakima Valley.

This will be a warmer weather-maker than the last few we’ve experienced in our area. I wouldn’t be surprised to see some pass-level snow in the Cascades on Friday morning, but the bulk of the precipitation moves in later that day along with warmer temperatures that will raise the snow level to around 4,500 feet (Snoqualmie is 3,022 feet). Our highest chance of precipitation in the Tri-Cities is a 50% chance of rain showers Friday night, with higher chances in the Blue and Cascade Mountains.

If there’s decent model consensus that this low will move across the Pacific Northwest, why is it only a possible inversion buster? This weak system is not expected to bring much low-level wind with it, which could limit vertical mixing in the atmosphere. While there will be solid airflow above the ground, if there’s no mixing between the ground and this warmer air above our inversion could stick around (though it could be not as strong for a little while). If this were to happen, freezing rain would be possible in the cooler spots. This would be because the warmer air aloft would melt the falling snow into rain, but this rain would freeze on contact with the ground if it is below 32°.

Even if the inversion sticks around, freezing rain isn’t a guarantee. Like I said above, we only have a 50% chance of measurable precipitation as it is. Secondly, even in this scenario, it’s entirely possible that we would be warm enough during the precipitation that the rain wouldn’t freeze on contact with the ground. With the inversion in place this week we have still been able to make it above freezing during the day. If it looks more likely that this will evolve into a freezing rain event, I will have more coverage on it over on Tri-Cities Weather on Facebook.

If the inversion is broken, we can expect afternoon highs this weekend to reach the mid-40s with decreasing clouds on Sunday. If it persists, high temperatures are more likely to remain in the 30s with low clouds and fog continuing.

Beyond this system, a ridge of high-pressure rebuilds over the region so that even if the inversion is busted, it is possible that it would return as early as Sunday evening. Another system out over the Pacific Ocean is then expected to shove this ridge eastward sometime midweek, but the strength and timing of this event is quite uncertain at this point. Better information about that can be expected as we move closer to it.