Colder temperatures are swinging into the Pacific Northwest out of British Columbia and Alberta with a brisk north to northeasterly breeze forecast on Tuesday. Those winds die down early Wednesday morning and despite mostly sunny skies temperatures aren’t likely to hit 40° that afternoon.
Again under mostly clear skies, temperatures are forecast to get quite cold Thursday morning, possibly close to 20°. With not much activity going on at the regional scale and the long nights, this leads to ideal conditions for the typical Columbia Basin temperature inversion to develop.
Under typical daytime conditions, the temperature drops with higher altitude in the atmosphere (at least, until you hit the tropopause around seven miles up). Without sunshine to heat the ground and no wind to mix things up on a calm night, cold air sinks to the ground to develop a reversal, or inversion, of typical conditions so that the temperature climbs with altitude in the lowest part of the atmosphere.
Lower temperatures can produce fog or low clouds as the temperature approaches the dew point, which is the point where the air becomes saturated with water and it starts to condense out in the form of dew/fog, fog/clouds, or precipitation. This occurs because colder air can hold less water than warmer air before becoming saturated.
Lingering wind Tuesday night into Wednesday morning will keep the temperature enough above the dew point to keep clouds and fog from forming though it will be plenty cold – in the mid 20s. Wednesday night into Thursday morning look much calmer and therefore a bit cooler. The modeled dew point depression, which is the difference between the dew point and the temperature, reaches 3°F during this period and is close enough to risk cloud formation.
During the winter (and close to winter) months in the Columbia Basin, the sun angle is too low to penetrate cloud layers enough to heat the ground, cause vertical motion, and clear out the fog/low clouds. When this happens, temperatures will rise during the afternoon on nearby ridges but the basin floor will stay cold and cloudy. Many times this lasts several days and is the primary reason why the Tri-Cities does not see 300 days of sunshine annually.
All this said, I’m still not 100% convinced the fog will form. The air coming south out of interior British Columbia and Alberta will be very dry. Furthermore, strong inversions do best under a strong ridge of high pressure because the high pressure will help trap the cold air in the basin. A transitory ridge is likely Thursday, Friday, and into Saturday but beyond that it becomes messy as other weather systems move near the region.
The very dry conditions and weak short lived ridge may still be enough to generate an inversion though it might be weak. During very strong inversions cities like Spokane (1800 feet elevation) even get into the action. During a weak inversion, however, you can get out of the inversion by climbing Badger Mountain and the clouds will sometimes clear into the afternoon hours before reforming the next night.
Right now, I say there’s a 30% chance of a thick inversion, 40% chance of a weak inversion (both including low clouds and fog), and a 30% chance of remaining sunny through the end of the week. Under any of these cases, the second half of the week will be very winter-like with highs in the 30s. How low the lows get will depend on whether or not clouds form.
