Lowland snow in the Pacific Northwest on Wednesday?
Parts of the Pacific Northwest could see a wintry mix of snow, hail, and graupel today as part of a dynamic system. This is on top of heavy rain west of the Cascades and heavy snow in the mountains.
A cold front will sweep through the region on Wednesday afternoon and evening. This comes one day after the most recent cold front and with the airmass coming in off the Gulf of Alaska the atmosphere is primed for excitement in the Vancouver, Seattle, and Portland metro areas.
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Unfortunately for snow lovers in the Columbia Basin, not enough of this activity will make it over the mountains to produce snow in lowland areas like the Tri-Cities1 or Yakima.
Now, before you board the hype train, it is important to note that lowland snow accumulations will be minimal if there is any accumulation at all and what does accumulate will melt quickly. WSU AgWeatherNet reports soil temperatures in the low-40s (near 5ºC) throughout Western Washington. The same can be expected for nearby areas.
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The isolated snow chance begins around 14:00 in most of Western Washington, the Lower Mainland, and Western Oregon but not everyone will see snow. Heavier rain showers could (but are not guaranteed to) lower the snow level down to sea level briefly.
This is because evaporation is a cooling process and with heavy rain comes evaporation. In addition, strong storms pull down a modest amount of cool air from up above. Because the snow chance will be tied to shower activity it is difficult to say who will be the recipient of frozen precipitation this afternoon.
The highest chance of snow reaching down to sea level is in the Lower Mainland of British Columbia where wet bulb temperatures are forecast to be near 0ºC. The wet bulb temperature is essentially the lowest temperature that can be achieved due to evaporational cooling (though that description is admittedly a little oversimplified).
The low freezing level will also foster small hail development. We’re generally looking for pea sized hail or even smaller, not something that would cause any damage. Isolated thunder is possible in some of the stronger storms.
This kind of weather pattern is common during and after a strong frontal passage in the Pacific Northwest during the cold season. Wintertime cold fronts don’t often lower temperatures all that much in Vancouver, Seattle, or Portland because at the surface we are still pulling in relatively warm air off the Pacific where water temperatures are around 50ºF (10ºC).
Temperatures a few thousand feet up do drop, however, amplifying convective activity. When we talk about convection in meteorology we’re usually talking about how quickly warm air can rise into colder air. When there is a large temperature difference between the relatively warm surface and the relatively cold mid- and upper-levels this action is more pronounced.
The convective activity isn’t particularly impressive but could still cause some travel headaches. This is especially true if snow or hail ends up falling in an urban area. Remember that bridges ice before other roadways so use caution if you observe one of these snow or hail showers.
The featured image is HRRR modeled new snow in cm for Washington and nearby areas through 17:00 PT Wednesday. (WeatherBell)
- Kennewick, Pasco, and Richland in Washington. Not the Tri-Cities in the Lower Mainland of British Columbia. ↩︎
