Atmospheric River To Bring Heavy Rain and Mountain Snow To Pacific Northwest

Around this time of year, we have to closely monitor any atmospheric rivers that decide to make landfall along the West Coast. Under the right conditions, the influx of moisture brought in by an atmospheric river can result in a multi-day period of heavy rain and mountain snow. For those unfamiliar with atmospheric rivers, briefly read over this infographic from NOAA. Essentially, atmospheric rivers are narrow bands of anomalous water vapor content that are connected to the tropics.

North America from space. Elements of this image furnished by NASA

We can monitor these atmospheric rivers in real-time by using precipitable water imagery derived from microwave sensors on satellites. Higher precipitable water values equate to higher amounts of water vapor contained within the atmosphere over a given location. Basically, precipitation water maps tell us how much liquid water we’d have if the water vapor within the atmosphere above a given location at a given time were condensed into liquid. In the precipitable water loop, you’ll notice the tongue of high values that have already reached the Pacific Northwest. The next question becomes whether or not the atmosphere is currently primed to condense out a lot of that moisture from the atmosphere. If you’re in the Pacific Northwest, you probably already know the answer to that.

The Pacific Jetstream currently extends to the Pacific Northwest, with a jet streak just offshore. Basically, flight-level winds are currently anomalously strong over the region, and we monitor these winds because they indicate where surface lows might develop. Also, it’s important to note that these winds are mostly oriented west-to-east (zonal flow). In such a pattern, storm systems generally move in a zonal manner, so the Pacific Northwest will continue to capitalize on the injection of moisture from the atmospheric river.

A surface high pressure system currently sits offshore of California, shielding most of the state from any effects from the atmospheric river. In the Northern Hemisphere, wind flows clockwise around surface high pressure, and you can actually see in the precipitable water loop the atmospheric river moving along the periphery of the high pressure in a clockwise manner.

From tonight through New Year Day, basically the entire Pacific Northwest will be under the gun for heavy rain and mountain snow. We’re talking 4-6+ inch rainfall totals along and west of the Olympic Mountains and Cascades (as far south as most of Oregon) through tomorrow night. Along and just west of the Rockies in Idaho and Montana will also be at risk for higher rainfall totals over the period. Snowfall totals will easily exceed 1-2 feet around and above 4,000 feet through Wednesday night.

A shortwave ridge is going to begin building into the Pacific Northwest on Thursday and Friday, as a surface low treks toward British Columbia. In effect, the Pacific jet stream will become less zonal, and the atmospheric river will nudge farther northward into British Columbia near Vancouver Island or just to the north. Thus, the Pacific Northwest will temporarily become less active late week, although the western third of Washington State will still manage to get some precipitation during the period. If you want to see A LOT of snow, head to the Coast Mountains in British Columbia later this week.

By the weekend, the ridge will move out of the region, and the pattern will become active again across the Pacific Northwest. Surface high pressure will once again position itself offshore of California, directing the moist airmass away from the state toward the Pacific Northwest. In other words, the western third of Washington and Oregon will have to deal with more heavy precipitation this upcoming weekend.