Atmospheric River Update: 1-2” In Lowlands, Rising Rivers Expected

FastCast—Thursday, June 9 to Monday, June 13:

The atmospheric river will begin impacting the Pacific Northwest on Thursday. The lowlands will receive most rain on Thursday and Friday, with additional showers possible on the weekend. Rain by early Sunday morning will total 1-2 inches around the lowlands. The foothills will pick up 1.5-3 inches, there will be 0.5-1 inches in the rain shadow (NE Olympic Peninsula to Everett area), and the coast and mountains will receive 2.5-5 inches. Many rivers draining into Western and Eastern Washington will rise, with many rivers draining into Western Washington reaching “action” stage (bankfull), and the Naches and Stehekin Rivers in Eastern WA will likely reach minor flood stage. Muggy weather is possible during this atmospheric river event, especially on Friday, when dewpoints in the upper 50s to low 60s are expected. On Thursday and Friday, expect highs in the low to mid 60s and lows in the mid to upper 50s (warm nights for early June). Partly cloudy conditions and a general break from the rain is expected on Saturday, with highs reaching the mid to upper 60s. Highs plummet on Sunday and Monday, only reaching the low 60s. Some locations may only top out in the upper 50s. Lows from Saturday night to Monday night will be in the upper 40s to low 50s.

——————————————————

Continue reading the full blog below!

A rare June atmospheric river is almost upon the Pacific Northwest. Substantial rain is expected around the region, along with rises on many area rivers. Before diving into high-resolution forecasts, let’s take a look at expected totals through Sunday morning from the European model.

The general synopsis is that the lowlands will receive 1-2 inches of rain, with even more in the foothills (1.5-2.5”) and the most on the coast and in the mountains (3-5”). 

For a broader view of the atmospheric river, take a look at the UW forecast for 5 PM Thursday, showing the massive plume of moisture arriving in the Pacific Northwest.

The blue/dark blue shades denote a strong atmospheric river, with subtropical moisture streaming across the Pacific Ocean. This setup is rarely found in June, and is very similar to fall and winter atmospheric rivers.

Let’s take a look at two high-resolution forecasts. First, the NAM model for precipitation through 5 AM Saturday.

The NAM shows that rain in the lowlands will decrease from south to north, with the least in the rain-shadowed areas near Port Townsend. Totals outside the rain shadow are 1-1.5 inches. The NAM shows 2-3 inches on the coast and isolated areas in the mountains with 5-10 inches! (You can see why rivers will be rising)

Next, the UW-WRF high-resolution forecast through Saturday evening (a few hours later than the NAM).

The UW forecast is the most aggressive with this atmospheric river, (basically the NAM forecast with higher amounts), giving the lowlands (south of Everett) 1.5 to 2.5 inches, with the highest amounts in the foothills. This forecast also gives the coast 2.5-3 inches and the mountains 3-8 inches. 

The bottom line with all these forecasts is that 1-2 inches of rain is a very likely possibility in the lowlands. This means that over 75% of June’s average rain will fall within 3 days.

As expected with this much rain, rivers will rise. The map below from the NWS Advanced Hydrologic Prediction System shows what to expect on the rivers.

Each yellow dot is a river near action stage (bankfull…just below flood stage). The Naches and Stehekin Rivers in Eastern Washington will reach minor flood stage. Note that the Snoqualmie, Snohomish, Skagit, and Nooksack Rivers will all reach action stage as well.

This much rain is uncommon in June. Remember that ponding, standing water, and potential urban flooding is possible, along with rises on small streams and creeks. 

One final note with this atmospheric river…conditions will be muggy at times. This is shown by forecasted dewpoints on the European model on Friday evening.

Dewpoints in the upper 50s to low 60s are expected. These are quite high for Western Washington, and you’ll likely feel the mugginess. 

Stay tuned to Twitter and our local meteorologists for updates about this June atmospheric river!