June Atmospheric River Heading for the Pacific Northwest

FastCast—Tuesday, June 7 to Saturday, June 11:

A nice day is ahead on Tuesday as a transitory (temporary) high pressure ridge moves through. Expect lowland highs in the low to mid 70s, with partly to mostly cloudy conditions around the area. However, a big change is ahead. An atmospheric river will impact the Pacific Northwest from Thursday to Saturday. This will be like the atmospheric rivers we experienced this past winter, but dialed back. Still, rain totals could be quite substantial for this time of year. As of Monday night, models are showing a total of 1-2.5 inches for the lowlands! This is a very large total for June. Additionally, the mountains and coast will receive 2-5 inches of rain. This system (generally split between 2 rounds…on Thursday and then on Friday & Saturday) will drive some rivers around the region toward bankfull, but not to flood stage. This may change, so stay tuned. Conditions will be mild and muggy during the atmospheric river, with highs in the mid 60s, and lows in the mid to upper 50s. Stay tuned for more information as the forecast becomes more accurate in the coming days.

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A rare June atmospheric river is heading for the Pacific Northwest. This rain event will generally be split between a system on Thursday and then on Friday/Saturday. 

The UW forecast below shows integrated water vapor transport, or the amount of water vapor being moved through the atmosphere, on Thursday evening, as the atmospheric river approaches the region.

This setup is reminiscent of fall or winter. This specific atmospheric river will bring less rain than its fall or winter counterparts. However, it will still be substantial for June, likely with more rain than this past weekend’s system.

Let’s take a look at the forecasts. The European model is most aggressive with the total rain from this system, as seen below in the total precipitation forecast through Sunday.

The European model shows a whopping 2-2.5 inches for the lowlands, which is actually more than the entire month of June’s total average rainfall. In this forecast, the mountains and coast would receive 2-5 inches, and Eastern Washington would receive 0.2 to 1.5 inches of rain.

The European model forecast is on the high side, so let’s take a look at another forecast. The GFS (American) model’s forecast for rain through Sunday is below.

The GFS is a lower resolution than the European, hence the very gradual changes in precipitation amounts. The GFS gives the lowlands 0.75 to 1.5 inches, still quite a lot for a June rain event. The mountains and coast receive 1-4 inches, and Eastern Washington receives up to 1 inch. 

One interesting aspect of this upcoming atmospheric river is the potential for some rivers around the region to reach bankfull. The NWS Advanced Hydrologic Prediction Service forecast below shows expected flood stages on area rivers.

Every yellow dot on a river is “action stage”, or bankfull. This rise on rivers would be due to the combination of warmer weather and the atmospheric river. Stay tuned for more information about the potential for flooding.

So…as of Monday night, the bottom line is that there will be substantial rainfall from Thursday to Saturday. What is uncertain is the amount of rain, and that will become more defined as we move closer to the event.

On a nicer note…a nice day is ahead on Tuesday, with a brief high pressure ridge bringing highs in the low to mid 70s, as seen below in the high-resolution NAM model.

Enjoy the nice weather on Tuesday as the atmospheric river heads toward us…and stay tuned for my next update on Wednesday!