First “atmospheric river” of the season approaches western Washington

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A few days of quiescent weather have followed last weekend’s excitement but a different flavor of unsettled weather is on the way for the second half of the week.

As of Wednesday morning, a jet streak (i.e. a region of strong winds within the jet stream) was stretched across the central Pacific to the south of a broad area of low pressure in the Gulf of Alaska. This is a common setup for an approaching storm during rainy season. In mid-August it is much more exciting to see at a time when we could really use some rain.

250 hPa wind speeds as depicted by the GFS model on Wednesday morning August 19, 2020. Note that the USA is on the right side of this map.

Running the model out 48 hours to Friday morning shows a fairly typical evolution of the above pattern–the jet streak rotates around the southern end of the low pressure system and aims its energy directly at the PNW.

250 hPa wind forecast for Friday morning August 21, 2020

Looks good from a large-scale dynamics perspective. What about moisture?

An expansive atmospheric river can be seen on the latest observed precipitable water image. Check out the long, narrow corridor of yellows/reds stretching all the way across the Pacific between roughly 40 and 50 degrees latitude.

Observed integrated precipitable water (TPW) on Wednesday morning August 19, 2020

The moisture is already being pulled into a cyclone centered just to our west, near 48N/135W.

So the ingredients for a decent rain are in place, now how will this all play out?

Lets make use of a couple products from the Center for Western Weather and Water Extremes (CW3E) out of California. First, the forecast for IVT, or integrated vapor transport, defined as the horizontal flux of atmospheric moisture at a specific point. IVT is a commonly-used indicator of the strength of atmospheric rivers.

Two quantities matter for IVT–strength and duration. The forecast strength is nothing impressive. 500 kg/m/s is classified as “moderate”, or a 2 out of 5 in terms of intensity. But the duration is on the longer end of the spectrum, nearly 48 hours.

Integrated Vapor Transport (IVT) forecast timeseries for coastal Washington beginning Wednesday August 19, 2020.

Also of note in the above plot are three “spikes” in IVT during the event. Each of these spikes corresponds to a shortwave trough embedded within the larger-scale jet stream. Generally each of these manifests as a brief increase in the intensity of rain, followed by a lull.

It turns out we can see each of these waves approaching the coast on a model forecast of IVT. I labeled the three waves in the image below. All three are embedded within the broader atmospheric river to the south of the aforementioned parent cyclone. The below forecast is for Wednesday evening when the first wave will be approaching the coast.

Model forecast IVT for Wednesday evening August 19, 2020

Putting this all together, we are looking at a fairly weak, but long duration rain event.

How much rain are we expecting? The GFS shows very impressive precipitation of 1-2 inches over the Olympics and central-north Cascades. Excellent for knocking down any recent wildfires that have formed.

GFS forecast total precipitation (inches) through Saturday night.

The ECMWF shows similar values over the mountains but less in the lowlands, only around a third of an inch. Eastern Washington will remain mostly dry.

ECMWF forecast total precipitation (inches) through Saturday night.

Timing? In Puget Sound, the first wave is expected Wednesday night into Thursday morning followed by a lull. The most significant rainfall is expected with the third wave during the day on Friday.

The best part is that the rain should be mostly out of the way by Saturday, with nice weather expected Saturday afternoon and Sunday.

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