Rain finally arrives in Seattle, models hint at a more significant storm next week

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It might be the most anticipated rain in the history of Seattle.

Radar screen capture at 2 PM Friday 9/18/2020

The rain was a bit scattered in nature, with some areas only receiving a few sprinkles. I was one of the lucky ones in West Seattle, with around a third of an inch ending up in my rain gauge.

My rain gauge at 2:15 PM Friday 9/18/2020

One disappointing feature of the rain is the smell, which resembles an ashtray or a smothered campfire. The rain did scour out a bit of the smoke, as the AQI dropped 30-40 points to the low-100s. One would think that a steady rain would be more effective at clearing out smoke, but those tiny PM2.5 particles have proven to be very stubborn.

Seattle-Duwamish PurpleAir AQI trace

The weather and smoke models agree that the air quality will continue improving through Saturday before a big push of westerly flow clears almost all of Washington State on Saturday night into Sunday. Areas downwind (east) of the fires may still have smoke to contend with.

The next big weather event — an atmospheric river

Looking farther ahead, model forecasts are indicating a good chance for a more significant storm system arriving next Wednesday or Thursday.

The latest deterministic GFS model output shows a deep longwave trough of low pressure forming over the Gulf of Alaska by early next week.

GFS forecast 500 hPa heights and anomaly for Wednesday AM September 23, 2020, map via Alicia M Bentley

A Gulf of Alaska low in this position favors mild and wet weather in the Pacific Northwest as the jet stream and associated weather disturbances tend to ride around the southern periphery of the trough. Such a situation can easily be seen by looking at the jet stream forecast (250 hPa) for the same time as the above 500 hPa image.

GFS forecast 250 hPa wind speed for Wednesday AM September 23, 2020, map via Alicia M Bentley

The real fun happens when the disturbances tap into the moist, subtropical air to the south of the trough. The integrated vapor transport (IVT) forecast shows a long corridor of IVT > 800 kg/m/s (max > 1000) aimed at Washington from the southwest on Wednesday morning. IVT values of this magnitude are usually associated with strong atmospheric rivers, as is the case in this forecast.

GFS forecast Integrated Vapor Transport for Wednesday AM September 23, 2020, map via Alicia M Bentley

What is the probability of getting a significant atmospheric river? The above maps are just one run of one model–we have to look at the ensembles to assess the probability. The California Center for Western Weather and Water Extremes (CW3E) produces some really nice IVT ensemble forecast visualizations. The image below includes both the GFS (American model) and ECMWF (European model) ensembles, 70 model runs in all (20 GEFS + 50 ECMWF).

IVT ensemble plume forecasts for the WA coast at 47.5 degrees N. Note that this image is from models initialized 12 hours earlier than the other images in this post. Image via CW3E.

Comparing the solid purple line and the sold green line shows considerable differences in intensity between the GFS and ECMWF ensemble means. But generally there is fairly strong agreement in an atmospheric river reaching peak intensity late Wednesday into Thursday.

A major part of the forecast uncertainty is model differences in the location of where the core of the atmospheric river will make landfall. A simple (but somewhat ugly) way to visualize this is with a spaghetti plot that outlines the location of IVT > 750 kg/m/s from all of the GEFS ensemble members:

“Spaghetti” plot for IVT > 750 from the GEFS, image via CW3E.

What’s interesting about the above plot is that most of the GEFS members appear to be a bit farther north with the peak IVT compared with the GFS deterministic run that I showed above. So it’s possible that the most significant impacts will end up being in British Columbia. If that happens, Washington will still see a period of moderate rainfall as the cold front moves south, but will miss out on the heaviest rain.

When we look at the actual probability of > 1″ of rain in 24 hours from the GEFS, we see that Vancouver Island is indeed favored for the heaviest rain.

GEFS probability of > 1 inch of rain in the 24 hours ending Thurs morning 9/24/2020. Image via WeatherBell.

However, the ECMWF ensemble gives much higher probabilities of significant rainfall over western Washington.

ECMWF ensemble probability of > 1 inch of rain in the 24 hours ending Thurs morning 9/24/2020. Image via WeatherBell.

These type of model uncertainties are fairly common for an event that is still 5+ days out. But it is certainly within the range of high confidence that a strong atmospheric river event will occur somewhere in the vicinity of western WA/southern BC. While rain would be much appreciated, too much rain on recent wildfire scars often leads to flash flooding and mudslides. So it is a situation worth monitoring as we move into early next week if impacts in Oregon are stronger than currently forecast.

The weather will stay interesting into later next week, as strong westerly flow embedded in the colder air behind the front will bring a chance of postfrontal convective activity and a convergence zone.

But lets not get too far ahead of ourselves, there are several days worth of wonderful clean air to enjoy before we get too excited about a big fall storm.

One comment

  1. I’m very happy for you over there in the west, but here in Wenatchee I still can’t see more than about a mile and the AQI is currently still above 180! Will have to wait another day for that westerly push before planning any outside trips.

    Like

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