First Storm of the Season: Bomb Cyclone Offshore Bringing Heavy Rain, Gusty Winds, and Large Waves

FastCast--Sunday, Sep. 24 to Tuesday, Sep. 26:

The first storm of the season is very close to impacting the Pacific Northwest. The main storm, a large and deep "bomb cyclone" (which means the storm's low pressure rapidly decreased 24 millibars in a 24 hour period) will remain offshore, but will push an atmospheric river into the Northwest, combined with gusty winds and large waves on the coast. For the lowlands, expect 1-2" of rain through Tuesday night, most falling from Sunday afternoon to midday Monday, followed by more rain from a secondary storm on Tuesday. Areas from Sequim northward to Bellingham will likely have less rain due to the Olympic rainshadow (0.25-0.75" possible). Some areas in the foothills may get up to 2.5". Winds will be gusty at times in the lowlands on Monday, mainly SE winds gusting 20-30 mph, except stronger (35-45 mph) for the Northwest Interior and San Juan Islands. More wind is possible on Tuesday, but details still remain uncertain. In the lowlands, expect our chilliest weather in months, with highs in the low to mid 60s and lows in the mid 40s to low 50s. Also, be prepared for a chance of isolated thunderstorms & heavy showers on Monday and Tuesday. Continue reading below for more storm information!

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Continue reading the full blog below!

The first storm of the season is ahead for the Pacific Northwest, and it will be a multi-faceted system with a few different impacts. 

Let's start with the storm itself. The main storm will remain offshore, but will be a massive system. Below is the European model forecast for the storm.

This will be a very deep storm, deepening explosively (categorizing it as a "bomb cyclone") to 959 millibars, extremely low for September!

While the storm will remain offshore, its impacts will reach quite far. The most noticeable impact will be an atmospheric river aimed at the Pacific Northwest. Below is the UW WRF forecast for water vapor transport, showing the atmospheric river at 4 AM Monday.

Notice the current of highly concentrated moisture aimed directly up the Pacific Northwest coastline. This will bring the biggest rain event since late spring.

Let's take a look at expected rain totals, comparing the European Ensemble (EPS) versus the American GFS. We'll start with totals through Monday night, seen below on the EPS.

Through Monday night, the EPS shows 0.5-0.8" for the lowlands, with more (0.8-1.5") west of Puget Sound. The coast will get 1.2-2", the mountains will get 0.75-3" (most in the Olympics), and the rain shadow will drop San Juan Islands totals to 0.3-0.5" per this forecast.

Let's compare this to the GFS forecast, also through Monday night.

The GFS shows more rain through Monday night than the EPS. This forecast shows 0.8-1.4" in the lowlands, up to 3" on the Kitsap Peninsula (likely somewhat overdone), and 1.2-2" on the coast. The mountains get 0.75-2", with up to 4-5" possible in the Olympics. Another note here is that the rainshadow covers from Victoria BC to Whidbey Island, and is stronger, with 0.05-0.25" in the center of the rainshadow.

Now, let's see what these forecasts show for total rain through Tuesday night, seen below starting with the EPS.

The EPS shows lowland totals increasing to 1-1.5" (an additional 0.4-0.5"), with coastal totals up to 2-3" and mountain totals up to 1-2.5", except up to 5: in the Olympics. Overall, this forecast shows an additional 0.4-0.75" for the region on Tuesday.

Let's compare that with the GFS forecast, also going through Tuesday night.

The GFS forecast shows the lowlands getting 1.1-2.9" of rain through Tuesday (an additional 0.3-1"), with the coast getting 1.8-2.5", and the mountains getting 1-3", except up to 7" in the Olympics.

Overall, expect rain from Sunday afternoon through Monday evening, then again on Tuesday.

The next aspect of this storm that we'll cover is the wind. Even though the bomb cyclone is far offshore, it is very large and will spread winds into the Pacific Northwest. These winds will predominantly be southeasterly. 

Below is the European model forecast for peak wind gusts from Monday afternoon to early evening.

This forecast shows winds on the coast gusting 40-45 mph, with gusts of 25-45 mph for the Northwest Interior, especially the San Juans. The remainder of the area will have gusts of 20-30 mph. 

Let's compare this to the high-resolution NAM forecast, showing wind gusts at 4 AM Monday (earlier peak than the European).

The NAM forecast shows winds peaking earlier and stronger. This forecast shows SE winds gusting 40-50 mph (possibly up to 55 mph) at the coast, 30-40 mph from Everett northward, and 45-50 mph in the San Juans and Northern Whidbey Island. The lowlands will likely peak at 25-35 mph in this forecast.

Remember, despite relatively average wind speeds for a PNW storm, power outages can occur much more easily at this time of year due to trees being fully loaded with leaves.

The final impact of this storm will be large waves. Since the bomb cyclone is very large and deep, it will churn up the ocean for a long time. This will cause large swells and large breakers along the coast, all the way from  Northern British Columbia to Northern California. Below is the European model forecast for wave heights at 7 PM Monday.

Waves of 15-25 feet will hit the coast from Coos Bay northward to Haida Gwaii in the Queen Charlotte Sound of Northern BC. From San Francisco to Coos Bay, waves will be elevated, peaking around 8-15 feet. These waves are unusual for this time of year, so be prepared for changing beach conditions, hazardous breaking waves, and waves running far up the beach. Minor coastal erosion could be possible around high tide near the strongest waves.

This is quite a storm moving into the Northwest to kick off storm season, and another (smaller) storm is ahead on Tuesday, which could bring more impacts. Stay tuned & stay safe!