In the wake of Superstorm Sandy, which has rewritten weather history across the northeast, I've had quite a few people ask me whether something similar could ever strike the Pacific Northwest.
In a word, no -- at least not in the same way Sandy affected the northeast, but there are certainly some worst case scenarios that could bring together a confluence of rare weather events to create our own mega storm, it just wouldn't act the same way.
First, let's recap Sandy's journey to set up the comparisons:
What made Sandy so unprecedented in modern weather record keeping was that it took some events that were pretty rare individually, and combined them all together at precisely the same time -- a hurricane, an arctic storm, a strong jet stream, a strong blocking pattern in the western Atlantic, and a full moon.
Put that together with the unique topography of the coastline, the storm track, the storm's landfall, the storm's arrival time, and the population density where it happened. The probabilities of what happened would happen were astronomical and likely why we've never seen anything like it in our lifetime -- or even centuries. Hollywood could not have scripted a more daunting disaster scenario.
Let's recap Sandy's path to history: It began as a hurricane that went through the Eastern Caribbean. It wasn't even a particularly strong hurricane -- in the category 1-2 range. It curved around North Carolina, missing making landfall and thus not getting any weakening effects. It then ran into a ridge of high pressure to the east that blocked any more eastward movement. Meanwhile a significant cold front with a blast of arctic air was coming to the northeast from Canada. Third, a strong jet stream was blowing over the Appalachians and through the zone where Sandy and the arctic front were about to meet. A weak upper low pressure center then interacted with Sandy to pull it back to the west and merge with the jet stream and arctic storm to create the mega storm.
The storm was massive in size -- hurricane force winds were 350 miles across the storm, with tropical storm force winds over 900 miles across -- and had a record-low pressure for the northeast of about 946 mb at landfall (under 940 at times) which is about 27.9". The storm's landfall was in New Jersey at the full moon's extra-high tide -- at the precise location to bring the peak of the record storm surge to the Jersey shores, Manhattan and Long Island. Wind gusts were frequently 70-90 mph along there.
And don't forget those winds spread as far inland as Cleveland and Boston, and the storm's arctic component brought blizzard conditions and over 2 feet of snow to the West Virginia and Maryland mountains.
What could happen here?
The Pacific Northwest has some important differences with the northeast that would prevent any kind of similar-scripted storm from striking here.
First of all, our ocean water temperatures are much colder (50-60 degrees) than off the Atlantic (60-75, warmest as you go south) so a hurricane/tropically-fed storm would never get even close to our shorelines. No tropical-fed storm has ever struck the U.S. West Coast (a tropical storm almost made it into L.A. years ago) and it's because the cold ocean temperatures act like a concrete barrier that would destroy any storm before it got close to here.
But that's not to say the remnants of past tropical storms can't affect us. Some of our greatest storms have had moisture and energy left over from typhoons that have struck Asia. Usually those storms undergo "Extra-tropical transition" where they transform from getting their energy from the ocean heat and instead get it from the battle between cold air from the arctic north and warm tropical air to the south. So no hurricane-type system here, but we can certainly have the energy from an old one.
Building our own mega storm:
Let's try to build a hypothetical scenario to where a storm here might attempt to match Sandy.
I'll start with winds, the easiest component to attempt to match Sandy since windstorms here are not uncommon. We could certainly someday get a mega record windstorm if a very deep low pressure storm made landfall into northwestern Washington or southern British Columbia. This is the typical way to get strong winds here -- create a large pressure difference from north to south along the I-5 corridor and have the winds race north. The deepest storms to strike the Northwest have had central pressures of around 955-970 milibars. Sandy at its deepest point was in the upper 930s and made landfall at 946 mb (setting all sorts of all-time low pressure records across the southern New Jersey/Philadelphia areas). Normal pressure is 1012, with typical variations around 1000-1020.
So, let's say an old super typhoon merged with a cold storm dropping out of the Gulf of Alaska and morphed into a monster 946 low that made landfall across Forks -- and was fairly close to that pressure when it passed due north of Bellingham. That would likely create an all-time record wind event here that would make the Columbus Day and Hanukah Eve storm pale in comparison.
One benchmark of a Northwest storm is taking the pressure difference between Bellingham and Portland. The Hanukah Eve storm set the record at 23.2 mb. At landfall, Sandy's pressure difference over a similar distance was an incredible 37.7 mb. Obviously you'd be breaking our record by 50 percent so that would really be something to fathom, and if you put that kind of difference here, the entire I-5 corridor would be looking at hurricane-force winds and them some with I'd bet well-over triple-digit wind speeds along the coast.
If our windstorm had some tropical heritage and was part of a Pineapple Express storm, this potential storm could bring over 10" of rain to the mountains and perhaps 2-4" in the lowlands. Seattle's all-time wettest days are in the 3" range except for one day in 2003 when we got just over 5". Let's say this storm brings 15" of mountain rain and 5-8" of rain in the lowlands. That would bring extreme, record river flooding. Urban flooding would be an issue too; landslides and mudslides would be frequent and widespread.
But Seattle would not get the storm surge-type flooding that Sandy brought. The coast would likely get pounded with 30-35 foot (40 foot?) waves and cause some coastal flooding -- especially if the storm hit at high tide but our coast is ragged and barren and much more used to these types of storms than the sandy and flat beaches of the Atlantic Coast. Westport and Ocean Shores-type oceanfront communities would have some serious issues but the inland waters of Puget Sound would also be protected from these kinds of waves. And the storm makeup would be different than a hurricane and thus not quite as adept at creating a surge. But it'd still be quite the miserable and dangerous day to be on the coast. And with winds blowing 80-100mph plus, you wouldn't be there anyway.
West Virginia had a blizzard with Sandy. But if we're playing this up as Pineapple Express mega wind/rain event, snow would not be an issue -- even in the mountains -- because snow levels would be sky high. But let's play this game: What if this 946 storm hits in January...after an arctic storm has just passed through and a large Canadian arctic air mass was in place before the storm arrived?
Playing out the original scenario, it would be a December 1996-type event of very heavy snows at first -- several inches to feet -- then a change to soggy rains as the warm air mixed in. This would cause considerable additional damage due to the heavy wet snows being soaked with rain -- on top of our 80+ mph winds, don't forget. So we've upped the ante here.
Yet this would be very difficult to achieve -- the strong wind storm would very quickly scour out the cold air and change it to rain and a storm of this magnitude would certinaly have plenty of warming winds before the bulk of the storm arrived. So it'd have to snow a lot before the main storm cell arrives with its heavy rain and wind.
But if the storm did manage to somehow bring enough moisture to snow, but not the warming winds for a while, that is probably the most disastrous scenario I can come up with as far as wind, snow and rain -- 20 inches of snow turns to 4 inches of rain on top of hurricane force-winds. The power outages would be immense. If you wanted to toss in another wrinkle - have a freezing rain event interject the transition from cold to warm (a la January 2012) but again, the wind storm portion would warm things up rapidly.
Now if you wanted to do something on more of a regional scale of destruction that seriously factored in all the weather possibilities, you keep the same set up above, but have the 946 low move into Astoria and then down the Columbia River with an intense arctic high centered over BC. In this case, a blistering windstorm devastates Oregon with extreme flooding, and Washington is left with a massive snowstorm that cripples the area, while Bellingham gets both due to a screaming hurricane-force wind out of the Fraser Valley, which also reenforces the arctic air. This is sort of what happened in January 1950 blizzards only the storms weren't 946 strong -- just regular strong lows moving in south of the area but a whole lot of cold air in BC and moisture in the storm.
But in our megastorm case, you have 80-100 mph winds in Eugene and Portland with several inches of Oregon Cascade rain, a place of rain-freezing rain ice storm in the transition in southwestern Washington, several inches to feet of snow in the Puget Sound region with strong north winds (but probably only borderline damaging), and temperatures near zero in Bellingham with locally 70-90 mph winds out of the Fraser River Valley and wind chills -30 or worse. Roads are flooded on the coast, covered in debris in the Willamette Valley, and under two feet of snow in Western Washington. Power is out to millions.
But even that won't bring the scale of destruction Sandy saw. Now, have Mt. Rainier erupt on the same day? Maybe we're close...