If you are taking an Alaska cruise starting this weekend, bring an umbrella and heavy rain gear: you will need it.
Why? Because a powerful atmospheric river will strike the BC/southern Alaska coast, dumping very heavy rain on the coastal terrain of the region.
What really got my attention today was the forecast of the movement of water vapor in the atmosphere from off the Pacific, something called "water vapor transport" in the weather business (see forecast for 11 AM Sunday, below).
Wow. This graphic shows the movement of water vapor integrated through atmospheric depth, although most of the action is in the lowest 10,000 ft. The colors show the magnitude and the arrows indicate both the direction and magnitude.
Immense amounts of water vapor will surge toward the coastline embedded in a strong airflow from the west in what is known as an "atmospheric river." And when the moisture-laden air hits the substantial coastal terrain it is forced to rise, cooling as it ascents, leading to water vapor condensing out into clouds and rain.
Heavy rain.
A group in San Diego has an atmospheric river rating system that parallels the 1-5 scale used by hurricanes, with AR-5 being the strongest atmospheric river (see scale below)
According to this scale and the latest model runs, this event will be an impressive AR-4 (orange color). Here in Seattle, only an AR-1. You won't be bragging to your grandchildren about it.
Want to see the latest precipitation forecasts for the next few days?
Through Tuesday morning, some coastal areas of BC will enjoy up to 15 inches of rain.
And coastal Alaska will be more than a little damp, with 3-6 inches being widespread.
Why will we get this damp situation? I am authorized to tell you since I have written several research papers on such features.
The upper-level map (from about 18,000 ft) shows the classic pattern, with a strong trough (of low pressure) over the Gulf of Alaska and a ridge of high pressure off Oregon. This contrast in pressure (or heights) produces a strong current of winds from the southwest that moves a lot of moisture to the coast.
So if you want to experience the torrential rains of a powerful atmospheric river, you might be able to secure a last-minute ticket for a cruise leaving on Sunday.
Tell them that I sent you.
_________________________
NOTE: I will do a special online zoom session at 10 AM on Saturday for Patreon supporters. I will discuss the forecast for the upcoming winter and answer your questions.
Reminder UW students: I will be teaching Atmospheric Sciences 101 this fall:
And if you are over 60, you can take the class for $5 through the UW Access Program...either online or in person.
"We're due," that's for sure. The wild birds have been absolutely ravenous for a week, note-worthy. Thanks for explaining so much about the mechanics of these things.
ReplyDeleteTiming is about right as typically, atmospheric rivers and pineapple express storms start to penetrate en mass in October anyway. While technically a tad early, it's not too far off the mark.
ReplyDeleteCurrently in SE Alaska, but will be returning to Seattle on Saturday - sad that I will be missing this one.
ReplyDeleteNorth and central BC get their strongest atmospheric rivers in September and October. Then once the jet stream sags south usually by late October it will be our turn. It's a good thing that the BC Central coast is sparsely populated. 15 inches will cause major flooding.
ReplyDelete