The Strongest Atmospheric River on Record for the Gulf of Alaska?

 The atmospheric river currently over the Gulf of Alaska may be the strongest on record in that area.

The latest model runs show extreme values of the key measure of atmospheric river strength, vertically integrated moisture transport (IVT), which describes how much water vapor is being moved over a period of time.

Below is the map  of IVT for this morning, with values exceeding 1900.  I have personally never seen anything like it.

An estimate of the strongest atmospheric river  over observed for 1990-2019 by Dr.  Lexi Henny of NASA/Goddard suggested this is the strongest atmospheric river during that period of record.  Impressive.

The plume of moisture was very apparent in water vapor satellite imagery this afternoon (below). Do you see the southwest-northeast directed plume aimed at SE Alaska and BC? Some of the associated clouds have moved into Washington.

This atmospheric river is predicted to hit land as an AR-5, the strongest level observed.  We can be thankful that the center of the river will make landfall north of Washington State, where only AR-2 levels should occur on the northern Olympic Coast.

By Tuesday, as much as a foot of rain will fall over the coastal terrain of BC

Strong atmospheric rivers are not infrequent over SE Alaska and BC this time of year, with the path of the moisture plumes reaching Washington State in November.  

The fact we are in a neutral to La Nina year means that there is a higher probability of more rain than last year (an El Nino year)...something shown for the western Washington lowlands below.

My advice.  Make sure you have a decent umbrella.

A Powerful Atmospheric River Takes Aim at British Columbia and Southeast Alaska

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.

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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.