Learning something new – AR Scale

Last week I learned about a new-to-me scale used to classify Atmospheric River events. Here’s the chart!

It’s a pretty cool, and detailed, chart. It uses the “Maximum Vertically Integrated Water Vapour Transport” and the length of an event to come up with a category from 1 being beneficial (because rain is good!) to primarily hazardous (because torrential rain for a really long time is not good for most things!)

If you’re wonder what Maximum Vertically Integrated Water Vapour Transport means think of it as how much water is in the atmosphere *and* how fast it is moving. It’s acronym is IVT.

As I wrote on Twitter, last week’s event was probably a Cat 1 or Cat 2 at most. Mostly beneficial.


We call something an Atmospheric River when it presents on the forecasts and observations as a long area of moisture connecting our mid-latitude location to tropical moisture from the Central Pacific or Hawaii.

You can see forecasts for Water Vapour Transport at the NOAA.

A small AR on Thursday

We are getting more rain on Thursday, and yes, it is also going to have AR characteristics. Here is the image for Thursday night (our time).

This looks like fairly strong river pulling moisture from being the Dateline in the Central Pacific. The good news, though, at least for us, is it appears to be only brushing Vancouver Island on Thursday. By Friday night the focus of the moisture has moved down to Washington and Oregon and it continues to move south and weaken.

For Vancouver Island this again looks like a Cat 1 AR event, possibly up to Cat 2. What happened last year with these events if they did not shift south and instead stalled over us, pumping all of that moisture straight into a very small area.


Expect the rain to start on Thursday afternoon, continue and intensify all night and into Friday and then start to taper off on Friday afternoon.

Snow? Possible

Yes, we already have cool air around us and while these AR events are generally warmer, this one will be quick enough that the cold air will stick around. Expect snow (in green below) to appear particularly on Thursday on the mountains and there might be a chance on the highway passes as well. Rain (in grey below) will be all you see near the ocean.

By late Thursday night and Friday the air should have warmed enough that it is rain in all areas.

Real Snow on Sunday/Monday!?

Um… we’ll talk about this on Friday. The model needs to chill out!

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