May 31, 2021

Strong Atmosphere River Heads into British Columbia and Southeast Alaska

While the Northwest has been drier than normal, it has been drenching wet a few hundred miles to the north.

An unusually strong atmospheric river....a long current of moisture stretching southward to the subtropics...has been in place for days, bringing large amounts of water vapor to Alaska.   And the atmospheric river will be in place for several more days, slowly moving southward.

To illustrate, the scale and beauty of the atmospheric river, here is a water vapor satellite image for 11 AM today. The lighter areas have more water vapor.  You can see the current of moisture extending from near Hawaii into central BC.  This current is the result of low pressure over the Gulf of Alaska and high pressure offshore of California.  

A forecast of total water vapor content in the atmosphere for 11 PM tonight shows a healthy atmospheric river (white and blue colors indicate the higher amounts of moisture).

As this atmospheric river slams into the mountainous West Coast, the air is forced to rise, cool, and release its moisture, resulting in massive amounts of rain.  Looking at totals over the past week, some locations of SE Alaska have gotten as much as 6-8 inches!  

The European Center weather model is predicting lots of future precipitation from this atmospheric river through 5 AM Thursday, with some locations being hit by 6-8 inches of additional accumulation (see below).   This graphic is a great illustration of the enhancement of precipitation by the terrain.

The Northwest should stay dry during the week, but our turn for rain is coming over the upcoming weekend.  Interestingly, the extended weather forecast has shifted, with the latest European Center 1-month projection going for wetter than normal conditions from the Cascade crest westward (see below).  Good to keep the fire danger down.


  1. It is probably a toss-up. but frequently we have a few warm days in May, and then June fizzles out into a cloudy/rainy early summer, until the 4th of July!...I do not mind at all, as I grew up here, and do not know better.

  2. Yeah except for the occasional June heatwave... take 2019 for example, mid nineties in the first half of the month! You gotta love the weather here...

  3. This does not totally surprise me in the least as a Puget Sound native. Am enjoying the warm summer like temps and weather right now, but the west streak that often is seen in mid June is also not unusual.

    Have had some cool, damp 4ths over the years, but lots of warmth and sunshine too that day. There are years that the grass begins to turn brown as early as mid to late May and says brown all summer before starting to green up in Sept.

    But we've also have had summers that were very late to arrive, such as 2012, it was wet and not very warm until mid July then it went all hot and dry until mid October before the first rains began to reach the area.

    It can definitely be all over the map around here.

  4. I'm sucking up the sunshine and heat while I can get it! I know June gloom will return in all its 'glory' (BLEH!) all too soon.


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