Aloha Winds Over Puget Sound

If you walk outside and close your eyes, you might imagine your were in Hawaii, with strong trade winds bringing in pleasantly warm air. 

This evening, many western Washington stations are around 60F with a persistent breeze of 10-15 mph and some higher gusts.

And strangely enough it is basically dry near Puget Sound, while heavy rains are falling in the foothills.

You doubt the Hawaiian connection?  Let me show you.

Here is a wonderful image from a weather satellite that measures the amount of moisture in a vertical column of air (taken earlier today).  Lots of water vapor in the tropics (red and purple colors) because the warm air can hold a lot of water vapor.  But look closely--do you notice a tendril of higher water vapor heading towards the Northwest?

 Here is a blow-up of the relevant section.  That narrow current of large amounts of water vapor is an atmospheric river and you will notice that it starts around Hawaii.    That is why such atmospheric rivers are called Pineapple Expresses.

But we can have more fun....we can use our model output to run back-trajectories, which shows us where the air over us now came from.   I did this using the very nice NOAA Hysplit system, showing trajectories ending at 100 meters, 3000 meters, and 5000 meters above SeaTac Airport. You can see the air at these lower levels is coming from the region northeast of Hawaii.

With this origin, the air is relatively warm, with lots of water vapor.

But why is Seattle dry while the Cascades foothills are getting inundated (as demonstrated by the precipitation over the past 24h shown below, click to expand)?

Because the winds approaching the region are westerly, with Puget Sound being in the rainshadow of the Olympics.

And yes, one more question.  Why are we so windy tonight, with gusts to 25-40 mph at some locations?  To illustrate, look at the winds at West Point in Seattle.  Winds are gusting up to nearly 40 knots and still increasing!

The explanation:  A low center is passing through BC to our north, resulting in a large pressure gradient (difference)  over western WA (see map).  The large N-S pressure gradient is producing quite strong southerly winds.

eʻoliʻoli i ke aniau o ke ao

from Cliff Mass Weather and Climate Blog https://ift.tt/2AIMfPV