May 20, 2014

Was there a mini-hurricane off the Northwest coast on Sunday?

This weekend there was an amazing sight in the satellite imagery off our coast, from roughly 8 AM through 2 PM.  Here is a close up shot around noon from the NASA MODIS satellite.   Looks like a hurricane with spiral rain bands. Or a spiral galaxy.

It actually was quite small.  To get some perspective, here is a wider view.  See it due west of the NW tip of the Olympic Peninsula and southwest of Vancouver Island?   It was about 60 km (36 miles) across.

A few hours later it started to dissipate but still had an other-worldly look! (again, giving you both views)

Want to see an animation of this "mini"hurricane?  Check out this animated gif (special thanks to Jeff Knecht for bringing it to my attention):

Or go to this link.

Earlier in the morning the Windsat sScatterometer satellite passed over the region (scatterometers useicrowave radiation to measure capillary wave amplitude and from that wind speed/direction).  This image, or roughly 8 AM, suggests some rotation off the northern Washington coast (click on image to make it larger)

And here is the image from another scatterometer around can see two areas of rotation, with the NE one associated with our little hurricane.
Impressively, high resolution numerical forecast models had a handle on this feature.  To prove this, here are the surface winds and sea level pressure forecast for 8 AM Sunday from the UW WRF model (this was a 3hr forecast).   Look closely and you will see the circulation, admittedly with a small error in the location.

Our little hurricane had a very week pressure signature and very modest (roughly 10 mph) winds.  The signature was limited to low clouds (unlike real hurricanes) and its origin could have been either from a weak upper level disturbance or from some kind of instability process associated with horizontal wind shear (although the amount of shear was very weak).

Anyway, an interesting phenomenon

Lunchtime Climate Series 

Finally, if anyone is interested, the Seattle Chamber of Commerce is continuing its lunchtime climate series in downtown Seattle next week.

More more information, the link is here.


  1. Might be like the coastal eddies they get down in the Channel Islands ( Convergent surface wind flow can do some interesting things.

  2. Wait a minute...don't you need the Coriolis Effect to spin up a hurricane?

    Ting Ting and Po

  3. Forget the off shore vortex, watch the cloud flow overland! I stood out in the powerful showers in my Tacoma area home and giggled to my weather geek heart's content!

  4. Here's a bigger, stronger example off our coast from 2006:

  5. Circulation in the relatively cooler mP air mass may have formed in a similar manner to a polar low that we see occasionally over the Bering Sea.


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