February 27, 2018

Graupel Storm Hits Puget Sound

Sunday afternoon something strange hit the area between Seattle and Everett, from the Sound into the foothills.

A graupel storm.

Small white pellets starting coming down, first slowly and then in an immense wave in places, piling up until some locations had accumulated ..5 to 2 inches of the white balls.

Here is a nice video by Jamie D. that shows the fun:

Now some folks caled it hail and others thought it was sleet.  But it really is graupel, and  was caused by a strong Puget Sound Convergence Zone working on unstable air.

First, what is graupel?  This is what they look like close up.  White pellets.  Relatively soft.  Opaque.

Graupel is heavily rimed snow crystals.   Snow crystals form in clouds whose temperatures are generally cooler than about -15C.  At lower levels, where the temperature is warmer, but below 0C, there can be supercooled water (liquid water that is below freezing).

Some of the ice crystals fall into the supercooled water, where the supercooled water freezes on to the crystals in a process called riming.   The process is also known as accretion.  As the rimed particles get larger and heavier, they fall faster, gathering more supercooled droplets.   Finally, the heavily iced up particle---the graupel--reaches the surface. 
That is what happened Sunday afternoon.   The formation of graupel requires lots of supercooled water and that generally requires strong vertical motion, which is often associated with convection, towering cumulus and cumulonimbus being well known examples.  Convection occurs when the air is unstable and buoyant parcels of air ascend rapidly.

And everything was in place on Sunday afternoon.  A front went through that morning resulting in unstable air and cumulus convection moving into the area (see visible satellite image at 1 PM).   In the satellite image the convection is shown by the bright clouds, with dark spaces between them, coming in off the ocean.

At the same time, the wind were northwesterly on the coast, with air moving around the Olympics to the north and south, and then converging over Puget Sound--thus producing a Puget Sound Convergence Zone (see surface map at 3 PM).   As air converges together, it is forced to rise, which can really rev up clouds and precipitation, particularly when the air is unstable.

As a result, strong convection formed along the convergence zone between Seattle and Everett, something clearly shown by the radar image at 4 PM.

And that convection produced the graupel showers that caused the landscape to whiten.   Now to be clear, there was some snow mixed in at higher elevations and where the precipitation was heaviest, but graupel was dominant in most locations.

Finally, graupel is different than sleet or hail.  Sleet occurs when rain drops fall into a sub-freezing layer near the ground, resulting in freezing of the rain drops into little, hard ice pellets.  Sleet is generally dense and clear.   That is not what happened on Sunday.  And hail is multi-layered and dense, resulting from ice particles moving up and down several times in intense convection.   Not what we had two days ago.



  1. I'm glad you posted about this- we were driving north on I-5 from Seattle to Bellingham and ran into this extremely unusual storm north of Mountlake Terrace! As we looked to the north, the sky was dominated by very dark clouds that looked like summer thunderstorms I've encountered in other parts of the country. While I was driving, my wife showed me the radar image of the convergence on her phone, and I knew we were in for an interesting experience, especially as it was clear that there was a lot of instability involved. The temperature right before we encountered it was 42, and what started off as big raindrops quickly turned to sleet- or as you've explained- graupel and snow that piled up quickly as the temp dropped to 32! In fact, it almost immediately stopped traffic and made conditions very slippery. It was such a weird experience and the ground was covered for about 10 miles, in fact visibility was reduced and we accidentally pulled off an HOV exit and had to stop for a few minutes before reentering the interstate. And then, just like that, we made it through, the temperature rose back 10 degrees and the clouds broke up and it turned into a gorgeous evening in Skagit and Whatcom Counties. Definitely an unusual event!

  2. Yeah grauple is a common type of shower in the northwest this time of year. Just like you describe, usually with enough accumulation one can make a snowball and it packs like real snow. It's consistency taste like dip n dots ice cream. Dip n dots is essentially the grauple of ice cream. Whatever way you want to look at it.

  3. Fascinating, Cliff! We had graupel come down one afternoon last week in Burien around 470' above sea level. I knew that it wasn't hail and it felt more like compressed snowballs... very cool!

  4. Cliff I think we experienced graupel Saturday morning, 6:50 a.m., in and around Pt Townsend. We were awoken by the loud noise on the roof and looked out to see what we thought was hail. It went on for about 5 minutes. At about 7 a.m. there was a loud clap of rolling thunder. The weather radar showed a narrow band of yellow/red and also blue. There was quite a discussion about it on Nextdoor.com. When I went out to look at it, it was uniform in size, about pea size, whereas the hail I have seen is often different sizes. Your post now makes be think it was graupel, thanks!

  5. About an inch of graupel at my house (N. Bothell). The local kids wasted no time trying to build snowmen. The temperature dropped about 5 degrees in a half hour or so.

  6. Same thing happened in Sacramento (CA) area on Sunday (Monday?) It was quite interesting to experience (I am in Port Ludlow).

  7. Monday storm in Sacramento CA just as described.
    K - former Bellingham :)
    Winter is a choice, I choose No!

  8. Cliff, the conditions for graupel sound terrifying for icing (you might call it riming) for small airplanes. Rime ice is bad (for little airplanes) as it produces rough shapes which greatly increase drag and disrupts lift. Clear ice is bad, though while it accreted in a smooth layer, it “only” adds weight, sometimes quickly, but follows the airfoil shape. Graupel conditions look to be the worst of both. Could you please show a vertical slice from a model through the graupel incident? If one has stumbled into freezing rain, “they say” that you may be able to climb into warmer air above to escape. (This doesn’t sound feasible for graupel conditions because of the vertical development.) Also, what sort of adverse weather warnings would one look for as to avoid these conditions?

  9. It's officially snow pellets, not graupel. Graupel is never mentioned in official weather observations


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