June 19, 2013

Super Shear

Some days one can look at a time lapse cloud animation and come away amazed...and today is one of them.  A day where the winds are simultaneous blowing at many different directions and speeds above us, producing an effect that either is startling or gives you a case of meteorological disorientation. 

Today (Wednesday) was one in which there was substantial directional wind change (shear) with height.  Our mountains often produce major differences in direction in the lowest 5-10 thousand feet, due to the channeling and blocking influences of the terrain.  But today there was even more going on, with a low center sitting over the region (see map for 500 hPa...roughly 18,000 ft at 11 AM this morning).

Ready to see what I am talking about?  Here is a video from Greg Johnson, taken from his his dual-cam today from 9 AM to 2 PM.  This view is looking north from north Kitsap, with Whidbey Island on the horizon.   Watch the flag, which shows you the near surface winds, and the clouds above, and pay particular attention after 11 AM. (click on image to view or go to the link)


After 11 AM, the winds are easterly (from the right) near the surface (shown by the flag), westerly in the lower atmosphere (check out the clouds over Whidbey Island), and easterly again farther aloft (in the high clouds).

You can get a closer view by just watching the western cam.  Here is is:

You can see the origin of these complex winds by looking at a few weather maps---all valid at 11 AM.  An upper level map (300 hPa pressure, about 30,000 ft) shows the trough south of Washington and strong southeasterly flow over Washington.  That is why the upper clouds were streaming towards the NW.

A  map at 850 hPa (about 5000 ft) shows a very different story,  with northwesterly flow over the area, a deep low over western Montana, and only a weak trough along the coast.

The observed surface winds show weak easterlies near the surface over the cam location.  The easterlies were probably due to weak troughing to the northeast of the Olympic Mts.

 The vertical shear of the wind was quite apparent still at sunset (and very beautiful as well)

Large directional  and wind speed shear in the vertical is not unusual around our region, and as noted above, our terrain is a major contributor.

One final thing...large vertical wind shear can produce turbulence for those flying through it.  There was major vertical wind shear (mainly in speed) between roughly 15,000 and 25,000 ft over our region today (the southeasterly winds strengthened rapidly with height) and this led to many reports of light to moderate turbulence.  Here is an example pilot report (pirep):

SEA UA /OV SEA104035/TM 1726/FL250/TP B737/TB LGT OCNL MOD CHOP 250-210/RM -ZSE
(translation.  Turbulence light with occasional moderate chop between 21,000 and 25,000 ft)


  1. An expanding low, with both ridge crest effects together with wrap-around convergence influence factoring in, am I right. ?


  2. My son was at sailing camp at Sand Point on Wednesday, where they experienced a dramatic shear effect in the early afternoon.

    A sudden gust picked up equipment, papers and chairs, like a giant hand, and tossed everything into Lake Washington.

    No one was hurt, but all the parents had to fill out new forms because the paperwork ended up in the water.

  3. Hey Cliff,

    Watching the western cam video was indeed fascinating. I noticed one thing that particularly caught my eye, though.

    Starting at roughly 0:10, just to the right of the flag, despite all the obvious wind-driven movement there's one fixed spot that consistently has a smallish, fairly discrete cloud. If we were looking at a creek or river, I'd think there was a rock there disturbing the flow - but that explanation doesn't really work, up in the air. However I'm guessing there is some explanation for why a cloud was continuously generating right there, other than random chance...

  4. I noticed this while it was happening, and sort of wondered WTF about different cloud layers CLEARLY traveling in different directions under the influence of different wind flow.
    @Westside guy, Cliff has talked about that feature before, and has posted awesome videos from those same cameras... there's a big ~400-500 feet cliff at the edge of Whidbey Island that builds its own clouds because it forces the air up so high so fast; when the wind is from the correct direction, that cliff puffs like a steam locomotive. My five-year old train-obsessed son asks to watch "Locomotive Island"... it really is compelling visually.
    Mt. Rainier does this too, sort of, but because of the shape of the mountain, it makes flying-saucer lenticular clouds instead of a steam train. :-) We live in an amazing place.


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