Tuesday, November 16, 2010

Windy Western Washington

Time for me to return to Pacific Northwest weather for a day.  Unfortunately (or fortunately if you like highly dynamic weather) for us, this impending "arctic outbreak" isn't the only major pattern change going on. Here's a look at our current hemispheric plot like we've been looking at over the last several blog posts.
Fig 1 -- Northern Hemispheric plot of 500 mb heights (shaded) and mean sea-level pressure (contoured) from 12Z, Nov. 16, 2010.  From the HOOT website.
First, I'd mention that the really cold air mass is continuing to slowly move south from the pole in our direction.  But turning our attention to the northern Pacific, we can see some very broad areas of high pressure at the surface which are mostly mirrored in the 500 mb height shadings above.  With high 500 mb heights in this region, we expect the mean temperatures in the lower atmosphere to be relatively warm.  However, as cold air continues to slowly creep southward over the continent through Canada, it's going to be increasing the temperature gradient between the relatively warm ridge over the eastern Pacific and the cold continental polar air.

See the band of green shading where the heights (and consequently temperatures) rapidly change from warm (the yellows and oranges) to cold (the blues and purples)?  This is the region where we would expect to be find the strongest winds aloft, also known as the jet stream.  What have we seen that connects temperature gradients with winds aloft?  If you guessed the thermal wind, you'd be correct.  But I'm not ready to return to that topic yet...

Basically, you can see how the height (or temperature) gradient seems strongest over the Pacific Northwest, and it would only be expected to strengthen as cold air moved south.  This region is an area where shortwave storm systems are likely to form.  It only takes a small pocket of colder air perturbing that interface to start cyclogenesis at the surface and eject a shortwave.  Last night the Pacific Northwest saw one of these shortwaves move through (you can see in figure one above how the surface cyclone associated with this has already moved inland over Idaho and Montana).  While not the biggest precipitation producer, this little storm did bring some powerful winds.
Fig 2 -- 24-hour meteogram from UW rooftop weather station.  As of 1847Z, Nov 16, 2010.
The figure above is what is called a "meteogram" from the weather station on top of the University of Washington atmospheric sciences building.  It shows a time series of several different meteorological variables over the last 24 hours, with the most recent time at the right.  This allows us to see how several different variables have evolved over time and also identify any correlations between them. (Click the image to get a full-size version.)

In the top panel, we can see the wind speeds over the last 24 hours.  Note how winds remained relatively strong, varying between 10-25+ knots from 00Z to 06Z last evening (from about 4 PM to 10 PM).  Gusts were often well over 30 knots.  Strong winds like these were seen throughout the Puget Sound region and were responsible for some moderate damage and power outages last night.

Let's notice one additional thing about this data.  Many of the news sources last night were saying that the strong winds were in association with a cold front moving through as this cyclone came on shore to the north.  Is this so?  Because of the effects of mountains, wind direction is not the best indicator of frontal passages out here (take note of this, meteorology students!).  Instead, we remember two other features of fronts:
  1. A cold front typically lies within a local pressure trough such that pressure falls as the front approaches and rises after the front has passed.
  2. Fronts are technically defined as regions of strong potential temperature gradients.  But, since we're at the surface and near 1000 mb, ordinary temperature gradients will do.  So we'd look for temperatures to cool behind a cold front  Common sense.
Where do we see those two features occuring?  Right around 530Z (930 PM, PST).

This is the most likely time that the cold front passed through the area--toward the end of the time of maximum winds!  This means that these strong winds were in the warm sector of the cyclone ahead of the cold front.

Another interesting feature of this meteogram is how the temperature profile changed (or didn't).  Note in the bottom panel of these meteograms there is a "solar radiation" chart showing how much radiation was received by the sensor on the roof.  We can guess where the sun went down by seeing where that graph finally went to zero--around 100Z.  However, even though the sun went down, the temperatures did not drop much at all until the front came through four and a half hours later.  Three factors probably contributed to this:
  1. Since we've already concluded that these strong winds were in the warm sector of the cyclone, there was probably some good warm air advection going on which canceled out much of the cooling.
  2. Cloud cover over the warm sector helped to insulate the lower atmosphere so that longwave radiation from the surface did not escape to space.  This would help keep the near-surface layers from cooling significantly.
  3. The strong winds through the lower atmosphere kept the boundary layer well-mixed, preventing a strong surface inversion from forming by mixing warmer air (in terms of potential temperature) aloft down to the surface.
So what about these crazy wind directions and speeds? What caused them?  The biggest factor seems to be the interaction with the terrain, particularly the Olympic Mountains to the west.  Below is a 3-hour forecast graphic from last night's UW WRF model run.
Fig 3 -- 3-hour forecast of 950 mb temperature, MSLP, and 10 m wind barbs from UW WRF model initialized 00Z, Nov. 16, 2010.
There are two areas where winds in the Puget Sound region were particularly strong.  One was over and to the east of the Strait of Juan de Fuca  and the other was over the south Puget Sound and Seattle areas.  Why were the winds stronger in these regions?
  1. To the north, there is a relatively narrow gap over the Strait of Juan de Fuca between Vancouver Island and the Olympic Peninsula.  Both of these land masses quickly rise to mountains, and as such any westerly flow approaching them (like we see here) is going to be channeled and accelerated through the gap.  Thus we see really strong winds through the strait and any land mass downstream of that gap.
  2. As strong westerly flow approaches the Olympic Mountains, a lot of the low-level air is forced to split and go around the mountains.  On the other side (the eastern side), this creates an area of relatively low pressure (which we see very, very nicely in the figure above).  As such, once air gets around the mountains, it is accelerated toward that area of lower pressure (the air wants to fill this relative "vaccuum").  Therefore, we see strong winds across southern Puget Sound being turned from westerly to more southerly as they are accelerated by this pressure difference.
So there is a brief look at some of the details surrounding this little shortwave passage through western Washington.  With this particular upper-air setup, though, we're expecting even more action as another shortwave moves through on Wednesday...then again on Friday...then again over the weekend...  It promises to be an exciting week.

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