55mph in Portland; A Strange Windstorm Last Night

February 17, 2014

The peak gust at PDX of 55 mph was the strongest southerly wind gust (our storm wind direction) in 14 years!  Yet only 15,000 or so power outages in the metro area and Clark county?  How is that possible?  There’s more to this story.  Take a look at the peak gusts from last night’s system:

PLOT_Wind_Metro_PeakGusts

The center of town sure sticks out doesn’t it?   Except for a swath from Milwaukie to the Columbia River, peak gusts were all well below 50 mph.  For most of our area the gusts were no more than we’ve seen a few times this winter already.  We had very strong wind a few thousand feet up in the atmosphere overhead and some of that must have “mixed down” to the lowest elevations in the middle of town.

That  gust to 55 mph was the strongest at PDX since a windstorm in January 2000.

windgust

So was it stronger than the December 2006 storm which had a gust to 53 mph?  No.   That’s because the wind measuring equipment at NWS recording sites changed around 2007 from a 5-second peak gust to a 3-second peak gust.  There was about a 10 year period where Portland’s observation equipment used the longer duration gust requirement; basically the wind would have to peak for a bit longer to get that high value registered.  So the 55 today would probably be equivalent to a 46-48 mph peak gust during (the 5-second era) based on Wolf Read’s exhaustive comparison of past/present windstorm readings here:  http://www.climate.washington.edu/stormking/StormRanksASOSadjusted.html.   For comparison, the Davis line of instruments that many of us have at home, use a 2 second gust.  Those would tend to read a little higher than the 3 second gust.  Without all those numbers, the outage numbers probably tell an even better story.  During the December 2006 storm in which we saw widespread gusts 50-60 mph, almost 300,000 people in Western Oregon lost power!   Compare that with the 15,000 today.

Also note the 55 mph gust a few years ago in March was with a squall line from the west, and the 56 in 2007 was a downslope east wind type of event.

Tomorrow’s wind will be much more reasonable, with 35-40 mph gusts most likely peaking around midday with another bout of rain.

Chief Meteorologist Mark Nelsen