Will There Be A Windstorm Monday?

December 20, 2015

8:15pm Sunday…

The short answer is…possibly.  If anything does happen, it’ll happen  between 11am-1pm.

Here’s what’s going on:

  • A deep area of low pressure will track almost straight west to east just to the north of us tomorrow.  From around Willapa Bay on the Washington coast late morning to around Pullman on the Idaho border by late afternoon.
  • Central pressure should be around 985 millibars, plus or minus 5 mb.
  • Theoretically there should be a strong rush of wind up the valley as the isobars align east/west at midday when the low passes by north of us.
  • It’s moving very quickly, so it’ll occur quickly and leave quickly.

You can see the WRF-GFS forecast for Noon:

slp.20.0000

One would think that this could produce 50-65 mph gusts in the northern Willamette Valley with this setup.  But there are some problems with that:

  1. We have very few situations in the past where the low pressure takes this west-east path, so not many past examples to learn from.
  2. Last time we saw a track like this we were totally “burned” overforecasting windspeeds…December 17, 2012  That low was even deeper, but farther north.
  3. There are other examples where the same track has given us wind gusts to 60 mph.  Early January 1990 is one good example.
  4. Almost all models are producing very weak wind fields in the very lowest part of the atmosphere (where we live).  Interesting don’t you think?  Are they all wrong, or are human forecasters going to lose on this one again like in 2012?  Our RPM would say gusts only 30-40 mph…just windy for a few hours like we’ve seen 15 times this month already!
  5. This is likely due to the low level southerly pressure gradient being southerly like in a typical windstorm, but all wind from just a few thousand feet on up is straight westerly at that time.  The two don’t line up like last winter’s 3 windstorms where the surface low travelled almost south to north.  We won’t see strong wind from above mixing down.

You can see the sounding from the WRF-GFS for 1pm…notice all westerly except right at the surface?

kpdx.21.0000.snd

Now I am still on vacation…and I hopefully don’t have have VACATION-BIAS* (see below) right now, but…

THIS IS WHAT I WOULD FORECAST

PORTLAND METRO & WILLAMETTE VALLEY:  Southerly wind 20-30 mph with gusts 40-50 mph between 11am-1pm.  Then dying off.  Similar to what we saw about 10 days ago.  Plenty of power outages, but not a huge wind storm.  I know this is way below the NWS forecast of 65 mph gusts, we’ll see how that works out.  For betting purposes…I bet no higher than a 46 mph gust at PDX.

COLUMBIA RIVER GORGE:  This is a spot that just about has a guarantee of seeing some high winds…not sure why there isn’t a high wind warning out from Multnomah Falls to Carson area.  SW wind 30-40 with gusts 60-70 mph suddenly arrive around Noon-1pm.  It’s a similar event to the damaging wind about a month ago.  Wind gusts 50-60 mph possible from Hood River all the way out to Arlington/Hermiston.  This could be a big windstorm all across North Central Oregon too.

Alright…we’ll see what happens now.

Chief Meteorologist Mark Nelsen

* VACATION-BIAS DEFINITION:  When forecaster hopes that nothing exciting happens when on vacation in a warm place so he/she doesn’t miss it.  Thus he/she forecasts less exciting weather than he/she would otherwise.