December 01, 2017

The Upcoming December "Drought" in the Northwest

A stunning transition is going to happen on Sunday.  

Precipitation over the Northwest will end. 

And we might not see another drop for over a week.  Maybe more.  An unusual occurrence in December.  You will see sun....but as we will note there is a threat:  low clouds and fog.

Let's take a look at the forecast upper level (500 hPa)  maps this week from the NOAA GFS model...and be prepared to have your jaw drop!

At 10 AM Sunday, a trough has moved passed us and a ridge of high pressure has started to build in the eastern Pacific.  The rain will be ending at this time.

One day later, the eastern Pacific ridge has amplified.   We are totally dry.

The ridging increases by Wednesday.... the ridge is HUGE and extends up to Alaska.

 Saturday at 10 AM.  The ridge is still there, but a bit weaker.    Plenty strong enough to keep us dry.

 It revs up even more on Sunday!
 And it is STILL THERE on Tuesday 12/12.

 And it continues after that for several more days.

Folks...we are talking about a total shut down of precipitation over us for 1-1.5 weeks.  In December.

OK, I know what you are saying.  What do the ensemble systems using many runs show?  Here is the anomaly (difference from normal) of 500 hPa heights for the mean of the NOAA GFS ensemble system for 4 PM Thursday.  Much higher heights than normal (high pressure) over the NE Pacific and our area.  BIG positive (high pressure) anomaly.

The forecast a week later IS THE SAME THING, just a little weaker.  Wow.

Next lets look at the precipitation forecasts from the vaunted European Center ensemble.  There are 51 different forecasts in this ensemble system.

No precipitation for virtually all of them for Dec 5-9th, and then nearly nothing through December 12th.  After that, a few members have some light rain--very light rain. And yes, some rain this weekend.

So we should be confidence in the forecast: after the rain stops Sunday morning it will be absent for a week, and maybe more.

There  will be sun ... plenty of it.  But after a few days of high pressure there is a good chance that low clouds and fog may form, at least in the mornings.  

Why?   Because clear skies from the high pressure will allow the earth to radiate into space, resulting in low-level cooling.  That cooling can bring the air near the surface down to saturation, producing fog/low-clouds.  The weak winds associated with the high pressure won't mix it out.  And the sun is so weak now, it will have a hard time burning off the low clouds.   We will see.

And a major tip:  if the low clouds form you can easily escape them and enjoy sun by going an elevated park (e.g., Cougar Mt) or into the Cascades/Olympics.  Leaves will be nice and dry...making raking easy and fun.


  1. It's beginning to look that this December could end up like Dec '85: very dry and stagnant air quality.That winter season started out with a weak LA Nina too,but returned to neutral ENSO conditions in December.It will be interesting to see what the ONI Index is, come next Thursday when the next update comes out.
    Also,I've never seen such a complete reversal in a monthly temperature forecast as there was between the original December CPC outlook on Nov 16th,and the revised one on the 30th.Astounding!
    Cliff,how about a post on the MJO,and how the different phases of it affect Northwest weather? Like how it is influencing the weather patterns occuring right now.

  2. Very much looking forward to some sun, but... raking will be fun? Will a temperature inversion trap the marijuana smoke at ground level?

  3. This should be a nice break from the relentless rain. I remember a similar pattern in 2009 and 2013 in December. Both dry spells lasted around 2 weeks if I remember correctly.

  4. How cool do you think will the cooling be? My recollection is that sunny spells this time of year usually mean temperatures in the 20s and 30s, but I see forecasts that show highs in the upper 40s for next week. Is that really unusual or is my memory just faulty?

  5. Oh my, one of the most dull weather patterns possible this time of the year. Don't worry though, there's a terrible snowstorm and winter weather on the way. It will hit right around December 22. How do I know this? No meteorological skill required. I need the passes to be clear to get east of the Mountain for the holidays and my bad luck for travel is at "epic" level. ;)

  6. I very much forward to the dry, but if the low clouds do form and stay, that is my least favorite winter weather pattern! Not only boring, but the gloomy fog keeps high temps in the low 40's. Really hoping there is enough cross cascade pressure difference to mix down to the lowlands, then it will be amazing December sunny days and cool, frosty nights... Perfect!

  7. La Niña bust? Funny the super El Niño two winters ago it snowed to lower elevations all of. dec.

  8. what about open sky cold, will we see freezing temps?

  9. Fine with me! I can get the outside Christmas lights up. Right now I've got strings laying around the rec room ready to go.

  10. Cliff - I thought we usually got our coldest, driest winter weather in December? Temps down in the teens for a week or two (in Redmond at least) with cold, clear pattern, and frustration if there isn't enough snow yet to ski. - Matt

  11. I am originally from Arizona so I welcome the sunny days with open arms. Also, I really hope that there will be enough cold air or a diving low (like the one in early Nov.) that will bring snow.

  12. The sun n lack of no snow ain’t a big deal, it’s the fear of an extended / prolonged alpine inversion of above freezing temps that could cripple the high elevation snow pack after the previous weeks warm weather n rain melted the low elevations... starting to have fears of 2014/2015

  13. Matt, I seem to remember it being January. Anyway, it does mean that I can get some car projects done in the driveway, instead of having to play musical cars just to get into the garage.

  14. Less chance of fog if you park at the mouth of the Fraser or Columbia Gorge.

  15. Vanishing Arctic ice could drive future California droughts, ( - pertinent here?


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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