It's been 9 days since I last posted. Part of the reason (other than keeping busy at work) is the lack of stormy weather. As mentioned in the previous post this first half of December was looking drier than average. Sure enough...the 14 day anomaly across the western USA shows a very dry month SO FAR...
Temperatures have been running near average over the Pacific Northwest. Although wintertime inversions in some valleys/basins have given cooler than average (Pendleton, Tri-Cities)
Again, nothing too exciting. We had a pretty good easterly wind episode the first week of the month; Troutdale saw 7 consecutive days with gusts 35 mph or higher. That said, it is December and that sure isn't unusual. We've seen snow in the central/eastern Gorge twice now, although real marginal both times... the 2nd was last night. I had a wonderful day of light rain showers with the temperature between 33-36 degrees all day long (at home). Snowpack IS running near average in the Cascades which is great news. As I write this I see Hwy 26 through Government Camp has turned snowy again
A couple of notes...nowadays I spend time putting out shorter thoughts/maps etc... on Facebook, Twitter. It's much easier on a busy night to quickly post a model chart or other weather information there as opposed to a long blog posting. You can find me on Facebook as @marknelsenweather or Twitter as @MarkNelsenKPTV Also, we have a Northwest Weather Podcast.
Lots of fun! Just this past week the four of us (Brian MacMillan, Jeff Forgeron, Anne Campolongo, & I) discussed La Niña so far, Vista House wind, Solar Eclipses, & holiday plans for the weather center. Several weather folks having a jovial conversation...what could be better? You can find it here: https://www.kptv.com/podcasts/weather/
1- We are turning wetter now and should see more regular rainfall over the next 10-14 days. I don't see a significant dry spell (2+ days) between now and at least the middle of next week.
2- There's NO sign of real cold weather or lowland snow west of the Cascades for the next 10+ days. That bumps us up right against Christmas of course. My gut feeling is that we won't be seeing snow anywhere near the lowlands for Christmas Week (next Sunday through Christmas Day).
3- I don't see flooding or an especially stormy pattern for the next 7 days. Most of the weather systems moving inland are relatively weak; I'm not seeing big deep low pressure areas in the eastern Pacific. Of course that could change, but not this week.
I can explain both #1 and #2 just by showing you the 500 millibar forecast from the GEFS (GFS ensembles) for the next two weeks. Red is above average heights, blue is below
It's a continuing train of weather systems moving west to east in a "zonal" flow. That's as opposed to a "meridional" flow which would lead to more north-south movement. Also notice there are no prolonged ridging episodes (dry weather). In this pattern it never gets cold because the chilly arctic air is bottled up to the north of the jet. You aren't going to get lowland snow in this setup. Look at the 850mb ensemble chart from the ECMWF model.
I'm amazed that through the entire 2nd half of December there are almost no members (each line is one of 51 ensemble members) that drop below -7. That's about what we need for lowland snow in onshore flow. Very consistent westerly flow = mild.
Confirmation comes from the ECMWF ensemble forecast of Portland snowfall the next two weeks (through Sunday the 28th). Not a single member tries for 2" or more snow.
There are hints that Christmas Week might be a bit drier than this week and coming weekend. You see quite a few more gaps in 24 hour precipitation totals during the 2nd week.
What About La Niña? Shouldn't Weather Be Wilder at This Point?
No, not necessarily. Remember that La Nina doesn't guarantee lowland snow, and it sure doesn't guarantee lowland snow and stormy weather between November 1st and January 1st. For all we know we've got crazy wild weather action coming in January or February! For fun I looked back through the last 8 La Nina winters to see what happened. Some start just like this...mild and dry-ish. My brief notes on each.
2017- A boring November, dry ridging 1st half of December. No decent skiing until Christmas Vacation
2016- Crazy and wild ride. At this point we had just gone through an ice storm and were preparing for a 2pm arrival (December 14th) of a snow storm. Everyone hit the road at the same time and you remember what happened. I always remember the date 12-14-16.
2011- Stormy November, Dry December. Then a big atmospheric river wipes out quite a bit of snow on Mt. Hood right after Christmas. January 2012 following was stormy
2010- Stormy November, arctic air at Thanksgiving
2008- Boring November, dry December start. Arctic blast arrives right about now (Dec. 14th). Two weeks of snowy mayhem followed...
2007- Boring/dry/mild November, huge coastal storm and widespread flooding just north/west of metro early December. A series of colder storms began in mid December bringing feet of snow to the foothills and many feet the rest of winter to the Cascades
2000- Nothing all that interesting, cold/dry November, in fact entire winter was dry-ish. Didn't fit La Nina pattern at all
1999- Warm & wet November
1998- Very wet November and December, arctic blast around December 17th
1995- Warm and wet November, stormy December included the last major regional windstorm (12-12-95)
You can see this current year isn't a very good fit for any the last 10 La Nina winters. Maybe similar to the last one 2017-2018? Interesting...
That's it for now, enjoy the rainy weather. Best chance for some dry is tomorrow and maybe sometime between Thursday showers and renewed rain on Saturday...possibly Friday.
Chief Meteorologist Mark Nelsen