It’s been a chilly December thus far!  Through the first 15 days of the month, PDX is running 2.6° F below normal. That’s not wildly cold by any means; we’ve had no “exciting” weather save for a very marginal “conversation snow” event two Sundays ago. It’s just been very consistently cool the past 3 weeks; not a single 50-degree day since Thanksgiving! Most days have had temps in the mid 30s to mid 40s during the waking hours, with a handful of freezing nights. We’ve been under cool North Pacific troughs or brisk offshore flow the entire time.  It definitely feels like classic “West Side Winter” conditions.

Now on to the big story. Wow….I don’t think I’ve seen such a messy forecast in many years, as what we have showing for next week!  Models are completely uncertain as to what’s coming just 4-5 days from now. The lowlands of NWOR and SWWA could be looking at temps in the 20s with snow….or it could be 25 degrees warmer with rain at the Cascade ski resorts. (Huh?)

I’m not going to post a bunch of maps; you can go to https://www.tropicaltidbits.com/analysis/models/  and click on the “Northwest U.S.” tab.  Then peruse the last several days of GFS, ECMWF and CMC (GEM) maps to get a picture of the Great Confusion of December 2022.

Here’s a basic summary of what’s going on:

1.  We are going to have VERY cold arctic air to our direct north (southern BC & Alberta) much of next week. The cold air moves into position on Sunday. 

2.  HOWEVER…we don’t know how much of that cold air (if any) will swing southward into Washington and Oregon. That’s the source of the confusion…and the reason for so much disagreement in local forecasts next Monday through Thursday. 

At the beginning of this week, models were much more bullish at bringing cold air south. Then by Wednesday it looked like the plan had completely fallen apart, with any snow confined to points north of Seattle. But in the last 24 hours some models (especially the ECMWF) have reverted to colder/snowier solutions. Interestingly the GFS is showing less chance of cold than the other models; normally it’s the silliest of all when it comes to arctic wishcasting!  But even the 18z GFS has decent disagreement among its members:

See how the red squiggles split apart above the 21DEC caption?  This shows that beginning Tuesday, there is major divergence in the ensemble members. About a third of them show cold air diving down, while about half turn extremely mild with snow levels up to 6-7,000 feet by Wednesday evening.

We don’t know exactly what will happen next week, even if the cold air makes a run for us.  But if you live between Longview and Salem, now is a good time to mentally prepare for the possibility of a significant snow or ice event at some point between Monday night and Thursday. Don’t panic and buy all the milk and avocados at your local grocery store, but DO have some basic winter home supplies (cat litter, shovels, etc) on hand in case wintry conditions pay us a visit.

Stay tuned!

Karl

P.S.  Here is the most outlandish map of the week, the Tuesday morning Euro.  850mb temps of -21c for Portland and -25 to -30 on the east side?  Not going to actually happen but it would mean overnight temperatures in the -10s to -20s across most of Central and Eastern OR/WA.  Fun fantasy!