How cold will it get in the PNW next week?

Most seasonal forecasts for the winter of 2023-24 predicted mild conditions for the PNW, largely because of the ongoing El Niño event. When all is said and done, the average winter temperature will probably be above normal as expected. But it now appears that winter will make at least a brief appearance across all of the PNW lowlands at the end of next week.

Anyone looking at an extended forecast on their phone has seen some shocking numbers for next week. As of Friday morning, my iPhone forecast showed temperatures as low as 15°F in Seattle.

Where does these numbers come from? They are likely a bias-corrected versions of model ensemble means. The GFS ensemble mean temperatures for next weekend are actually colder than this, representing the usual cold bias of the GFS during our arctic air intrusion events.

The European ensemble mean represents a more reasonable consensus given that we are nearly a week away from this event — although even those numbers are considerably colder than anything we have experienced so far this winter.

The biggest challenge for meteorologists is communicating the uncertainty of an extreme event at such a long lead time. Here is the 2-meter surface temperature at Sea-Tac for all 50 individual ECMWF ensemble members.

You can readily see that almost all of the ensemble members agree on the temperatures through next Wednesday, then the members all diverge into a variety of forecasts spanning 10°F to 50°F.

East of the Cascades, the numbers for Spokane have an ever great range: -20°F to 40°F.

So what can we make of this uncertainty?

1. At least 80% of the individual ensemble members above are showing at least some arctic air reaching the PNW. So we have very high confidence that individuals and communities should begin preparing for sub-freezing temperatures.
2. Around 10% of the ensemble members are cold outliers and another 10% are warm outliers. For now, these should be disregarded, but if those percentages increase we may have to give the more serious consideration.
3. Let history be our guide — most of our significant arctic outbreaks in the past 10 years have seen minimum temperatures dropping no lower than 15-20°F in Seattle. It would be a real shock to see temperatures lower than that — especially given that the average minimum temperature in the European ensemble is right around 20°F as well at the end of next week.

What about snow?

I’m hesitant to speculate on lowland snowfall amounts at this time. The next two storms impacting the region over the weekend and early next week will both bring significant beneficial snowfall to the mountains.

Over the lowlands, there will probably be at least some snowfall with the arctic front passage next Thursday or Friday. This arctic front will probably be a quick mover and I doubt there will be significant accumulations in the Seattle metro area from that event. But most people are likely to see at least a few flakes at the end of next week.

There will probably be additional snowfall threats once the arctic air is entrenched next weekend, but that is far to far in the future to speculate on.

Appreciate the beauty of this pattern

Given that this winter season has been largely mild and devoid of impactful weather, fans of winter can simply admire the beauty of this ECMWF ensemble forecast for next Thursday.

I’ll write another post in a few days once there is better model consensus on some of the details.