It’s a rare long November dry spell, plus lots of gusty east wind ahead

You have to admit, the bright sunshine was spectacular today! As I was driving into work, the parkway was line with bright yellow/orange maple trees, finally donning their fall outfits 2-3 weeks later than normal. We should see 5 more days like today...that’s Tuesday through Saturday. Then the normal November rain returns

November started with a soaker, but now we’ve gone 7 days without rain, and 5 more are likely. That means we’ll end up with at least 12 consecutive dry days, the November record is just 13. So this long dry spell in November is quite rare. It’s interesting that happened in the 3rd year of a La Nina...November 2000.

The reason for the unusually dry weather is the return of that pesky upper-level ridge along the West Coast. Here’s tomorrow’s forecast of 500 millibar heights, about 18,000′ overhead. The flow is going well around us, north into Alaska and Yukon, then heading south into the Midwest...brrr!

By Friday, the pattern isn’t much different

But a well-advertised change occurs Sunday through Tuesday. Wet westerly flow punches through the ridge...the view on Tuesday

I think it’s safe to say the first half of Thanksgiving Week will be wet, but it’s too early to know if that will continue through the holiday weekend. The ECMWF ensemble 24hr rainfall chart shows the change very well. Each horizontal line is one of the 51 ensemble members...but they are all wet aren’t they?

So we’re headed into mild/wet weather NEXT week, but this week we’ve got the sunshine, plus a gusty east wind.

November through February IS the “east wind season” in our area. What does that mean and why? In the cool season the sun angle is very low and we don’t get much energy from that sun. So when weather systems with clouds/rain are blocked from moving overhead, long nights lead to cold air pooling in the lowest elevations. There happens to be a bit of a “bowl” in Eastern Washington and north-central Oregon. This is the Columbia Basin. That “bowl” tends to fill with cold air and then the weak sun is not able to warm it up due to short days. That cool airmass can grow quite deep, often up around 3,000-4,000′ deep. The dense/heavy air means an area of high pressure develops. This is a pretty good depiction of that cool/heavy airmass.

Of course it’s trapped by the Cascades, like a dam holding back water (except that it’s heavy air). BUT there is just one gap below 3,000′ through the Cascades between Canada and California. That’s the Columbia River Gorge. So that heavy air surges west through that narrow gap and then spills out into the flatlands west of the Gorge. There’s an additional item, remember the cold airmass in the central Gorge is typically 3-4,000′ thick, but it’s usually less than 2,000′ once it reaches the east metro area. So it’s a bit of a “waterfall of air” too. Wind accelerates and peaks just inside the west entrance to the Gorge. Peak gusts reach 50-70 mph many times in this area during the cool season and can peak up around 100 mph during very cold easterly flow when a low pressure area approaches from the southwest. The strongest wind I have ever seen in that area was the day before the big February 1996 flooding rains arrived. Temperatures around 10-15 degrees were accompanied by a 101 mph gust in Corbett on a high-quality wind sensor. Strongest wind is always a combination of large pressure gradients plus a strong inversion and unusually warm temperatures overhead. The 2nd highest wind event I’ve seen was in January 2009. Peak gusts reached 50-60 mph even into Gresham.

As you can see, the eastern metro area gets the brunt of the cool season easterly wind. Strongest gusts are concentrated east of I-205 near an south of the Columbia river in a typical setup. The wind goes away in spring when the stronger sunshine starts heating land effectively east of the Cascades; that’s in mid-late March.

Tonight the satellite picture shows the “cold pool” has developed nicely east of the Cascades (clouds in white). The X marks Portland.

That’s another feature of the cold season...low clouds and fog (depends on your elevation) stuck in the cold air eastside. Places like Hood River, The Dalles, Maupin, and Madras can sit under gray low clouds for a week at a time. Yet Timberline and Mt. Hood Meadows will be in warm sunshine. During these inversion events you want to head UP to find warmer temperatures.

By the way, there’s no sign of a cold spell or lowland snow in the next two weeks. I think we are clear for Thanksgiving Weekend. Notice almost no ECMWF (Euro) model members produce any snow in Portland through the end of the month.