9pm Monday...

Time is flying, or maybe I’m just getting older. We’re quickly heading toward mid-November so let’s do a quick recap of the season so far.

Meteorological Fall includes September/October/November, so we only have 3 weeks left this season:

1. Temperatures have been near normal, or slightly below since September 1st. September and November (so far) slightly warmer than normal, but October slightly cooler than average. Nothing interesting here

2. Wet! After an incredibly dry spring/summer, rainy weather arrived in mid-September. We are very fortunate it didn’t wait a few more weeks. It’s always amazing how we go from desiccated/dry ground and brown vegetation to green within just a few weeks each fall. Since that time, 8.87″ rain has fallen in Portland! As always, the mountains scoop up lots more rain. Mt. Hood Test Site at the bottom of Timberline’s Pucci Lift has seen 15.30″. North Fork SNOTEL south of Multnomah Falls at 4,000′ picked up 27.70″. A SNOTEL station named “Sheep Canyon” is on the southwestern slopes of Mt. St. Helens. In that notoriously wet location, over 3 FEET of rain has fallen since the wet season began; 37.40″. It’s hard to believe, but those numbers are about normal for this point in the fall. The Cascades are very wet (or white) late September through springtime!

Here are some other numbers showing just the last 30 days. Screaming message here is that you shouldn’t complain about rain in the I-5 corridor…tons more rain regularly falls west and east in hills/mountains surrounding the lowlands.

Rain Coast Cascade Range.png

SUMMARY: THIS FALL HAS BEEN QUITE “NORMAL” SO FAR

November is here and it’s one of the 3 wettest months of the year west of the Cascades. November, December, & January are close to the same rain-wise. It’s also the month that we slowly slip into our “winter” conditions west of the mountains. High temperatures drop down through the 50s.

November Monthly Averages.png

WHAT’S AHEAD?

More wet weather. Sure, today was amazing with early blue sky and comfortable temps in the 50s. But a relatively weak system is sweeping north along the coastline; we’ve got a wet night ahead. The Cascades picked up 4-12″ snow over the weekend, and I expect another 4-6″ through tomorrow above 3,000′. But don’t get excited about an early start to the ski season yet…

PINEAPPLE EXPRESS

Starting Wednesday, upper-level ridging pushes warmer air north along the West Coast. That ridge LOOKS strong enough to give us warm & dry weather Thursday based on this map

Mark Jet Stream.png

But plenty of warm & subtropical air will be flowing northeast from north of Hawaii. Check out “Precipitable Water” on Thursday. You can see why these narrow ribbons of warm/moist air are named “Atmospheric Rivers”.

Precipitable Water ECMWF.png

When it’s a warm one like this, freezing levels soaring over 9,000′, we also call it a “Pineapple Express”. Same idea, a warm, cloudy, and very wet weather pattern for us.

Since this is like a narrow firehose of rain that flips around easily, forecast rainfall can be all over the place as well. The ECMWF forecast through Friday morning seems a bit excessive showing 2-4″ in the western valleys of Oregon and SW Washington just through that time.

ECMWF 90HR Rain Accumulation.png

It’s better to use “model ensembles” in a situation like this. For Salem, the 51 members of the ECMWF model show an average of 2.50″. That’s the bottom half of the chart. Each of the horizontal lines on the top half show accumulating rainfall the next 6 days (at Salem)

ecmwf-ensemble-KSLE-indiv_qpf-6394400.png

For this reason (and at this moment), the event late Wednesday through Friday does not appear to be a significant flood producer. But, always something to watch as we get closer.

Snow levels will be very high Wednesday night through Sunday, mainly above 8,000′.

7 Day Forecast Cascades Mt Hood Government Camp.png

Add 3-6″ of rain to a fresh foot of snow later this week and you can guess what will happen. There won’t be much left. But hopefully a bit colder next week and MAYBE some ski terrain opening for the weekend before Thanksgiving…we will see!

To sum things up…it’s going to be very wet this week, especially Wednesday night through Friday. But models are also implying the ridging pushes most or all of the rain north of us next weekend. I’m skeptical…more later in the week

Chief Meteorologist Mark Nelsen

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