9pm Sunday...

What a nice early fall weekend; partly cloudy (& dry) sky plus comfortable temperatures. I drove through a couple spots of drizzle midday on the way to work, but it was not measurable. Yesterday we were a few notches above average; today a few below.

Highs Today DMA.png

The next two days will be similar with more sunshine than cloud cover. Wednesday a weak/dying cold front moves overhead. The main result will be cloudier skies (like today) PLUS spots of morning drizzle or a shower. Thursday should be uneventful as well. So other than a shower/drizzle chance Wednesday morning, not much happens through Thursday and your life will be un-affected by any real weather. Then on Friday things change…rain arrives (yes, real rain) and we may have 4 wet days on tap.

HIGHLIGHTS

  • The endless dry days we’ve seen for 3 months (since mid-June) come to an end this coming weekend
  • Expect whatever you have outside to get soaked Friday through NEXT Monday…around 1.00″ rain in the lowlands
  • This won’t help the drought much, but at least the top layer of soil will finally moisten up
  • All Cascade wildfires will see chilly temps, high humidity, and a good soaking.
  • After next weekend, Fire Season 2021 will be on life-support over/west of the Cascades; we are going to avoid a long episode of smoke and huge fires west of the Cascades this year. A good ending considering how bad things were looking by late June (drought + driest spring + extreme heat). A lack of lightning for most of August plus a cooldown late that month was a game changer.

The reason for a cooler/wet forecast next weekend?

Our first strong upper-level trough of the season. That means a significant dip in the strengthening early fall jet stream. Cool air pushing farther south than it has the past few months, spinning up several disturbances on the boundary with the warmer air. You can see the relatively flat flow now…no hot ridge of high pressure nearby

Mark Jet Stream.png

But check out next weekend. Our typical wet/cool weather pattern in both spring and fall

Mark Jet Stream2.png

How confident am I that this is a real pattern change? Just one example…ALL of the 12z ECMWF 24-hour precipitation ensemble members produce some sort of significant rain Friday-Monday. That’s unusually high confidence. If only half of them were showing this, I’d be a bit more skeptical. Also notice about half of these members bring in more rain as we head toward the end of September

euroens24hrpcp.png

Ensemble forecast temps also show we are headed into cooler weather

euroenstemps.png

 

How much rain?

The operational high resolution runs of the GEM, ECMWF, & GFS produce anywhere from 0.70″ to 1.20″ during this time in the western valleys (Portland, Vancouver, Salem). In this case it’s better to use the averages from the ensembles…which are in good agreement… Somewhere around 1.00″ or so in the lowlands of NW Oregon and SW Washington. Not a drought-buster, but enough to moisten things up quite a bit.

You’ll notice in the comments above I’m quite optimistic about putting this fire season partially to rest. That’s because those ensembles also push 2-3″ rainfall into the Cascades, right over those 5 fires burning from north of Detroit down to east of Grants Pass. That’s enough to eliminate most smoke from those fires. That’s why I’m feeling good about that last few weeks of this fire season. Last year just under 2 million acres burned on the Oregon and Washington landscape. About a million of that was in the one week after Labor Day (easterly windstorm leading to mega-fires). So far, we’re at about 1.4 million this year. We will likely end up with less acreage burned this year, even with a hotter summer and much drier spring conditions. By the way, last year on this date we were in the middle of that incredible 8-11 day stretch (depending on location) that featured the worst air quality we had ever seen in NW Oregon and SW Washington. Here are the numbers that we will be avoiding this year

Air Quality PDX September 2020.png

That’s it for now, I just wanted to give a "heads up" about the wet weather arriving later in the week. I’ll plan on another blog post midweek with an update. 

Chief Meteorologist Mark Nelsen

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