Rain Is Back This Evening As A “Wet” September Wraps Up

9pm Sunday…

I was on vacation and missed the big downpours last weekend so I’m happy to see my first rain of the season tonight. I took a much-delayed “Graduation 2020” trip from New York City to Washington (DC), then down to very sweaty/hot Florida with part of the family. It was soooooo nice to fly back into PDX on a clear evening late last week and feel the low humidity. It’s not perfect, but we live in a nice climate don’t we? Of course it’s “pics or it didn’t happen” nowadays, so here you go

Alright, moving on to the rain…models were right on showing a nice day today followed by light evening rain. We hit 75 in Portland today, down about 10 degrees from the very warm mid 80s yesterday. Notice the 60s up in Puget Sound with showers off/on all day

We may not have broken the all-time record for 90 degree days, but Portland has never seen so many 85+ or 80+ degree days. Yesterday was the 90th day at/above 80, a new record

Radar shows one band of rain moving through the I-5 corridor right now and another offshore. That should arrive around 2-4am. This is the leading edge of a cool upper-level trough. It swings through the region Monday night and Tuesday. It’s a pool of cool air dipping south, you can see the lower-than-normal heights (cool colors) on the Monday night 500mb map

Then by Thursday the trough is gone, replaced by weak high pressure and higher than normal heights (warmer colors)

So we have two days of the usual spring/fall showers and sunbreaks pattern. Those are the days when we get sunbreaks, then a downpour 10 minutes later, then sun pops out for 30 minutes, repeat… This is also the setup (Monday and Tuesday) for scattered thunder/hail showers too. I noticed Lifted Index (a measure of instability in the atmosphere) is around zero or below both days along with CAPE up a bit too. A pretty obvious signal for isolated thunderstorms in our climate west of the Cascades. During the event last weekend some areas picked up 3-4″ of rain! This time it’ll be more like 1-2″ in the wetter parts of the metro area near the Cascades, and .50-1.00″ in the lowest elevations. Unfortunately with westerly flow you won’t see much in the lee of the Cascades. Less than one tenth of an inch for some of you in Central and north-central Oregon. The GRAF model gives us these numbers for rain by Tuesday night

Notice the Cascades and Coast Range could get 2″ or so in the wetter locations.

Portland has experienced quite a few “wet” Septembers lately; six consecutive years. Actually two of them were close to normal. Most often the rain has fallen in the 2nd half of the month like it did this year.

And you can see PDX and northern parts of the metro area are running well above the Willamette Valley totals for September rain due to the downpours early Sunday morning last week

Last weekend’s rain put fire season on life support. This rain event will put the “nail in the coffin” for sure. Even after six days of dry and warm weather, those Cascade fires were barely putting out any smoke yesterday/today and we’ve seen almost no increase in acreage burned. This event will finish things off. About 200,000 acres have burned in the Oregon Cascades this summer! Much of it started from those thunderstorms that moved north along the mountains the last few days of July and August 1st-3rd.

Are we entering a days/weeks long rainy period? I don’t think so. We’ll be dry Wednesday, then a weak warm front gives us sprinkles/showers Thursday. By next weekend, heights are rising and the early fall jet stream has been pushed far to the north again. It’s possible we have a very warm first weekend of October on tap.

The ECMWF 24 hour precipitation ensemble chart shows good agreement on dry (circled) Friday through Monday, then possibly some showers return next week at some point. Both GFS and GEM models are similar. This is about as normal as it gets for early October. A little wet, then dry, then a bit of wet again…

SUMMARY

  1. Some nice wetting rain falls the next two days, probably up to 1″ in valleys and 2″ or so in Cascades
  2. Mainly dry Wednesday through NEXT Monday except for some light showers Thursday
  3. A mix of sun and clouds is likely Wednesday and beyond.
  4. Temperatures average near normal the next 7-10 days
  5. I’ve been waiting for a cool/wet day to clean my garage and workshop…it’s time.

Chief Meteorologist Mark Nelsen

34 Responses to Rain Is Back This Evening As A “Wet” September Wraps Up

  1. Opie says:

    I looked up the 15 wettest Septembers at PDX. 11 were followed by drier than average Octobers.

  2. Anonymous says:

    Those 80 degree temps that keep showing up on the KPTV forecast haven’t been supported by the operational runs over the last few days. I wonder where they’re getting those numbers from because as it stands, it doesn’t seem like they’ll verify. 75 maybe, but 80? Neither Euro or GFS have shown those numbers for me.

    • Zach says:

      Agreed. I think they are overdoing it, and mid 70s are more realistic.

      • Anomynous says:

        Wishful thinking from Mark. He always likes throwing very warm and sunny days at the end of the 7-day, especially when the short to mid range is awful and rainy and way too Pacific Northwest like. We all need some California to look forward to at the end of the week right? 😃 🌞

        • Anonymous says:

          I don’t think that’s fair to Mark given he’s been hoping for rain activity for months now. I doubt it’s based on personal bias, but more likely he saw strengthening high pressure or something. I guess that’s my real nitpick here though: I wish they’d elaborate on what’s driving those numbers.

          Rod Hill has Monday listed at 83 degrees too lol

  3. tim says:

    The pnw is having a very wet September which is not normal, growing up as a kid we had to wait until november before the rainy season started but now it’s September I suspect October and November will be dryer then normal, even mark mention how unusual it is.

  4. Ken in Wood Village says:

    The NWS issued another severe thunderstorm warning on that storm and I’m watching the 8 o’clock news and they are reporting that there could have been a possibility of a tornado with that storm. They said they have a video but are trying to upload it to show it.

  5. Zach says:

    This storm has exceeded my expectations far more than the first one a week or so ago.

  6. Ken in Wood Village says:

    There is a storm heading to Yacolt that looks like it has a tornado hook signature (at least it looks like it). My radar velocity shows different wind directions 🤔

  7. Snomanski says:

    We westsiders don’t usually get the best stuff, but a heavy line just trained through, bringing about .70 in the last 2 hours. 1.49 total since 8 or so last night. Bull Mountain hits the bulls-eye. To crib from Josh, a virtuoso weather performance. A break in the action now.

    • Anonymous says:

      Yup, in LO and got hammered over the past couple of hours. From right around when they sent the flash flood warning out to about 10 minutes ago.

      • Joshua Lake Oswego says:

        I am too far SW in LO to have gotten any of the heavy showers except for the one at 7pm or so. Radar peeping was very frustrating.

        But, I work downtown and saw heavy shower after heavy shower this afternoon. Right in the bullseye.

        Today definitely over-performed except for the far western metro area.

        Let’s see if we can get in on some of the juicy action on Thursday. Could miss us altogether to the north. Looking pretty dry for a week to 10 days after that.

        • tim says:

          I’m not gonna complain about a week of dry weather after all this rain since its still September the real fall weather is yet to come hopefully.

  8. Ken in Wood Village says:

    I’m looking at the radar and I’m seeing some intense showers (maybe thunderstorms soon) from Newberg, NE to Troutdale. I wouldn’t be surprised to see hail with it too.

  9. Zach says:

    PDX averages about 1.5″ of rain in Sept. We are going to at least double that this year.

  10. Roland Derksen says:

    A total amount of 1.2 inches of rain here for the last system- that’s a bit lower than was forecasted, but it makes this September my second wettest in 46 years of observation- and with 3 more days left of rain, we’re in a good position to see a record high wet September.

  11. Anonymous says:

    Wonderful south wind and light rain here on the south coast of Oregon tonight.

  12. Anonymous says:

    Some pretty steady rain right now, looking forward to the next couple days of it.

  13. Anonymous says:

    Welcome back Mark! I’m always looking forward to your next blog, but I’m glad you get some good vacation time here and there.

  14. Michael says:

    Thanks so very much Mark. Always enjoy reading what you have to say.

  15. W7ENK says:

    Rain finally got underway in Milwaukie about 20-30 minutes ago. Was out cleaning up the yard right up until the first drops!

    It smells so good outside… ☺️

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