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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

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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

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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

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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

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

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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. an 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

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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. 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.

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And you can see PDX plus north part of the metro area is running well above the Willamette Valley totals for September rain due to the downpours early Sunday morning last week

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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.

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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.

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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…

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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

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