October Begins Cool, & Water Year Ends

September 30, 2018

6:30pm Sunday…

It’s been a real quiet weather weekend with just a few sprinkles/showers yesterday and today. Officially we’ve had all of…wait for it…  .01″ in Portland.  That’s weekend total.  I know a few of you had more, but overall boring for weather geeks like me.

Tonight (the end of September) wraps up the “Water Year”.  If you’re new to the Pacific Northwest there’s a very good chance you’ve never heard such a phrase.  Here’s the deal…our rainy season is late October through March; during the cool season.  As you know, we have a very dry warm season here compared to areas east of the Rockies.  That’s May through September.  All that winter rain (in the ground) and snowmelt (coming down rivers) is used to sustain our lives/properties/crops through the dry season.  So it’s important to know how wet our rainy season is each year.  But each rainy season crosses from the end of one calendar year into the first part of the next calendar year.  So for the purposes of ranking our wet seasons, we use a “water year”.  That year begins on October 1st and ends September 30th.  Make sense?

How did we do this year?  We ended up drier than average by 6″or so.  Here’s a nifty little chart from the folks over at Portland NWS.  Winter rainfall was just about normal until we hit February.  But that month and March were drier.  Then a very wet first half of April brought us back to close to average again.  And you know the rest of the story…we’ve pretty much flat-lined since that time.


Comparing this water year to recent years shows we had two very wet “wet seasons” and now we’ve experienced a dry one.  You can see the big year to year variability.

Rain WaterYearPDX

The water year gauge “resets” at midnight and we begin a brand new year.

Here’s what I see for this first week of October:

  1. Next few days feature a splitty flow with a southern system moving into California giving them a much needed rain.  We get a weak system dragging through here tomorrow night and Tuesday morning; sprinkles/showers at most during that time
  2. A large upper-level ridge wants to develop out in the Gulf of Alaska (just to our west) later this week and through early next week.  This means a real lack of storminess in the eastern Pacific but also quite chilly air (for early October) just to our east.  Notice all 3 big models (GEM/ECMWF/GFS) show the same pattern for NEXT Monday, the 8th…one week from tomorrow

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A slight move westward and we’re into an unusually chilly (early frost?) pattern all across the Pacific Northwest.  A slight move east or a weaker ridge (ECMWF) would mean cloudier, but mild and occasionally wet weather.   We’ll see how it works out and how much rain we get next weekend.  ECMWF was somewhat wet so I went with that model.

3. No sign of a stormy October weather pattern.  With a large ridge out there we’re sure not going to see an extra-early start to the Pacific storm season here on the West Coast.

4.  Cool temps, or at least cooler than average.  We’re done with 80 degree temps and at this rate there’s no reason we couldn’t be done with 75 too.  Check out those ECMWF ensemble forecast highs the next two weeks…pretty cool this weekend and early next week


So the next week will be cooler than average with more showers toward the weekend but not a “start of the wet season”.

Chief Meteorologist Mark Nelsen

Beautiful Weather; Last of the 80s?

September 26, 2018

6pm Wednesday

Today has been another fantastic fall day.  A chilly start with lows in the 40s and lower 50s, then all areas west of the Cascades in Oregon made it well into the 80s.  Check out those lower 70s on the coastline as well

Todays Observed Highs OrWa 2017

I saw someone mention somewhere on social media that this is summer weatherwise.  Most definitely not.  The long nights and low sun angle give a totally different feel.  It gets warm between about 2 & 6pm in this pattern, but cool during the lengthening nights, mornings, and evenings.  Even here in the city it was still only 69 at noon.

Todays Observed Temps in Portland 3D City Earth Scene 2017_2

Strong high pressure centered to our west over the Eastern Pacific remains in place through Friday and that guarantees us two more days above 80 degrees.  Then lowering upper-level heights plus a strong onshore push drop us 10-15 degrees Saturday for a more typical late September day.

So is this the last of the 80 degree temps?  It could be.  Check out the past few years…specifically the last date we hit 80 each of those years.

Last 80 Degree Day Heatwave

The average for Portland (in any year) is September 29th.  Last year we hit our last 80 on this date (the 26th).  After this Friday there’s no sign we’ll hit 80 in the following week, although the ECMWF ensemble forecast highs for Portland do hint at warming again late next week.


That said, models have not been doing real well handling a split-flow setup Sunday and beyond so I have low confidence beginning that day.  I’m not a fan of putting question marks on the 7 Day forecast (and we don’t do that here) but early next week would be the time to use them!

Here’s a plot I use regularly…the ECMWF 850mb temperature ensemble plot.  This is temperature in celsius up around 5,000′.  Each thin line represents one of the 51 members of the “ensemble forecast system”.  Basically each line is one run of the model and it’s done 51 times every 12 hours.  The blue line is the actual “operational” model run and has a bit higher resolution.  Red line is the average of all the ensemble members.  You can probably guess what the yellow/green line is…climatology.  Temperatures up around 5,000′ drop quickly the first two weeks of October.  You can glean a bit of info from this chart; I noticed two items today.

tseries_850t_000-360_Portland (1)

  1. Good agreement among ensembles (all the lines close together) through Sunday…the 30th, then they are all over the place.
  2. Temperatures are well above average through Saturday AM, then plunge to below/near average after that time.  Looks like we’ll be all done with the 80s down here at sea level after Friday.
  3. BUT, look at that range the middle of next week!  That could give us a high of 85 or as cool as 58, best to just keep the 68-70 for now until the solutions come together.

By the way, today is the 71st day we’ve hit 80 this year in Portland

80 Degree Days Yearly

As we all know, the past 5 warm seasons have been VERY warm; quite a contrast with those much cooler years 2010-2012.

Chief Meteorologist Mark Nelsen

A Warm & Sunny Week Ahead

September 23, 2018

6pm Sunday…

It’s been a pleasant early fall day outside but somewhat chilly.  Portland only hit 68 which ties for the coolest day so far this season.  The average for this date is 74.

We have a lot of sunshine ahead for this work week as very strong high pressure builds just west of the Pacific Northwest.  Here’s the 500 millibar height forecast (in pretty TV colors) for midweek.  Heights get up to around 588dm, that’s summertime warm.
Jet Stream Forecast 2017

850mb temps don’t reach “heatwave” levels though, topping out around +16 to +17 Tuesday-Thursday.  This time of year we CAN hit 90 or even higher, although it’s rare.  In those cases those 850mb temps get up above +20.  We will get some light offshore flow Tuesday-Wednesday, maybe 2-4 millibars worth of easterly wind through the Gorge those days.  Not exactly a big start to the east wind season, that’ll come later in October.  That plus the warm atmosphere overhead should be enough to push temperatures west of the Cascades into the low-mid 80s.  Models seem to be running a little cool for some reason with surface temps.  Notice the ECMWF shows highs only around 80, this is the EPS (ECMWF ensemble system)….


You can see a pretty clear end to the warm weather next weekend too.  As a cold upper-level trough drops out of Canada next weekend, another wave moves east across the eastern Pacific and merges with it…looking like this by next Sunday

Jet Stream Forecast 2017_b

That’s the GFS, but the ECMWF is similar.  This is the setup we get in winter when we go from cold northerly flow quickly to mild southwest flow.  Of course the jet stream, cold air, and moisture flow is all far less dramatic this time of year.  So we’ll go from very warm weather Friday to cool and somewhat showery by Sunday/Monday.  It appears October may start cool.

The operational run of the ECMWF gives us a ton of rain the first few days of October, but the ensembles say hold on…not quite as wet.  0.60″ instead of 2.50″ as you can see on the ensemble plot (blue is operational run, green is ensemble average)


So enjoy the warm sunshine this week, because it may be the last time we see several (or any) 80 degree days.  Even in September in this “cool-ish” pattern we have gone two weeks without hitting that number.  A cool pattern in early October means even low-mid 70s will be hard to do.

By the way, a quick update on fire season.  Fire danger WILL go up west of the Cascades this week with dry east wind so be careful with any burning.  Of course it’s not going to dry out like it does in summer but a fire CAN spread in the weather Tuesday-Friday.  There are only 3 large fires left burning in Oregon.  All are showing minimal behavior/spread.  The two on the map below west and NE of Grant’s Pass both started during that lightning storm on July 15th…still going 2.5 months later!

Mark Fire All Oregon

Chief Meteorologist Mark Nelsen

A Cool Weekend, Then Warmer Weather Ahead

September 20, 2018

7:00pm Thursday…

Today has been another very nice early fall day with temperatures reaching the lower 70s.  Here’s the late afternoon view from our camera looking over the Tilicum & Ross Island bridges


Weather the past 11 days has been amazingly consistent; cooler than normal.  Every day has been below average; topping out between 68 & 72 degrees.  Typically in September we see more up & down movement.  For example very warm days in the upper 70s and 80s and then quick drops into the 60s.

High Temp Last 13 Days

Tomorrow should be even a bit warmer, likely reaching the mid-upper 70s. That’s due to increasing southerly wind just ahead of an approaching cold front.  So enjoy a warm and mainly sunny Friday.  A few light showers arrive around sunset in the western valleys of Oregon and SW Washington; but it’s a weak front so don’t expect a soaking.  A few light showers may pop up Saturday, but Sunday looks dry.  Overall kind of a slow weather weekend; not much happening and no one should get soaked either.

Now about that warmer weather ahead; remember 5 days ago I posted that we were in a cool pattern for the next 10 days.  Apparently it will stop at around 8 days.  It’s pretty obvious that we’re going back into a warmer pattern starting Monday.  You can see the cool upper-level trough over us this weekend

Jet Stream Forecast 2017

then the big ridge coming back next week

Jet Stream Forecast 2017_2

This is the classic early fall warm weather pattern that many of us like.  Lots of sunny days, along with warm temps.  Bright blue skies since the fire smoke is long gone too.  This time of year nights are so long that we drop well down into the 50s and 40s at night so there’s no need for air conditioning.

Here’s a glance at the 24 hour rainfall forecast from the ECMWF ensemble.  Each horizontal line on the upper chart is one ensemble member.  The bottom chart is the average of all those members.  There appears to be almost no chance for showers from the 24th-28th (next week)


How warm will we get next week?  We know 850mb temps will reach into the +15 or higher range.  That can put us into the lower 80s in late September.  But to get warmer than that (closer to 90) we would need a bit warmer temps overhead plus easterly offshore flow with a thermal trough.  The GFS indicates that will happen at some point next week but ECMWF is keeping the ridging a little to our west.  That doesn’t give us a significant offshore flow; you can see the effect on the model surface temp forecast:


In a nod to the EURO I didn’t go wild on high temps, keeping them around 80 or so most of next week.

By the way, mark your calendar for SATURDAY, OCTOBER 27TH.  It’s the 26th annual WINTER WEATHER FORECAST CONFERENCE at OMSI.  It’ll be starting at 10am.  You can see last year’s conference presentations here:  https://oregonams.wordpress.com/2017/10/28/presentations-25th-annual-winter-weather-conference/ 


Chief Meteorologist Mark Nelsen


First Cool Month Since March; Feels Like October!

September 16, 2018

9pm Sunday…

It’s been a very quick switch to not just early Autumn weather the past week, but it feels more like October with highs struggling to reach 70 some days.

Month Climate Temps Calendar

We’re running slightly below average for the month with no sign of warmer weather over the next 10 days.  That tells me we could actually end up with a cooler-than-average month.  That hasn’t happened since March.  And the rain is starting to (slowly) add up; we picked up another .35″ in the city today for a monthly total of 1.55″

Rainfall so far this month_compared to normal

That puts us above our typical September rain of 1.47″.

There were people worried in mid-summer that “Summer would continue into November” or it “would never rain again”.  Always keep in mind with weather that we have short-term memory.  The rain ALWAYS comes back.  And temperatures ALWAYS cool in the fall.  Even after the hottest summers we cool off.

We had a few heavy showers around the metro area earlier this afternoon but they are gone now.  I think we’ll be dry through Thursday, although a weak system might give us a sprinkle Wednesday night.  Otherwise our next wet system appears Friday.

It’s been much cooler this month due to an upper-level “dip” or trough in the jet stream.  You can see it’s still there on the GFS ensembles showing upper-level heights on Wednesday


The cool colors represent below average 500 millibar heights.  Then the same model for next Sunday shows the continuing cool pattern.


The ECMWF is similar and doesn’t show the upper-level trough dissipating until at least 10 days from now.  See the ECMWF ensembles showing the trough just starting to leave.


Very interesting since we were dominated by higher than average upper-level heights from late April through mid August.  I’ve always found it fascinating how a “switch can flip” and we go from one weather regime to another.  Typically it’s that late spring wet-to-dry switch.  But sometimes it goes the other way too and that appears to be the case for Autumn 2018.

Enjoy the 3-4 days of dry weather this week!

Chief Meteorologist Mark Nelsen

Fire Season 2018 Winds Down…A Lucky Year in NW Oregon & SW Washington

September 13, 2018

5pm Thursday…

(First a quick note about our website & apps:  Thanks for all your patience over the past week as we switched providers for our website and apps.  It was a bit rough but I think everything is now back to normal on all platforms.  Note that on the website we have a new menu; “current conditions” on the left side; it has a few new images…check that one out.)


This has been a tough fire season again across the West.  Although some spots have been spared in our region.

In Oregon there have been two “hot spots”through the fire season.  That’s the numerous lightning-caused fires across SW Oregon that have burned for two months.  And then several very large fires (some from lightning, some human-caused) across north-central Oregon.

Assuming some huge new fire doesn’t break out in the next few weeks, (unlikely looking ahead weatherwise) I think we are well past the big danger period for this season.

Who got lucky in our area?

There were no major fires across most of NE Oregon, the northern Oregon Cascades, all of NW Oregon, and all of SW Washington.  As of this moment total acreage burned across the Pacific Northwest is significantly DOWN from last year as well…around 700,000 acres vs 1,100,000 or so last year.

Considering late spring and early summer was the driest or 2nd driest on record in our area I think we were very lucky.  It’s most likely due to lack of lightning.  For a second season it seems we had very little thunderstorm action both in the mountains and down here in the lowlands.  I don’t have the stats to back that up (yet), but I’ll find them eventually.

Fire Fuel Moisture Oregon2

The showers and cooler weather most of the time since late August has helped tremendously in the Cascades.  Check out the Log Creek “1000 hr fuel” moisture level this summer:  the black line is this year, yellow is last summer.   Blue is the average…you notice the woods in that area are typically driest in late August and then moisture increases in September.

Fire Fuel Moisture Oregon

This year we were running exceptionally dry through the third week of August, but then some showers brought us back up to average for that time of year.  Then you see showers the past few days have brought fuel moisture back ABOVE average for early September.  With more showers through the weekend and again later next week I have no reason to believe those fuels will dry out again.  Notice that huge dump of rain last September that put an end to the fire season.

To summarize:

  1. Much of Oregon remains dry, but cooler temperatures have lowered fire danger even in those spots
  2. Fire Season 2018 is winding down quickly west of and on the west slopes of the Cascades.  It’s not over yet, but the chance of large fires developing is going downhill.
  3. For the 2nd consecutive year fire season is ending a bit early; I doubt anyone is complaining!

As for weather…looking ahead I see a continuation of below-average temperatures through next week but Monday through Thursday should be dry.  Note the ECMWF ensemble high temperature forecast looks like early October


I love this graphic…the 24 hr accumulated precipitation product from the ECMWF ensembles.   You can glean lots of information quickly.  First the bottom section.  That’s the ensemble “average”.  You clearly see the best chance for significant rain is centered right on Sunday.  Looks like every single ensemble member (each is a horizontal line above) shows a tenth of an inch or more.  Maybe 20% give PDX more than 0.50″.  Then there is good agreement on a dry spell Monday through at least Wednesday and likely into Thursday as well.  We’ll dry out a bit.  But some decent hints also that late next week and the following weekend could be showery.  Good meteorological stuff!


Enjoy the mainly dry weather Friday, I think there will just be a few light showers popping up late in the day…many of us stay dry.

Chief Meteorologist Mark Nelsen

Wettest Day Since Early April In Portland

September 12, 2018

11pm Wednesday…

What a soaker for the central part of the metro area this evening!  Here’s the radar image from 6:20pm as thunderstorms were dumping huge amounts of water on inner NE and SE Portland.

KPTV 2017 Default Earth

Officially at PDX it’s the wettest day since April 7th and we also broke a record for the day

Rain Record PDX

Of course this time of year the record daily rainfall isn’t that hard to break…the old record was only 0.23″.  But wait, apparently even heavier rain fell between the official gauges.  There were several stations in inner east Portland that showed more than 1″ of rain; that would include the early morning rain too.

Rain Portland HYDRA Gauges.png

Tomorrow’s showers should be much more tame with most action over the Coast and Coast Range.

Chief Meteorologist Mark Nelsen