Hottest September Start In Years; Late Season Heat On The Way

August 31, 2020

8pm Monday…

Meteorological summer ends tonight in the Northern Hemisphere. That’s June/July/August. For much of the hemisphere those are the three warmest months of the year. The longest days are in late June plus the strongest sunshine of the year. The sun in September definitely doesn’t have the same “bite” as June and July!

Each September is different in our area. Some years the 80s and 90s keep going through early-mid September. In others (last year), “summer” just suddenly comes to an end. September was wet last year as the fall rains arrived early. We only made it to the 80 degree mark ONE DAY after the 6th of September. This September (at least the first half) promises to be much different.


  • A long spell of warm to hot weather begins Tuesday across the Pacific Northwest. It will last for at least 8 days, maybe longer. Expect mainly or all sunny days during that time.
  • There’s no sign of rain through at least the 8-10th of the month
  • Labor Day Weekend will be the hottest 3 day holiday weekend of the summer. It’s possible we approach 100 degrees Sunday and/or Monday west of the Cascades (not on the coastline).
  • Tuesday through Saturday those high temperatures range from warm to hot, but not record-breaking “crazy hot”. Our current 7 day forecast numbers look like this.

This has happened in the past. Back in 2011 we saw 9 days at/above 87 degrees in the first 10 days of the month. In 2017 we had 5 days at/above 90 in those first 10 days of the month, part of it filled with thick smoke and record warm nights due to the Eagle Creek Fire in the Gorge. Then in 1944 Portland saw 7 days at/above 90 degrees in those first 10 days. So it’s rare, but sometimes we do get a late season hot period. Good for your tomatoes, peppers, & squash, not great for sleeping.

Why are temps going to soar? Take a look at the 500mb map for today. Upper-level ridging to the west while a cool trough is slipping by to the east.

The result was our coolest day in almost three weeks…75 in Portland…nice.

That ridge moves right in over the West Coast, looking about like this by Thursday.

A sharp-looking ridge like that typically gives us some offshore flow and that will be the case on Thursday. Add in very warm 850mb temps (temperature around 5,000′ elevation) and that’s why I pushed highs into the lower 90s on that day. Then by Friday night that ridge has weakened a bit and upper-level heights are a bit lower. We could conceivably stay below 90 either Friday or Saturday for that reason, but I decided to keep temps right around 90 based on those very warm/hot 850mb temps. Models are keeping us in the 20-23 degree (Celsius) range Wednesday through Saturday.

You may noticed something else going on, another ridge is popping up just to the west of us Friday night. This one is going to be a real monster, check out the heights by Labor Day! A 597dm ridge over us, or just offshore, is quite rare. The maximum 500mb height over Salem based on many decades of atmospheric soundings is 598dm; this is close! It’s going to be a hot Labor Day Weekend…bit of an understatement there.

BTW, the colors represent the anomaly; you can see how unusual the ridge is up there in the Gulf of Alaska.

At this point (Sunday/Monday) models are in very good agreement showing 850mb temps around +26 to +27. That’s very close to the all-time record of +28 over Salem. If it was late July or early August, we’d be up in the 105+ range for a high temp in Portland. But it’s early September, and everything has to be just right (or wrong) for us to get to 100. In fact we’ve never gone above 100 after September 5th (PDX) or 7th (Downtown). That’s because nights are getting so long, sun angle is lowering quickly = it’s tougher to fully mix down the hot air overhead. The exact orientation of that upper-level ridge will determine whether we get easterly wind Sunday/Monday. Right now models are implying that will happen. That’s why I’ve got a record-breaking or record-tying numbers in the forecast for both Sunday and Monday.

Beyond Labor Day, models all agree the ridge sticks around for at least a few days, possibly much longer. The ECMWF model shows a full 11 days of very warm to hot weather through the 11th. These numbers are actually the ensemble average; the average of all 51 ensemble members

GFS ensembles are similar…

We will see how this all plays out, but prepare for a hot start to September!

Chief Meteorologist Mark Nelsen

10th Consecutive Warm/Hot August Wraps Up Soon

August 27, 2020

10pm Thursday…

Today was another fantastic day…as the most consistent/stable weather pattern of the entire warm season continues. Each of the past 8 days in Portland we’ve topped out between 78 and 84 degrees. Keep in mind a typical high temp this time of year is 80-81 degrees; this is about as normal as it gets folks!

Humidity has been low, skies have been mainly sunny, & nights have cooled well down into the 50s. Many would say we are having a perfect summer weather pattern. That’s because hot high pressure has stayed to the south, allowing just enough onshore flow to keep us comfortable

Somehow my pool has remained in the mid-upper 70s, even with lows in 40s and highs only 75-80 out there. Must be the constant sunshine each day. We are living through another warmer-than-average August.

Does it seems like late summers have been consistently warm lately? Sure enough, Portland hasn’t seen a cool August since 2010! Although 2011/2013 were only a little above average. Of course the Portland/Vancouver metro area has an increasingly warm “urban heat island” at night which leads to ever-increasing nighttime temps under clear skies. But daytime highs (a more reliable indicator of a warming climate in a city) have been steadily marching uphill too. 1940-2020 August high temps at PDX, make sure you click on it for a MUCH better view.

After a slightly warmer day tomorrow, in the 85-88 degree range, we’ll cool off over the weekend with more onshore flow. Then a weak upper-level trough passes nearby Monday

Models say we could get lots of clouds and even a few light showers out of this system late Sunday night and Monday morning. I have a feeling the last day of August will feel more like September. But then things heat up again the first week of September. All models agree a flat, but strong ridge of high pressure in the upper atmosphere parks over the Pacific Northwest. You can see the very warm ridge on Tuesday’s 500mb chart…this is the pretty one we use for TV.

The ECMWF ensembles say summer is coming back the first 7-10 days of September. Check out the rain outlook, little/no chance for rain through the 10th, after the showers this coming Monday

And the surface temp forecast shows 10-15 degrees above average daytime highs. The Euro, GFS, & GEM all hint there could be some sort of heat event during this period. I mean several days at/above 90 degrees. At least hot weather in September is accompanied by much cooler nights than July and early August!

To summarize

  1. Nice summer weather continues through Sunday
  2. Monday is THE ONE DAY in the next 10 we may see some showers
  3. Sunshine and much warmer weather returns the first week of September.
  4. There is no sign of an early start to fall rains and/or cool/wet September weather like we saw last year.

Chief Meteorologist Mark Nelsen

Perfect Late August Warm & Dry Weather Pattern Continues

August 23, 2020

8pm Sunday…

I took a full week off right when the heat wave began last week. 6 days of camping in the Cascades included Diamond & Paulina lakes (excellent!). One night at Still Creek campground too (not as excellent). Of course we ended up right around 100 degrees the first day of that heat wave, then cloud cover from the south kept us in the 90s the 2nd day. Then a third day in the 90s this past Monday before cooler marine air made an appearance. That first day all official observing stations in the metro area hit 100 except Portland & Vancouver. Troutdale was offline; inconvenient timing. Aurora and Eugene both hit 101.

The result is that we’ve just about hit our yearly “allotment” of 90 degree days. One more than last year…so far.

Porltand DOES get more 90 degree weather than in the past. Check out the average for each decade. The 1950s and 60s saw far fewer hot days than the last 20 years.

DECADE# of 90 Degree DaysAvg per year

Today I came into work and realized it was going to be the most boring forecast of the entire summer! This is GREAT news for all of us though; we are in a very stable late summer weather pattern through the foreseeable future. Many of us will consider these last 8 days of the month just about perfect:

  1. ABUNDANT SUNSHINE Models aren’t showing much marine layer west of the Cascades = very little morning cloud cover, if at all
  2. NO HOT WEATHER Just enough onshore flow to keep us in the 78-88 degree range
  3. COOLER NIGHTS Longer late August nights + dry airmass = overnight lows well down into the 50s, hardly a need for air conditioning.
  4. NO FIRE SMOKE All that smoke from California and central/eastern Oregon wildfires remains south of Medford and east of the Cascades

We are seeing a pleasant weather pattern because upper level heights are a bit above normal. Here’s the ECMWF ensemble forecast of 500mb heights for later this week (Friday), note the above average heights (warm colors). Click for a larger view

Next weekend a cool upper-level disturbance drops down just to our east, bringing early Fall temps to the Rockies, but leaving us dry. This is Sunday the 30th.

Notice that new ridge developing just to our west; models show that ridge becoming the dominant feature overhead in the week leading up to Labor Day. Here’s Wednesday the 2nd

Looking WAY out there, to Labor Day Weekend, models show that ridging sticking around; we may yet see additional 90 degree weather in the next 2-3 weeks. Here’s the ECMWF & GEM models for Labor Day, the 7th of September. Depending on your browser, this may or may not show with a “slider” to move between the two.

The main point?

There’s no sign of an early rainy spell as we head into the first few days of September. Warm and dry late summer weather continues until further notice.

We are now regularly producing a weather podcast here at KPTV. The 3rd episode this month was just posted online. This week while I was on vacation, Brian/Anne/Jeff got together. Hear about the “firenado”, record heat, and Anne’s chicken fears… Make sure you subscribe and rate/review if you get a chance.

Chief Meteorologist Mark Nelsen

Heat Wave This Weekend

August 13, 2020

8pm Thursday…

We are entering the final stretch of Summer 2020. Just a little over two weeks left in August. So far this summer has been near normal temperature-wise. Notice no real anomaly (much warmer or cooler than average) since mid-May

Of course the first half of June was very wet, but since that time it’s been VERY dry. PDX has only seen 0.26″ rain in the past 8 weeks! But that’s not unusual and I sure don’t see any sort of real rain in the next 10 days.

We do have at least a four day heat wave on tap.


  1. Expect hot weather Saturday through Tuesday. The first two days will feature high temperatures somewhere between 98-102 degrees in the western valleys of Oregon and extreme SW Washington
  2. Next week looks warm/hot even after we get a “cool down” Monday.
  3. Very warm temps make it to the ocean beaches Saturday and at least part of Sunday.
  4. There’s no sign of thunderstorms west of the Cascades through this stretch
  5. By early next week we will have reached our yearly “allotment” of 90 degree days. That’s 13 days, but about to become 14-15 when new climate normals come out next year. For the 2011-2020 period, we’ve averaged 17 per year! This year we’ve seen one in May, June, six in July, and one in August (so far)

Weather Geek Talk

A strong ridge of high pressure has begun developing over the Western USA. By Sunday/Monday it looks about like this around 18,000′

Models have been trending with a stronger ridge and hotter atmosphere overhead this weekend and beyond. Today there is excellent agreement that over Salem we’ll see 850mb (C) temps jump to +19 tomorrow, +25-26 Saturday PM, +22-24 Sunday PM, then down to +18-20 Monday/Tuesday. I just checked my “August Chart” for 850mb temps. In the 10 year period from 1999-2009, the highest afternoon temp in August was 24.5 degrees. Folks, this is a “top tier” heat event coming up!

Alright, so what about surface wind direction? That becomes very critical from mid-August into fall. No offshore flow and we’re not going to hit 100 degrees. Tomorrow the pressure gradient goes flat across the Cascades/Gorge. No windsurfing at Hood River and calm conditions for that fire out in Mosier. But a “thermal trough” develops along the coastline tomorrow night and Saturday. Here’s a classic sea level pressure map for a heat wave; click for a closer view. Saturday at 5pm

I see 3-4 millibars easterly flow through the Gorge Saturday. Add in the crazy hot atmosphere overhead + all sunshine = we’re headed to/above 100 degrees. In theory we could be as high as 104, but I decided to go with a 102 Saturday, assuming easterly flow won’t be TOO strong. That’s after checking the 2008 and 2016 heat waves that produced 100+ temps in mid-August. You can check out blog posts from each of those events (lower right side of this page). Maybe most interesting is that there’s no reason to panic if it’s only 80-82 at noon Saturday; we can easily jump 20 degrees after noon in this pattern.

Saturday night should be a VERY warm night; possibly only dropping to around 70 well after midnight in the city. Easterly flow goes away Sunday, but with such a warm start + hot atmosphere we should still hit 100 in the afternoon. Of course if we have any significant high cloud cover (some models show that), we won’t hit 100. Here’s 2pm Sunday as the thermal trough is about to push east of the Cascades

Finally, a strong push of marine air should drop us 5-10 degrees Monday; “only” a high around 90 or so. But notice this chart shows 850mb temps (from ECMWF model) remaining above average through most of the next two weeks. Green is the average temp (notice it goes downhill in late August), blue is average of all 51 ensemble members. Each thin line is one ensemble member

Enjoy the cool temps tonight, and try to stay cool this weekend. Here’s our ocean beaches forecast; the left side forecast temp for each day is the central Oregon coast. Right side is up on the north coastline, including Long Beach peninsula. In general it’ll be quite a bit warmer the next few days up around Cannon Beach and Seaside

I’ll be on vacation tomorrow through most of next week so probably no posts until Saturday the 22nd.

Chief Meteorologist Mark Nelsen

How Hot Can It Get? Checking Out Oregon’s Hottest Days Plus A Heat Wave Ahead

August 11, 2020

7pm Tuesday…

Yesterday we hit 90 in Portland, and of course it’s been much cooler today. That was our 8th day at/above 90 this summer so far. Looks like we’ll end up close to normal this year since I see three more 90+ days by next Monday. By the way, the last cool summer? Way back in 2012.

Yesterday also happens to be one of the three times we’ve hit our all-time high in Portland…107 degrees. It happened TWICE during the big 1981 heat wave

and previously in July 1965

The all-time high at Salem and Eugene is 108, and 109 in Roseburg. I remember back in 2009…as we approached that big heat wave in late July I wondered if we were finally going to break Portland’s record. Instead we hit 106 for two consecutive days…a close call!

Wondering what the all-time record high is in Oregon? Most likely it has happened in one of three spots. The three hottest parts of our state are: Rogue Valley and surrounding low elevations in SW Oregon, the lower Columbia Basin in north/northeast Oregon, and the Snake River area right along the Idaho border. Some stations in these areas see an average July day at 90 degrees or a bit higher. You can see these areas (circled) on a PRISM map showing average July high temperatures in the region


Official records show both Pendleton and Prineville hit 119 degrees during a heat wave in late July and early August 1898.

But there has been some meteorological debate over at least one of those numbers. I remember the head of Portland NWS, around 1991 when I started my career, cast doubt on the Prineville reading.

  1. I don’t doubt the Pendleton number much, since that area has seen 114 (July 1928) and 115 (August 1961) during the 122 year span following the record.
  2. It’s unlikely Prineville ever hit 119, Why? In the 122 years since that time, Prineville has only been above 105 on four days! 106 in 1915/1922 and 107 in 1998. Nothing even close to that 119 degree reading, while other stations (like Pendleton) have made it to within a few degrees of their all-time records. For example, during the 1928 and 1961 heatwave (when Pendleton made it to 114/115), Prineville only hit 102. Suspicious for sure. Something strange was going on with the readings in 1898 at Prineville IMHO.

The other numbers seem reasonable; Umatilla is right at the lowest elevation along the Columbia River. Pelton Dam is down in a hot canyon location in north-central Oregon near Warm Springs. Pilot Rock is just south of Pendleton. Monument and Spray are both along the John Day River, near or below the 2,000′ elevation. Monument hit that number twice Beyond these numbers, there are a few places that have hit 115 including: Medford, Arlington, Hermiston, & The Dalles. Keep in mind there were not many weather reporting stations during the 1898 heat wave. In fact before that time there were very few reliable weather records. Umatilla stopped reporting in 1965. Weather records are always a challenge…

Looking ahead, we have this summer’s 2nd heat wave on the horizon; it looks very similar to what we just experienced in late July. A hot ridge of high pressure pops up over the Western USA late Friday through early next week. Models, lead by the ECMWF, are pushing 850mb temps up around +23 to +25 (Celsius) overhead along with some easterly flow over the Cascades and through the Gorge. That’s for Saturday afternoon through Sunday. That’s a recipe for 100 degrees again; or we’re going to be close! Right now our forecast looks like this

We’ll up that to 100 degrees Sunday if models look the same tomorrow. The good news is that extreme heat becomes uncommon after mid-August. So this may be our last heat wave of the season. Weakening sun energy + lower angle + longer nights/shorter days start to take a toll in late August.

Enjoy the cool nights and comfortable days through Friday!

Chief Meteorologist Mark Nelsen

A Few Showers Thursday; After A Very Dry Six Weeks

August 4, 2020

7pm Tuesday…

Time is flying this warm season; our 7 Day Forecast takes us into the 2nd week of August.  September and fall will be here in no time.  After a cool start to July, we ended up just slightly warmer than average and very dry.  Of course the heat wave last week sealed the deal.

Two consecutive days at/above 100 degrees doesn’t happen very often here, last time was three years ago

By the way, in case you’ve wondered about how many 100 degree days we get compared to the past?  Salem has a great long-term record that goes back to the late 1800s. 100 was a rare visitor from 1900 to 1920 and again in the 1950s. It’s a bit more common now; averaging about 1.5 days each year since I was a kid.

July ended up very dry, only .05″ at PDX. The dry season started the third week of June with the “faucet” pretty much just shutting off at that time. Portland has seen less than .10″ since late June (the past six weeks). Compare that with the same six week period the past three years…

What’s ahead? More typical August weather the next 7+ days. For now we have a weak upper-level ridge overhead. That means no hot weather, but warmer than normal.

But a cool upper-level disturbance moves by Thursday, accompanied by a low-level cold front.

You can guess what this means… lots of clouds and a few light showers. It won’t be like spring (or fall) downpours. We’re talking really light stuff. For example the ECMWF model gives the metro area .05″ to .20″. Enough to wet the ground, but it won’t help your yard or garden

Notice very little rain falls south of Salem, east of the Cascade crest, or in the eastern Gorge. This is all about far NW Oregon and SW Washington.

By Friday the trough will have moved east and ridging takes over for the weekend = back to summer! Sunday…

If you are headed into the northern Oregon or southern Washington Cascades, expect showers during the daytime Thursday. Some chilly camping until the sun comes out Friday afternoon. Weekend camping should be just fine

Looking farther ahead, models are trying to bring a bit of a “downturn” a week from now. Not much rain shows up on the ECMWF ensembles for the next two weeks (through mid-month), but it’s not totally dry as we’ve seen the past few weeks.

There doesn’t seem to be any hint of a big heat wave either. The ensemble high/low temps from the same run show temperatures running near normal after the Sunday/Monday warm up.

To summarize:

We’ve got typical early-mid August weather on the way; no significant rain and temperatures near normal. Keep enjoying the summer of 2020!

Chief Meteorologist Mark Nelsen