Still On Track For Drying Halloween Evening

October 29, 2015

10pm Thursday…

The next 45 or so hours are going to be a real soaker across the region.  Models are in unusually good agreement that we’ll see a strongly “orographic” rain event.  That means the mountains will be favored for the heaviest rain, even more so than normal.  Take a look at our RPM rain forecast and then the UW WRF-GFS accumulated rain forecast:



In general the mountain totals will be around 3-5 times what we see in the lowlands!

By Sunday evening, I expect:

  • 1.50-2.50″ rain in the western valleys of Oregon
  • 2.00-3.50″ from Longview up through Olympia in the valleys
  • 5-10″ in the Cascades and Coast Ranges.  The usual very heavy rainfall areas in the southwest Washington Cascades could see 10-12″.

This shouldn’t produce any significant river flooding since reservoirs are pretty much empty and rivers are very low.  I see Detroit Lake is lower than at any time last winter!  The first few inches will be soaked up by the dry ground too.

There WILL be some local/urban flooding Saturday midday/afternoon as models are showing a very intense band of rain with the cold front.

Check out our RPM midday Saturday:


Whew!  That’s intense…not a good time to be having an outside party.

Then check out the 5pm image:


This is even faster than model runs yesterday.  The WRF-GFS is not quite as fast, showing the rain just leaving the metro area around 5pm.

Either way I think we’re still in good shape in the metro area for Halloween

Behind this system I see a colder airmass with some snow in the mountains.  This isn’t a really cold airmass; at best we get a dusting or 1″ at Gov’t Camp by Monday morning.  But higher up around 5-6,000′ we’ll get a good dumping, the first of the season.  I could see a foot up around 7,000′.  That’ll be it for another 5-7 days though.


Enjoy the weekend, I’ve been working a bunch of weekends this fall so I’m taking tomorrow off.  No blog postings until Monday unless I get real inspired to do it from home.

Chief Meteorologist Mark Nelsen

There is HOPE for Halloween Evening! Dry For Some

October 28, 2015

A glimmer of good news is starting to emerge from our numerical weather models for Halloween evening!

As far as I recall, we have not had a “washout” (pouring rain) during the critical Trick-or-Treat time since at least 2001.  That’s when I started taking my kids; in fact I never went candy collecting with the kids in anything other than a sprinkle.  I do remember a washout with strong wind and rain in 1994 and then it turned to snow in Sandy behind the front that year.

All that really matters is the 5pm to 8pm period that evening.  For anyone under age 10 it’s pretty much over by that time.  So it’s not like we need the whole day dry.

A cold front will be traversing north to south across our viewing area from afternoon through the evening.  Under the front it’ll be pouring anywhere from the Cascades west.  What has changed is that models seem to be speeding up the movement a bit, which means the steady/heavy rain will likely come to an abrupt end first up north and eventually late in the evening farther south.

Take a look at the 12z ECMWF 6 hour rain forecast from 11am to 5pm Saturday:


That means the total ENDING at 5pm.  This says the heaviest rain is just arriving in the metro area around 4-5pm.  Now the 5pm-11pm total:


During that period a lot of rain from Portland to Albany (and along the north-central coastline).  But notice there is very little rain from about Tillamook north along the coast and north of Portland.  The current run of this model says the little kids will be in great shape on the far north coast and north of the metro area; wet elsewhere.  The previous run of this same model was a bit farther north (by about 20 miles), so the trend is a bit faster…very good.

How about another model…the GFS?  Even better news.  Here’s the 5pm-11pm rainfall:


Look at that!  Pretty much dry from Pacific City north along the coast and north of Woodburn along I-5.  This run says the main rain has finished by 5pm, especially west and north metro areas.  Very good.

The UW’s model (WRF-GFS), which is based of the GFS model shows something similar:


Mainly dry once the main cold frontal band passes by overhead.

To summarize:

If model trends continue, kids in the metro area could again escape with a mainly dry Halloween or at least just showers.  There is a better chance for dry conditions the farther north you go.  Salem and Albany still look wet for now.

It’s still 3 days away so it’ll be perfectly normal to see some movement in that main rain band forecast as we get closer.

Stay tuned!


Chief Meteorologist Mark Nelsen

A Great Blog Post By Cliff Mass

October 27, 2015

The current state of the “Climate Change” discussion is a mess; at least that’s my opinion.

Here are the problems:

  • A scientific issue has somehow become a political issue
  • Advocacy groups routinely claim almost any extreme weather event is caused by global warming (not true)
  • Skeptics claim humans have nothing to do with the warming, or deny there is any warming (not true)
  • American public isn’t too concerned even after a constant drumbeat of negative news related to CC (polls show this)

I have no interest in doing a long posting related to global warming/climate change, but I sure found a GREAT piece by Professor Cliff Mass today.  I highly recommend you read it.

His main argument is this:  It’s going to warm up quite a bit more, so we’d better adapt!  I think it’s a realistic view, not a doomsday view.  We’ve “made our bed” so now lets figure out how to live in a warmer world.

What do YOU think about his posting?


Chief Meteorologist Mark Nelsen

Be Careful When Reading Modeled Rain Forecasts

October 26, 2015

I see this statement (or something like it) winter after winter…“the models were showing X rain this weekend and we only got X much…they suck!” .  More than 50% of the time I think it’s misinterpretation of the model output.

Take a look at the 00z WRF-GFS output this evening for total rain ending Sunday morning.  This is the 36km resolution model:


Definitely a big soaker on the way!  Looks like Somewhere between 2.50-5.00″ at PDX right?  Wow, that could cause some flooding. Salem maybe 2.50″ as well since it’s right on the line between pink and black.  Likely over 5″ in the Southwest Washington Cascades.  Now take a look at the 12km version of the same model:


What?  It says 2.50″ at PDX, less at Hillsboro, and a bit more at Troutdale.  Salem is about the same or a little less than 2.50″.  Now take a look at the GFS model output on our broadcast software:MarkRain_GFS_ECMWF_Compare

That is extreme!  Looks like PDX could easily see 6-7″ here, Maybe 4″ out in Hillsboro and a crazy 8″ at Troutdale and Battle Ground right?  Nope.  Here is what I estimate from looking at each of these 4 models this evening:


Why so low?  Two things are going on:

  1.  In a coarser resolution model, the space between data points is larger.  That means in areas where terrain changes quickly (PDX to Larch Mountain WA anyone???), the amounts are going to be unrealistically high near the rising terrain and often shifted a bit to the west of where it really is.  I’ve seen that on our RPM model many times.  So you generally want to find the highest resolution output you can.  That also applies to going to a place like IGES and looking only at output for PDX.  It’s always too wet in a strong orographic flow (like what is coming this weekend).
  2. Specifically in the case of the ECMWF and GFS we get on our system at FOX12, WSI (our vendor for wx graphics) only gives us grids spaced 100 kilometers apart!  Yikes.  That means there’s only a point to contour from every 60 miles…really bad.  There may be one grid point in the Cascades and another over the western valley for example.  The contouring program is just averaging between those two points…pure interpolation.  Thus the inaccurate extreme amounts seen over and east of I-5.  Our RPM that goes out to 72 hours is 12 km resolution so that’s much better, like the 12 km WRF-GFS run above.

So how did I get those numbers I used on-air?  I generally look for the driest part of the northern Willamette Valley , even if it appears to be way over at Hillsboro or Forest Grove on a map.  That works well in these orographic events.  Keep in mind this applies to meteograms produced by picking a specific point as well.  Here’s the PDX 00z GFS meteogram:


But Hillsboro is far more reasonable…this is what I would expect in most of the metro area:


Of course almost nothing I’ve written applies in showery weather patterns or precipitation coming from a different direction, but our big heavy rain/flood events are almost always under westerly flow.

By the way, yes, we’re going to get a soaking this weekend, but I don’t think 6″ or more in the valleys will be happening.  The Cascades will get a real pounding with 5-10″ possible.  Luckily rivers are low and the ground is dry…the first 3-5″ should soak in nicely.

Chief Meteorologist Mark Nelsen

Blob Update

October 26, 2015

Lots of talk the past 2 years about unusually warm water in the northeast Pacific.  It has been named “The Blob”.  I’ve seen some discussion on different forums recently that it has gone away.  Yes and no.  The positioning of warmer than normal sea surface temps has changed, but there is still a LOT of “warm” water offshore.  Note a month ago:


Warmest water relative to normal is way out there…looks “blobish”.  But big picture is warmer than normal water across all areas to our west.

Now the current image:

sst now

It has cooled quite a bit way out there, but the anomaly has strengthened within the 500 miles of the coastline…more reds showing up.  If you want marginal snow events here this winter you’ll want that to go away!  Plus that has to have some effect on our weather as it has in the past two winters.  Very “Warm PDO” look.

That’s all.  Maybe a posting later on a soaking coming this weekend and first mountain snow (to the passes) about a week from now.

Chief Meteorologist Mark Nelsen

Strong Wind Gorge Now, Then This Afternoon In Valley

October 25, 2015

8am Sunday…

Looks like some real windy fall weather coming up today.  I’m not working, but with morning coffee I see two things going on:

  1.  Strongest east wind of the season so far is howling through the Gorge.  Over 8 millibars pressure gradient is “cold season worthy”.  It’s flowing from high pressure east of the Cascades to an approaching surface low off the central Oregon coastline.  Peak gusts likely 60-70 at Vista House and 50-60 in the windier areas around Cape Horn and Corbett through the morning hours.  Then it’ll go calm as a south wind arrives around noon or so.  Which leads us to…
  2. Gusty south wind for all of us this afternoon.  South wind gusts 25-40 mph will likely cause a few outages 3-7pm in the metro area and the valley.

A surface low is cozying up against the coast this morning and moves into western Washington this evening.  Note the mid-afternoon (2pm) surface map from the 12z NAM (latest model available).  You can see the pressure gradient turning southerly south of Salem at that time


Then at 7pm the low is weakening, but in a perfect position to cause a rush of southerly wind up (technically down) the valley


Not a big windstorm by any means, but enough to remind us November is only a week away!  In fact I noticed the cross section from the WRF-GFS last night has that “winter look”.

By that I mean strong easterly flow, then southerly flow, then back to strong easterly again Tuesday.  My son has his last high school soccer game in Corbett Tuesday afternoon…looks like that ball is going to have a hard time getting into the goal when they are kicking into the east end!

Rain will be arriving for most of us soon, enjoy that and the refreshing fall breezes this afternoon.  Oh, and possibly a thunderstorm with the convective showers mid-late afternoon as well.

Chief Meteorologist Mark Nelsen

A Record Warm October Again? Probably Not, but Close

October 22, 2015

Since we now have only 9 days left in the month, I figure it’s time to take a look and see how we are running compared to last October.  You know, the warmest on record here in Portland.


In fact it was the warmest on record in all of Western Washington and most of Western Oregon.  There’s NO WAY we would do that two years in a row…right?  Well, it’s going to be close but I think it’s most likely this will go down as the 2nd warmest.

So far the entire West is running well above normal this month with the core of the warm anomaly over the Intermountain region and Rockies.


Last year PDX had an average October temperature of 60.1 degrees.  Again, that was the warmest October on record at PDX.

SO FAR (through the 22nd) this month is running at 61.5 degrees; that’s 1.4 degrees warmer than last year’s record!

Seems like a slam dunk right?  Well, that’s with the coolest 9 days of the month to go.  Or at least the average is coolest at the end of the month.  I see two possibilities

1. If we were to have 9 perfectly normal days to end the month

Temperature would be 58.6 degrees…A tie for 2nd warmest on record.

2. If our 7 Day Forecast is correct plus I add two “normal” days at the end

Temperature would be 59.8 degrees…2nd warmest on record

Considering my forecast if anything seems a little warm (warm-biased) I think it’s unlikely that we’re even warmer than that.

7 Day

Chief Meteorologist Mark Nelsen