Urban Flooding Saturday: A Good Forecast, But Still Problems

Noon Monday…

Saturday afternoon was a perfect example of urban flooding. Streets and a few homes/businesses saw flooding in parts of the metro area…apparently mainly in the downtown and eastside areas since almost all the pics/video seems to be from there.  Yet there were no flood warnings for area rivers (none were needed) nor was there any flooding on any sizeable streams in our area.  The NWS did issue an Urban & Small Stream Flood Advisory.  The event was almost perfectly forecast by our models…take a look at our RPM forecast from Saturday morning with the hourly rain accumulation highlighted:

CSqD9zzUYAAkeTq

Sheets of rain from 2-3pm (it was an hour or so late), and for several days models had been advertising this very intense band of rain passing through the metro area in the afternoon.  I didn’t work Friday-Sunday, but highlighted in the middle of the Thursday evening blog post was this:

Capture

8:51am Saturday I posted this on our FOX12WEATHER Facebook page:

Capture2

Seems pretty clear-cut…Portland airport was doused with .67″ rain from 3-4pm, and .50″ in the two hours leading up to it.  Great forecast by models and forecasters.

But you can’t have that much rain within such a short time in urban areas and not expect flooding.  Urban locations are covered in concrete and water can’t do down into the soil.  Plus we had leaves blown off trees that were blocking drains.  I’m pretty sure it wouldn’t have been so bad to see the same thing anytime after mid-November when the leaves are mainly gone.

Notice I keep saying URBAN flooding?  I drove about 12 miles during the peak of the rain (kid shuttle) in far east Gresham and Corbett areas and only saw some small pockets of water here/there.  In areas where there isn’t much concrete it generally isn’t a huge problem unless something is blocking drains.

So a great forecast, but I think communication could have been a bit better.  As forecasters we tend to focus on widespread flooding, when rivers and streams rise and flood large areas.  In the few times around here when it’s obvious that we’re going to see extremely heavy rain in a short period (this time it was obvious) we need to hit that harder and make a clear distinction between the localized urban flooding and the big-time river stuff.

Now of course the elephant in the living room is the missed cutoff time for the rain.  The rain was about 2 hours later to leave on Halloween evening than forecast.  We probably should have told kids to delay 2 hours and most of them in the metro area would have been dry.  Note the 8pm radar image, just about the entire metro area was fine by then:

radar_8pm

This is what that same Saturday morning RPM model was showing for 5pm Halloween evening:

rpm_5p_sat

and the 5pm actual radar image:

radar_5p

Models were too fast with the movement of the main rain band.  Our models are far better than 10-15 years ago.  Back then we would have just said DECREASING RAIN IN THE EVENING, which would have been quite a bit more vague.  Nowadays we’re often able to pin things down to within an hour or two, but by giving these more detailed forecasts we’re opening ourselves up to a miss once in a while.  Too bad it was during the prime Trick-or-Treat hours!

By the way, total rainfall accumulation was a spot-on forecast as well.  Looks like metro area was in the 1.50-3.00″ range and Cascades were 6-12″…not so bad for an extreme rain event!  Also note how well all the mountain rivers handled the rain.  If rivers/reservoirs would have been full already, we would have seen big flood issues!  As expected, the big totals were in the Cascades with lighter amounts in the lower Coast Range.

Rain totals are from 5pm Friday to 5pm Sunday (72 hr totals)

Portland:  2.45″
Troutdale:  2.94″
Hillsboro:  1.65″
Salem:  2.43″

Cascades

June Lk (nr Mt St Helens)  12.00″
North Fork (nr Mult. Falls)  11.40″
Mt. Hood SNOTEL (Tmbrlne Ski Area)  6.60″
Larch Mtn (WA)  6.70″

Coast Range

Saddle Mtn (nr F. Grove)  6.80″

Chief Meteorologist Mark Nelsen

16 Responses to Urban Flooding Saturday: A Good Forecast, But Still Problems

  1. JohnD says:

    Two things:
    Is there a time limit while posting?
    Very frustrating when trying to render a thoughtful post, only to have it time out!

    Secondly, Like a lot of us, I am a lifelong NW Oregonian. We have seen a lot of intense rainfall episodes over time. The one on Saturday seemed exceptional. We were out and about in the city and got caught up in a localized flooding event under an overpass. Our Jetta stalled–but fortunately regained solvency after a few minutes.

    I understand that this was certainly not a 24 hr. rainfall record. But what about a 2–3 hour record? Does anyone have some stats about that?

    Thanks.

  2. Farmer Ted says:

    Friday afternoon through Monday morning rain total 4 3/4 inches.

  3. Boring Oregon says:

    Just looked at the 7-day forecast and it says there’s another wet weekend. Why does the rain always have to happen on the weekends? Why not during the week when we’re all at work and aren’t affected by it? Disappointing I say.

    • Mark Nelsen says:

      Because it’s November and it rains almost every day. So let it be done…

    • Tyler Mode in Battle Ground says:

      I’m off Wednesday 🙂 Perfect!

    • Boring Oregon says:

      Ok yeah, it rains almost everyday. The thing that gets me is there are a few wimpy showers during the week then it POURS starting Saturday and ending Sunday.

    • Get old like me and retire 😉

    • W7ENK says:

      I commute to downtown Portland by bike from Milwaukie. That’s 8-1/2 miles each way, 17 miles a day, 85 miles a week — and I’d rather not ride in the rain. Unpleasant I say! But, it’s inevitable…

      It’s either that, or I pay TriMess to ride their stupid new train. That damm thing more than tripled my commute time from 23 minutes to an hour and 10. I can bike it in 30-35, and it saves me $100 a month.

  4. Paul D says:

    I say you did a fine job on the Forecast, Mark! After all the dry Halloween’s we’ve had the past few years, we were due for a wet one. I took my daughter out at 7:30pm and we stayed dry (except for the dripping trees!).

  5. pappoose in scappoose says:

    Heard a stray wander on by a few minutes ago.

  6. WEATHERDAN says:

    We received our first serious rain around 4:00 PM on Saturday afternoon. Then it poured until around 7:00 PM. Then the rain pretty much came to an end. Perfect timing on the rain. It started just after the end of the Willamette University vs. Lewis & Clark football game. Now on to Winter. Peace.

  7. runrain says:

    Even had a bit of time out on the deck Sunday afternoon in the sun before the next band of showers moved in. Always enjoyable to be able to do that in November.

  8. oenos123 says:

    Mark, Good forecast and hopefully for us, the storm was much more intense up north than the Portland area.
    Also, for information, based on the national building codes, any construction, road and sewage system MUST handle 1.4″ of rain/hour in the PDX area. This storm was well below the standard and still, some people saw some floods. A good remainder that everybody should have an habit to take care of their street.

  9. W7ENK says:

    Overall a great forecast, Mark. In terms of rainfall totals, I think you were pretty much spot on. You even discussed the disparity between model output and reality in terms of projected rainfall early last week, and this setup was a prime example of what you were talking about.

    As for the timing, if it were(almost) any other day of the year, I don’t think anyone would even bat an eye at a 2 hour delay in the rainfall cutoff. Unfortunately, as this situation proved, and as you said earlier, trying to nail things down to an exact hour doesn’t always work out. Just to be constructive, I think when time-dependent outdoor activities are at stake, i.e: taking the kids out trick-or-treating, it might have been better to forecast a broader window of say “between 5 and 9pm”, or perhaps a more detailed explanation of the rain clearing out from the North and West side of town sooner, but with an emphasis on those areas further South and East (in this situation) still being stuck with the rain longer/later than everyone else. Of course, those are things most of us weather geeks already know and can anticipate, but the general public not so much. I heard a lot of people saying “I thought they said this rain was supposed to be gone by now!!”, even in some of the public comments left on your FB posts…

    Either way, the kids still had fun. As a third generation native Oregonian, I’ve just come to expect the occasional wet Hallowe’en. It reminded me of when I was a kid — I can remember trick-or-treating a number of times in the pouring rain. That didn’t stop any of us back then!

    Again, kudos on the great forecast! 😀

  10. A similar amount of rainfall here in Vancouver BC for 72 hours, but I found yesterday to be especially dreary. 1.45 inches on the 1st alone. That’s my highest amount for a day since March 25th. Also quite cool- 46F through the afternoon.

    • W7ENK says:

      Being South of the front pretty much all day on Saturday, it felt ridiculously warm down here in the Portland area. A cloudy and muggy 65F degrees all day, after a low at my place of only 62.2F Friday night… I remarked several times how odd it felt for practically November.

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