Flood Update: La Nina Making Up for Lost Time?

What a mess this morning, not here in the metro area, but down in the Willamette Valley, here are the highlights:

  1. Heavy rain is ending west of the Cascades, now it’s light showers through tomorrow afternoon
  2. It doesn’t matter, the damage has already been done rain-wise
  3. A few rivers have major flooding, most have minor
  4. The smaller creeks and streams and localized flooding are a much larger issue today

Check out the 3-day rain totals:

Portland: 2.61″
Salem: 6.71″
Corvallis: 5.93″
Eugene: 5.00″

Now you can see why almost all the flooding issues are down in the valley!

Here are the rivers with MAJOR flood warnings from the Northwest River Forecast Center:

ALSEA RIVER
LUCKIAMUTE RIVER
MARYS RIVER (Record Flooding)

Rivers with MINOR flood warnings

JOHNSON CREEK
CLACKAMAS RIVER
MOLALLA RIVER
S. YAMHILL RIVER
PUDDING RIVER
SANTIAM RIVER

Something that sticks out here, all the rivers draining the Cascades either have no flooding or minor flooding.  Minor flooding means just a foot or so above flood stage by the way.  There’s a good reason for that.  The flood control reservoirs (Detroit, Green Peter, Cougar etc…) are all less than one quarter of capacity!  Some have very little in them.  Check out the reservoir “teacup” diagram:

So no major flooding expected on the Willamette River itself.  I see Detroit Dam and Fern Ridge are holding back about 90% of their river’s flow…pretty impressive!  So where is all the water coming from?  It’s from the Coast Range (no dams there) and the water falling in the foothills and valley itself.  6″ of rain falling over the wide, flat expanse of the Willamette Valley plus the foothills (dams don’t capture that water) is enough to cause the widespread flooding down there.  So yes, it’s possible to have 1996 style flooding in Willamette Valley towns but not have major river flooding at the same time.

Up here in the Portland Metro Area, other than ares of water from this morning’s downpour, I don’t foresee any signficant flooding issues showing up in the next 24-36 hours.  But we are still under a Flood Watch.

Meteorologically interesting this morning as a surface low has moved onshore a bit north of the forecast.  It’s moving right into the Portland Metro right now and will just die overhead!  Strong south wind to the south of it over Salem  just compounding the flooding.  To the east, a very thin layer of that cold arctic air is bringing mainly freezing rain to the central and eastern Gorge.  That will come to an end only with the precipitation tapering off.  The cold air isn’t going anywhere this evening; no west wind to scour it out.

Two more things to talk about:

1. Possible strong south wind tomorrow night.  A deep low tracks towards SE Alaska tomorrow evening, but models are definitely hinting at a secondary low or at least “triple point” moving by just to our north.  Interesting to note that both MM5-NAM, WRF-GFS, and our RPM show the tightest southerly gradient right around midnight-2am tomorrow night.   The WRF-GFS would suggest gusts 40-50 mph since it shows 60 mph wind just a thousand feet or two above the surface.

2. Long range:  Possible heavy rain on snow event next Tuesday-Thursday, timing and amount of rain uncertain.  Not good of course.

3. Longer range: Models have definitely been trending towards colder flow/troughing over the West Coast about one week from now onwards:  Check out the last two ensemble 850mb charts from GFS and ECMWF. 

The signal is there, especially on the GFS.  But they both have us back in the “close to arctic blast, low elevation snow possible” pattern.  We’ll keep an eye on it.

Chief Meteorologist Mark Nelsen

104 Responses to Flood Update: La Nina Making Up for Lost Time?

  1. chiefWright says:

    A very interesting “big picture” analysis in the NWS this morning:

    A VERY ATYPICAL PATTERN THIS MORNING WITH A WESTWARD MOVING COLD FRONT MOVING THROUGH JAPAN AS A PART OF A LARGER COLD REGION THAT ENCOMPASSES MUCH OF ALASKA. WHY IS THIS GOING TO BE IMPORTANT FOR OUR WEATHER OVER THE NEXT WEEK? WELL FIRST OF ALL IT SETS UP A VERY LARGE BAROCLINIC ZONE EXTENDING OVER MUCH OF THE PACIFIC…INTENSIFIES THE WEST TO EAST MOVING JET…AND CONSTRICTS THE FLOW TO ROCKET MOISTURE LADEN SYSTEMS QUICKLY ACROSS THE PACIFIC. THUS IN MY OPINION THE REASON WHY WE ARE A SUCCESSION OF VERY WET SYSTEMS IN CLOSE PROXIMITY TO ONE ANOTHER. THE ATMOSPHERE ALWAYS TRIES TO REMAIN IN EQUILIBRIUM AND WITH THIS STARK CONTRAST IN AIR MASSES…THE WAY IT ACHIEVES THIS IS TO GENERATE THESE LARGE AND ENERGETIC SYSTEMS TO REMOVE THE ENERGY.

  2. David B says:

    Meanwhile, up in Seattle, it really looks like our winter storm doesn’t have much longer to live. Not only have surface winds from the north died since this afternoon, Cliff Mass just blogged that the profiler at Sand Point shows the cold layer rapidly being eroded by mild south winds aloft.

    I rather suspect I will awaken some time in the wee hours of the coming morning to the sound of dripping water outside.

  3. i think its the east wind thats just keeping it pretty cool around here!

  4. IN troutdale temps look like they might be cool enough for snow overnight!!! currently at 36 degrees it could snow and stick at 34 which is not far from 36!!! WHAT DO U GUYS THINK???

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