Reminder About Posting Comments

November 21, 2011

Lots of new people lately; everyone is welcome to post comments and discuss weather.  Just remember that if you’re a first time user or change location (post from a coffee shop?), your comment gets sent to moderation until I get a chance to approve it.  Generally I am “offline” from midnight to 9am or so, so be patient!

Flooding and Wind Update: Monday Evening

November 21, 2011

Some good news this evening, 12z & 18z models have backed off slightly on forecast rainfall intensity and shifted the band of heavy rainfall to the north.

If this trend continues, the chance for flooding in the western valleys is much lower than previously posted.  Maybe just 2-3″ total at best.  We do get our RPM model output every 3 hours, and the 21z was similar.  It’s always good to be prepared; but right now I wouldn’t worry about anything dramatic flood-wise occurring in our area.

So let’s talk wind.  A fascinating baroclinic zone (boundary between two different airmasses) dropping down the north Oregon Coast tomorrow morning with an unusually sharp cutoff to the south wind on the north side of the front.  For example, it’s possible Astoria will have an almost calm wind in the hours after sunrise while it’s gusting 60-80 mph at Rockaway Beach.  Should be interesting to see it develop.

Here in the western valleys, a windy night tonight (especially after midnight) as the next wave moves along the frontal zone.  Strongest wind should be around daybreak or just beyond with the tightest pressure gradient south to north up the valleys.  How windy?  40-50mph gusts should just about do it.  I’d like to see the isobars oriented more east-west across the valley for peak gusts higher than that.  That would be a better setup for very strong wind with the 70 mph southwest wind surfacing.  We’ll see how that turns out, but 40-50 mph here in the Portland metro area will give us quite a few power outages and some trees down, especially considering how few south wind storms we’ve see the past 4-5 years.

Chief Meteorologist Mark Nelsen

Flooding This Week; Confidence is Growing

November 21, 2011

Thanksgiving Week 2011 might be one to remember…because  the big weather story the next 3 days will be….RAIN and WIND.  Get the sandbags ready!

Forecast models have come into quite good agreement on a significant flooding event for NW Oregon and SW Washington.  Here’s the basic plan:

  • Steady rain commences Monday night and continues through Wednesday morning, then it’ll cutoff.
  • It appears now that heaviest rain will be Tuesday afternoon through Wednesday AM.  That will be the most likely time for urban flooding, when we get water all over the place.
  • Our models show anywhere from 3-6″ in the Valley between Salem and Longview, and up to 10″ in Central/North Coast Range and South Washington Cascades from Mt. St. Helens down to near the Gorge.
  • Strong wind develops on the Coast from Monday night through Tuesday midday, possibly lasting through Wednesday AM south of Newport.  Peak gusts maybe 70-90mph at the most exposed places and 60-70mph in the coastal cities.  Here in the Valley, Monday night through Tuesday midday we could see gusts of 40-50 mph.

After this mornings’ system, it’ll be relatively quiet until later tonight.  So no signficant weather today after the coastal wind dies down. 

I just checked the maps this evening and it’s eerie how little they’ve changed in 24 hours, showing the wettest system we’ve seen in years setting up over Northwest Oregon the next 3 days.   

Two things make it really stand out.

1.  Very strong 850mb winds (4,000′) from the SSW from early Tuesday to early Wednesday (about 24 hours).  Up to 70mph+ right over Portland and stronger along the Central Coast.  This should allow strong wind to surface along the coastline Monday night through midday Tuesday as the front works its way south.   After that it’ll be much lighter at the beaches, or it’ll at least move south of Newport.

2.  At the same time plenty of moisture to work, and that means lots of rain, nothing TOO unusual there.   But something is different with this front; models are showing a very narrow line of extremely heavy rainfall rates (possibly up to .40″ per hour) for several hours gradually dropping south through Northwest Oregon overnight Tuesday into Wednesday morning.  I remember an urban flood event we had within the past two years where lots of intersections and small creeks flooded.  I think those were rainfall ratest of about .20″ for 3-4 hours.  So this is definitely going to cause some trouble.  Imagine what the 70mph  850mb wind running into the south-facing slopes of Silver Star Mountain or the elevated terrain of far northern Clark and Cowlitz counties can do for rainfall?  The WRF-GFS shows up to 10″ of rain in those spots.  Our RPM tends to overdo rainfall totals in some storms; and it shows a large area of 5-6″ rainfall centered on a line from Lincoln City to Mt. St. Helens.  Hard to believe we would see that much in the lowlands, but even 3-4″ will be a big rainstorm.

So, we know the timing of the main rainfall and wind; now is time to prepare.  If you live near a small creek that floods easily, be ready.  The large rivers?  Unlikely anything significant will happen with the Willamette or Columbia because this will only affect part of their basins.  Watch out on the rivers draining the north and central Oregon Coast Range and the Lewis, Cowlitz, and Washougal rivers.  Also, Salmon Creek, Johnson Creek, Tualatin River…we’ll be watching those for sure.  Sandy River might avoid big trouble if the heaviest of the rain stays north of Mt. Hood.

I’ll give another update this evening…Chief Meteorologist Mark Nelsen