10am Update: Rain Too Far South

November 22, 2011

Everything is going MOSTLY according to plan this morning; models did an excellent job showing the front moving down the Coast and cutting off wind north of about Tillamook.

Big problem with the placement of the main rain band.  Here’s the problem, the front (and center of heaviest rain) has been about 50 miles farther south than expected, leaving the heaviest rain band directly over the top of the Metro area.  In 3 hours (ending at 10am) we’ve received 3/4 inch of rainfall here in town.  Another 2 hours of this and we’ll start seeing flooding.  This afternoon the band of heavy rain SHOULD lift north.  If the rain doesn’t back off by noon, WE will be the hotspot for flooding, not up north.  There is no other populated area seeing more rain than we are seeing here.

 COASTAL WIND: Coastal wind is raging south of about Pacific City.  Peak gusts were 60-70 mph most spots, little less than I expected.  The wind did go just about calm from Tillamook up to Astoria; that’s why we sent our television crews to Lincoln City.  Brian MacMillan getting blown away was a great choice for the morning show.  It’ll continue to blow all day long; more of the same.  Probably not any stronger than what we’ve already seen.

METRO WIND:  Hasn’t been too strong (over 40 mph) since the front has been a little closer to us than expected.  So it may pick up some this afternoon as the front lifts north.  Gusts 40-50 still possible.  A few power outages still likely here and there.  Then it’ll go calm late tonight or tomorrow morning as the front finally pushes through.

Chief Meteorologist Mark Nelsen


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

Flooding…Is It Finally Our Turn?

November 20, 2011

Just a moment or two to blog this morning, but what a set of weather maps to wake up to!  For several days we’ve been expecting a frontal zone to set up right over the Pacific Northwest Monday-Wednesday with rain, wind, and mild temps.

Now the last few runs of most of the models have shown this setting up directly over NW Oregon instead of Western Washington.  In fact all the flooding events in the Pacific Northwest the past few winters have been just to our north.  Well now they are showing the flooding rain zone directly over us.  I couldn’t believe my eyes when I saw the 12z RPM rain totals.  A large swath of 6″ plus totals not only in the Coast Range but Salem to Longview in the lowlands get 4″+ as well.  The 12z WRF-GFS shows 5″+ as well in the same general area.   If this actually occurs, it’ll be our most significant flood threat in many years in the Northern Willamette Valley.  I’ll post again late this evening (real late) on my strong wind thoughts as well.

Chief Meteorologist Mark Nelsen

Snow Showers Tonight

November 18, 2011

Some excitement across the Portland Metro Area this evening as the band of heavy showers moved through.  Sticking snow on the hills, cooling temps, chunky rain even in the lowest spots…meteorological winter has arrived apparently.

The good news is that the coldest atmosphere of this event is moving in now through tomorrow morning.  The bad news is that precipitation intensity doesn’t look too high the next few hours. 

The earlier models had shown the surface low coming inland almost right over us.  The brand new 00z NAM and our 00z RPM both show the low either over us or to our south overnight.  Keeping most precipitation down in the Valley.  Due to the movement of the low, the south wind is going to totally cutoff very soon.  It’s already gone light east here at the station.  That always helps to get snow a bit lower.  But our RPM shows little or no precipitation after about 11pm.

So the rest of the night through tomorrow AM is really going to be about watching the radar.  If you get a heavy shower to pass right over you, maybe a dusting?  Sure feels like December out there with the chilly air and snowflakes mixed in!

Chief Meteorologist Mark Nelsen

A Cold Friday; Big Snow in Cascades

November 18, 2011

Mountain snow has been even heavier than expected; Automated sensors show about 22″ new at Mt. Hood Meadows since yesterday morning and possibly 26″ up at Timberline!  That’s 2 feet already with more to go! 

The snow level did drop a bit further overnight, and the freezing level right now is around 1800′ over Portland based on the top of our transmission tower (1900′) at 31.8 degrees this morning.  That’s a very chilly airmass.  When you get heavy showers at that temp, sticking snow can push a good 1,000′ lower.   In fact take a look at the 10am ODOT cameras at Shorty’s Corner (1200′) just east of Sandy and Quartz Creek (1,100′) in the Coast Range.  Both had or have snow on the side of the road. 

For the rest of today, sticking snow should generally remain well above 1,000′ with the minimal daytime heating.  But anyone could see flakes mixed in the rain or just plain flakes with such a chilly atmosphere.  There are no travel concerns in the lower elevations through this evening here in Western Oregon.

Tonight there is a little “fly in the ointment” as they say.  First thing I noticed on the satellite looper this morning is that low pressure center offshore is farther south than models showed it placed.  It’s a minor but important detail. 

I’ll be at work a bit early this afternoon, eagerly awaiting our 18z RPM run to see if it shows the low coming inland farther south as the two UW mesoscale models show.  More later…

Chief Meteorologist Mark Nelsen