Forecast Disaster Possible: Next Weekend

May 30, 2011

Will it really get to 85 degrees this upcoming weekend?  Is that possible, considering we haven’t even hit 75 yet?  Let me explain.

The very last thing a forecaster in the Pacific Northwest wants to do is predict the warmest and sunniest weekend of the year so far, especially after a cold and wet spring, and have it turn into a cloudy and cool weekend.  This ranks up there even higher than a busted snow forecast since we are all in a delicate emotional state about the 3 month long cold/wet spell.

So I walk in today and see the potential for 80+ degree temps Saturday and 75+ temps Friday and Sunday.  The reason isn’t quite what you would expect.  There is no big huge ridge of high pressure developing over the West Coast.  Instead, another upper-level low (with it’s cool weather and showers) dives south towards us.  That’s nothing new the last few weeks.  However, this time around, models are insisting it’ll stay off the coastline and park itself off the California coast Saturday.  You can see it on the 12z ECMWF, 12z GFS, and 12z GEM (Canadian) models here…all 3 upper-level maps are for 5am Saturday morning.   Click on each for a view you can actually read.  Note they are all extremely similar and all are from the same “initialization” time:

The key is the location of the upper-low.  Note the upper-level winds are then easterly over Washington and Oregon, causing pressures to fall west of the Cascades.  A nice setup for easterly flow and sunshine.  Combine that with 850mb temps up around +15 C and we could easily hit 84-87 degrees here in Portland on Saturday.  It only occurs for one day, but the easterly flow actually starts Friday afternoon and ends early Sunday, so upper 70s would be possible on each “side” of that day.

Here’s the problem…5 days ahead of time we have to assume models are correct with location of the upper-low and the pressure pattern. to get those temperatures.  If the low is a bit closer, right over us, or to our west, forget about real warm weather.  Thus, the high potential for a change in the forecast.  A real pain.  So we decided to keep highs in the 70s on our 7 Day forecast until models are more clear, or at least continue to show the exact same pattern over successive model runs.  Even that is better than we’ve seen so far this season.

In the short term, an active pattern the next two days with southerly or southeasterly flow bringing moisture up over us for showers.  Our RPM model shows a cluster of showers turning up into NW Oregon overnight, leaving us with a very wet start to Tuesday.

On a far happier note:  here are the wedding photos from Brian MacMillan and Sophie Soong.  They both got married Sunday (but to different people…just want to clear that up).  Those are some good looking people!


Chief Meteorologist Mark Nelsen