A Brief Hot Spell Saturday & Sunday

August 2, 2012

We haven’t been above 88 degrees this summer in Portland; and July only averaged 78 for a high (cooler than average).  That’s going to make the sudden onset of heat this weekend somewhat dramatic; although this will just be a brief intermission in another cool summer (so far).

The past couple weeks have seen no significant changes in the weather; just below average temperatures with more morning cloud cover than normal for late July/early August.  Here are the highlights:

  • Friday will be much warmer and sunny, but definitely not hot
  • Saturday will be on the one scorching hot day we haven’t seen so far this summer west of the mountains
  • Saturday will  be 95-99 in the western valleys
  • Saturday will be the ONE real warm day at the coast; 85-90 on the North Coast
  • Slightly cooler Sunday, but still 90 and more humid; it’ll feel the same
  • Back to cooler Monday and beyond
  • No rain in sight…a dry summer so far

After today’s weak weather system slides by to our east tonight, we’ll see high pressure in the lowest 5,000′ or so in the atmosphere develop over the northern Rockies.  When that happens we get easterly flow across the Cascades.  Air flowing across the mountains produces a downwind area of low pressure at the surface that develops west of the mountains.  We call that a thermal trough in this case because air then flows from the east at the surface too; thus an easterly or offshore wind through the lowest elevations of the Pacific Northwest.  It’s most noticeable in the lower gaps of the Cascades and Coast Range (near Columbia and other rivers).  There’s always been a bit of a chicken vs. egg debate on the thermal trough.  But a nice paper/talk at last winter’s Pacific Northwest Weather Conference showed that the easterly flow over the Cascades is what produces the “Heat Low” or thermal trough west of the mountains.  The presenter had run the WRF-GFS with several different versions of terrain and found that with flat terrain, there is no heat low.  In fact with no Cascades or Coast Range the next few days, we’d just see a northerly wind tomorrow switching to an east, then southeast, then southwest wind.  Or something along that line.  The mountains intensify our warm spells or hot weather.

Of course most of the public doesn’t care about this; it just means an abrupt change to hot weather.  It starts Friday with light easterly flow through the Gorge the entire day and 850mb temps climbing up around +15 by afternoon (warm by this summer’s standards). 

Then check out Saturday’s surface map from the MM5-NAM:

It’s shows the most well-developed thermal trough we’ve seen since late last summer.  Several millibars easterly flow across the Cascades, wing gusts at Vista House will be up around 40+ mph.  Add around a +22 to +25 850mb temp, and we should see a high between 95-99 degrees.  That’s a good 10 degrees warmer than anything we have seen so far this summer.  It also appears the Coast will get in on the action for 1 day.  Highs of 85-90 are possible Saturday on the North Coast (north of Lincoln City).

Do you hate real hot weather?  Don’t worry, as I mentioned before it won’t last long.  In fact we lose the offshore flow by Sunday afternoon and we’re back to cooler than average temps next Tuesday or Wednesday with strong onshore flow again.  Take a look at the roller-coaster  850 mb temps on the ECMWF 850mb chart, from +11 this afternoon (cool trough) to +25 Saturday afternoon (100?).  Then back to +11 by next Wednesday!  That’s a 25 degree change at 5,000′ from this afternoon to Saturday afternoon.  Great time to be at a Cascade lake.  We just spent 3 days up at Olallie Lake; perfectly clear (no low clouds), but nighttime temps were well down into the 40s.  This weekend will be significantly warmer up there.


I’ll be back at work Saturday afternoon.

Chief Meteorologist Mark Nelsen