100 Degree Heat Again? Looks Likely

August 13, 2012

This is the warmest week of summer (as mentioned on Friday’s post), and it’s looking very likely that we’re headed for the hottest two days of summer late this week.

We reach 100 degrees about 60% of summers here in Portland, and we’ve done it once this year already.  I think it’s quite likely we will hit or exceed 100 degrees on Thursday and/or Friday.  The perfect setup is there on the maps:

  • Very warm upper-level ridge (hot air mass) moves overhead Wednesday-Friday.  The actual temperature at 5,000′ only warms 10-12 degrees from the “cooler” weather of today and tomorrow, but it’ll warm more than that down here in the lowlands as you’ll see below.  850mb temperatures are forecast at +23 to +25.  They rarely get warmer than that, even for 100 degree days.
  • On Wednesday, a strong (very Fall-like) surface high pressure slides down into the Northern Rockies and Northern Plains behind a departing upper-level trough.  It’s a shot of cool air and taste of Fall for them.  But this quickly turns surface wind offshore (easterly) later Wednesday through at least Friday evening.  This should be the strongest offshore flow we’ve seen since late Spring.
  • The inverted surface “heat low” sets up right on the coastline Thursday, which means extreme heat will spread to the Coast Thursday and possibly Friday, more likely just Thursday.
  • The combination of hot upper-level air mass, strong offshore flow, and total sunshine means we’re really going to bake.

How warm could we get Thursday or Friday?

1. Western Valleys, including Portland Metro:  98-104 degrees is definitely a possibility now.  For sure up to 98, but I think we’ll see some low-mid 100s.  I don’t think we’ll be near our 107 all-time record because upper-level heights aren’t high enough.  I’d prefer to see +25 to +26 degrees at 850mb. and 592+ heights at 500mb for that.

2. Oregon Coast:  85 guaranteed at least one day (Thursday) even on the central Coast this time.  95-100 is possible on the North Coast that day!  Not sure about Friday, warm for sure, but maybe not hot.  You know just a quick wind direction change to west or southwest and suddenly it’s 70 degrees out there.

By the way, the records for Thursday and Friday are 102 and 105 here in Portland, a bit hard to beat.  That was from a heat wave in 1977. 

I’m not working this week until Saturday, it’s comp time for some of those 12 weekend and holiday days I work when the rest of you are relaxing at home.  You know, President’s Day, Memorial Day, a snowy weekend or two in January etc…do I sound defensive about those days off?

Chief Meteorologist Mark Nelsen