Warm Summer Nights & Improving Weekend Outlook

August 28, 2013

I’ve heard a few complaints this summer.  Not about the hot days (there haven’t been many), but the warm and humid nights.  A good reason for this…3 times we’ve seen a humid air mass overhead and/or light showers in the area for several days at a time.  It happened just before the 4th of July, a couple of weeks ago, and now it’s happening again this evening.  Dewpoints are way up in the 60s again.   Tonight and two weeks ago we have been in a pattern with moist flow coming up from the south on the back side of an offshore trough of low pressure.  It’s sitting just close enough to give us moisture, but not close enough to give us a big marine push with the cooler air.

Take a look at the 60 degree or warmer nights over the past 8 years:


We will end up quite close to June-August of 2009.  Of course that summer saw tons of hot days, but not much of the humid weather.  Brian Schmit put together this chart showing that this August will end up #2 for 60 degree nights here in Portland since 1981.


The urban heat island effect is more dramatic the last few decades due to the growth of the Portland Metro Area, but it seems to me we’ve had more of these “humid nights” the 2nd decade of my career compared to the first.  I can’t believe that this fall I will have been doing television here in Portland for 20 years!  Add on 2 years of non-television forecasting before that and I’m becoming an old man…22 years in meteorology.

Thursday could be a real interesting day.  Lots of tropical moisture, good lifting as the upper-low offshore kicks out, and some afternoon sunbreaks.  I think we’ll see thunderstorms around that could be real juicy.  00z WRF-GFS and our RPM are both showing that.  They are also showing some “training” of shower bands later tonight and early in the morning over our area.  That’s when a line of showers moves parallel to the upper-flow, allowing one shower after another to move over the same area.  With such a juicy atmosphere overhead, SOMEONE IS GOING TO GET SOAKED.  Not sure exactly where that will be.  We’ll know within 12 hours.

Beyond tomorrow, the big story is the 2nd upper-level low’s movement this weekend.  Once again models have slowed things down.  Plus now they are showing some offshore flow for Saturday = hot for the last day of August.  Labor Day Weekend is looking better and better!

Chief Meteorologist Mark Nelsen