Another Cool Summer?

July 18, 2012

Let’s talk long-term weather, then discuss some possible fun action tomorrow evening/night.

First, it’s a pregnant woman’s dream, or so I’ve been told by a co-worker.  Long range models show average or cooler than average temps will continue most likely through the rest of the month.  A 90 degree temperature in Portland appears unlikely during the next 2 weeks.  If so, it’s going to be the first time in PDX history that we haven’t hit 90 through the end of July for 2 consecutive years:

The reason?  We had 10 days of warm ridging nearby; that began on the 4th of July.  Now it appears upper-level troughing is going to linger closer to us (or overhead) for the next 10+ days.  At this point I don’t see an excessively cloudy and chilly pattern, but definitely one that will keep the heat away.  When those upper-level troughs linger nearby, most of the time that maintains a thicker than average cool marine layer; basically the cold Pacific Ocean dominates our weather a bit more than in a “regular” summer.  Get used to lots of 70-82 degree high temps; many would say those temperatures are just perfect.  But this also likely means more gray than we are used to during this normally sunniest time of year.  Models are in very good agreement on the general pattern.  Check out the 850mb temperature ensemble charts from the 18z GFS:

and the 12z ECMWF:

Both models, and their ensemble members, have a chilly upper low just to our north Sunday and Monday, the recover to slightly above average much of next week, then below again after a week from now.

Now lets talk short-term.  Some severe thunderstorm action today, but only over Union County, around La Grande.  Reports of golfball size hail and even a funnel cloud spotted too. 

Real quiet west of the Cascades with just one or two weak storms over the Cascades.  West of the mountains we had a very deep marine layer that kept us socked in low clouds all day long.

Tonight and tomorrow the upper low offshore moves slightly farther offshore, allowing warmer air aloft to move in along with weak easterly flow above about 3,000′.  This should break up the marine layer and bring us a much warmer day (80-85 degrees).

The the fun might begin, all models show the upper low suddenly moving north tomorrow evening through Friday morning.  The strong diffluence in the upper atmosphere on the northeast side of the low (over us) tends to promote rising motions in the atmosphere below it.   Not unlike a wood stove or fireplace starting much quicker when a breeze or wind is blowing over the top of the chimney, pulling air up through it.  The WRF-GFS is showing a large are of storms and showers developing starting tomorrow evening and into the night.  This COULD be a nice little overnight thunderstorm outbreak.  Now our RPM has the action and upper low moving slightly farther east.  It produces nice storms in a line from about Madras to Mt. Rainier.  I trust the WRF-GFS a bit more.  Either way SOMEONE in the Pacific Northwest is going to get a nice light show tomorrow night!

Chief Meteorologist Mark Nelsen