After one heavy rain event during the 3rd weekend of September, the Pacific Northwest is currently wrapping up a second wet spell.  This one was not quite as wet for most of the Portland metro area; however, it was definitely more “stormy” as weather nerds would define it.  We had a particularly massive downpour yesterday (Monday) from a squall line that moved through the area around 4pm.  I was working indoors when it happened; but could hear the heavy raindrops on the store’s roof.

Later in the evening, a strong thunder cell developed in northern Clark County near Battle Ground.  Several people on the local WX discussion boards mentioned that it had some of the “hook” appearance that tornado-producing cells often produce.  Sure enough, around 7pm a tornado touched down on the outskirts of town.  Fellow weather observer Tyler Mode caught some excellent footage of the event, including this video clip of the rotating cloud.  Not to mention some damage reports the following morning!  He mentioned that he’s never seen a tornado in person before; quite the treat to get one just a couple miles from his home!

Most of our Pacific Northwest tornadoes occur during spring and fall, much like the south-central U.S.  These are the two times of year that we have cool northern airmasses interacting with a strong sun angle…the result of which tends to be more instability in the atmosphere. Spring and autumn showers, especially the quick and heavy ones, are usually caused by puffy cumulonimbus clouds.  Steady winter rain, on the other hand, generally is associated with gray and gloomy nimbostratus instead.

Unlike the Midwest and South, however, our tornadoes are pitifully weak.  So weak, in fact, that many of them would probably go unreported were it not for smartphones, social media, etc.  One infamous exception was the April 5, 1972 Vancouver tornado.  It was classified as EF-3, which is a pretty typical “Midwest-type” storm.  It killed 6 people and injured hundreds.

Rain totals for the past 2 days are not as heavy as the previous rainstorm.  We didn’t have the kind of steady/heavy overnight totals we did a week ago.  But the entire metro area still picked up between 0.5″ and 1.5″ of rainfall, which is pretty impressive for a single event in September.

That will bring the September rain total for PDX to at least 3.6″, putting it slightly behind 2019 but still far behind the record-setting 5.62″ from 2013.  We may see a little more rain either tomorrow, or more likely Thursday, before we turn the calendar into October.

Tomorrow…we have a weak warm front passing overhead for lots of clouds, but not much rain if any.  There’s a slightly better chance for rain from the cold front on Thursday, though still not extremely soggy for Portland.  The heaviest rain will be confined to Vancouver Island and the Olympic Peninsula.

As we begin October, we are expected to return to a sunny and ridgy pattern along the West Coast for a few days.  Earlier model forecasts showed light to moderate offshore flow developing, which would have meant a weak “thermal trough” for Western Oregon and high temps in Portland likely hitting the low 80s.  But more recent runs have taken most of the easterly flow away and cooled the airmass a little bit, so we might stay in the mid/upper 70s instead.  Keep in mind that the sun angle is back down to early March levels now, AND the lower atmosphere is saturated with a lot of moisture from the heavy rain.  Both of these increase the likelihood of fog in the lowlands as we move into the weekend; fortunately, we’re not quite to the time of year when these inversions allow cool air to remain trapped in the valleys all day long.

As we progress through the month of October, two things happen:  the odds of wet weather rapidly increase, and the normal daily temperature drops more quickly than any other month. In fact it makes some sense to think of Early October and Late October as two separate months, meteorologically speaking. Even if rain doesn’t arrive on schedule during the month, the dry days become much less warm in late October as the continental airmass cools rapidly with the long nights and weaker sun.  Cool inversions also become a bigger factor in valleys and basins.  The same warm upper-level pattern that would push us to 85 degrees at the very beginning of October, for  example, might only get us to 65 by the time Halloween rolls around.

Enjoy the fog and sun this weekend! We are already seeing things green up considerably outside, and the warm weather coming up should really give that feeling of a ‘second spring’ as new growth takes advantage of nearly 4″ of rainwater.  -Karl