The sunshine and warm (not hot!) temperatures have been nice this month.
But now it’s getting a bit weird. Where’s the rain? So far only .05″ rain in Portland, but take a look at how wet our recent Septembers have been
Most of the region has been relatively dry, but the southern half of Oregon HAS seen a nice soaking the past 3-4 days due to an upper level low sending clouds/rain through California and a bit farther north. Look at the areas that have picked up at least 1/2″ the last 3 days
Now in most years the bulk of our first soaking falls in the 2nd half of the month. Take a look at the increase in rain chances as we go from August to November. A bump up in mid-September with about a 30% chance that any one day at mid-month will give us measurable rain. Then you see a 2nd jump in late October. By November we are IN the rainy season with a peak occurring in late November and early December. During that time about 70% of days (on average) will feature measurable rain; a drastic change from that 5-6% chance in early August!
Another way to look at this is: WHEN DO WE GET OUR FIRST “SOAKING” OF THE SEASON?
Let’s consider 1/2″ rain our first soaking at PDX. Notice in the graphic below that in many years it happens in mid-September. For some reason September 17th/18th has been popular lately! Not this year of course. Back in 2012 and 2011 (not on the graphic) there was no soaking until early October. In fact in just under 1 in 5 years that happens. So although it’s not unheard of to wait until October for real rain, it IS not a common occurrence. Then, every couple of decades, the rains REALLY hold off. By that I mean in 4 years (since 1940), Portland didn’t pick up 1/2″ rain until the 2nd half of October. I remember 1987…no soaking until Halloween and the days following. So I suppose the good news is that the rain ALWAYS arrives at some point; it always has by Halloween at the latest.
The big message I’m seeing on all our modeling is that there’s no soaking in sight; at least through the end of September. Looking at the big picture, this loop of 500mb heights goes from now through the first day or so of October. It’s the 5 day average anomaly…warm colors are higher than normal and cool colors show below normal heights. Notice ridging is dominant along the West Coast through the 5 day period ending around the 3rd of October. That’s about 12 days out in time
If we just take a look at 3 different ensemble forecasts (many runs of the same model), the GFS, GEM, & ECMWF models all have a similar theme…much drier than normal through October 1st. The ensemble averages all give us 1/2″ or less rain through the rest of September (a week from Friday)
THE BIG MESSAGE?
This is one of those unusual years where you need to keep watering through all of September. Many years you can stop earlier in the month, but not this year. Luckily, even in drier years, plants/lawns stop using much water in October as temperatures cool. In much of the region the official irrigation season ends October 1st too.