3 More Sunny Days, Then Back to Wet & Cool

May 7, 2017

9pm Sunday…

It’s a new workweek for me but for most of you it was a fantastic end to the weekend.  Sunshine covered the entire Pacific Northwest this afternoon and we ended up right at normal here in Portland = 66 degrees.

If you’ve been outside the past couple of hours you know temps are cooling off quickly.  With high pressure overhead, a dry airmass, and clear skies, temperatures will plummet tonight.  Lows drop way down into the 40s with even a few upper 30s in the colder outlying parts of Portland.  Hopefully you didn’t plant your warm weather veggies yet…it’s a bit too early.  Not just because of another chilly night but we see quite a change in the pattern after Wednesday.

The good news is that we have 3 warmer days ahead.  We should be somewhere between 70 and 80 degrees each of the next 3 afternoons.  But then the cold showers return.  Take a look at the ECMWF ensembles showing 24 hour precipitation amounts the next 15 days:

You can see the very good agreement among the ensemble members showing pretty solid .10 to .30″ 24 hour rain totals for at least an 8 day period beginning Thursday.  This goes along with the 850mb ensemble chart

A nice warmup for 3 days, and then those pass-level temperatures (in the free atmosphere) plummet down to around freezing during that 8 day period.  Of course with the strong May sunbreaks we’ll get very little snow down to 4,000′, but the point is snow returns to the higher elevations late this week and the valleys will feel more like early April again.  Lots of showers, hail showers, and probably cold thundershowers here and there.  Also note there is a tick upward about 12 days out.  You see the decreasing rain totals on that previous chart as well.  The GEFS (GFS ensembles) have more ridging near the end of the 16 day period too.  You can see the upward trend in temps when average.    Of course the blue line is the operational run and it appears to be much warmer than the mean (average) of the ensembles.  Remember these charts show the temperature around 4,000′ or so.  The GEFS surface temps look like this, showing the same 8-10 day cool spell ahead:

So buckle up, enjoy the 3 warm days ahead, put off all your indoor tasks and enjoy the sunshine!

One more bit of good news…ALL Willamette Basin reservoirs are full and it’s just the first of May.  You can see it on the Army Corps of Engineers “teacup diagram” page.

There will be PLENTY of water for this summer, including irrigation (more of an issue typically east of Cascades) and water recreation.

Chief Meteorologist Mark Nelsen