Well…that was sure a depressing start to May! A high of only 60 degrees in The Dalles with cloud cover, and 54 at PDX. It was a rather cool weekend as well, though yesterday was quite a bit sunnier. I actually got cold yesterday evening at a potluck and bonfire party in White Salmon. About 50-52 degrees at 8:30pm there, with a biting wind. The last two years we did this, it was much warmer with evening temps probably close to 70 degrees, so we got spoiled…

All of that is about to change in a dramatic way. Remember a few days ago, how I said it would probably be in the 70s for a few days this week? Well it turns out I was wrong…we have a bona fide May warm spell coming up. Wednesday should be not only the first 70-degree day of the year, but the first 80-degree day as well. I kind of figured that we couldn’t hold this back much longer, and sure enough it’s come with a vengeance!

But this is not going to be a “normal” springtime ridge. Usually we see skies clear out and winds turn offshore, before the really warm air builds up overhead. This ‘first-dry-then-warm’ pattern keeps dewpoints down at the surface, ensuring that it still cools down at night. Instead…we’re going to see the warm air sneak up from the south, behind a warm front associated with a system out in the northern Pacific:

We don’t get offshore flow in this pattern, so there’s no chance for dry air to move in at the surface. Instead…we will have plenty of southerly and southwesterly flow, pumping a HUMID airmass into the region. The fact that we’ve had so much rainfall in the past 2 months, will help humidify the air still further.

Check out these dewpoints for Thursday afternoon, courtesy of the 12z NAM:

I’m pretty sure the southern Willamette Valley hasn’t seen dewpoints quite like that, since the May 2008 heatwave. Though they probably came close, a few times in 2013-2014. Yesterday on Wunderground, I remember seeing surprisingly warm nights for The Dalles Wednesday and Thursday night, in the mid/upper 50s. But I didn’t think of humidity until I saw the NAM this morning.

So here’s what happens:

Tomorrow should be another gloomy day like today. Maybe just 3 or 4 degrees warmer, and you might notice the humidity creeping up a bit as the day progresses. Then Wednesday the clouds break, the real warm airmass arrives, and dewpoints continue to climb. After the cool, clammy April it’s going to be a real shock to have 80-85 temps. The fact that it won’t drop much below 60 at night, will give things a sultry summer feel.

Then by Thursday afternoon we have a historically humid airmass (for this time of year anyway), and some secondary energy riding up from the south, in advance of a marine cold front on Friday. That’s a great recipe for thunderstorm activity. More on that later…