5th heat wave of summer will be more reasonable, plus some overnight lightning on the way

The past few days have been very nice, although high level wildfire smoke from Canada brought us quite a bit of haze this afternoon. Don’t worry, I don’t see thick low level smoke again the next few days. Sunday wasn’t fun with AQI values up into the UNHEALTHY category for many of us.

WHAT’S AHEAD

1) Hot weather returns tomorrow afternoon and continues through Sunday. This time the airmass will be about 10-12 degrees cooler which keeps us in the 90s; luckily in the low-mid 90s too.

2) Overnight thunderstorms! A nearly perfect setup for nocturnal thunderstorms is in place for 5-8 hours. Many of us may wake up to lightning or thunder at some point tomorrow night.

3) Widespread showers are possible Tuesday/Wednesday next week, although at this point they don’t look like real soakers. But we haven’t seen anything more than sprinkles or light showers in over two months!

HOT WEATHER

One reason the heat wave won’t be too hot this time? We don’t have a big upper level ridge building overhead. Instead, a cool trough sits offshore, staying just far enough west to allow a warm/hot airmass to move into the Pacific Northwest. For the meteorologists and geeks, 850mb temperatures will be climbing up to around +20 this time around. Remember we peaked at +26 during last week’s heat wave. Here’s the view up around 18,000′ for Friday. Doesn’t look like a big heat wave does it?

The reason we’re able to get above 90 in this pattern is the offshore flow down at the surface. The surface map for Thursday afternoon shows a nice “thermal trough” of low pressure west of the Cascades. Maybe 2-3 millibars easterly flow through the Gorge for gusts 25-35 mph at Vista House tomorrow. After this time, we bounce between weak offshore and weak onshore flow through Sunday. This tells me we won’t see wide variations in high temperatures these next four days.

The setup is about the same in the upper atmosphere by Sunday afternoon, maybe a little stronger ridging to our east.

Then that entire trough shifts “kicks out” and reenters the main westerly flow Tuesday and Wednesday. A big cooldown will be the result along with at least a few showers.

So it appears we’ll pick up another 4 days at/above 90 for the season, boosting our total way up to 24.

And with increasing humidity Saturday and Sunday, we’re assured of more warm/sweaty nights. So far this month we’ve seen 6 record warm lows at PDX.

THUNDERSTORMS THURSDAY NIGHT

Sometimes we go an entire summer without seeing nighttime lightning west of the Cascades. When that DOES happen, it’s always a little treat for the weather geeks like me. Maybe some of you too. Well it appears we might see some thunderstorm action Thursday night. A weak upper level wave moves north across the Pacific Northwest tomorrow night through Friday morning.

Notice in the middle of the night the flow is southerly or even southeast. That is a CLASSIC setup for us west of the Cascades. The flow is “diffluent”, which means ahead of that wave the flow is “diverging” a bit (opposite of converging). When that happens, low level air must move upward to replace the higher level airmass. Upward moving air in a moist airmass leads to showers and thunderstorms. In the setup tomorrow night ANYONE can get a thunderstorm, even over the cold coastal low level airmass. Lightning can occur over a 50 degree fog; unrelated to surface heating. These will be “high-based” thunderstorms. They often produce abundant lightning (a great light show!) but sometimes don’t produce much rain. Regardless, cover anything that shouldn’t get wet before you go to bed Thursday night. And let’s hope we don’t get many fire starts because it’s right back to hot weather Friday behind the system. You can see the 3km NAM model producing the popup storms early Friday morning

These storms will be scattered, which means you might get knocked out of bed (not literally) by loud thunder at 3am, but your friend 15 miles away says “what thunderstorm???”.