Strong Thunderstorms Likely Tomorrow

So many things to blog about… so little time.

Today was by FAR the warmest day of the year for the Pacific Northwest. The Portland Airport hit 81 degrees, making this year the first year since records began back in 1940 that we’ve had an 80 degree high before a 70 degree high. Of course, we did indeed hit 70 degrees on our way up to 80 degrees today, but I think ya’ll get what I’m saying. Thank you KOIN 6 meteorologist Steve Pierce for the find!

Under any “normal” weather pattern, my blog today would simply be about how tomorrow will be even hotter than today. But tomorrow’s weather pattern is anything but normal. Tomorrow afternoon and evening, models show an extremely unstable airmass over the region, giving us a chance for strong thunderstorms.

I know I’ve probably got everybody hyped up now, so I’ll preface the forecast with an important note: thunderstorms tomorrow will be strong for our area, but they still won’t hold a candle to the long-lived springtime supercells experienced over the Great Plains. We’re not expecting any tornadoes, 100 mph downbursts, or baseball-sized hail coming out of these storms. Moreover, the “cores” of these storms will be scattered in nature, meaning the vast majority of people won’t experience the “brunt” of a given storm and many may even stay dry. But if a strong storm does in fact grace your domicile tomorrow, you could see brief periods of torrential rain, gusty winds, frequent lightning, and small hail. That’s what’s got meteorologists so excited – it’s extremely rare to see even marginally strong thunderstorms in the Pacific Northwest.

If you were outside today, you probably noticed that things felt really muggy by Northwest standards. It wasn’t just your lack of acclimation to the warmer days of summertime – it actually was uncommonly humid today. Just take a look at some of the dewpoints around the region this evening, and you’ll see what I mean.

Thanks to the moderating influence of the cool Pacific Ocean, dewpoints rarely get above 60 degrees in the Pacific Northwest, even in the summer. In contrast, areas east of the Rockies can see much higher dewpoints due to warm, humid air originating from the Gulf of Mexico/Gulf Coast moving into the area. If the Pacific Ocean isn’t responsible for our high dewpoints, what is?

The answer? An atmospheric river stemming from Hawaii, also known as a “Pineapple Express!” Yup,  the same thing that gives us flooding rains in the winter is making today uncomfortably humid under bluebird skies. We are far enough south that we are still getting the humidity from this Pineapple Express, but none of the clouds and precipitation. Instead, we are under a strong upper-level ridge, with a thermal trough making its way from California into Western Washington.

The warmth and humidity at the surface brings us to the concept of CAPE, an ingredient that will play a key role in the development of tomorrow’s thunderstorms. CAPE is an acronym for “Convective Available Potential Energy.” Technically speaking, it is the amount of energy an air parcel would have (that is why it is potential energy, not kinetic energy) if lifted throughout the atmosphere. It is very important parameter for convection because it gives us an idea of how buoyant an air parcel is, and when we know this, we can calculate how intense convection will be. The higher the CAPE, the more buoyant the air parcel, and the better potential for thunderstorms.

To get a large amount of CAPE, you need to have an unstable atmosphere – one in which temperature decreases rapidly with height. Moreover, you need moist air (particularly at the surface), because when moist air rises, cools, and eventually becomes saturated, the phase change from water vapor to liquid water droplets releases heat into the atmosphere, allowing the air to cool more slowly than its surroundings and making it more positively buoyant.

Because of the moderating influence of the Pacific, it is very difficult for us to see significant amounts of CAPE here in the Pacific Northwest. More often than not, there is no CAPE, meaning an air parcel cannot freely convect – it has to be forced upward by terrain, a front, or some guy waving a fan really really hard. When we do see CAPE, amounts are generally pretty marginal – it is rare for us to get over 500 Joules/kilogram of CAPE. As the image above and sounding below shows, we had quite a bit of CAPE today but still didn’t see any storms – in fact, here in Portland, skies were the bluest they’ve been in a long long time. With an upper-level ridge over us, we’ve had sinking air that stifles convection, so the CAPE hasn’t been “realized.” But that will change tomorrow.

Tomorrow, this ridge will move to the east, making way for an upper-level trough to come into our area. Like all midlatitude cyclones, this upper-level trough is a “cold core low,” meaning it, well, has cold air at its center, both aloft and at its surface. However, the cold air aloft will spill into our area before the cold air at the surface, leading to a very sharp decrease in temperature with height and an even more unstable atmosphere tomorrow than today. Moreover, this trough will help pump more moisture at low levels into the region, resulting in an even warmer and muggier day tomorrow than today.

The latest GFS puts us well into the 80s, but even more startling are the dewpoints it has over us, which are in the mid-upper 60s over much of the Willamette Valley!

Unfortunately, I don’t have any dewpoint climatology info handy, but as a lifelong weather nerd living in the Pacific Northwest, I can only remember a couple times when dewpoints have gotten this high, and they’ve always been during summertime heat waves when moisture is coming in from the southeast. And it’s not just the surface that’s seeing lots of moisture. Take a look at how high precipitable water values are over the region at this time! Moreover, notice how the ridge will have moved east, resulting in colder air filling in aloft.

So with extremely warm air at low levels, moist air throughout the atmosphere, and cool air at mid and upper levels, let’s take a look at our CAPE at 5 pm tomorrow.

Yowza! EXTREMELY high values over the region. The main reason why values are so high is because of the freakishly humid air that this model (the GFS) is showing. It’s worth noting that the models pretty much nailed the dewpoint forecasts today, but I still think the dewpoints on the GFS are too high, and that’s why we are seeing such astronomical amounts of CAPE.

And if you can believe it, there’s yet ANOTHER factor that will help us see strong thunderstorms tomorrow. If you look at the sounding forecast for 5 PM near Salem below, you’ll notice that there’s a decent amount of wind shear, with winds increasing steadily with height. Wind shear allows thunderstorms to become longer-lived and more intense because it helps prevent the downdraft from choking off the updraft. If wind shear is really strong, you can get rotating thunderstorms and tornadoes, but thankfully the wind shear profile tomorrow isn’t favorable for tornadic thunderstorms. I cherry-picked this sounding because it had the highest CAPE… it is extremely unlikely that we’ll see 4000 J/kg of CAPE, but I still may frame this sounding and put it on a wall because it’s just such an incredibly unstable atmosphere. Salem launches a radiosonde (weather balloon) at 5 pm tomorrow… I am EXTREMELY excited to see what it measures!

In conclusion, the atmosphere is predicted to be very unstable tomorrow because we’ll have very warm air at low levels, relatively cooler air aloft, and an extremely moist atmosphere. This will lead to thunderstorm development in the afternoon, and because we have decent wind shear in the atmosphere, some of these storms could be long-lived and have gusty winds at their outflow. Storm activity should peak around 5 pm. Even though individual storms could last a long time, activity will sharply decrease Thursday evening as cooler and drier air spreads into low levels, reducing our CAPE. As noted, this will not be a “Midwest event” in any way, shape, or form… the reason why meteorologists are so excited about it is simply because it is so rare to see something like this in our neck of the woods.

I’m looking forward to tomorrow, it should be fun to watch these storms develop. Use common sense if you decide to go storm chasing, seek shelter if you hear thunder, and enjoy a very rare Pacific Northwest weather event!