Sunday, April 12, 2015

"April Showers Bring May Flowers"

Some beautiful convecting cumulus clouds in the
post frontal zone. Edmonds, Washington (4/1)
As we make our way into the 2015 Spring season, I start to get really excited about the weather patterns we'll see here in the Pacific Northwest. It only just hit me that the phrase we all learned in first grade: “April showers bring May flowers” sums up a very large weather concept that dominates our coastal region. The keyword in that phrase: showers. As we moved into April, we’d already had a number of shower events here where it would pour for 10 minutes and then become sunny again and folks all around were like “what the heck planet, make up your mind!" I remember it was always risky walking to class through the UW campus because you just might get soaked if you had to walk from the Fishery Science building all the way to Smith Hall. But Spring time in the PNW is known for these showers and us Seattleites are known for owning top notch technical rain gear so in the end it all works out.

First of all, it would be nice to be able to differentiate between a couple precipitation terms that weather forecasters use. The two to compare: rain vs. showers. Rain is a widespread event with persistent falling of precipitation whereas showers are a brief, sporadic, and scattered falling of precipitation also accompanied by sun breaks. It all seems so elementary, but there is a huge difference between the two. Getting hit by one or the other lies in the timing and position of a cold front when one comes into the region. Believe it or not, more precipitation can come after a front has passed and in the post frontal convective zone, than it does when the cold front arrives with its rain bands*. At least this is the case in the PNW and it all has to do with our Pacific ocean.

When a cold front comes in over our region, it's bringing in a huge mass of cold air behind it that stems from the north and travels over our Pacific Ocean. As that air passes over, it not only picks up moisture, but also creates an unstable situation. Convection will occur here because this cold air wants to move down while the warm air just over the surface of the ocean wants to rise up, leading to the formation of those really gorgeous puffy white clouds against a bright blue sky, aka the cumulus cloud. With these cumuliform clouds around, we'll often see heavy rain and then sun breaks, and then a repeat of that as they grow and move inland. Sometimes these clouds can keep convecting and grow into larger cumulonimbus and give us the occasional thunder cell with showers. I go absolutely crazy when thunder and lightning occur in Seattle! Seeing so much growth and energy stem from what was once a little puffy cloud is, for a lack of a better word, amazing.

"Open Cellular Convection" - Vis Satellite (4/9 3PM)
Let me show you what open cellular convection looks like on a satellite image. I pulled this visible image from Thursday April 9th at 3PM. Here you can see two low pressure systems intertwining a little, I put red "L"s where they are. The main thing to notice are all those little white dots. This is where the cold air is coming in, and convective instability lies, or what we call open cellular convection. Those little white dots are cumuliform clouds. Showers form here as the clouds grow and move west towards land. The dark spots are just clear areas, so no clouds there.

What's also pretty cool is that as these low pressure systems come in during the Spring season, they can contribute a significant amount of mountain snow pretty late in the game. It was enough to reboot Steven's Pass over on Hwy 2 this weekend and get some people up there to ski. Check out some new snow totals here from the system that just came in over the weekend. Sorry Snoqualmie, looks like this season just didn't workout. 

Anyways, I got the idea to write this blog because we had a thundercell roll through a week ago to bring thunder, lighting, and heavy showers here. And I went crazy... Also, the Skagit Valley Tulip Festival is going on now and EVERY single photo on social media is currently featuring a tulip selfie but they are very beautiful flowers. Talk a look at some photos shot by my friend, Mitch Pittman, who visited down there a couple weeks ago. Even though it's not quite May, you get the picture. Check out more of his amazing work here. ⤲

*Dr. Cliff Mass at the UW has an excellent weather blog that's been running for years and gives me inspiration to write weather stuff on mine. Of course he's already written a blog on this concept, god damn it. Except for the occasional typo though, everyone should give his blog a read. You can find his link here.

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