Wednesday, July 18, 2012

Portland and Thunderstorms

Often times when we forecast the chance for thunderstorms here in the Portland-metro area, we get e-mails from viewers who would like to know very specific details when it comes to when, where and why. In an attempt to provide you with a better understanding of how we get these thunderstorms (and how we get the weather we see in general), I will be writing a series of posts that break down these weather set-ups.

Summer-time thunderstorms are not uncommon for Portland. In the Willamette Valley, we average less than 10 thunderstorm days per year. Since 2012, we have averaged 5 thunderstorm days. The "stormiest" month of the year is actually July. July 3rd, 13th and 22nd are actually three of the stormiest days of the year over the last 13 years! So why during the summer do we see thunderstorms? There are a few ingredients.

In order to get summer thunderstorms in Portland, we need daytime heating. The hottest time of year for us is mid to late July when we have a daily average of 82 degrees. When the sun is out and heating the earth's surface, that causes air to bubble up from the ground and rise in the atmosphere. As the air bubble rises, it cools. As long as the air bubble is warmer than the air it is rising through, that bubble will continue to rise on its own. The typical temperature profile of the atmosphere is for temperatures to cool as you go higher. You can imagine that on a 80 or 90 (or occasional 100) degree day, the bubbling of air is going to rise quite high on its own. This gives the air instability.

Now that we have a rising bubble of air, we must add some moisture to the atmosphere or thunderstorms will not occur. How do we get moisture into the air? We often see a general southerly flow through the atmosphere on thunderstorm days. That southerly flow provides us with moisture from the more mild Pacific waters off the coast of California as compared to the waters off of the Oregon coast. The southerly flow helps push that moisture north into our area. So we have added moisture to our rising, unstable air bubble and now we can create a cloud that has potential to create lightning.

Our final step is a trigger needed to agitate the atmosphere and spark the thunderstorm. There are several different "triggers" that can help us out. One is a front, be it warm or cold. During the summer we don't see a lot of warm or cold fronts so this isn't a common summer-time trigger. Strong winds in the higher levels of the atmosphere can also aid thunderstorm development. The winds help evacuate those rising air bubbles at the top of the atmosphere and prevents air from sinking back down. A "vorticity maximum" can be a good summer-time trigger. A vorticity max is basically a kink in the overall flow of the atmosphere. Think of it as a mini-low pressure system. As this passes over head it would be the last little push that would give our rising bubble that last ingredient and a thunderstorm would develop!

Over the last few days, Portland has had the threat of some afternoon thunderstorms. Yet, we have not seen those develop over the metro area. Here's why:

This is our 500 millibar chart valid for Wednesday at 8 am and is a snapshot of the happenings half way up the atmosphere. The main feature is obvious, a closed low pressure system off the Oregon/California coast. Notice the wind barbs flowing around the low. This has been the weather setup for the last couple days. A generally southerly flow winding into the Portland area. With the summer-time heating, south flow and some good moisture (you may have noticed a "muggy" feeling the last few days) thunderstorms looked like a strong possibility. Our problem? We lacked a trigger! This upper low would have been the PERFECT trigger for us but due to its location being so far away from the metro area, that doesn't help! This setup did have some strong upper level winds but they were located over central and eastern Oregon thus helping fire off some severe storms in those parts.

Thursday's setup looks a bit more promising for storms here in Portland.

 To the left is a different view of the 500 mb chart we saw above. This shows the low moving back to the north around Thursday evening. The feature to look at however is the dark red dot just east of Portland. That is a vort max trigger. With the low closer to us, plus this vort max, I see a good setup for storms.

The next map shows rising and sinking motions at nearly 10,000 feet. There is a dark red dot right on top of Portland at the same time as our vort max. The red dot is telling us that we have rapidly rising air occurring at that time. This probably due to the forecast high of 83 degrees Thursday. Now we have two ingredients: rising air and a trigger. Just need that moisture.
Finally, our precipitation map is showing a very heavy cell just southeast of Portland at the same time as our rising air and trigger mechanism. The models are showing plenty of moisture as well but the models are not always right! However, as I see this on multiple forecast charts, I would definitely keep an eye to the sky tomorrow late afternoon and into the evening. 

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