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. 

Oklahoma!

Recall spring and early summer here in the Pacific Northwest....
Portland had one of the worst springs of all-time! It ranked as the 4th coldest and 2nd wettest spring on record. The average temperature for the 2011 spring period (March-May) was 49 degrees. We received 14.41" of rain as well, 0.9" shy of tying the all-time record. Given the two, I'd say that's a pretty miserable stretch.

While we "suffered" here in the Pac NW, it compares very little to what the Sooner State has endured since January 1. Oklahoma has gone through a variable mixed bag of extremes this year. Let's review:

February 10:
In Northeast Oklahoma, Nowata recorded the state's coldest temperature ever when the mercury disappeared to minus 31 degrees Fahrenheit! That record stood for 64 years and previously was minus 27 degrees.  Also ending that day was the new state record for snowfall in 24 hours. Just to the Southeast of Nowata, 27" of the white stuff fell in Spavinaw! Winter Canadian air was to blame for these winter records.

Let me draw your attention to the top left map. This is the setup for February 9th. Notice that over Oklahoma, the pink colors are moving from the north to the south. This indicates very cold temperatures in the atmosphere above, an "arctic outbreak". Those temperatures above translate into cold temperatures at the surface. The map on the bottom right displays relative humidity through roughly half the atmosphere. The pink colors indicate a "dry" atmosphere, or low humidity levels. The brighter blues tell us that the atmosphere is saturated and wet. The more saturated the air is, the more likely precipitation is to fall. There is a bulls-eye of high saturation over Oklahoma on the 9th. Combined with the cold temperatures, we get the beginning of a record snow event!

February 9 setup

On the 10th, notice that Oklahoma looses it's moisture source. The record 27" of snow fell out of that saturated air but dried out quickly after. When the atmosphere is dry, that typically means there are no clouds in the sky. A clear sky allows heat from the day to escape out to space and rapidly cool temperatures at the surface. This is how you drop temperatures to minus 31 degrees!

February 10 setup

May 23:
Oklahoma lies in the middle of what is known as Tornado Alley. More tornadoes occur here than anywhere else in the world! The threats from tornadoes includes hail and high winds. State records were set in both categories in a two day span beginning on the 23rd. A record 6" hailstone fell in Guetbo. Below damage to a roof of a car that collected the hailstone.

May 24:
A tornado outbreak near El Reno caused a scientific wind gauge to record a wind gust of 151 miles per hour as the tornado passed. That gust crushes the previous record of 113 miles per hour that was set back in 1994.







July:
A majority of the country baked this summer in record heat. Oklahoma was at the top of that list.

Temperatures Compared From Average

 Oklahoma averaged a July temperature of 88.9 degrees. This is not the average high temperature. It is the average temperature felt at any point in Oklahoma for the month! It has never been that hot in any state for any month, ever! Truly remarkable.

Summer: 
The summer never provided relief for Oklahoma. Mark Shafer, director of climate services for the Oklahoma Climatological Survey said that at one point 99% of Oklahoma was experiencing severe drought.

November 5:
From the non-weather department but still significant and record-setting: Oklahoma experienced its strongest earthquake on record. A 5.9 magnitude earthquake struck near Sparks. It was a shallow earthquake, which allows the earthquake waves to travel longer distances. Earthquakes are nothing new to Oklahoma. Given the states proximity to one of the United State's major fault lines, the New Madrid fault, earthquakes from that seismic zone probably impacted Oklahoma in the past. There are several known fault lines in Oklahoma and while it is not in a perceived "earthquake zone", its location still can produce sizable quakes.

2011 has been a torture on Oklahomans. What does all this extreme record-setting phenomena mean? I wish I had an answer for that but I'll let you draw your own conclusions!