Enhanced risk of severe thunderstorms for the Tri-Cities Saturday

The cover photo was taken by Dennis Brown during Saturday morning’s storm. Thanks for sharing, Dennis! Information in this post is accurate as of 8:00 am PDT Saturday.

The Storm Prediction Center has upgraded much of Eastern Washington and Oregon to be in an enhanced risk of severe thunderstorms Saturday. This is for areas to the east of the Cascades, north of Bend, and west a line that runs approximately from Pendleton to Pullman to Spokane and includes the Tri-Cities area. Surrounding this enhanced risk is a slight risk area that includes the rest of Eastern Washington, parts of Eastern Oregon, the Idaho Panhandle, and far Western Montana.

Before getting into the details of the forecast, I thought it would be good to explain what these risk areas mean and what a severe thunderstorm is.

It is important to note that lightning is not one of the criteria for a severe thunderstorm. The idea behind severe thunderstorms is that the National Weather Service uses them to warn for storms that are strong enough to produce damage. Lightning can and does cause damage, so since every thunderstorm has lightning the NWS has chosen to not include that in the criteria. Heavy rain is also not part of severe thunderstorm criteria. Instead, storms producing heavy rain may be accompanied with a flood advisory, flood warning, or flash flood warning depending on the severity of the situation.

The Storm Prediction Center in Norman, Oklahoma issues severe thunderstorm forecasts for the entire contiguous United States, doing so with a five-category system.

Today, the Tri-Cities and surrounding areas are in an enhanced risk for severe thunderstorms, meaning that we are expecting to have widespread thunderstorms with scattered storms reaching the severe criteria. This is thanks to a number of parameters, including high moisture content in the atmosphere and an upper level low moving in from California. For this forecast to verify, we need the sky to clear from this morning’s storms to help raise temperatures this morning and early afternoon.

THE FORECAST

Thunderstorms are expected to start firing in and south of Central Oregon (Bend area) in the early afternoon and progress northward. It looks like these will become a line of storms to move through the Tri-Cities area and the Yakima Valley around dinner time before heading further north towards British Columbia. After that there might be a storm or two meandering around the region, but things will be much more mellow with scattered rain showers resuming after midnight.

These storms will be moving quickly, perhaps 40-60 mph at times, so while things may get intense it won’t last all that long. Nobody is guaranteed to see a severe thunderstorm, but this is the highest single-day chance we’ve had in the Columbia Basin in quite a while.

All of the percent chance values you see below will seem quite low, but we care about these because they are high impact events. A 30% chance is a 3 out of 10 chance, and a 3 out of 10 chance of damaging wind is a lot different and more impactful than a 3 out of 10 chance of getting 0.01″ of rain. If these storms do happen, they will deserve our respect.

SEVERE RISKS SATURDAY

We have a risk for all three severe weather types on Saturday at differing levels. Here’s a run down of what I’m watching.

SEVERE WIND
30% chance within 30 miles of any single location.

Our biggest chance of meeting severe criteria is from wind. As the thunderstorms move from Oregon into Washington they are expected to become more linear in fashion. This combined with the heavy rain they might have will create conditions for strong, damaging wind gusts and this is the reason the Storm Prediction Center upgraded the region to an enhanced risk. It is possible that when storms move through they could be accompanied by winds in excess of 75 mph. It would be good to secure anything you have outdoors that would not handle this well (including trampolines).

SEVERE HAIL
15% chance within 30 miles of any single location.

The second largest risk is from severe hail. The upper level low moving into the Pacific Northwest will allow for more upward motion in storms. This coupled with daytime heating (which will be best if we get sunny) looks to be enough for isolated severe hail events.

TORNADOES
2% chance within 30 miles of any single location.

Yes, there is a chance of tornadoes today, though that chance is low. Tornadoes don’t often form in linear storm events like we’re looking at today, but there are a few scenarios where a tornado could develop. There is a chance that the eastern end of the line of storms could have individual storm cells that would be capable of tornado action. Furthermore, weak tornadoes can sometimes form in strong outflow boundaries. It is possible that the models are wrong and that these storms end up not being linear, which would raise our tornado chances slightly, but there is also a chance that the models are wrong and we won’t get anything at all.

If a tornado warning is issued, you should be notified by FEMA’s system on your phone (the same system that sends out AMBER alerts). Now would be a good time to go into your settings and make sure that is on. The chance of tornadoes is quite low, but tornadoes are very high-impact events so it’s important to be as ready as possible.

SHELTERING IN A SEVERE STORM OR TORNADO

Because of the nature of these storms, you should shelter for severe thunderstorms like you would shelter for a tornado. If 75+ mph winds are produced in a thunderstorm it won’t make any difference whether it was a tornado or straight line winds. The best place to be in storms like this is in a basement away from windows. If you don’t have a basement, further sheltering ideas can be found at this link. I’m not sharing this information to try to scare anyone, I just firmly believe that better and safer decisions are made when a plan is prepared before the event rather than during.

HEAVY RAIN

Beyond severe storms, the Weather Prediction Center has us in a slight risk for flash flooding. This means there is a 10%-20% chance that rainfall will exceed what is called flash flood guidance. What this means for you is that you need to be aware of the possibility of street and flash flooding, especially in low lying areas. Remember that most flood deaths occur in vehicles, so if you come across a flooded road turn around, don’t drown.

RESOURCES

I will be updating Tri-Cities Weather on Facebook as well as my personal Twitter periodically throughout the day, but since I will be driving around storm chasing my posts will be limited. You can also use the following websites:

• Ingalls Weather current Radar and Lightning
• National Weather Service Pendleton website, Facebook, and Twitter