The Storm Prediction Center has placed parts of the region under a marginal risk for severe thunderstorms on Sunday. For the Columbia Basin, this includes areas that are along and east of a line stretching from Condon and Irrigon north to the Tri-Cities and Othello. More broadly speaking, it also encompasses most of northeastern Oregon, the Palouse and Spokane areas of Washington, the Idaho Panhandle, and Montana west of about Missoula. Outside of this area, non-severe thunderstorms are possible.
A marginal risk is the lowest of the Storm Prediction Center’s five-category system for describing severe weather that is expected. The other categories (in order from lowest to highest) are slight, enhanced, moderate, and high. Because of the nature of our thunderstorms, it is very rare to have any forecast higher than a slight risk in the Pacific Northwest. A severe thunderstorm is defined as having winds of at least 58 mph, quarter size hail (1 inch diameter) or larger, and/or having a tornado.
Those in a marginal risk can expect scattered strong thunderstorms with small hail and gusty winds, along with isolated instances of severe thunderstorms. Typically these thunderstorms would just barely cross the threshold of being severe. For this event specifically, this marginal risk is in place for a 5% chance of severe hail and strong wind happening within 25 miles of any location within the risk area. Less likely than that, but still possible, is a landspout tornado (like Richland had in March) in the development region of these storms, which should be in and around the Blue Mountains south of Heppner and Pendleton. I can’t stress enough how low of a chance it is to have one, but if one forms it will be weak and short lived. General thunderstorms with heavy rain, gusty winds, and small hail are likely to be scattered throughout the region, with the highest chance for our area being (as usual) along the Blue Mountains.
This weather event is resulting from a combination of several factors. There is a decent amount of moisture in the area, and as the ground and air heat up during the day Sunday it will cause these to rise into the atmosphere. That upward motion will be accelerated thanks to relatively cool temperatures in the mid-levels of the atmosphere. This is because warm air is less dense than cool air, and the cooler things are the faster the warm air will rise through it and bring the moisture along to form the clouds necessary to create the rain showers and thunderstorms.
Further up in the atmosphere, the jet stream is being forced to the south of a deep low pressure system centered offshore. Think of it as if the stream is being squeezed as it tries to move around the low. We will be downstream of this in what’s called the exit region. In the exit region, the jet stream is able to spread out again, which lowers the pressure up there and pulls more air up – accelerating the lift described in the previous paragraph and helping foster even better storm development. In the map above, the squeezing can be seen where the black lines are close together to the west of the Bay Area, and the exit region can be seen where the lines are spreading out over the Pacific Northwest. I hope this all makes sense, but if not just know that what’s going on about 18,000 feet above us makes a more favorable environment for storm development.
Finally, the reason why the chance of thunderstorms is greatest near the Blue Mountains is because the varied topography of mountain areas can force air up as wind moves across it. The taller the mountains are, the better the upward motion becomes. We can sometimes have this kind of movement (called orographic lift) from the Horse Heaven Hills or Rattlesnake Mountain, but it is definitely better in the Cascades and Blues.
As with most of our thunderstorm events, not everyone will see a thunderstorm (let alone a severe thunderstorm). Patterns like this make it hard to tell well in advance who will have rain and who won’t, so the best I can give you is that there will probably be storms moving around the area Sunday and some of them might produce hail and gusty winds. Models are indicating that we could see a round of showers in the morning and early-afternoon with more isolated (but stronger) showers later on during the day.
As these form, I will be watching them and posting updates on my Twitter page. Usually I would have you head over to Tri-Cities Weather on Facebook, but because of how the different algorithms work, Twitter is the best for getting out frequent updates for weather patterns like this. You can also keep tabs on my radar page to see current doppler radar and lightning data. Beyond Sunday, we will continue to have unsettled weather with periodic rain showers thanks to two low-pressure systems interacting in our region. Hopefully we can receive enough rain to improve our drought situation.
