It’s that time of year when our local weather forecasts are filled with watches, advisories, and warnings.
But what do those different levels of weather impacts mean?
KLCC’s Love Cross called up the National Weather Service in Portland and spoke with meteorologist Jon Liu to find out.
Love Cross: I imagine there's a hierarchy of events that kind of help you determine which of these kinds of weather alerts to issue. So, if we can start with the lowest of those, which I understand are the watches, what constitutes a “weather watch”?
Jon Liu: A watch, just using as an example, what you’re most likely to see a Winter Storm Watch. The watches tend to be issued earlier than our other products. You're probably going to see those a few days before whatever impactful weather is expected to occur. Our threshold for issuing a watch is when we see at least a 50% chance of hazardous winter weather. Just because you see a watch doesn't mean something is going to happen. But once you see a watch, that means, on our side, we're sort of on alert and monitoring the situation closely because there's at least as good a chance as not that hazardous conditions are expected. And these often can get upgraded to different products later as time progresses as we get closer to an event that we could potentially end up with higher certainty or we're more certain that a hazardous winter event is going to happen. Then, in that case, these would likely get upgraded as we get closer.
Cross: So the next step would be the “advisory”. What is the criteria for that for the advisory?
Liu: Our criteria sort of varies based on, for example, elevation. Some areas just tend to get a lot more snow. So, for example, in the Cascades if you get six inches of snow, that really doesn't mean anything- that's pretty typical. But for a lowland area in the Willamette Valley, that's pretty bad. I guess speaking as to lowland areas, just as an example, our advisory is, out of our two winter weather products, the slightly less severe one and our criteria would be one to three inches in 12 hours of snow accumulation. So, these tend to represent still hazardous conditions but not that would cause severe impacts or grind daily life to a halt.
Cross: And so when things get to that next level, I understand that's a “warning”. Can you break that one down for us?
Liu: So for warnings, that's our higher end product that we issue for lowland areas. What that indicates is we're seeing either high confidence of either more than three inches in 12 hours of snow accumulation or more than six inches in 24 hours of snow accumulation. And these tend to be more severe warnings that have a lot more dangerous conditions. Potentially these could cause life threatening travel conditions, widespread power outages and/ or a major economic disruption like closing down major roads. So, these tend to be the more severe higher end impacts.
Cross: And we get these not only for winter weather events, like storms and ice, but also things that can come at other times of the year, like for floods and for wind too. Is that right?
Liu: Yes, that's correct. The system of “Watch – Advisory – Warning,” that order of progression that tends to be in place for most of our products.
Cross: And is this the same criteria that all National Weather Service offices use?
Liu: I believe that probably varies a bit just because what may be considered severe weather for one area may not be considered severe weather for another. Like if California sees two inches of rain in a day, that's pretty bad, potentially. But if we see two inches of rain in a day, that's just business as usual.
Cross: Just another day.
Liu: Yeah. Not too out of the ordinary a Portland winter. Right.
Cross: That’s John Liu, a meteorologist with the National Weather Service in Portland. Thanks so much for breaking this down for us and happy forecasting, I guess.
Liu: Thank you very much.
This transcript has been edited for clarity.
