Low humidity values and the elevated risk for fire danger


PORTLAND, Ore. (KOIN) – You can hear summer blowing across Portland this Tuesday as a warm offshore breeze is moving the tree tops around.

Not only is it warm and sunny again, but it sure is dry out there. We’ve been living in the months of June and even at times July the last four to five days. The clear sign that it isn’t summer yet, outside the date, is the fact that the mountains still have a low level of snow digging towards the bottom slopes. Mt. St. Helens looks lovely in the cityscape today.

Portland, Oregon 4-20-21 – Mt. St. Helens

Before we get into some of the fire conditions, I do want to note that we are one of the warmer spots in the country right now; in fact, it’s snowing through the Midwest today as cold air rushes through the mid section of the United States. Notice in the map below, that deep blow color extending through southern Michigan, Indiana, Illinois and Missouri. Cold winter air is funneling in from the northwest and temperatures have been at freezing. Snow has been coming down for some communities there, so it’s summer here and winter there.

You can see this all occurring on the visible satellite at mid-day. Clear conditions over in Washington and Oregon, meanwhile, cold continental air is coming down from the northwest with the clouds out of the northern states. Plenty of snow on the mountains through the Rockies at this time. I’m not sure about you, but I’ll take the warmer weather right now. That warm weather comes with a price.

Visible Satellite 4-20-21

We know that it has been unseasonably warm and dry during this stretch of summer weather. As of mid-day in Portland, temperatures are in the lower 70s (10 degrees or so above average today). I want to note, the humidity value this afternoon. We are currently sitting at 21%, which is quite low. We use relative humidity as a gauge for moisture content. Typically we will use dewpoint to have an understanding of the moisture, but in the context of wildfire conditions, the use of relative humidity is useful because of the exchange of moisture with fuels (vegetation).

For reference, in the month of April, our afternoon humidity value is usually around 55%. It will continue to get lower as the summer months come because of the increase in temperature. A reminder that the relative humidity drops as the temperatures get warmer. Clearly, if our relative humidity level is down in the 20s, that is much lower than our normal standard. That is one of the main reasons why we have fire concerns this week. Conditions are just not typical for this time of the year and the threat for fire spread and combustion is much higher.

The wind will continue to blow this afternoon. Coming from the east and northeast around here in the Valley. That isn’t the case for the Oregon coast, where there is more of a northerly to offshore flow. Overall, the wind will be gusting to the 20 mph range for just about all. That offshore flow that will help promote warming off the mountains can warm us up and it will also be dry. The combination of the wind and dry conditions has many areas under a burn ban.

Today the Aurora Fire District put out this tweet about the high fire danger for Clackamas and Marion counties. This has been the trend across the valley this week, which is extended from last week. No backyard or agricultural burning until conditions improve and the forecast is reevaluated. I suspect this will happen late in the week or by the weekend.

Right now, there is a wildfire down in south-central Oregon in the Klamath region. This is the Ponina Fire, which is 5 miles north of Bly. We don’t typically use our wildfire map graphic until fire season comes. The wildfire, at last update, is sitting at 1,641 acres and the containment is at 40%. Improvements for all come this weekend as the weather pattern shifts. We may even get some rain and much higher relative humidity levels by Saturday for all.

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