They Have Been Posted, But What Do They Mean?

Hot Weather | Photo by

As you drive around town there are many signs advising you to do many thing such as Yield, Slow School Crossing, S-Curve Ahead, Deer Crossing, Construction Ahead, and even Stop.

Deer Crossing Sign
Deer Crossing Sign | Photo by

Most of those advisories are pretty self-explanatory. We are in the middle of summer and here in the Pacific Northwest that means hot, dry weather that prompts many advisories and warnings. Whenever the National Weather Service issues them you will see them posted here on EDN in the Advisories section of my weather forecasts. This summer has been a particularly busy one for these due to many wildfires which have sprouted up like weeds in your front yard.

Since we have had so many Advisories, Watches, and Warnings I feel this is a good time to review what they actually mean. I’ll start with the strongest warning issued lately.

Red Flag Warning
Red Flag Warning | Image by

A Red Flag Warning, according to the National Weather Service, is: “A term used by fire-weather forecasters to call attention to limited weather conditions of particular importance that may result in extreme burning conditions. It is issued when it is an on-going event or the fire weather forecaster has a high degree of confidence that Red Flag criteria will occur within 24 hours of issuance. Red Flag criteria occurs whenever a geographical area has been in a dry spell for a week or two, or for a shorter period , if before spring green-up or after fall color, and the National Fire Danger Rating System (NFDRS) is high to extreme and the following forecast weather parameters are forecasted to be met:
1) a sustained wind average 15 mph or greater
2) relative humidity less than or equal to 25 percent and
3) a temperature of greater than 75 degrees F.
In some states, dry lightning and unstable air are criteria. A Fire Weather Watch may be issued prior to the Red Flag Warning.”

Many Red Flag Warnings have been issued recently and will continue to be issued as long as the fire danger risk stays at such high levels.

Fire Weather Watch means that conditions are such that temperatures are getting hot and relative humidity is lowering which means grass, timber, and debris are becoming increasingly dry and becoming easier to burn weather due to the natural cause of lightning or by human misuse of flammable materials.

Subsidence High Pressure
Subsidence High Pressure | Image by

With the fires burning all over the state of Oregon the smoke has been trapped near the surface by subsidence due to upper air high pressure which has been siting over us. Particulates from that smoke ride on the airflow from east to west and get trapped in the Southern Willamette Valley.

Air Quality Alert
Air Quality Alert | Image by

The result has been really high air pollution levels causing an Air Quality Alert to be issued by the National Weather Service. “Pollutants in smoke can cause burning eyes, runny nose, aggravate heart and lung diseases, and aggravate other serious health
problems. Children, the elderly, and individuals with respiratory
illnesses are most at risk of serious health effects. If you
experience respiratory distress, you should speak with your
physician. Limit outdoor activities and keep children indoors if it
is smoky. Please follow medical advice if you have a heart or lung

A Heat Advisory means that a period of hot temperatures is
expected. The combination of hot afternoon temperatures and warm
overnight temperatures will combine to create a situation in
which heat illnesses are possible.You need to drink plenty of fluids, stay
in an air conditioned room, stay out of the sun, and check up
on relatives and neighbors especially those who are most vulnerable due too age or illness.

An Excessive Heat Watch is issued by the National Weather Service when “the heat index is expected to be greater than 105 °F (41 °C) across the northern states or 110 °F (43 °C) across the southern states during the day, and/or nighttime low temperature will be at least 75 °F (24 °C) or higher for two consecutive days.” Local areas often have their own criteria due to their specific normal temperature ranges for the season.

Excessive Heat Warning
Excessive Heat Warning | Image by

An Excessive Heat Warning is issued by the National Weather Service within 12 hours of the heat index reaching one of two criteria levels.” In most areas, a warning will be issued if there is a heat index of at least 105 °F for more than 3 hours per day for 2 consecutive days, or if the heat index is greater than 115 °F for any period of time. Note that local offices, particularly those where excessive heat is less frequent (such as the Pacific Northwest) or in areas with deserts or mountainous terrain, often have their own criteria. High values of the heat index are caused by temperatures being significantly above normal and high humidities, and such high levels can pose a threat to human life through conditions such as Heat Stroke, Heat exhaustion, and other heat-related illnesses.”

We’ve had a respite from the excessive heat and bad air, but remember that it only takes another high pressure ridge to turn the winds again, heat us up, and send more smoke our way. That means we all need to be prepared to keep cool and take all of the necessary precautions.   Keep appraised of the conditions and you will know if and when they will return.

With the lumber mill fire near Junction City this past weekend, Sunday August 13th, more smoke is in the area and depending on the local winds could spread throughout Eugene-Springfield and beyond.

Let me know what you would like me to talk about or explain. You can comment below or email me at: [email protected].

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