The Red Flag Warning for all of Clark County has been extended until Tuesday, September 16, 5 am
The National Weather Service (NWS) Portland has extended the Red Flag Warning until Tuesday morning at 5 am. Previously it was set to expire September 14 at 8 pm. This includes both of our fire weather zones 604 and 660. The combination of low relative humidity (12%-25%), mid-level instability (mid-level *haines 6 conditions) and critically dry fuels will maintain extreme fire weather conditions through Monday. There is also a threat of dry thunderstorms through Monday night adding another element to the extreme fire weather.
*Haines Index - This is a fire weather index based on the stability and moisture content of the lower atmosphere that measures the potential for existing fires to become large fires (although this is not a predictor of fire starts). Orange will indicate Haines index values of 4 (low), dark orange will show Haines index values of 5 (moderate), and red will depict Haines index values of 6 (high). Values of 4 and above are plotted on each map even though the overall Haines index is from 2 to 6, with six being the highest potential for large fires (see table below). It is calculated by determining the sum the atmospheric stability index (term A) and the lower atmospheric dryness index (term B). The stability index is determined from measurements of the temperature difference between two atmospheric levels and the dryness index is determined from measurements of the dew-point depression.
Due to large variations in elevation across the United States, the index is calculated for three different pressure ranges: low elevation is 950-850mb; mid elevation is 850-700mb; and high elevation is 700-500mb. It is named after its developer, Donald Haines, a Forest Service research meteorologist, who did the initial work and published the scale in 1988. http://www.spc.noaa.gov/exper/firecomp/INFO/hainesinfo.html