Evans

The heat wave from July 25 through Aug. 1 provided yet another round of record-breaking temperatures for Eastern Oregon, with La Grande seeing multiple days of new daily record high temperatures set.

In fact, La Grande matched recorded new daily records on July 30 and Aug. 1, recording a high of 105 degrees each day to set new high water marks. While this heat wave wasn’t quite as severe as the historical one during June of last year, such events are a sobering reminder of what a future climate may hold for Eastern Oregon. Both NOAA and the Intergovernmental Panel on Climate Change anticipate that heat waves will become more frequent and severe with a warming climate.

But what exactly is a heat wave? The criteria for determining a heat wave changes depending on the context, but is generally considered to be a period of excessive heat that lasts multiple days. Here at the National Weather Service, we’re interested in advising the public about the heat when we anticipate adverse impacts to health due to abnormally high temperatures.

At what point that exactly is will depend heavily on the geographic region of interest. In the eastern United States, humidity plays a large role in the heat, and the Heat Index is often utilized when defining a heat wave. The Heat Index factors in humidity’s effect on temperatures and human health, but out here in the West, where the air is much drier, the Heat Index is often similar to or even less than the actual temperature.

As a result, the NWS offices of the western United States use a prototype product called Heat-Risk (www.wrh.noaa.gov/wrh/heatrisk/?wfo=pdt), which is designed to put a period of hot temperatures into a climatological context. The main advantage of HeatRisk is that it takes a multitude of factors into consideration, outside of just how hot temperatures may reach during the peak of expected heat.

Factors such as the overnight low temperatures, time of year, duration of the heat and thresholds of health-related impacts from the heat based on CDC data are all utilized in tandem with the forecasted high temperature. All of these serve as inputs into the HeatRisk calculation, which determines a category score from 0 to 4, similar to the UV Index or the Air Quality Index. A 0 category denotes no heat impacts expected and 4 suggests the entire population is likely at risk for heat-related impacts, not just the more heat-sensitive members of the public such as the elderly.

The National Weather Service may issue either a Heat Advisory or an Excessive Heat Warning based on the HeatRisk. In simple terms, a warning is more severe than an advisory both when it comes to the magnitude and duration of the extreme heat, with temperature records more often at risk under a Warning. But HeatRisk is just one of many tools NWS forecasters utilize when advising the public about the potential of a heat wave.

Out here, east of the Cascade mountains, where the public is a bit better equipped to handle long spells of heat than our neighbors on the west side, the character of the heat and the time of year is also assessed. This is done in part to be sure heat events are assessed on a case-by-case basis and to ensure more severe events are addressed thoroughly.

For metro areas such as Portland and Seattle, however, where many homes lack air conditioning, even marginal events may require an elevated notice in order to better advise the public, at-risk populations, and emergency responders on the potential for a heat wave.

Regardless, HeatRisk offers the National Weather Service and its core partners a simplified tool to communicate the potential impacts a heat wave may pose on the public.