Thursday is expected to be the hottest day of the year so far in Fresno at 99 degrees – the record is 100 degrees set in 2004 – but at least it won’t be a magenta day.
Magenta is the color that designates the hottest and most dangerous weather under a new method created by the National Weather Service, which is updating the way it warns Californians about heat danger.
Starting May 15, the Hanford weather office will begin using the new Potential Heat Risk warning system, which is already in use in some parts of the western United States.
Other weather service offices in California will also start using it.
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The system crunches data, including 30 years of weather records, to come up with guidance. It is still being fine-tuned, which is why the weather service calls it experimental.
The system is not your father’s heat index map that turns red when the weather gets exceptionally warm.
Rather, it’s a seven-day forecast based on temperature and factors such as average weather conditions for the time of year, and will rate the potential heat risk to the public.
“Before, we relied solely on an apparent temperature or the heat index,” said meteorologist Kris Mattarochia, science and operations officer at the Hanford office.
“When it reached a certain number, we would warn the public,” he said. “But the public is affected at different temperatures. The whole population doesn’t react the same. Everybody has different impacts.”
For a really healthy person, a 90-degree day might not mean anything. But an older person on medication – it may mean a lot to them.
Kris Mattarochia, National Weather Service meteorologist
For instance, “for a really healthy person, a 90-degree day might not mean anything,” he said. “But an older person on medication – it may mean a lot to them.”
Another factor is time of year. A hot spell on May 1 might be more worrisome than one in July when the public is more likely to expect it and be ready, he said.
To give the public a quick view of heat risk, each weather service office will rate the expected heat impact in its forecast area on a five-point scale, zero to four, and assign colors to each number – green, yellow, orange, red and magenta.
Each color, or level, means a different kind of advisory or warning.
Green (level zero) means no elevated risk, and yellow (level 1) means low risk to those extremely sensitive to heat, especially those who lack cooling systems.
Green and yellow will not warrant a special warning to the public, but those who are extremely heat sensitive can take appropriate action.
Orange (level 2), means moderate risk for those sensitive to heat, especially without cooling or adequate water. When covering at least half the forecast zone, it will trigger a heat advisory if it’s close to the next level.
Thursday’s Potential Heat Risk forecast is mostly orange for the Valley and yellow for the foothills.
Red (level 3) mean high risk for most people, especially those without air conditioning or adequate water. It will trigger a heat warning if 30 percent or more of the forecast zone is affected, or if it affects less than that but it’s a highly populated area such as Fresno or Bakersfield.
Magenta (level 4) means two or more days in a row of heat risk, including a very high risk for everyone due to little or no overnight relief, and will merit a public warning.
The latter three levels could warrant an excessive heat watch five days in advance.
If it’s Monterey and it’s 90 degrees for two or three days, that could trigger a warning. In Fresno, it’d be higher.
Laurence Kalkstein, University of Miami professor
The idea is to provide heat risk guidance to decision-makers and heat-sensitive people.
“We don’t want to warn people for just the most extreme events because there are people that are impacted by heat events that are not considered extreme,” Mattarochia said.
The Potential Heat Risk system is based on research into “relative thresholds” at the University of Miami, including by Professor Laurence Kalkstein in the Department of Public Health Sciences.
“If it’s Monterey and it’s 90 degrees for two or three days, that could trigger a warning,” Kalkstein said. “In Fresno, it’d be higher.”