You may think: it’s not the heat; it’s the humidity, but it’s both. NYC meteorologists were informing our citizenry of dangerous dew points where the thermometer would read 95 but it would feel like 107. Once relative humidity level gets above 45% it becomes increasingly uncomfortable, and if relative humidity reaches 100% that means the sweat can’t evaporate.
Many have lost count of the heat waves we’ve had this summer, and as of this writing August 13th brought clocked in as the hottest day of this summer. Mayor de Blasio regularly advised those without AC to take advantage of the city’s 500 cooling centers, and extended hours of public swimming pools during the heat waves.
NOAA’s seasonal forecasts start with the assumption that for any random summer or winter, there are three possible climate outcomes–temperature or precipitation that is well above normal (upper third of the climatological record), near normal (middle third), or below normal (lower third)–and they are each equally likely. Together, all probabilities must add up to 100%, so the “default” probability for each outcome is 33.3%.
The goal of the forecast is to estimate how the actual conditions in the weeks or months before the season–like the presence of El Niño–tilt the odds toward or away from “equal chances” for all categories. If models predict that the chances of well above average temperatures are 60%, then the chances for the other two outcomes (near-average and well below average temperatures) must be split between the remaining 40% (100-60=40).
How do we divvy up the remaining fraction? Physically speaking, it makes sense to say that if the odds have tilted toward one climate outcome (say, unusual warmth) by a given amount, they’ve tilted away from the opposite outcome by an equal amount. So, by convention, forecasters divvy up the remaining fraction by holding the chances for near-normal conditions at the “default” 33.3%, and subtracting it from the remainder. What’s left (6.7%) is the probability of the least favored category.
When the odds of one climate outcome are so high (70 percent or more) that the remaining fraction is smaller than 33.3 percent, forecasters set a minimum 3.3 percent chance for the opposite outcome, and give the larger portion of the remainder to the near-normal outcome. (Otherwise, the least favored category would wind up with a negative probability, which doesn’t make mathematical or physical sense.) The Climate Prediction Center has a chart showing all the possible combinations, and their own description of the forecast format.
Since only one color can be assigned to any location on the map, the most favored category decides the color, but the other categories are still possible. So, in this summer’s season outlook for the Northeast and the West Coast, for example, the full suite of probabilities is 50-60% chance of well above normal temperatures, 33.3% chance of near-normal temperatures, and 6.7-16.7% chance of well below normal temperatures.