While showers and thunderstorms have kept afternoon temperatures to around normal in these parts, heat has been making headlines in the west. Excessive Heat Warnings cover at least part of three states – Arizona, California, and Nevada – and Heat Advisories are in effect in an additional four – Idaho, Montana, Oregon, and Washington, in addition to other parts of Nevada.
But just what does a Heat Advisory or an Excessive Heat Warning mean? These advisories, watches, and warnings are issued by your local National Weather Service forecast office. And while these advisories can be issued everywhere in the country, the criteria for just what “hot” really is varies from place to place.
Meteorologists use a variable called the “Heat Index” to determine whether or not a particular spell of hot weather will be severe enough to justify warnings or not. On-air meteorologists harp on the Heat Index frequently, and often unnecessarily (like when a temperature of 84 has a Heat Index of 86). But the tool, which combines heat and humidity to produce an approximation of what the air actually feels like, is actually quite useful for determining whether or not heat advisories are needed.
But even the actual numbers that warrant heat advisories vary from city to city. For instance, the Sterling, Va. Office, which covers Baltimore and Washington, issues Heat Advisories for when the Heat Index is expected to fall between 105 and 109 degrees Fahrenheit. Heat Warnings are issued by that particular office when Heat Indices are expected to be above 110 degrees.
While Sterling is generally consistent and specific with their criteria (although they may budge a bit for early-season heat episodes), other offices have much more lax guidelines.
The Phoenix, Ariz. office, on the other hand, does not institute official numeric guidelines. After all, if they issued Heat Advisories for every day where the temperature was expected to rise over 100 degrees, they might as well sign their Forecast Discussions with “Capt. Obvious.”
But what about Philadelphia? The Mount Holly, N.J. office does have a system, unlike Phoenix, yet it is not simple, like Sterling’s.
Philadelphia is a special case on its own, it turns out.
Researchers Kalkstein, Jamason, Greene, Libby, and Robinson authored a paper in 1995 that described in full detail the Philadelphia Hot Weather-Health Watch/Warning System, or PWWS for short.
The Mount Holly office, together with the city of Philadelphia, implemented PWWS on July 12, 1995. It combines the obvious meteorological variables – temperature and humidity – with less-obvious ones, like cloud cover and wind speed. Additionally, PWWS also took into account non-meteorological variables, like building construction. The paper notes that,
“In cities such as Philadelphia and St. Louis, the prevalence of multifamily structures characterized by red brick walls, tar roofs, and poor air flow contributes to inferior ventilation and increased solar load on the building when compared to similar inner-city housing in the Southeast and the West.”
PWWS used MOS forecasts generated by the NGM (which has since been retired) to classify days as one of 11 synoptic categories, of which some were then considered "oppressive." These categories accounted for the overall synoptic weather pattern (like whether the Philadelphia area was under cyclonic or anticyclonic flow), as well as wind speed and direction, and cloud cover (as measured by how many tenths of the sky were covered by clouds).
Two of the four categories with the highest frequencies were also correlated to the highest and fourth-highest mortality levels.
Based on the synoptic category, human forecasters can know to anticipate a mortality rate that is higher or lower than that of the average hot weather day, and based on that data, decide whether or not to issue an Excessive Heat Warning. The actual process as outlined in the paper, beginning from the 48-hour forecast of the NGM, is reproduced below.
Philadelphia's heat advisory and warning criteria shift based on time of the year, duration of heat plus humidity, and intensity. Heat warning thresholds in the early season (May) are lower than in July, and it's largely based on the above flow chart but there are benchmarks (they run along the lines of a heat index of 96 in May, 98 in June, 100 starting in July). Forecasters do have some sway to nudge a heat advisory to warning if the flow chart above suggests that need arise.