WeatherMatrix (Jesse Ferrell)

How can we quantify the coming heat wave?

By Jesse Ferrell
6/26/2018, 2:31:24 PM

We all know there's a big heat wave coming for the East this week, but this is the first week of summer, so what's the big deal? Meteorologists sometimes have trouble communicating extreme events such as these; people say "it's supposed to be hot, it's summer!"

Quantifying the quantifiers:

Eric Fisher tweeted a great map yesterday quantifying the threat, but I wanted to go a step further and explain it, to give you a behind-the-scenes look at what we meteorologists think when we see a map like this:

Here's the translation that I posted to Facebook:

At the 850 mb pressure level (generally 4,000-5,000 feet up in the atmosphere, a layer which is often used as a proxy for comparing "normal" surface temperatures without worrying about geographic effects), if the GEFS ensemble forecast (the same model run dozens of times with slight adjustments to the initial conditions) is right for 00Z July 1 (Sunday evening), it would have NEVER been hotter at that time, at that level, and so probably on the Earth's surface, on that date between 1985 and 2012, in the red area.

Given that we're close to the peak temperatures during summer, that's pretty impressive. In fact, anything yellow or above (above the 97.5 percentile) is impressive. That means that out of 100 years, on average 97.5 percent would have been cooler (since we're only looking at 27 years, it wouldn't have happened for 26.3 of those years -- in other words, very rare).

Each hour of the forecast, that area shifts, but I assume he picked this one because it was the largest.

It's not the heat, it's the humidity:

The humidity is also a big deal with this weekend's heat wave. We don't generally have much past data, and no "normal" data for the Heat Index (Apparent Temperature) or AccuWeather RealFeel temperature, so it's hard to gauge how this heat wave will feel, compared to normal, but what about the timing? It won't be super hot everywhere every day through the weekend. As a result, we have these two very unique maps that can help you prepare to sweat. Because the heat and the humidity won't hit at the same time, we've created a map showing arrival times for both:

Humidity Forecast 6/26

Heat Forecast 6/26

We can also look at the forecasts, and they are literally off the charts with the NWS predicting Heat Indexes over 115 degrees F over a significant area Thursday (shown below) through Sunday (when it moves into the Northeast). I'm told we'll have a RealFeel map soon.

NWS Heat Index Forecast 6/29

And finally, we can ask ourselves how unusual this heat wave will be for any particular day by looking at the number of potential daily records assuming the NWS forecasts are correct. For Sunday, nearly every station in New York state is likely to set a daily record high (i.e. it had never, in recorded history, been that hot in that place on that day).

NDFD Potential Records 7/1

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or


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WeatherMatrix (Jesse Ferrell)