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History Lesson: Fourth of July Rain Chances

28 Jun 2017, 4:07 am

The holiday weekend is fast approaching! WeatherNation will have a complete look at your forecast on air and online. One of the many tools factored into a forecast is past events. Let’s see what previous Fourth of Julys might say about the chances of “rain on your parade.”

If all we had to go on was history, the map below says the best chances of clear skies in the contiguous United States (CONUS) would be found, not surprisingly, in the West. Based on past Fourths of July, chances for rain are 10 percent or less for a large area from interior Washington to northwest New Mexico.  Check out this awesome interactive map for a closer look.

In the CONUS, the highest chances for rain on your parade would be in southern Florida (40-50%), followed by the highest elevations of the Appalachian Mountains in North Carolina and West Virginia, and the Northeastern border with Canada, where chances of rain on July 4th have been 30-40 percent. These places reflect locations with, historically, the combination of abundant available moist air and ways to lift that moist air, in the form of mountains or strong daytime heating.

But the highest probability of rain on Fourth of July for any U.S. state is in Hawaii, where the eastern slopes have a 50 percent chance or more of precipitation that day. On the lee side of the islands, however, the chances for rain drop to 10 percent or less.

How do we come up with these maps?

These kinds of historical-chances maps—the historical chance of a white Christmas; the chance of a Pennsylvania rodent seeing his shadow, and so on—are based on the nation’s “normals.” Normals are calculated every ten years at NOAA’s National Centers for Environmental Information. These are the same normals that your weather forecaster is talking about when she or he says “today was 4 degrees above normal.”

How much rain are we considering here?

These maps are based on the historical likelihood that at least one-tenth inch of rain will occur. Why one-tenth inch? Because that’s plenty to drench your crepe paper float, make your hot dog buns soggy, or drown out your fireworks. If you’re interested, there are also chances calculated for a hundredth of an inch, a half-inch or an inch.

Which should I use: normals or forecasts?

If you’re planning for an event that’s within 7-10 days, use the forecast. Forecasters track patterns that will move the chance of precipitation for a specific, upcoming day up or down from the historical norm. However, if you’re planning for next year’s July 4th, or any event beyond a dozen or so days, normals are a great way to gauge things.

So, are we only considering July 4th here?

Actually, no. Climatologically speaking, July 4th isn’t all that different from July 3rd, or 6th, or even June 30th, for that matter. These normals capture a range of dates surrounding July 4th to help broaden the sample for a specific date. This has the effect of smoothing out some of the “noise” that would be associated with only focusing on a single, specific date.

Author: Rebecca Lindsey
Cover Photo: dvs

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