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Sneak preview: Very hot weather possible for Fourth of July in Washington

Fireworks viewed from near the Potomac River on July 4, 2013. (Ian Livingston)

Computer models are advertising scorching hot weather for Washington on Independence Day. It may be somewhat humid as well. And, as is customary, it seems we’ll have to keep our eye out for late-day storms.

While still a week away, the GFS and European models are forecasting highs of 100 and 98 degrees, respectively, for the Fourth.

If we manage a high of 100 degrees, it would tie 1919’s record for the hottest Independence Day in Washington.

But before anyone becomes too alarmed by these simmering forecasts by the GFS and European models, consider that some other forecasts aren’t as oppressively hot. The ensemble members of these same models — which are alternate versions with slight tweaks to their inputs — forecast highs only in the low 90s, which is not far off the average high of 89.

Examining the overall weather pattern, I am not convinced it would support triple-digit heat. I’m not seeing the kind of intense heat dome over the region that is typically in place during our big heat waves. Also absent is an area of surface high pressure over Bermuda, which acts like a heat pump when it’s unusually steamy.

Based on available information, a high temperature in the low to mid-90s seems most likely, although nailing down specific temperatures seven days in advance is difficult, and adjustments to these numbers, upward or downward, is likely.

Irrespective of exactly where air temperatures end up, it will probably feel several degrees hotter thanks to elevated humidity brought on by winds from the south and southwest. The National Weather Service projects there is a 17 percent chance that the heat index will exceed 100 degrees.

Rain chances are even more difficult than temperature to pin down this far out. But with the moist airflow from the south and lower pressures to our west, it seems like we’ll have a chance — though probably a small one — of late-day storms. This is more or less par for the course in early July.

The stakes are always high for the July 4 forecast given the tens of thousands of people that gather outside in Washington. We’ll continue to update this forecast as details come into focus.

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