Today’s weather set-up. Dew points, a measure of humidity, shown on the left. Green lines are dew points 60 or higher (humid). Blue lines are below 60 (dry). Upper level winds are shown on the right. Jet stream (region of fastest winds) is positioned in the blue and green shaded areas. (NOAA, left, and Unisys weather, right. Adapted by CWG)

But by this evening, humidity starts trickling back in. And by Thursday morning, it’s a sauna once again. In the afternoon, temperatures surge to around the century mark and the heat index spikes to 103-108! What’s going on?

The jet stream is moving fast - with its peaks (hot weather) and valleys (more pleasant weather) rippling through the region every couple of days.


Temperature simulation for Thursday afternoon from the NAM model (StormVistaWxModels.com)

OVERALL UPPER AIR PATTERN HAS SOME MOVEMENT TO IT...WHICH IS NOT ALWAYS THE CASE DURING LATE JULY/EARLY AUG.

Often times, the pattern at this time of year is more stagnant and we can be stuck with hot, humid weather for days. That’s what we experienced in late June through mid-July. But the weather pattern has evolved into one that’s more dynamic.

The fast-moving weather set up in place now can be a good or bad thing, depending on your perspective.

On the plus side, it doesn’t stay too hot or too unsettled too long. If you don’t like the weather one day, chances are you will the next.

On the negative side, the weather is highly variable and more difficult to predict. And nice days, like we’re having today, are a tease rather than a fixture.

For better or for worse, a fast moving, variable weather pattern may well continue into August if long range models are correct.

Jason is the Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science, and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.