A heat dome is a ridge of high pressure that traps hot air like a lid on a pot, pushing temperatures roughly 30°F above normal and lingering for days. Instead of letting hot air rise and disperse, the high pressure pushes it back down, compressing and warming it further, while also blocking clouds and storms that might otherwise bring relief. The result: day after day of intensifying heat with little to break the cycle.

WASHINGTON - If you’ve been outside in the DMV this week, you already know something’s brewing. By Wednesday, that "something" has a name: heat dome. And this one is shaping up to be a doozy, arriving right as the region gears up for a big Fourth of July weekend.

A heat dome is a ridge of high pressure that traps hot air like a lid on a pot, pushing temperatures roughly 30°F above normal and lingering for days.  Instead of letting hot air rise and disperse, the high pressure pushes it back down, compressing and warming it further, while also blocking clouds and storms that might otherwise bring relief. The result: day after day of intensifying heat with little to break the cycle.

Extreme Heat Watch, Heat Advisories issued as dangerous conditions grip the DMV

The buildup has been gradual but is about to accelerate fast. Monday brought near-seasonal conditions with highs in the 80s, but temperatures climb into the 90s Tuesday before the heat dome settles over the mid-Atlantic starting Wednesday.

From there, things get serious. Wednesday’s forecast high of 100 degrees is just two degrees off the all-time record of 102, set back in 1901  — and forecasters expect Thursday and Friday could also see record-breaking temperatures. Some projections put Thursday and Friday even higher: highs of 103 degrees Thursday and 105 degrees Friday would break daily records if the forecast verifies.

Dangerously hot conditions with extreme heat index values are expected across Washington, D.C., Maryland and northern Virginia from Wednesday through the Fourth of July holiday weekend.

The humidity is what really makes this dangerous. Forecasters expect the region to fall short of breaking 105-106 degrees outright, but with humidity, 103-105 degree air temperatures could feel like 110-112 degrees.  And there’s little overnight escape — overnight lows may only dip into the low 80s for a few hours, offering very little relief before the heat builds right back up the next day.

Extreme Heat Watch, Heat Advisories issued as dangerous conditions grip the DMV

It’s easy to focus on the scorching afternoon highs, but meteorologists are just as worried about overnight temperatures. When overnight lows stay in the high 70s or low 80s, the body never gets a chance to cool down - which is when heat waves turn truly dangerous.  Normally, nighttime is when your body and home get a break from the day’s heat. This week, that break barely exists.