We touched the 90-degree mark this afternoon in D.C. (at 2:35 p.m.), likely commencing the first heat wave of the summer. A heat wave, by definition, is three consecutive days at or above 90 degrees. We should have little trouble reaching 90 Tuesday and Wednesday; after that, how long the streak continues is unclear.

This promises to be the hottest weather of 2014 thus far, and the hottest stretch since July 15-21, 2013 when the high temperature averaged over 95 degrees (it was 93 or higher every day in this period, and temperatures remained above 80 for a record 138 hours).

A couple other reasons not to take this heat wave lightly:

1) We are least acclimatized to heat early in the summer and, therefore, most vulnerable to heat-related illness
2) The combination of heat and humidity will lift the heat index or “feels like” temperature to the mid-to-upper 90s Tuesday, and to around 100 Wednesday.

But whether your measuring stick is intensity or duration, this heat wave in D.C. won’t be setting records. Wednesday, the hottest day, should see high temperatures peak in the mid- to maybe upper 90s. Triple digit heat is unlikely (although some 98 or 99-degree readings are possible Wednesday, especially south of the District) and the streak of 90+ days may be challenged to exceed three days. Model data are suggesting a cool front will make enough headway by Thursday or Friday to lower temperatures a bit.

By comparison, consider that in 2012 we had 11 straight days at or above 95 degrees, with a punishing average high of 99.5 degree! Of course, the summer is young…

Let’s take a quick look at the model forecasts for high temperatures over the next three days:


NAM model: 90-93

NAM model high temperature forecast Tuesday (WeatherBell.com)

European model: 89-91

European model forecast high Tuesday (WeatherBell.com)

GFS model: 90-97

GFS model forecast high Tuesday (WeatherBell.com)


European model: 93-96

European model forecast high Wednesday (WeatherBell.com)

GFS model: 95-99

GFS model forecast high Wednesday (WeatherBell.com)


European model: 83-89

European model forecast high Thursday (WeatherBell.com)

GFS model: 91-96

GFS model forecast high Thursday (WeatherBell.com)

The cause of the heat wave is your classic Bermuda high  (area of surface high pressure off the Southeast coast), helping to pump warm air towards the region from the Southwest.

Hot temperatures about one mile up in the atmosphere and strong flow from the southwest  around an area of high pressure off the Southeast coast sets up hot weather mid-week ; simulation from the GFS model (WeatherBell.com)

This surface high pressure is complemented by a heat dome or area of warm, sinking air at high altitudes which compresses and heats up on its way to the ground.  These are common ingredients for a heat wave here in the D.C. area, but the hot pool of air looks to get shoved to our south  late in the week before it can really take hold, mercifully.