I’m not complaining, but gardeners might be. Over the last month or so, the immediate D.C. area – time and time again – has missed out on opportunities for heavy rain.

During this stretch, most of the area has received just 1.5-2.5 inches of rain, somewhat below normal.


Rainfall estimated by radar over the last 30 days in the D.C. area and Northeast (NOAA)

Yet, the area has been in a rather wet pattern. An active jet stream has brought front after front through the region, and showers and storms have been in the forecast on numerous occasions. But downpours have eluded us, and – instead – we’ve been grazed by lighter showers or, sometimes, nothing at all.

Part of the reason we’ve avoided more substantial rain is just luck. But there’s a legitimate scientific explanation as well: the stronger dynamics (energy) associated with  fronts moving through have been just to our north – anywhere from around the Mason Dixon line into southern New England.

For example, the Philadelphia area was deluged by over 2 inches of rain this morning and had its wettest day on record on July 28. Philly has achieved a staggering summer June-August rainfall total of 27.56″ of rain  – its wettest summer on record (with over two weeks left) – whereas D.C. has had just shy of 15″ (note: this is a healthy total; we’re not near a drought).

Watch this morning’s radar loop (courtesy Jesse Ferrell at AccuWeather) covering northeast Maryland into New Jersey and notice the eruption of heavy thunderstorms.

The radar view over D.C. (not shown), by comparison, was much more tame – with mainly light to moderate showers.

The recent severity of the weather in Philadelphia and New Jersey relative to D.C. is likely related to the tendency for a core of much stronger upper level winds to pass through that area, enhancing wind shear and thunderstorm development.


High altitude winds as simulated by the high resolution NAM model at 7 a.m. this morning (WeatherBell.com)

Link: Why wind shear is needed for thunderstorm development

In short, the D.C. area has tended to be far enough north to benefit from the cool, dry air on the heels of fronts passing by, but far enough south to miss out on the more intense rains.