In reality, Reagan National Airport and many areas inside the Beltway, received about 1.5 inches.

The 72-hour observed accumulated rainfall (above) and 72-hour National Weather Service forecast (below), based on models, reveal some of the glaring differences between forecast and reality. The models tended to forecast a maximum near the District, but that’s where the minimum ended up.

The main reason the heaviest precipitation and convection ended up to our south and east was likely related to how far south of us a warm front — where the rain was predicted to focus — ended up.

On the image below I’ve annotated where the front was meandering during the event — all the way down into North Carolina. The heaviest thunderstorms and rainfall are usually north of such a frontal feature but close enough to tap into the moist and unstable air to the south. Had the front lifted into Virginia, the D.C. area would’ve more likely been deluged.

Instead, we were stuck on the cool side of the front and temperatures in the 60s to near 70, which helped to keep down any instability that could fuel heavy storms. While the air above us was still unstable enough to generate some thunderstorms, the more concentrated bands ended up closer to the best moisture and instability toward central Virginia and the southern Delmarva (and even farther south).

The two maps below, which show convective available energy during the event, illustrate how far to the south the juiciest and most unstable air remained — indicated by the green and brighter shades.

The rain was not only held back in our area by the position of the main warm front but also — perhaps — by a coastal front that may have been trying to form along the Delmarva coast on Thursday.

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On the map showing precipitation totals at the top of this post, note the heavy rain accumulated over southern Delaware. The rain was likely caused by a developing front at the intersection of warm air coming in off the ocean and cool air off the land. To the west of that area of rain and storms, there was an area of compensating downward motion near the Interstate 95 corridor that inhibited the development of storms and precipitation. That helped save the Nationals game but ruined the forecast.

I spent the bulk of my career trying to forecast precipitation amounts. This storm illustrates some of the difficulties in trying to nail down the location of an extreme rainfall event. Given the atmospheric setup, it was a no-brainer that there would be an extreme rainfall event somewhere in the Mid-Atlantic. But the question was always where it would focus. Extreme rainfall events often manifest themselves at small-enough scales that models often have difficulty in forecasting their exact locations.