While out on my usual evening run, braving the midsummer Maryland mugginess, I captured the eerie scene shown above, using my iPhone 8 Plus. My jaw dropped. As a Washingtonian (the state), I rarely see thunderstorms back home, let alone picturesque shelf clouds. The map below indicates zero severe thunderstorm watches per year on average for most of the West Coast. On the other hand, Washington D.C. (the capital) receives around a dozen watches each year.

Thunderstorm map from the Storm Prediction Center

So, what is a shelf cloud? What is the science behind it? It is commonly defined as a low, horizontal, wedge-shaped cloud at the leading edge of a thunderstorm gust front. Although this means that an intense line of thunderstorms is approaching, often bringing high winds and heavy rain, one should not expect a tornado. This article uses simple diagrams to explain a storm’s updrafts (warm/unstable rising air) and downdrafts (rain-cooled sinking air). We know that warm air is less dense than cool air, but it is the tilting of the updraft and resulting condensation of the warm/moist air above the outflow boundary (or gust front) that creates this unique formation. The shelf cloud was likely attached to the base of a cumulonimbus cloud, which has great vertical depth, thus blocking incoming sunlight and giving the cloud a dark appearance. You may notice a little sunlight in the background, where there were breaks in the clouds, because the photo was taken facing west near sunset.

Earlier that day (July 2, 2019), I visited Howard University’s Atmospheric Research Site in Beltsville, MD. At 2 PM, I helped launch an ozonesonde, which is essentially a weather balloon that also measures ozone concentration. Intense daytime heating of the ground (under mostly clear skies) and surface winds generally out of the southwest (bringing moisture from the Gulf of Mexico) led to hot and humid conditions.

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The contrast between surface high temperatures in the mid-90s and cold air aloft (steep environmental lapse rate) suggested that there was significant instability needed for convection to occur and thunderstorms to form. Just above the planetary boundary layer, the lowest layer of the troposphere (~1.3 km deep), the air was nearly saturated (90-100% relative humidity), suggesting that cumuliform clouds (including cumulonimbus) were forming. There were certainly aerosols on which water vapor could condense; the ambient air quality was even quite poor that day. The convective available potential energy (CAPE), higher values indicating a greater potential for severe weather, was above 2000 J/kg, as predicted by the 12z NAM for 00z (8 PM EDT). All the ingredients for storm formation were in place.

Mixed-layer CAPE from Pivotal Weather

At around 5:30 PM, a severe thunderstorm passed through the D.C. area, prompting severe thunderstorm warnings (bottom left screenshot). This storm brought heavy rain, high winds, and lightning, caused minor flooding, and increased the moisture in the air. However, in my opinion, these clouds were not nearly as impressive as those to come. Round two of thunderstorms arrived at around 8 PM (bottom right screenshot). On radar, it seemed to appeared to have a bow echo signature. Although longer-lived, this second storm did not seem to bring heavier rain to my location, compared with the first one.

More formidable storms do not always bring more severe weather conditions. Still, you should take cover, as I did after taking several photos and videos. I am sure that I will witness more impressive storms during the remaining weeks of my summer internship at the NOAA Environmental Modeling Center in the D.C. area. I should mention that the sunsets here are incredible as well! Feel free to leave a comment below, especially if you have had a similar experience!

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