Below is a sampling of the types of imagery that forecasters access to make decisions about warnings and storm evolution — all captured during the height of the squall line’s transit across the District, Maryland and Virginia.
1. Snapshot from space. Using the GOES weather satellite, a high-resolution digital camera captures the naturally reflected sunlight off cloud tops. Note the turbulent, bubbling roils of high-altitude cloud, which we call deep convective turrets, propelled by the strong buoyancy of updrafts.
Here’s also an animated loop:
2. Infrared cloud top temperature. Again from the vantage of the GOES satellite, this image is used to quantify the altitude of storm clouds, which correlates with intensity. This image, at 3:31 p.m., shows a significant degree of storm organization, with the black regions colder than minus-70 Celsius. At these extremely cold temperatures, clouds are composed entirely of ice and snow, even though torrential rain is falling at the surface!
3. Water vapor image. The black coloration to the south and east of the storm indicates very dry air, but the storm itself is cocooned in mid- to high-altitude moisture, indicated by the light-gray shades. The moist atmosphere helps protect the storm system from evaporation, enabling it to reach greater intensity.
4. Classic precipitation reflectivity. This is a familiar classic image. Oranges and reds mean heavy rain rates, on the order of more than an inch an hour up to several inches per hour. The squall line is taking on a bowed shape, and note the proliferation of cloud-to-ground lightning strikes.
5. Composite reflectivity. The previous radar scan sliced horizontally through the storm at the lowest possible radar beam altitude. This scan shows the complete range of scans, which step vertically through the storm, extracting the most intense radar return at each location within the storm volume.
Note that there is a lot more red here, and this helps meteorologists better assess the rain and hail producing a potential storm system. Magenta colors — not seen here — would be a tip-off for hail, but in general this system was a very heavy rainer, not a significant hail producer.
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6. Doppler winds. The colors here show winds blowing away (red, pink and orange) vs. toward the radar (green). In this image, the squall line has passed east of the Sterling radar. A strong, widespread current of cool, dense (rain-chilled) air, called the outflow, is prominent. Pockets of straight-line winds exceeding 70 mph are shown in orange, although it appears that these most intense gusts probably failed to descend completely to the ground.
Peak wind gusts recorded throughout the region from weather sensors ranged from 55 to 65 mph.
7. Vertical slice through the storm. Some companies, such as WeatherLab.com shown here, provide access to vertical radar slices through storms. A tremendous amount can be learned by studying the vertical storm structure. This image of the squall line, taken at 4:03 p.m., reveals classic squall line structure: a towering, leading convective line (with tops to 55,000 feet) and a broader, weaker, trailing region of moderate rainfall falling from layered (stratiform) clouds.
8. Cloud-to-ground flashes. There are several lightning detection networks. This publicly accessible site (LightningMaps.org) shows an exceptionally dense coverage of cloud-to-ground lightning strikes. Such prolific lightning is triggered by a very unstable atmosphere; cloud updrafts create trillions if not quadrillions of liquid droplets, snowflakes and ice particles that interact in the mid-levels of the cloud to build strong electrical fields.
Here’s another view from the GOES weather satellite:
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Broader view of the wall of lightning hitting approaching the East Coast. pic.twitter.com/tdYJ2jzx07
— Dakota Smith (@weatherdak) July 22, 2020
9. Cloud updraft buoyant energy. To get an intense storm system such as Wednesday’s, you need plenty of instability — which is a term describing the buoyant energy available for cloud updrafts. This image shows an analysis of that energy content (CAPE, or convective available potential energy) feeding into the nascent squall line at 3 p.m., as the storm approached the District. Note the pocket of extreme CAPE values, at 5,000 joules per kilogram — nearly three times the amount of energy that supports ordinary thunderstorms!
10. Precipitable water. The rains were torrential in spots, exceeding two inches in just 45 minutes. Feeding all that moisture into the cloud roots was a very humid atmosphere just ahead of the squall line. In this image, there is a shaded bubble where the humidity was maximized, feeding directly into the storm (2.1 inches of precipitable water — meaning if all the vapor in the air column condenses, you would get 2.1 inches of rain depth).