Severe thunderstorm warnings and reports of large hail and damaging winds across the D.C. area Thursday, June 18, 2015. (Jordan Tessler)

Ingredients came together early Thursday evening for the most intense thunderstorm outbreak the Washington, D.C. area has witnessed this year.

Western Fairfax County was hardest hit. Sections of Fairfax were bombarded by hail larger than golfballs and a violent downward thrust of winds, known as a microburst, downed large numbers of trees in south Reston into north Fairfax.


Close-up of wind damage and large hail reports in western Fairfax County (Jordan Tessler)

The damage over Fairfax County was created by an intense, multicell thunderstorm tracking from the northwest, while congealing into a line of storms across the D.C. region. A warm front had just pushed north that morning; storms were particularly intense because the atmosphere was rapidly destabilizing, and an unseasonably strong pocket of wind shear (winds increasingly with altitude) was traversing the region.

[June 18, 2015 storm coverage, as it happened]

How the large hail formed…

Wind shear in particular promotes the growth of more organized, longer lived storms with a geometry that separates the updraft from the downdraft. The updraft becomes particularly strong, lofting large amounts of supercooled water high into the cloud. Supercooled water remains liquid below freezing – but when water droplets contact tiny ice crystals, the water freezes on contact. This is called riming, and it leads to growth of hail embryos.

An exceptionally strong updraft is needed to suspend and grow large, heavy stones, the size (1.5-2”) that fell over western Fairfax County. Two inch stones are indeed rare in our region. The very high dewpoint air (low-mid 70° F) feeding this storm may have contributed to a large mass of supercooled water swept up into the cloud.

Inside the microburst

In western Fairfax County, a pocket of significant wind damage (large trees felled on several homes) was co-located with the region of large hail, over Reston. The source of the damaging wind was most likely a wet microburst.

A microburst is a concentrated region of downrushing air, embedded in the storm’s larger downdraft; when striking the ground, the air fans outward as an explosive blast.


Microburst diagram (National Weather Service)

The same wind shear giving rise to separation of a strong, tilted updraft also likely enhanced the downdraft portion of the storm’s circulation.

The fast descent of air is initiated by several processes – including the drag created by a large, sinking mass of rain water and hail. Additionally, evaporation of rain drops and melting of hail cool the air, increasing its density thus accelerating its descent. It is very common to observe wet microbursts embedded in pockets of heavy rain and hail.

Microbursts generate wind gusts on par with weak to moderate intensity tornadoes, and there have been instances documented wind speeds up to 150 mph. Unfortunately, there is no way to detect these small and highly transient regions of violent wind; they are generally too small to be resolved by Doppler radar (unlike their larger cousins, called downbursts) and they usually dissipate after 5-10 minutes.

Photos


Clay O’Neill helps cut a fallen tree in the middle of a residential street in Reston, Va. (Photo by Marlon Correa/The Washington Post)

Tree down from microburst in Reston (Tess Boyer via Facebook)
Power lines and trees down in Fairfax (Karen Karadimov via Facebook)
Power lines and trees down in Fairfax (Karen Karadimov via Facebook)

Rainfall totals

The storms were moving along at a rapid clip. Nevertheless, they produced extremely high rainfall rates and prompted the National Weather Service to issue a flash flooding warning for the District, due to the vulnerability of poor drainage areas.


Doppler estimated rainfall totals June 18, 2015 (National Weather Service)

Rainfall totals were generally around 0.5-1 inch in the immediate metro area, although much lighter amounts fell west and south of Fairfax County – areas just grazed by storms. Reagan National Airport picked up 0.53 inches and BWI 0.88 inches, while Dulles only logged 0.11 inches.

Reagan National now has 5.34 inches of rain in June, more than 3 inches above normal.