Massive hailstone that fell in Timonium, Md. June 23, 2015 (Nicka Pohl)

As a violent thunderstorm tore through northern Maryland Tuesday evening, it unloaded massive hail – large enough to rank among the biggest in Maryland state weather records.

Nicka Pohl shared with Capital Weather Gang the photo of a softball-size, spiked hailstone measuring approximately 4 inches in diameter, which fell near Timonium. Hail larger than golf balls fell throughout the Hunt Valley, Cockeysville, and Timonium areas in northern Baltimore County.

The hail dented cars all over the area and smashed back windows, Pohl said.

Christopher Strong, the warning coordination meteorologist at the National Weather Service forecast office serving Washington and Baltimore area, said Maryland records show there have been only two instances of hail larger than the 4-inch stone Pohl collected.  Hail measuring 4.5 inches in diameter was observed in LaPlata in 2002 when a large, violent F4 tornado swept through the area.  And a stone measuring 4.5 inches was reported in Baltimore in 1970.

The thunderstorms that produced the hail formed thanks to a very unstable air mass and strong wind shear (increase in winds with altitude) ahead of a cold front approaching from the northwest.

A nearly solid band of thunderstorms, called a squall line, swept through during rush hour (image below).


Infrared satellite image of the squall line striking our region yesterday. Red colors indicate the coldest, tallest thunderstorm cloud tops. (Weathertap.com)

Embedded in this line, just north of Baltimore, was a powerful storm cell rocketing up to nearly 65,000 feet. Convective cloud tops reaching this altitude are rare, likely the top 5 percent of the most vigorous cells in the D.C.-Baltimore region, as elucidated by weather radar.


Echo top height revealed by the Doppler radar at NWS Sterling. Magenta shaded regions indicate cloud tops in the 63,000-64,000 foot range. (RadarScope)

Can large hail be identified on weather radar?

The answer is yes, although indirectly. Radar works by sending out intermittent, brief pulses of microwave energy sideways through the atmosphere. Those pulses bounce off hydrometeors (rain drops, hailstones); the strength of the returned signal, called radar reflectivity, is displayed using a logarithmic scale (dBZ) and correlated to precipitation intensity. The radar operator uses a color enhancement scheme to distinguish among the intensity of return signals. Returned power levels of 50-55 dBZ indicate torrential rain on the order of several inches per hour.

However, interpreting radar signal power imposes a dilemma. High power returns can imply either a large number of small hydrometeors, or fewer but very large rain drops/hailstones – both situations scatter an equal amount of energy.

Values over 60 dBZ are uncommon and usually indicate a mixture of heavy rain and hail. When the power approaches 70 dBZ, large hail is a near-certainty. The image below shows a radar snapshot from the Cockeysville storm, contained in the white circle. Those pink and magenta colors represent 65-70 dBZ of returned power.


Radar reflectivity for the Cockeysville storm (white circle) indicating 65-70 dBZ. Composite reflectivity is shown, which delineates the highest returned power within the vertical radar scan. (RadarScope)

A 4-inch hailstone weighs close to a pound. To grow such a large stone requires levitation within a cloud updraft on the order of 70-80 mph. That, in turn, necessitates significant cloud buoyancy, like turning up the heat inside a hot air balloon.

The available buoyant energy yesterday afternoon soared to impressive levels, fueled by the Sun’s strong heat, hot air streaming northward on southerly breezes, and very high humidity. Condensation of water vapor releases a tremendous amount of heat energy within storm clouds; subsequent freezing of suspended liquid water releases another shot of heat.

Cloud updrafts are further enhanced by wind shear, which tilts these vertical currents sideways; then the heavy mass of rain and hail falls out beneath the current, not straight down through its core. The rising air accelerates, reaching velocities that can suspend large hailstones. The wind shear along the Maryland-Pennsylvania border was stronger than over Washington, D.C. enabling the most violent storm updrafts to develop close to Baltimore.