A cold front marching east since Sunday has been producing severe weather in the form of large hail, strong winds and isolated tornadoes across the central U.S. This same cold front is on track to impact our area overnight tonight into early tomorrow, bringing showers and the season’s first chance of thunderstorms (nothing too severe expected as of now).
When discussing severe weather, meteorologists refer to different types of thunderstorms as “convective modes.” Different convective modes can include anything from an ordinary non-severe thunderstorm, to a linear complex of storms such as a squall line or derecho, to a supercellular, or rotating thunderstorm. What are the main threats associated with each convective mode? What type is most common across the D.C. area and surrounding suburbs?
1) Ordinary Thunderstorms
These types of storms are also known as your summertime “popcorn” storms. Their life-cycle from the cumulus stage, to mature stage, to dissipation stage is only about 30 minutes. They typically blow up at the peak heating time of the day, or mid-to-late afternoon, and clear out by nightfall. They offer some nice rumbles of thunder, provide a nice drink for the flowers, then clear out rather quickly, causing no more than minor impact to outdoor plans.
Primary threats: Brief but heavy downpours, frequent lightning, gusty winds, small hail.
Subcategory: Pulse storms. These storms still have a relatively quick lifecycle, but they have higher cloud tops resulting in brief, but severe conditions (characterized by the National Weather Service as wind gusts greater than 58mph or hail greater than 1 inch in diameter). The stronger winds associated with pulse storms often come in the form of a downburst or microburst.
Ordinary thunderstorms are the most common convective mode for the D.C. area and surrounding suburbs during the warm months.
2) Squall Lines
Squall lines are long, linear complexes of thunderstorms often associated with a strong cold front. They can be hundreds of miles in length and travel great distances across the landscape. While the D.C. region experiences its fair share of squall lines each year, they are not the convective mode we experience most. One limiting factor is the disruptive influence of the Appalachian Mountains to our west. Less than half of the squall lines moving from the Midwest survive the mountains, and in order to do so they must typically cross during the peak heating time of the day. A good rule of thumb with squall lines is, “The worst is first.” In other words, the leading edge of the squall line usually features the strongest winds and heaviest rain.
Primary threats: Damaging wind. Some secondary threats include small hail, frequent lightning, and in rare cases weak tornadoes.
Subcategory: Derechos. Squall lines on steroids. This type of storm system is still VERY fresh in everyone’s minds after the major derecho last year on June 29, 2012. They are bigger, last longer, and travel farther distances than the typical squall line. Luckily, derechos are quite rare for the D.C. region occurring at a rate of only once every four years.
Supercells are severe thunderstorms with embedded rotating updrafts, called mesocyclones. These types of storms are incredibly dangerous, as they are the convective mode most likely to produce tornadoes (though only a small percentage of them actually do). These storms are very common across Tornado Alley of the Great Plains and Dixie Alley of the Southeast in the spring when tornadoes are more prolific.
Supercell thunderstorms are the least common convective mode for D.C., and that’s a good thing! Some of the most recent events that featured multiple supercells (and tornadoes) across the region included April 27-28, 2011, as part of the Southeast Super Outbreak, and Sept. 17, 2004, from the remnants of Hurricane Ivan.
Primary threats: Tornadoes, very large hail, torrential rain, frequent lightning, damaging winds.
A Storify post from last October provides an impressive collection of photos of supercells impacting the D.C. area.
Does each thunderstorm fit nicely into one of these three categories? Absolutely not! There can be embedded supercells within a squall line, and ordinary storms can occur simultaneously and in near proximity with supercell thunderstorms. There are also other convective modes not mentioned in this article, but these are the three most common and most recognized.
The most important thing to remember is that no matter the convective mode, any thunderstorm is capable of producing deadly cloud-to-ground lightning. So when a chance of thunderstorms is in the forecast, make sure you stay informed, keep a weather eye to the sky, and remember that “when thunder roars, head indoors.”