Understanding Severe Thunderstorms: SPC, Watches and Warnings

Understanding Severe Thunderstorms

Each year, the atmosphere undergoes a significant change from Spring into Summer. As the days lengthen and temperatures warm, thunderstorms become a focal point of weather discussion in NYC – and the entire Northeast US. Meteorological Spring, which begins on May 1, typically marks the beginning of severe thunderstorm season in the NYC Metro Area.

Severe Thunderstorms are complex, dynamic and powerful. Very intricate atmospheric interactions occur prior to, during and after their formation. These can be confusing to interpret, even for meteorologists. Understanding the basics of how organized thunderstorms develop, and how meteorologists attempt to predict and quantify their potential impact, can go a long way in communicating their risks effectively.

Thunderstorm formation in the Northeast US

Despite being directly next to a body of water, organized thunderstorm development is quite common in and around the NYC Metro Area. Organized thunderstorm events occur when there is sufficient instability, atmospheric lift, and ample shear. This means storms can form via lift, act upon the unstable atmosphere, and remain organized due to the present wind shear.

The process by which organized thunderstorm events occur can be broken down even further. First, the sun needs to heat the surface to warm the air above it. When this warm surface air is forced to rise (a front, mountains, a sea breeze boundary, etc) it will continue to rise so long as it stays warmer than the air surrounding it. As a storm rises into freezing air, different types of ice particles can be created from freezing liquid drops. The ice particles can grow by condensing vapor (like frost) and by collecting smaller liquid drops that haven’t frozen yet (a state called “supercooled”). When two ice particles collide, they usually bounce off each other, but one particle can rip off a little bit of ice from the other one and grab some electric charge. Lots of these collisions build up big regions of electric charges to cause a bolt of lightning, which creates the sound waves we hear as thunder. (National Weather Service).

Thunderstorm (n): A storm accompanied by lightning or thunder

The Life Cycle of a Thunderstorm

So we’ve formed a thunderstorm. Now, the same storm will go through a life cycle. While the duration of each stage will vary depending on the atmospheric setup, each storm goes through these eventually: Development, maturation, dissipation.

As the National Weather Service puts it, “The developing stage of a thunderstorm is marked by a cumulus cloud that is being pushed upward by a rising column of air (updraft). The cumulus cloud soon looks like a tower (called towering cumulus) as the updraft continues to develop. There is little to no rain during this stage but occasional lightning. The thunderstorm enters the mature stage when the updraft continues to feed the storm, but precipitation begins to fall out of the storm, creating a downdraft (a column of air pushing downward). When the downdraft and rain-cooled air spreads out along the ground it forms a gust front, or a line of gusty winds.”

The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft beginning the dissipating stage. At the ground, the gust front moves out a long distance from the storm and cuts off the warm moist air that was feeding the thunderstorm. Rainfall decreases in intensity, but lightning remains a danger.

What Makes a Thunderstorm Severe

A thunderstorm is classified as “severe” when it contains one or more of the following: hail of one inch or greater, winds gusting in excess of 50 knots (57.5 mph), or a tornado. The National Weather Service defines severe thunderstorms strictly by this criteria. That being said, in the NYC Metro Area, thunderstorms are often sub-severe. They can still feature major impacts, such as frequent and dangerous lightning and heavy rain, but don’t meet severe thunderstorm criteria.

Varying amounts of wind shear, instability, and lift can all impact the type of thunderstorm event that occurs. For instance, an environment with lots of instability but very little shear will lead to quick development of storms. However, the lack of shear will cause the storms to have little motion or movement. So the updraft will form, and essentially “use up” all of instability — eventually collapse on top of itself, and dissipate. This is often the case during the “popcorn” thunderstorms which form during the summer.

On the contrary, environments with less instability and more shear can lead to quick moving thunderstorms that can sometimes produce strong winds. These storms will be teetering on the edge of “severe” limits, but often will be moving rapidly. When they encounter any localized surface instability, they can quickly produce strong winds, or a downburst — maybe even a tornado if the low level shear is adequate.

Understanding The Storm Prediction Center

The Storm Prediction Center is a division of the National Weather Service. It is staffed by experts in the prediction of thunderstorms and organized convective events. The Storm Prediction Center issues thunderstorm outlooks for the entire country for Day 1 (today), Day 2 (tomorrow), and Day 3 (the day after tomorrow). They also issue extended outlooks which highlight potential widespread storm events 4 to 8 days in advance.

The Day 1, 2 and 3 outlooks from the Storm Prediction Center feature risk areas, or categories of risk for severe thunderstorms: Marginal, Slight, Enhanced, Moderate and High. These categories are triggered by probabilities of severe thunderstorms within a certain distance of a point. So, for instance, if the forecasters at SPC agree that there is a 15% chance of damaging winds from thunderstorms in a certain area, that area will be placed under a “Slight Risk”. The risk areas can be triggered by the potential for Tornadoes, Hail, or Wind. For a breakdown, click here.

What To Do During Watches and Warnings

The main source of confusion when it comes to communicating severe thunderstorm risks are watches and warnings. Here’s a very simple breakdown of what they each mean.

Typical Terminology

Severe Thunderstorm Watch

A severe thunderstorm watch (also referred to as a blue box, or sva by meteorologists) is issued when forecasters at the National Weather Service believe that conditions are favorable for the development of organized severe thunderstorms — which are capable of producing winds or hail of severe criteria. This does not mean that your area is guaranteed to get a severe thunderstorm watch; rather that your area is included in a broad brushed potential focus area for severe weather.

Tornado Watch

A tornado watch (also referred to as a red box, or tw by meteorologists) is issued when meteorologists at the National Weather Service believe that conditions are favorable for the development of organized severe thunderstorms which are capable of producing tornadoes. Similarly to a severe thunderstorm watch, this watch is meant to inform the area included that they are in a focus area for potential thunderstorms capable of producing tornadoes.

Severe Thunderstorm Warning

A severe thunderstorm warning is issued by the National Weather Service when a thunderstorm is imminent, and is likely to produce conditions which will meet criteria for a severe thunderstorm. This means that either large hail or strong winds are likely, and capable of producing significant damage. During a severe thunderstorm warning, get or remain indoors and stay away from windows and doors.

Tornado Warning

A tornado warning is issued by the National Weather Service when meteorologists or trained weather spotters have observed, or detected via doppler radar, an occurring or imminent tornado touchdown. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation. When a tornado warning is issued, regardless of the weather outside, you should immediately go to a shelter. View recommended shelters here.

Tornado Watch Example

Severe Thunderstorm Safety

During any Severe Thunderstorm or Tornado Warning, the safest place for you to be is in a sturdy room and away from windows. A Tornado Warning is obviously the more serious of the two, and should be your indication to get to a basement immediately. If you don’t have a basement, find the sturdiest room (surrounded by sturdy walls) near the middle of your house.

If you are outside, do everything possible to get indoors but do not put yourself in additional danger to do so. If the storm is already nearby, go into a hardtop automobile and wait the storm out. Get away from boats and bodies of water, and try to avoid being near trees. Never — ever — shelter under an overpass. Remain in a safe place until the storm has completely passed and the lightning, wind, hail or tornado threat is over.