-> Severe Thunderstorms
Severe Weather Center
Latest Severe Weather Outlook
Updated: 12/1 8:01 am
This page will not be updated until spring unless conditions warrant a special update.
Severe Thunderstorm Information
Severe thunderstorms can strike Central New York at any time of the year, but are most common in May through August. Typically, most severe thunderstorms in our area contain strong, damaging wind gusts. Large hail can also be a problem, especially to agricultural interests. Tornadoes can and do also occur, though they are typically weaker and short lived.
Development of Severe Storms
There are four main ingredients needed for the development of severe thunderstorms. If even one ingredient is missing, or is very weak, widespread severe storms may not be able to form. It is only when all four ingredients come together strongly that we get big severe weather outbreaks.
Watch or Warning?
Watch- Conditions are favorable for the development of thunderstorms capable of producing severe weather/tornadoes (severe thunderstorm and tornado watches, respectively). Be alert for dangerous weather.
Warning- A severe thunderstorm/tornado has been indicated by doppler radar or spotted. Take cover immediately!
The first ingredient is moisture. As moist air evaporates and rises, it cools and condenses into clouds. These clouds can then grow into thunderstorms. Two common ways of measuring the moisture are the surface dew point and precipitable water (PWAT for short). Dew points in the 60s and 70s are favorable for thunderstorms, as are precipitable water values over 1".
The next ingredient is instability. Instability is the ability for air that begins to rise to keep rising at a faster and faster rate. Instability allows thunderstorms to create updrafts, which in turn causes the clouds to grow. The higher the instability, the quicker and more vigorously a storm will grow. Instability is measured in a parameter called CAPE, which stands for Convective Available Potential Energy. CAPE values over 1500 are generally needed for robust severe thunderstorms. However, if the other ingredients for severe storms are strong, severe weather can occur with CAPE values under 1000. An atmosphere characterized by hot and humid weather at the surface and cool temperatures in the mid-levels of the atmosphere would have a high CAPE value.
When ingredients come together right, thunderstorms can develop so fast, you can literally see the cloud growing.
The third ingredient is a trigger. Something needs to happen in the atmosphere to begin the thunderstorm building process. It is possible to have a very moist, high CAPE atmosphere, yet have no thunderstorms if a trigger is lacking. A trigger acts to give the air the initial upward movement. Once the trigger moves the air up, the air will rise faster and faster due to the instability, and the moisture in the atmosphere will condense into a cloud as the air rises and cools. Fronts (such as a cold front, where cold air pushes warmer air out of an area), wind shifts, lake/sea breezes (winds coming inland off of a body of water), winds from other thunderstorms and even terrain can act as a trigger for thunderstorms.
The first three ingredients are all that is needed to develop thunderstorms. However, without the forth ingredient, the severe thunderstorm potential will be isolated at best. To get widespread and very strong thunderstorms, wind shear is needed. Wind shear is the change in both speed and direction of the wind with height. The stronger the wind shear, the more likely severe storms may be. If the winds are increasing in speed with height, but remain coming from the same direction, storms will tend to form lines with damaging winds and hail. If the winds change both speed and direction, storms will tend to stay more isolated, but will likely rotate. These rotating storms, called supercells, can produce large hail, damaging winds and tornadoes. Occasionally, a line of storms will change the wind field around it and tornadoes will also form within the line itself.
Severe Weather Threats
Lightning photograph sent to Grotonweather by Trent O'Shea on May 3, 2012.
- Every thunderstorm has the potential to be deadly. Lightning is caused by a build of of static electricity in the atmosphere. This static is caused in part by ice particles within the cloud rubbing up against one another. The stronger the winds are in a thunderstorm, the more the ice particles will bump. This is why strong thunderstorms often (but not always) have frequent lightning. Thunder is a shock-wave created when lightning strikes as the air is instantly heated to tens of thousands of degrees. The old rule-of-thumb that says a storm is 1 mile away for every 5 seconds thunder is delayed after hearing lightning is more or less true. If you are close enough to a storm to hear the thunder, you are close enough to be hit by lightning and should take shelter inside a building and away from windows immediately. Using land-line telephones, the sink or shower, and even computers or other devices that are plugged in can be dangerous during thunderstorms as well.
Damaging Winds- As a thunderstorm begins to produce rain, some of the rain will invariably evaporate on its way to the ground. As it does so, it cools the air beneath the thunderstorm. Since cold air is more dense than warm air, it sinks to the ground. This is called a downdraft. The same instability that makes rising air rise quicker also makes sinking air sink quicker. As a result, thunderstorm downdrafts can become stronger in unstable environments.
Cool air associated with the outflow from a thunderstorm condensed into a fast moving cloud near the ground north of Groton in 2008.
Once the downdraft reaches the ground, it spreads out. This is called outflow, and can often be felt as a gust of cool wind ahead of a thunderstorm. Outflow can trigger new storms in some environments. When a particularly strong and damaging downdraft strikes the ground, it is called a micoburst. Winds in microbursts can sometimes exceed 100mph. Squall lines with strong winds develop when the wind shear allows the thunderstorm to continue growing while at the same time produce destructive downdrafts. Winds over 58mph are considered to be severe by the National Weather Service. If a damaging wind event is possible, it is wise to tie down or bring in objects that may get blown away. Next to falling trees and power lines, most wind damage is probably caused by objects being blown into other objects. Staying inside, away from windows, and on the lowest level of the building are the best ways to stay safe from thunderstorm winds.
Cutting a hailstone open reveals rings of clear and opaque layers formed by the melting and refreezing process.
- Hail is formed in thunderstorms that have robust upward motion. As water droplets get pushed higher and higher into the cloud, they freeze. Eventually, they become heavy enough to fall or get pushed out of the main updraft. As the ice particles fall, the outside melts slightly and rain drops adhere to the ice particle. If they then get caught back up in the updraft, a new layer of ice forms and the process repeats itself. When reporting hail sizes, it is best to measure the diameter of the largest hailstone if possible. Otherwise, comparing the size to a common object (such as coins or specific types of balls) is used to estimate the size. Reporting 'marble size hail' is not overly useful though, as marbles come in a variety of sizes. Hail an inch or larger in diameter is considered severe by the National Weather Service and can damage crops, buildings, vehicles and injure people and animals. The largest hail typically seen in a Central New York summer is around 2", but larger hail has occurred.
Tornadoes- The exact process that causes a tornado to be formed is still a bit of a mystery to the meteorological community. Tornadoes come from rotating thunderstorms called supercells. Many thunderstorms rotate, but only a small percentage produce tornadoes.
Without video to confirm rotation, it is nearly impossible to tell if this is scud or a tornado. Sent to Grotonweather by Sara Powell. Taken July 26, 2012 near Conklin, NY.
Many people often mistake 'scud clouds' as a tornado due to their rapid movement and ragged appearance beneath thunderstorms. However, the clouds must be rotating for it to possibly be a tornado. If a tornado warning is issued, immediately go to an interior room in the lowest level of a sturdy building. Taking heavy blankets or even a mattress to cover yourself can protect you from small debris. Monster tornadoes, such as the infamous Joplin, Missouri tornado in May 2011, are extremely rare or unknown to most of the Northeast. Most tornadoes in our area are weak to moderate in strength. Calling a tornado 'weak' can be misleading though, as even weak tornadoes can have winds nearing 100mph and can be deadly.
Don't drive through flood waters. Really. It isn't worth the risk.
- Flooding kills more people in the United States each year than the previous four threats combined. Flash flooding can occur anywhere a thunderstorm produces torrential rain. Most commonly, urban areas and small streams will flood, creating dangerous driving conditions. If you approach a flooded roadway in a vehicle, do not attempt to cross it! There are two main dangers with this. First, just six inches of swiftly moving water can lift a car off the ground and sweep it downstream. Secondly, since flood waters are muddy, it is impossible to tell how deep the water really is. Many deaths due to flooding occur because the roadway has been washed out under the flood waters, but the victim had no way of knowing. If you live in a flood prone area, keep an eye on water levels, as they can rapidly rise.
If you have any questions regarding thunderstorms or severe weather, feel free to contact me!