Good evening from Southern Connecticut Weather!
Some love them.
Some hate them.
For some, they invoke good memories and a sense of nostalgia, a reminder of the change in season. For others, the memories were scarring, invoking fear and dread. Thunderstorms are a seasonal rite of passage in our part of the world where we have four distinct seasons.
Two severe weather days earlier this week, which produced wind damage in western and SW CT towns, heralded the arrival of summer weather. What leads to thunderstorm development in Connecticut? How often does severe weather actually happen? Why do some storms seem to fizzle when they get to my backyard?
Let's take a look.
Some love them.
Some hate them.
For some, they invoke good memories and a sense of nostalgia, a reminder of the change in season. For others, the memories were scarring, invoking fear and dread. Thunderstorms are a seasonal rite of passage in our part of the world where we have four distinct seasons.
Two severe weather days earlier this week, which produced wind damage in western and SW CT towns, heralded the arrival of summer weather. What leads to thunderstorm development in Connecticut? How often does severe weather actually happen? Why do some storms seem to fizzle when they get to my backyard?
Let's take a look.
Above: A 2023 photo from southeast CT--taken by one of my dear friends. Note how lightning is striking in a storm on the left, but there's clear skies just to the right! This is a great reminder that it doesn't necessarily need to be raining where you are to be very close to lightning. When thunder roars, head indoors!
What Makes A Thunderstorm
Thunderstorms are a critical element in nature's effort to bring balance to the atmosphere. If you notice, we almost never see thunderstorms occur on dry 70 degree and sunny days. Thunderstorms tend to be a summer phenomenon in Connecticut, during the season where we have the ingredients in the atmosphere for their development.
For a thunderstorm (convection) to develop you generally need four elements in play: Shear, Lift, Instability, and Moisture (SLIM for short). In Connecticut, some of these are easier to get than others, and timing and location in Connecticut matters a lot. You don't need me to tell you that NW CT most often sees strong to severe storms while it is less common in far SE CT.
What Makes A Thunderstorm
Thunderstorms are a critical element in nature's effort to bring balance to the atmosphere. If you notice, we almost never see thunderstorms occur on dry 70 degree and sunny days. Thunderstorms tend to be a summer phenomenon in Connecticut, during the season where we have the ingredients in the atmosphere for their development.
For a thunderstorm (convection) to develop you generally need four elements in play: Shear, Lift, Instability, and Moisture (SLIM for short). In Connecticut, some of these are easier to get than others, and timing and location in Connecticut matters a lot. You don't need me to tell you that NW CT most often sees strong to severe storms while it is less common in far SE CT.
Above: A National Weather Service image of the ingredients necessary for thunderstorm development.
The amount of these ingredients vary based on location. This is why thunderstorms don't just blow up over every part of the state at the same time. Usually, you need to reach a critical mass and a trigger (Lift) to initiate storms. When you have high enough amounts of each, you can end up with severe thunderstorms.
Severe thunderstorms are defined as a storm with 58 mph or greater wind gusts, one inch (quarter size) hail or greater, or a tornado. Of course, we have all kinds of "non-severe" hazards with thunderstorms, including flash flooding and lightning.
Let's look at each ingredient a little closer.
Shear
Shear is the change in wind speed (speed shear) and/or direction (directional shear) with height in the atmosphere.
Speed and directional shear are important in severe thunderstorm development. With speed shear, a significant increase of wind with height will tilt a storm’s updraft. This allows an updraft and downdraft to occur in separate regions of the storm and reduces the risk of water loading and thunderstorm collapse.
Directional shear helps initiate the development of a rotating updraft. This is key in the formation of a supercell—the most mature and dangerous type of thunderstorm. Supercells are thunderstorms consisting of one quasi-steady to rotating updraft (I.E. storm with rotation). These are the storms most likely to produce large hail, strong winds, and tornadoes. It doesn't need to look like the picturesque Midwest supercells to be dangerous. In order to get a severe weather day, we need some sort of shear. You can have a high CAPE/low shear or high shear/low CAPE day, but you need some level of wind shifting aloft for severe weather.
The highest shear days come with synoptic (large scale) systems that include areas of low pressure, deep troughs, and tropical systems.
The amount of these ingredients vary based on location. This is why thunderstorms don't just blow up over every part of the state at the same time. Usually, you need to reach a critical mass and a trigger (Lift) to initiate storms. When you have high enough amounts of each, you can end up with severe thunderstorms.
Severe thunderstorms are defined as a storm with 58 mph or greater wind gusts, one inch (quarter size) hail or greater, or a tornado. Of course, we have all kinds of "non-severe" hazards with thunderstorms, including flash flooding and lightning.
Let's look at each ingredient a little closer.
Shear
Shear is the change in wind speed (speed shear) and/or direction (directional shear) with height in the atmosphere.
Speed and directional shear are important in severe thunderstorm development. With speed shear, a significant increase of wind with height will tilt a storm’s updraft. This allows an updraft and downdraft to occur in separate regions of the storm and reduces the risk of water loading and thunderstorm collapse.
Directional shear helps initiate the development of a rotating updraft. This is key in the formation of a supercell—the most mature and dangerous type of thunderstorm. Supercells are thunderstorms consisting of one quasi-steady to rotating updraft (I.E. storm with rotation). These are the storms most likely to produce large hail, strong winds, and tornadoes. It doesn't need to look like the picturesque Midwest supercells to be dangerous. In order to get a severe weather day, we need some sort of shear. You can have a high CAPE/low shear or high shear/low CAPE day, but you need some level of wind shifting aloft for severe weather.
The highest shear days come with synoptic (large scale) systems that include areas of low pressure, deep troughs, and tropical systems.
Above: a diagram of a thunderstorm. Note how warm air in advance of the storm gets "sucked" in while the cool downdraft leads to the precipitation that brings us our weather. With shear, the structure is allowed to grow and the thunderstorm intensifies.
Lift
Lift is a critical ingredient for thunderstorm development. Even if you have an oppressively hot and humid day, that's usually not enough for a thunderstorm. If you don't have some lift, which comes in the way of a forcing mechanism, not much is going to happen.
Lift provides the atmosphere what it needs to rise and form convection. It happens a number of ways around the country, but in this region, if we're looking at statewide convection we’re usually looking one major mechanism: a front.
Lift
Lift is a critical ingredient for thunderstorm development. Even if you have an oppressively hot and humid day, that's usually not enough for a thunderstorm. If you don't have some lift, which comes in the way of a forcing mechanism, not much is going to happen.
Lift provides the atmosphere what it needs to rise and form convection. It happens a number of ways around the country, but in this region, if we're looking at statewide convection we’re usually looking one major mechanism: a front.
Above: an image showing the four major types of lift. Lift happens year round, but in the summer the type of lift helps determine how far along storms will get.
Convective lift happens on days where you may see the pop up shower or thunderstorm. This is when the sun heats the area so much the surface heating creates its own lift. For severe thunderstorm purposes here it's the weakest kind of lift and doesn't usually create severe storms or even strong thunderstorms in CT.
Orographic lift is something that actually helps in NW CT due to geography. The mountains may not be high, but the rising air from the Hudson River Valley into the Berkshires/NW Hills can lead to organized thunderstorm development. Once that lift is lost as storms move east, they can collapse absent another potent forcing mechanism.
Convergent lift is when low pressure moves into the region and the convergence of air near the low results in lift. As mentioned above, this can coincide pretty well with out high shear days.
Finally, we have our most common mechanism for widespread thunderstorm initiation--frontal lift. They don't just come from the north as a cold front passes, though that's probably most common. Along the coast if there is a strong enough sea breeze it can bring a frontal boundary inland that can become a focal point for thunderstorms. These boundaries don't go too far inland, but can lead to storms in southern CT. Fronts are the boundary between two air masses of different temperatures and air densities. When the cold front (or sea breeze front) rolls through, the colder and more dense air behind the front lifts warmer and less dense air abruptly. When all the ingredients are together in the right quantities, you have showers and thunderstorms.
In less frequent scenarios, we have warm fronts lifting north across the region. These are particularly noteworthy because they usually happen in an environment that is more favorable for supercells and tornadic activity.
Instability
Instability is another critical element of thunderstorm production, and the more instability there is, the more potential you add to a developing storm.
Instability is the tendency for air parcels to accelerate (especially upward) when they are displaced from their original position. You don’t need a lot of instability for a garden variety thunderstorm, but you need Instability and usually significant amounts of it for for severe weather.
An unstable air mass is usually characterized by warm moist air near the surface and colder drier air aloft. This allows air that is forced upward to continue to rise on its own, which eventually will cool and develop the clouds and precipitation that make up a thunderstorm. High instability also contributes to prolific lightning events.
Convective Available Potential Energy, or CAPE—is a measure of the energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft.
As the National Weather Service notes, observed values in thunderstorm environments often exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg. Having 5,000 CAPE around here is unheard of, but If we’re talking about a high CAPE day, your ears should perk up.
There are various measures of CAPE out there, including surface based CAPE (SBCAPE), mixed layer or mean layer CAPE (MLCAPE), most unstable or maximum usable CAPE (MUCAPE), and downdraft CAPE (DCAPE). Each tell us something a little different about the kind of thunderstorms we may see.
Moisture
The last ingredient, moisture, is often the first ingredient we look to for thunderstorm potential. Think of a hazy, hot, and humid summer day. If you asked people what they remember about the end of those days, many will answer that they remember the thunderstorms. Moisture is a key element in thunderstorm development. Without moisture, there's no precipitation.
We often have ample moisture available. On days with a southerly breeze, we are getting moisture directly from the Atlantic Ocean. With more humidity in the last 30 years, we've had even more moisture to wring out of the atmosphere on thunderstorm days. if you want to know what kind of moisture we’re dealing with during this time of year, look at the dew point—which is a measure of atmospheric moisture. A dew point is the temperature to which air must be cooled (generally) in order to reach saturation. The higher the dew point, the more moisture in the air. Generally, you need dew points 60 and above for thunderstorms. Storms generally don't happen in the winter and spring (among other reasons) because the air is too dry.
Why Do Storms Fizzle In My Backyard?
As I mentioned above, the ingredients for thunderstorms are not equal across the state at the same time. That's why often times it's harder for central and especially eastern CT have trouble getting big thunderstorms. Yesterday is a great example of this.
Convective lift happens on days where you may see the pop up shower or thunderstorm. This is when the sun heats the area so much the surface heating creates its own lift. For severe thunderstorm purposes here it's the weakest kind of lift and doesn't usually create severe storms or even strong thunderstorms in CT.
Orographic lift is something that actually helps in NW CT due to geography. The mountains may not be high, but the rising air from the Hudson River Valley into the Berkshires/NW Hills can lead to organized thunderstorm development. Once that lift is lost as storms move east, they can collapse absent another potent forcing mechanism.
Convergent lift is when low pressure moves into the region and the convergence of air near the low results in lift. As mentioned above, this can coincide pretty well with out high shear days.
Finally, we have our most common mechanism for widespread thunderstorm initiation--frontal lift. They don't just come from the north as a cold front passes, though that's probably most common. Along the coast if there is a strong enough sea breeze it can bring a frontal boundary inland that can become a focal point for thunderstorms. These boundaries don't go too far inland, but can lead to storms in southern CT. Fronts are the boundary between two air masses of different temperatures and air densities. When the cold front (or sea breeze front) rolls through, the colder and more dense air behind the front lifts warmer and less dense air abruptly. When all the ingredients are together in the right quantities, you have showers and thunderstorms.
In less frequent scenarios, we have warm fronts lifting north across the region. These are particularly noteworthy because they usually happen in an environment that is more favorable for supercells and tornadic activity.
Instability
Instability is another critical element of thunderstorm production, and the more instability there is, the more potential you add to a developing storm.
Instability is the tendency for air parcels to accelerate (especially upward) when they are displaced from their original position. You don’t need a lot of instability for a garden variety thunderstorm, but you need Instability and usually significant amounts of it for for severe weather.
An unstable air mass is usually characterized by warm moist air near the surface and colder drier air aloft. This allows air that is forced upward to continue to rise on its own, which eventually will cool and develop the clouds and precipitation that make up a thunderstorm. High instability also contributes to prolific lightning events.
Convective Available Potential Energy, or CAPE—is a measure of the energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft.
As the National Weather Service notes, observed values in thunderstorm environments often exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg. Having 5,000 CAPE around here is unheard of, but If we’re talking about a high CAPE day, your ears should perk up.
There are various measures of CAPE out there, including surface based CAPE (SBCAPE), mixed layer or mean layer CAPE (MLCAPE), most unstable or maximum usable CAPE (MUCAPE), and downdraft CAPE (DCAPE). Each tell us something a little different about the kind of thunderstorms we may see.
Moisture
The last ingredient, moisture, is often the first ingredient we look to for thunderstorm potential. Think of a hazy, hot, and humid summer day. If you asked people what they remember about the end of those days, many will answer that they remember the thunderstorms. Moisture is a key element in thunderstorm development. Without moisture, there's no precipitation.
We often have ample moisture available. On days with a southerly breeze, we are getting moisture directly from the Atlantic Ocean. With more humidity in the last 30 years, we've had even more moisture to wring out of the atmosphere on thunderstorm days. if you want to know what kind of moisture we’re dealing with during this time of year, look at the dew point—which is a measure of atmospheric moisture. A dew point is the temperature to which air must be cooled (generally) in order to reach saturation. The higher the dew point, the more moisture in the air. Generally, you need dew points 60 and above for thunderstorms. Storms generally don't happen in the winter and spring (among other reasons) because the air is too dry.
Why Do Storms Fizzle In My Backyard?
As I mentioned above, the ingredients for thunderstorms are not equal across the state at the same time. That's why often times it's harder for central and especially eastern CT have trouble getting big thunderstorms. Yesterday is a great example of this.
Here's the radar image of the storms moving through the state yesterday. The got further because there was more lift, but lift was strongest in western CT. Second, note the time. In NW CT, when you were coming off the peak heating of the day, there's more energy to work with at 6:30pm compared to eastern CT around 8pm and later.
Then there's a subtle feature around Waterbury. The severe storm produced an outflow boundary. Basically downward flowing air forced out cooler and drier air ahead of what were potent storms, leading to more stability in the atmosphere ahead of the storms and the storm collapsing.
Timing matters quite a bit. The earlier mid-afternoon storms tend to take advantage of greater instability. That tends to allow storms to get further in the state. But the southerly wind ahead of these fronts often pull in more stable air, making it harder for storms to sustain themselves as the propagate east.
Then there's a subtle feature around Waterbury. The severe storm produced an outflow boundary. Basically downward flowing air forced out cooler and drier air ahead of what were potent storms, leading to more stability in the atmosphere ahead of the storms and the storm collapsing.
Timing matters quite a bit. The earlier mid-afternoon storms tend to take advantage of greater instability. That tends to allow storms to get further in the state. But the southerly wind ahead of these fronts often pull in more stable air, making it harder for storms to sustain themselves as the propagate east.
Above: The life cycle of a thunderstorm. Storms are usually, not not always, maturing in western and central CT, and dissipating in coastal and eastern Connecticut.
Thunderstorm Climatology
Finally, it's no surprise that our severe weather really takes place in the summer. While May tends to be the kickoff of our convective season, by far the most severe weather takes place in Meteorological Summer--June, July, and August. The worst tends to be centered in July, when we have our warmest and most humid period of the summer. By August we're on the decline and not how quickly September falls off!
Thunderstorm Climatology
Finally, it's no surprise that our severe weather really takes place in the summer. While May tends to be the kickoff of our convective season, by far the most severe weather takes place in Meteorological Summer--June, July, and August. The worst tends to be centered in July, when we have our warmest and most humid period of the summer. By August we're on the decline and not how quickly September falls off!
This year, when we have the expectation of more troughs hanging around and frontal boundaries moving through the region, it looks like it may be an active year for storms.
Severe Thunderstorm and Tornado Watches are issued by the Storm Prediction Center (SPC) in Norman, OK. Severe Thunderstorm and Tornado Warnings are issued by the local forecast office of the National Weather Service. In Litchfield County, that means NWS Albany issues your warnings. In Hartford, Tolland, and Windham County that means NWS Boston is issuing your warnings. In our southern four counties, your warnings are issued by NWS NYC.
In advance of storm days, the SPC issues outlooks for the country. Here are the categories they use. For Connecticut, a slight risk tends to be the highest category we reach, though each season an enhanced can happen once or twice. A moderate risk is exceedingly rare in CT.
Severe Thunderstorm and Tornado Watches are issued by the Storm Prediction Center (SPC) in Norman, OK. Severe Thunderstorm and Tornado Warnings are issued by the local forecast office of the National Weather Service. In Litchfield County, that means NWS Albany issues your warnings. In Hartford, Tolland, and Windham County that means NWS Boston is issuing your warnings. In our southern four counties, your warnings are issued by NWS NYC.
In advance of storm days, the SPC issues outlooks for the country. Here are the categories they use. For Connecticut, a slight risk tends to be the highest category we reach, though each season an enhanced can happen once or twice. A moderate risk is exceedingly rare in CT.
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Thank you for reading SCW.
-DB
Thank you for reading SCW.
-DB
