With summer comes another season however, severe weather season. Much like last year, this post will give you the basics of thunderstorm development, severe weather climatology in Connecticut, and what we should expect this season.
Severe weather can happen at anytime. All it takes are the right ingredients. Last season was a decent one for the state, complete with outbreaks as early as February to ones right in the heart of summer. However, I usually define severe season as May through September. The peak of the season is usually June and July.
Why is May the start of severe weather season in Connecticut? Let’s take a look at climatology.
Average May 1 High: 67
Average May 1 Low: 43
Average May 31 High: 75
Average May 31 Low: 52
Average May 1 High: 63
Average May 1 Low: 46
Average May 31 High: 72
Average May 31 Low: 56
Usually, at some point in May we begin breaking toward more of a summer like regime. This provides early opportunities for the ingredients of thunderstorms to come together. With warmer temps, conditions become more favorable for the development of convection—the vertical transport of heat and moisture in the atmosphere, especially through updrafts and downdrafts in an unstable atmosphere. It is important to note that often, the terms "convection" and "thunderstorms" are used interchangeably, but thunderstorms are only one form of convection. The vertical transport of heat and moisture is critical to “boomer” formation.
Have you ever seen a cumulus tower? Of course you have! CU towers are an example of convection at play and is often a precursor to a summertime rain shower or thunderstorm!
Ok, so what do we need for thunderstorm formation? Essentially, you need four things: Moisture, Instability, Lift, and for severe weather, add an abundance of the fourth--wind shear.
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, you’re not going to have precipitation! Our most humid days usually occur on a southerly or southwest flow. Moisture comes straight off the Atlantic and sometimes even the Gulf of Mexico.
What to watch for: if you want to know what kind of moisture we’re dealing with during this time of year, look at the dewpoint—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 dewpoint, the more moisture in the air.
During the summer, dewpoint is directly related to how humid it is. On a 90 degree day, a 70 degree dewpoint is not something most people what to see.
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 when they are displaced from their original position, especially upward after being lifted. You don’t need a lot of instability for a garden variety boomer, but you need Instability and lots of it for for severe weather.
An unstable air mass is usually characterized by warm moist air near the surface and cold dry 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 precip that make up a thunderstorm. High instability also contributes to prolific lightning events, which isn’t characterized as severe weather by the NWS but is deadly in its own right.
What to watch for: 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. Now, 5k 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 normalized CAPE (NCAPE). MLCAPE is very important to me, and that is usually the first place I look before looking at other parameters.
Lift is our third ingredient for thunderstorm development. You can have moisture and instability, but if you don’t have lift, not much is going to happen. Lift provides the air the nudge it needs to rise and form convection. It happens a number of ways around the country, but in this region, we’re usually looking one major mechanism.
The most common mechanism around here for thunderstorm initiation is the 'ole (cold) front. Fronts are the boundary between two air masses of different temperatures and air densities. When the cold 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.
The Fourth Ingredient: Shear
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 shear for severe weather.
Shear—is the change in wind speed (speed shear) and/or direction (directional shear) with height in the atmosphere. Shear usually refers to vertical wind shear.
Speed and directional shear is 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.
What to watch for: areas of low pressure, especially tropical systems upper level lows, can bring high shear days.
The Wildcard: The Cap
Last year, I talked about the great wildcard--Elevated Mixed Layers, or EMLs. This year I want to bring things closer to home. Ever wonder why so many days with high humidity and instability don't seem to produce storms? Often times, it is the capping inversion, or cap, which is responsible.
An inversion is essentially a layer of warmer air aloft that effectively serves as a lid on convective development.
Ok, so we’ve gone over the big ingredients for thunderstorm development. Well, how in the world does all of that come together for the storms we see every summer?
Most thunderstorms develop and die in three stages. These stages are: the development stage—when you see the cumulus towers form; the mature stage—where the storm is fully formed and does its damage; and the dissipating stage—when storms weaken and die.
It begins when warm and moist air is lifted upward. This is the updraft that often creates our cumulus clouds and towers. The moisture in the air condenses into water droplets as it rises into a cooler environment. The cloud will continue to grow in height as long as warm air from below continues to rise.
As mentioned before, there are several ways that an updraft of warm and moist air can form. Fronts are typically the best mechanism, but often times, updrafts form without any guidance because warm air naturally tends to rise. Air near the ground heats up during the day as energy from the Sun heats the ground, which then heats the air. Because warm air has less density than cool air, the warmer air rises higher in the atmosphere.
The Mature Stage
As the cumulus cloud continues to grow, the water droplets within it grow larger as additional moisture from rising air is added to the droplets. The cloud will begin to look darker as more water is added to it. This process adds weight to the droplets, and raindrops will start to fall through the cloud when the rising air can no longer hold them up. For hail production, the updraft is so strong that the water droplets freeze in the higher column of the storm until the updraft cannot support the hailstone. At the same time, cool and dry air flows downward in the cloud, called a downdraft, pulling water downward as rain. With an updraft, downdraft, and rain, the cloud is now called a cumulonimbus cloud and the cycling of air up and down is called a thunderstorm cell. In this stage, the intense updraft and addition of shear can create a supercell, which is mesoscale cyclone (storm with rotation) that drops tornadoes, hail, and very strong winds.
The moving air within the cloud builds up electric charges as it slides past other air. This allows lightning to form, similar to the classic example of creating a spark after moving your feet across a carpet. Thunder is the sound that happens when lightning strikes, and happens after you see the bolt of lightning because sound travels more slowly than light. Lightning can strike many miles away from an actual rain shaft or thunderstorm cloud. A lightning strike is approximately one mile further for every five seconds between the flash and the thunder. When thunder roars, head indoors.
The Dissipating Stage
When the downdrafts in the cloud become stronger than the updraft, the storm starts to weaken. Since warm moist air can no longer rise, cloud droplets can no longer form. The storm dies out with light rain as the cloud disappears from bottom to top. The whole process takes about one hour for an ordinary thunderstorm. Severe thunderstorms and squall lines are much larger, more powerful, and can last for several hours.
Allow me to be Captain Obvious for a second: Connecticut is not Oklahoma. The major threats we face here are not EF-5 tornadoes, but rather large hail and strong winds. Unlike synoptic or large scale events like snowstorms, severe weather is inherently local, with isolated areas getting legitimate severe weather and places just a few miles away a lot of sound but not much fury. For property owners this is a good thing, generally, if you are on the right side of a supercell. For forecasters though, this is a nightmare, as predicting mesoscale (small scale) details far in advance is next to impossible.
Widespread severe weather outbreaks, especially in New England, are relatively rare. There are two reasons for this.
The first is the definition of a severe thunderstorm, which is a high standard to meet:
A Severe Thunderstorm is defined as a thunderstorm that produces wind gusts of at least 58 mph and/or hail 1 inch in diameter (quarter size) or larger. Any thunderstorm that produces a tornado (or waterspout) is a severe thunderstorm, but those are warned differently.
The second reason is location:
New England is located in a more stable (generally) environment. The ingredients and mechanisms that produce severe thunderstorms do not align here very often, and when they do, the parameters that create big outbreaks (high CAPE/high shear) are often on the lower end of the severe scale than in the midwest. This results in generally weaker events, but that does not mean CT cannot see high end severe weather events.
Connecticut is actually an interesting space when it comes to severe weather, in particular, tornado climatology.
With complex mountainous and valley terrain and the presence of boundaries (think lift) that form commonly along Long Island Sound, CT seems to be in a location that is more vulnerable than most to tornadoes in New England.
That said, wind and hail are the bigger severe producers around here. Wind is the most common severe weather report. Straight line winds can be as powerful as hurricane force winds!
Let’s take a look at how we define our watches and warnings.
-A Severe Thunderstorm Watch is issued by the Storm Prediction Center (SPC) in Norman, Oklahoma for large portions of the region when the potential exists for severe thunderstorms.
-A Severe Thunderstorm Warning is issued by the local National Weather Service Forecast Office when severe thunderstorms are imminent based on radar or already occurring based on spotter observations.
-A Tornado Watch is issued by the Storm Prediction Center in Norman, Oklahoma for large portions of the region when the potential exists for tornadoes.
-A Tornado Warning is issued by the local National Weather Service Forecast Office when a tornado is imminent/occurring based on radar or already occurring based on spotter observations.
Finally, it is important to know what the SPC outlook is. This is an outlook that extends from day 1 into the longer range to give individuals a sense of what to expect in each region with regard to severe weather.
As mentioned before, the peak for our region is June and July. A sharp drop off traditionally happens during August and into September. Although much of the country has been well above average in terms of severe reports, Connecticut has been quiet. Overall, I think this year has the potential to be near normal for CT with the expectation of a continued warm pattern and a ridge/trough orientation somewhat favorable for active weather.
Finally, I want to spend some time on how to prepare for severe weather season. Much of this information comes straight from the National Weather Service.
-If you don't have one already, a NOAA Weather Radio is a great investment for overnight hours. With warning alarm tones, a weather radio will alert you when severe weather headlines are issued, providing you time to bring in furniture and prepare if you are in a place that usually loses power.
Lifeguards at beaches and pools have devices that can track thunderstorms and lightning. The same is true for athletic coaches, camp directors, and parks and recreation workers. Even without equipment, you can protect yourself by moving indoors to a place of safety at the first rumble of thunder. If you can hear the thunder, the storm is usually close enough for you to have the potential to be struck by lightning.
Lightning is an underrated killer. Lightning also leaves many victims with life-long serious injuries. Lightning can strike as far as 10 miles from the side of the thunderstorm cloud. In fact, many lightning victims are struck before the rain arrives or after the rain has ended and the storm is moving away. Many victims also report that at least a portion of the sky was blue when they were struck.
While inside a home or building
-Avoid any contact with corded phones.
-Avoid any contact with electrical or electronic equipment or cords that are plugged into the electrical system.
-Avoid any contact with the plumbing system. Do not wash your hands, do not wash the dishes, do not take a shower, or do not do laundry.
-Do not stand next to a concrete wall and do not lie on a concrete floor.
-Stay away from windows, outside doorways, and porches.
Tips while outdoors
-Watch for falling debris while driving.
-Plan outside activities so that you minimize the risk of being caught outside in a thunderstorm.
-If you hear thunder, move inside a safe shelter immediately. Generally, if you can hear the thunder, you're within striking distance of the storm. Golf courses, beaches, and pools are places where there are higher concentrations of injuries.
-If the sky looks threatening, move inside immediately. Don't wait for the first stroke of lightning. It could occur anywhere under or near the storm.
-Stay inside a safe shelter for at least 30 minutes after the last rumble of thunder was heard. Many lightning victims are struck after the worst part of the storm has passed.
-If you are caught outside in a thunderstorm and can't reach a safe shelter, you can only minimize your risk of being struck by lightning. If lightning strikes near you, it will most likely strike the tallest object in your immediate vicinity. Don't be the tallest object in the immediate vicinity and don't be near the tallest object. Second, get as low as possible to the ground, but minimize your contact with the ground. Do not lie on the ground.
This was a long post, but we're only touching the tip of the iceberg of what makes severe weather happen. Hopefully this will be something you can refer to during severe weather season. Rest assured, SCW will be here on Facebook and Twitter @SouthernCTWX to track storms and help you plan ahead. Please like, share, comment and ask questions. That's what we're here for!