Severe Weather Awareness Week - 2025: New York and New England Severe T'Storms

April 20-26, 2025 is severe weather awareness week in New York and New England and in conjunction with the National Weather Service, this article will look at the various types of warm season localized severe weather that occurs with the science behind how it forms, the impacts from it, how the warning process works, and what to do to stay informed and how to stay safe. In addition to this article on severe thunderstorms, look for the articles listed below to publish here at cbs6albany.com on the corresponding dates.

• April 21: Severe Weather Awareness Week - 2025: New York and New England Severe T'Storms
  
• April 22: Severe Weather Awareness Week - 2025: New York and New England Tornadoes
  
• April 23: Severe Weather Awareness Week - 2025: New York and New England Lightning
  
• April 24: Severe Weather Awareness Week - 2025: New York and New England Flooding
  
• April 25: Severe Weather Awareness Week - 2025: How to Prepare and Stay Informed
  

SEVERE THUNDERSTORMS
The severe weather season across eastern New York and western New England generally runs from mid to late April through September with peaks during the second two weeks in May through the first two weeks in June, and sometimes again in August. However, it's important to know that severe thunderstorms with damaging wind, hail, frequent lightning, and flash flooding can occur at any time of the year, although far less frequently in the cold season. Despite the damage that sometimes can occur, thunderstorms are important in maintaining the earth-atmosphere electrical system with an estimated 16 million occurring each year globally and according to NOAA's National Severe Storms Laboratory about 2000 in progress at any one time. In the United states approximately 100,000 thunderstorms will occur in an average year with 10% of those classified as severe.

SEVERE THUNDERSTORM DEFINITION
A thunderstorm is classified as severe if it produces one or all of the following, 1" diameter hail (quarter size) or larger, wind gusts of 50 knots (57.5 mph) or greater, or a tornado. Note: Lightning is not included in the definition of a severe thunderstorm as all thunderstorms, severe or not, generate lightning, which always commands respect.

Severe thunderstorm straight line winds can gust up to 120 mph in extreme cases with gusts locally more typically ranging from 60-90 mph. Winds of this magnitude are highly destructive to trees which was the case in Coxsackie (Greene County) and Stuyvesant (Columbia County) when a severe thunderstorm produced 70-90 mph winds along a 2 mile by 1/2 mile wide path on Wednesday July 7, 2021. Up to 100 large trees were blown down with some of them crashing through power lines onto houses and cars.

Similarly and again in Columbia County, damaging microburst winds from a tornado warned supercell thunderstorm up to 100 mph occurred along a 1/2 mile path slammed Claverack between 7:35 and 7:38pm on July 13, 2023 producing significant damage to trees.

Hail in severe thunderstorms can reach up to sizes larger than grapefruits which is rare, with quarter to golf ball size hail more common in some of the strongest thunderstorms locally. However, thunderstorms have on occasion generated hail up to the size of softballs which can be extremely destructive.

The largest officially recognized hail stone, to date, to have fallen in the United States fell on the town of Vivian, South Dakota on July 23, 2010 measuring an incredible 8" in diameter and having an 18.5" circumference. In New York the unofficial largest hailstones to have fallen both measured 4" in diameter (softball sized) with the first falling in Otsego County on August 16, 1951 (source: Weather Underground) and the second more recently in Amsterdam with an isolated tornadic supercell at 3:25pm on May 22, 2014. (This storm went on to produce the EF-3 Duanesburg tornado.) The image below shows the baseball sized hail from the May 22, 2014 storm that fell in Amsterdam.

Ping pong ball sized hail in Greenfield Center, Saratoga County on June 6, 2023 - Victoria McCoy

HAIL FORMATION - Hailstones form in thunderstorms when the updraft lofts water droplets into the sub-freezing higher levels of the atmosphere where the droplets freeze. Hail then grows inside the storm when both horizontal and vertical currents cause the tiny ice particles to collide with supercooled water droplets which then freeze on the surface. Hail falls to the ground when it grows so large the updraft can no longer support it, or when the updraft weakens. The larger the hailstone the stronger the thunderstorm updraft.

THUNDERSTORM FORMATION
Thunderstorms need three essential ingredients to form; moisture, rising unstable air, and something to lift that air. In the spring and summer, simple heating by the sun can do the trick. If the air aloft is cool, which it often is, the warmer parcel of air near the ground will rise and continue to rise until it hits a layer in the atmosphere that is warmer than it is. Hills or mountains often act as elevated heat sources aiding the process as well as mechanical lifting mechanisms like surface cold fronts or low pressure troughs which allow air to converge at low levels in the atmosphere and then rise. As the warm moist surface parcel of air rises, the water vapor it contains will cool, releasing heat and allowing for condensation to occur causing a cloud to form and grow.

Intense thunderstorms can reach heights of 50 to 60 thousand feet up into the atmosphere but more commonly range in height from 35 to 45 thousand feet in the Northeast. As the updraft rises into the sub-freezing air at those lofty heights some of the water droplets will freeze and grow, while others will remain in a supercooled state. The ice and water within the turbulent storm will collide which causes electrical charge to separate and ultimately results in lightning.

TYPES OF THUNDERSTORMS

Singlecell: Singlecell thunderstorms typically form on hot and humid spring or summer afternoons and consist of a single updraft and downdraft couplet forced by the heating of the sun. These types of storms generally have a life cycle of an hour or less and can produce a brief and very localized area of torrential rain, strong gusty downburst winds, and occasional to frequent lightning. Severe weather (wind and hail) is possible with single cell pulse storms, but is generally brief in duration and localized.

Multicell: Multicell thunderstorms are longer lasting and more organized than singlecell storms. They can either be severe or non severe and propagate as new updrafts form along the leading edge of rain-cooled air, otherwise known as a gust front, with individual cells lasting up to an hour while the system as a whole in some cases lasting for several hours and tracking across a wide area. Multicell storms can produce damaging wind, large hail, torrential rain and flash flooding as well as brief spin up tornadoes.

Supercell: Supercell thunderstorms are efficient severe weather producing machines comprised of tilted rotating updrafts, which can be as large as 10 miles in diameter, and rise to 50,000 feet with vertical velocities in the updraft of 150 mph or greater. The center of rotation in the storm is called a mesocyclone which can sometimes lower to the ground in a narrow column of violently rotating air otherwise known as tornado. The vast majority of damaging tornadoes are produced by supercell thunderstorms along with damaging straight line wind and giant hail. 80% of supercell thunderstorms go on to produce some type of severe weather.

Squall Line: A squall line is a grouping of thunderstorms oriented into a linear formation. Squall lines are typically narrow, ranging from 10-20 miles wide, but can extend for hundreds of miles in length. Intense squall lines can produce damaging wind over large areas, hail, and spin up tornadoes. A subset of the squall line is the derecho which is a linear highly organized complex of thunderstorms that produces widespread significant wind damage over a long duration and over a great distance. By definition, if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph or greater along most of its length, then the event can officially be classified as a derecho. The most recent derecho, to date, to impact our area plowed through much of eastern New York and western New England on October 7, 2020.

Severe thunderstorms produce damaging wind gusts by either mixing high momentum air aloft down to the ground or through accelerations caused by a descending cold pool of air that develops aloft through the evaporative cooling process that occurs when rain falls through an elevated dry layer. So, in cases when there is mid level dry air being entrained into a thunderstorm, that parcel of air will cool when rain falling through it evaporates. As the air cools it gets heavy where gravity then takes over and helps to pull it down to the ground quickly. That big rush of downward moving air can accelerate up to 120 mph in extreme cases and come blasting out of the base of a thunderstorm causing wind damage on the ground. If that wind damage in the downburst is contained within a radius of 2.5 miles or less it's called a microburst. If the damage area extends beyond a 2.5 mile radius it's termed a macroburst. Both micro and macro bursts are common with the types of severe thunderstorms that occur locally. A portion of the City of Albany was hit by a damaging microburst on June 20, 2024.

Straight line wind damage from a rear flank dowburst (macroburst) on the hillside above S. Shore Road on the Great Sacandaga Lake in the Town of Day produced during the major July 16, 2024 severe thunderstorm and tornado event.

SEVERE THUNDERSTORM WATCH VS. WARNING - IT'S IMPORTANT TO KNOW THE DIFFERENCE

A SEVERE THUNDERSTORM WATCH is issued by the Storm Prediction Center in Norman, Oklahoma when conditions over a large geographical area either are or will soon become favorable for thunderstorms to form which could produce locally damaging straight line wind gusts to 58 mph or higher and/or 1" diameter hail (quarter sized) or larger. Once a severe thunderstorm watch is issued, the local National Weather Service offices in the affected region make the final determination on which local counties are included or excluded from the watch. Severe thunderstorm watches are generally in effect for up to six hours at a time and are used to alert you to the potential of dangerous weather conditions developing at a later time. A watch does not been severe weather is imminent, only possible. This is the period where you need to be weather alert and aware and be ready to move to a safe shelter in the event a severe thunderstorm develops in your area.

A SEVERE THUNDERSTORM WARNING is issued by local National Weather Service Forecast offices when Doppler radar indicates damaging wind (58 mph or greater) or large hail (1” diameter or greater) in a thunderstorm or ground spotters report damaging wind and/or large hail occurring. Severe thunderstorm warnings are issued for parts of counties along a forecast storm path and are generally in effect for thirty to sixty minutes at a time. A WARNING means that severe wind and/or hail is imminent. When a warning is issued, that's when you need to act and move to your severe thunderstorm safe place for shelter. The safest shelter from damaging wind and large hail is inside a sturdy structure in a location away from the windows. If you have a basement, I recommend you move there. If you do not have a basement, then an interior bathroom or closet on the lowest floor of the building that you are in is a good option. If you live in a manufactured home, which don't typically do well in strong wind, you should identify in advance a sturdy structure that you can move to when severe weather is developing. It's best to plan now, before severe weather threatens, so you're prepared and know what to do in the event it does.

The National Weather Service, in 2021, added damage tags to severe thunderstorm warnings to provide additional information on the destructive potential of the storm.

• Base: 60 mph winds and/or 1.00" diameter hail (quarter sized) is likely or being reported with the storm - This is the standard and most common warning that is issued.
  
• Considerable: 70 mph winds and/or 1.75" diameter hail (golf ball sized) is likely or being reported with the storm
  
• Destructive: 80 mph winds and/or 2.75" diameter hail (baseball sized) or larger is likely or being reported with the storm
  

UNIQUE CLOUD FORMATIONS ASSOCIATED WITH THUNDERSTORMS

MAMMATUS CLOUDS:
Mammatus clouds form under the anvil of cumulonimbus clouds as air descends out of the anvil either in advance of or after a thunderstorm has gone by. In this case pictured below, the mammatus clouds occurred on the flank of a set of storms that rolled through the Capital Region during the late afternoon and early evening on October 23, 2018. For this cloud formation to occur, the descending air must be colder and contain more water or ice than the environmental air that it is sinking into, allowing for that surrounding air to cool which causes condensation to occur and the cloud to develop. Mammatus are a rare cloud type that forms when air sinks versus most clouds which form due to rising air.

Mammatus clouds are often associated with severe thunderstorms as strong subsidence in the wake of such a storm can force down ice which evaporates or melts which subsequently cools the air allowing for mammatus to form. The cloud itself, however, does not indicate that severe weather is occurring or imminent. In the case pictured below, the thunderstorms that produced the mammatus clouds were not intense or severe. However, the air aloft was quite moist and cold allowing for the formation to develop as the parcels descended in the wake of the storms. Mammatus clouds can also form under cirrocumulus, altostratus, altocumulus and stratocumulus clouds, but are generally not as well defined in those cases.

SHELF CLOUD
The shelf cloud formation occurs at the boundary between a thunderstorm's downdraft and updraft. The formation occurs when rain cooled air flows out from the base of the thunderstorm acting as a mini cold front which then lifts the warm moist air in advance of the storm. As that warm moist air flows over the outflow boundary condensation occurs forming the cloud. Shelf clouds themselves are harmless, however, sometimes the wind that follows them can be destructive, but not always. The intensity of the outflow winds depends on the intensity of the thunderstorm downdraft. Shelf clouds can run well in advance of the thunderstorm or squall line that produced it.

WALL CLOUD
A cloud lowering will often occur in the updraft region of a thunderstorm as the rising air causes a lowering of the air pressure and thus allows for water vapor condensation to form at a lower altitude causing a cloud to form. Wall clouds can either rotate or not depending on the structure of the updraft. In the example below, the thunderstorm that produced the wall cloud over Albany was a supercell with a rotating updraft and thus produced a rotating wall cloud. If the circulation becomes intense enough a tornado can form from the wall cloud region. This storm, however, did not produce a tornado but did produce quite a bit of large hail and damaging straight line wind.