Northeast Severe Thunderstorms

The official National Weather Service definition of a severe thunderstorm that occurs in the Northeast is a storm that produces winds at or in excess of 58 mph and or three quarter inch or greater diameter hail. The definition of a severe thunderstorm does not include any criteria pertaining to volume and duration of rainfall or the amount of lightning it produces. Meteorologists are strictly concerned with damaging wind gusts and hail when considering whether or not a thunderstorm is severe. Often, but not always, severe thunderstorms will be prolific lightning producers as well. However, in the Northeast during the early spring, the fall, and even in rare cases during the winter, convection will occur producing wind or hail meeting the severe criteria while at the same time producing little or no lightning. In these cases, severe thunderstorm warnings will be issued to alert you to the danger of wind and hail even though lightning is not present.

THE OUTLOOK, WATCH, WARNING SYSTEM

The National Weather Service uses an OUTLOOK, WATCH, WARNING system to alert you to the potential and the occurrence of severe weather. The system seems to confuse many people, but it really is very simple

Severe Weather Outlook

Severe weather outlooks are issued by the Storm Prediction Center in Norman, Oklahoma out to three days in advance of severe weather in the form of severe thunderstorms and tornadoes, for all regions of the country. The outlook products are designed more for meteorologists rather than the public in that they give a general and somewhat technical overview of the prevailing patterns that could lead to severe weather in a given region. However, the outlooks issued by the Storm Prediction Center are publicly available on the Storm Prediction web site, at http://www.spc.noaa.gov. In cases when a widespread and particularly dangerous severe weather outbreak is forecast, a special public outlook statement is written and disseminated on both the Storm Prediction Center web site as well as through local NOAA weather radio.

Severe Thunderstorm Watch

A SEVERE THUNDERSTORM WATCH is issued in advance of severe thunderstorms breaking out in a given region. A WATCH covers a large multi-county area in one or more states, and is normally in effect for six hours. When a WATCH is issued by the Storm Prediction Center, in Norman, Oklahoma, meteorologists at the center have determined that enough atmospheric parameters have come together over a region to support the development of organized severe weather. The WATCH is your first line of defense because it alerts you to the potential of damaging thunderstorms in your area during a given time period. A WATCH, however, does not mean severe weather is happening now. Since severe thunderstorms are normally small and affect very localized areas with severe weather, it is common for large parts of a watch area to be unaffected by thunderstorms, which is unfortunately why many people pay little attention to severe thunderstorm watches when they are issued.

Severe Thunderstorm Warning

A SEVERE THUNDERSTORM WARNING is issued when Doppler weather radar is indicating a storm is on the verge of or is producing damaging wind and or large hail or when ground spotters report a storm is producing severe weather. A WARNING is issued for a county or part of a county and means you should take cover immediately because severe weather is imminent. Due to the fast developing and fast moving nature of severe thunderstorms there may be little time between the official warning and the storm hitting your location which makes it very important for you to make your storm preparations during the WATCH period when there is time. DO NOT wait for an official warning before you take precautions. If you see threatening weather approaching your location and a WATCH is in effect, take the weather seriously and head for cover. You can find the latest National Weather Service WATCHES and WARNINGS on this site, by listening to NOAA weather radio, or by watching WRGB Channel 6. The WRGB Channel 6 weather facility is the most well equipped broadcast weather operation in upstate New York and western New England. Our severe weather diagnosing and coverage equipment includes an advanced severe weather information crawl alert system that instantly triggers upon receiving a warning. When a warning is issued the system immediately displays an on air text alert as well as animating the warned county or counties, that the severe weather either is or will be affecting. To diagnose and track severe weather meteorologists at WRGB Channel 6 use the first live broadcast Doppler radar in the area, Instant Doppler 6, along with the Instant Doppler 6 Storm Alert street level mapping and storm tracking system, as well as the WeatherScan analysis system.

TYPES OF SEVERE THUNDERSTORMS

The Single Cell Storm

The typical garden-variety single cell thunderstorm develops on a hot humid day in an atmosphere featuring very little wind with height. We have all experienced the quick summertime thundershower that brings a fast dousing of rain and drop in temperature. Theses quick thundershowers are an example of the single cell storm. These ordinary thunderstorms form as a result of atmospheric instability. In other words the air aloft is sufficiently cold enough to support a rising thermal of air from the ground. As the thermal rises, the air cools causing water vapor to condense into liquid droplets which forms the cumulus cloud. The cloud continues to grow if its environment is cold and moist enough to further support development. If the growing cloud reaches a warm dry layer in the upper atmosphere it will stop growing and begin evaporating. On a good day, the cumulus cloud will continue to grow, ice crystals will form at the top of the cloud and water droplets at the bottom. As electrical charges separate, lightning will be initiated. The thundershower begins to die when its updraft is no longer able to suspend the growing water droplets in the cloud. Heavy rain then falls dragging down cool air that is felt as a strong gust at the ground. The downdraft cools the air at the ground killing off the thunderstorm's source of warm moist air. The storm then dies out. The entire life span of the typical garden-variety thunderstorm is about thirty minutes.

Typically, single cell thunderstorms do not produce severe weather. Once in awhile, however, the down draft in a collapsing single cell storm will produce a brief downburst or microburst of wind over an extremely localized area that will cause some wind damage. Severe Thunderstorm watches will not be issued to cover the potential of isolated downburst winds from scattered single cell storms.

The Multicell Severe Storm

The multicell thunderstorm accounts for the majority of thunderstorms that occur worldwide. A small percentage of these multicell storms will produce large hail and damaging winds. Multicell thunderstorms form in unstable atmospheric environments where the down draft from a dying cell converges with warm moist air to force another updraft on the flank of the storm, which forms a new storm tower, perpetuating the system. As long as a supply of warm moist air continues in a region, a multicell thunderstorm system will continue to grow, lasting up to several hours. The atmospheric conditions that will cause a non-severe multicell storm to become severe include strong wind fields aloft, as well as a pocket of dry air in the midlevels of the atmosphere. As cloud and rain droplets fall into the pool of mid-level dry air, the droplets evaporate cooling the air. The rain-cooled air becomes heavy and rushes downward as part of the storm's downdraft, increasing the velocity sufficiently for it to produce damage when it hits the ground. Strong winds aloft may also be mixed down to the ground as part of the downdraft, further enhancing the surface wind speeds. Severe thunderstorm wind gusts out of multicell severe storms can exceed 100 mph producing tornado like damage. Multicell severe storms may last several hours producing periodic severe weather over localized areas.

The Supercell Thunderstorm

The supercell thunderstorm is the least frequent to occur but the most efficient and most violent. Giant hail, damaging surface winds, and in some cases, long lived major tornadoes, are all common prodigy of supercell thunderstorms. Strong atmospheric wind shear, which is a change in direction and speed of the wind with height, is necessary for a supercell thunderstorm to form. Normally, a supercell begins as a multicell storm, forming in a wind sheared environment. The wind shear begins twisting the updraft as it develops. As the updraft continues to intensify in an environment where the wind speed increases and changes its direction with height, it begins to spin, usually in a cyclonic direction. The spinning updraft and simultaneous evacuation of air under the developing storm forms an area of low pressure at the center of the storm, called a mesocyclone. The mature supercell will have a strong rotating updraft in conjunction with several down drafts. In the case of a supercell, the storm's down drafts of rain cooled air flowing out into the updraft region of the storm actually intensify the updraft sustaining the storm. Because of the steady state relationship between the supercells's updraft and down drafts, the storm may last for up to six of more hours producing continuous wind damage, hail, and tornadoes. Hail sizes up to grapefruits have been observed with supercell thunderstorms. And tornadoes form in approximately 60% of all supercell storms. The tornado typically forms at the location of the mesocyclone from a lowered cloud base called the wall cloud. The wall cloud is the spinning wall of clouds surrounding the mesocyclone center.

In the Northeast, either a severe thunderstorm or tornado watch will be issued in advance of supercell thunderstorm development. Supercells in the Northeast are rare, but they do occur every season. The Memorial day 1995 tornadoes in Hillsdale, NY and in Great Barrington, MA were produced by a single supercell thunderstorm and the outbreak of severe weather on May 31, 1998 that produced the devastating Mechanicville/Stillwater tornado was comprised almost exclusively of supercell thunderstorms.

The Squall Line

A squall line is a narrow line or band of active thunderstorms containing any combination of severe or non-severe multicell or supercell storms. Squall lines normally form as individual multicell or supercell storms which grow and expand into a solid line. As the line intensifies, air pressure drops and surface winds increase into the line helping to sustain it. You may have noticed on some hot humid afternoons, the winds steadily and dramatically increasing from the south or southeast. That increasing wind is very often in response to the dropping air pressure associated with a developing squall line over central or western New York. As the individual storms in the squall line continue to organize into one solid storm front, a solid down draft forms along the leading edge of the line. If the squall line is severe, the winds in the down draft can produce widespread damage over a vast area as it moves forward. Hail and tornadoes are also possible in intense squall lines.

The Convective Complex

Complex thunderstorm systems, technically termed the mesoscale convective complex (MCC) or the mesoscale convective system (MCS), are a large grouping of thunderstorms which may cover several hundred miles over six to twenty four hours as they propagate. These thunderstorm systems are often spherical in appearance on a satellite picture and cover a large area. Most MCS's affect the central United States, rarely the Northeast. However, in July 1995 an MCS (The Great Adirondack Blow Down or as it is otherwise known The Super Derecho) brought widespread wind damage to New York and New England. Another MCS (derecho) moved down the Mohawk valley during the pre-dawn hours on Labor Day, 1998 producing widespread wind damage. A severe thunderstorm or tornado watch is normally issued when an MCS is expected to develop, as the potential for widespread wind and hail damage as well as tornadoes is great with these systems

Steve LaPointe

WRGB Chief Meteorologist